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L_0475 | cell biology | T_2802 | Most cells are so small that you cannot see them without the help of a microscope. It was not until 1665 that English scientist Robert Hooke invented a basic light microscope and observed cells for the first time, by looking at a piece of cork. You may use light microscopes in the classroom. You can use a light microscope to see cells ( Figure 1.1). But many structures in the cell are too small to see with a light microscope. So, what do you do if you want to see the tiny structures inside of cells? In the 1950s, scientists developed more powerful microscopes. A light microscope sends a beam of light through a specimen, or the object you are studying. A more powerful microscope, called an electron microscope, passes a beam of electrons through the specimen. Sending electrons through a cell allows us to see its smallest parts, even the parts inside the cell ( Figure 1.2). Without electron microscopes, we would not know what the inside of a cell looked like. The outline of onion cells are visible under a light microscope. | text | null |
L_0475 | cell biology | T_2802 | Most cells are so small that you cannot see them without the help of a microscope. It was not until 1665 that English scientist Robert Hooke invented a basic light microscope and observed cells for the first time, by looking at a piece of cork. You may use light microscopes in the classroom. You can use a light microscope to see cells ( Figure 1.1). But many structures in the cell are too small to see with a light microscope. So, what do you do if you want to see the tiny structures inside of cells? In the 1950s, scientists developed more powerful microscopes. A light microscope sends a beam of light through a specimen, or the object you are studying. A more powerful microscope, called an electron microscope, passes a beam of electrons through the specimen. Sending electrons through a cell allows us to see its smallest parts, even the parts inside the cell ( Figure 1.2). Without electron microscopes, we would not know what the inside of a cell looked like. The outline of onion cells are visible under a light microscope. | text | null |
L_0475 | cell biology | T_2803 | In 1858, after using microscopes much better than Hookes first microscope, Rudolf Virchow developed the hypoth- esis that cells only come from other cells. For example, bacteria, which are single-celled organisms, divide in half (after they grow some) to make new bacteria. In the same way, your body makes new cells by dividing the cells you already have. In all cases, cells only come from cells that have existed before. This idea led to the development of one of the most important theories in biology, the cell theory. Cell theory states that: 1. All organisms are composed of cells. 2. Cells are alive and the basic living units of organization in all organisms. 3. All cells come from other cells. As with other scientific theories, many hundreds, if not thousands, of experiments support the cell theory. Since Virchow created the theory, no evidence has ever been identified to contradict it. | text | null |
L_0475 | cell biology | T_2804 | Although cells share many of the same features and structures, they also can be very different ( Figure 1.3). Each cell in your body is designed for a specific task. In other words, the cells function is partly based on the cells structure. For example: Red blood cells are shaped with a pocket that traps oxygen and brings it to other body cells. Nerve cells are long and stringy in order to form a line of communication with other nerve cells, like a wire. Because of this shape, they can quickly send signals, such as the feeling of touching a hot stove, to your brain. Skin cells are flat and fit tightly together to protect your body. As you can see, cells are shaped in ways that help them do their jobs. Multicellular (many-celled) organisms have many types of specialized cells in their bodies. Red blood cells (left) are specialized to carry oxygen in the blood. Neurons (cen- ter ) are shaped to conduct electrical im- pulses to many other nerve cells. These epidermal cells (right) make up the skin of plants. Note how the cells fit tightly together. | text | null |
L_0475 | cell biology | T_2805 | While cells are the basic units of an organism, groups of cells can perform a job together. These cells are called specialized because they have a special job. Specialized cells can be organized into tissues. For example, your liver cells are organized into liver tissue. Your liver tissue is further organized into an organ, your liver. Organs are formed from two or more specialized tissues working together to perform a job. All organs, from your heart to your liver, are made up of an organized group of tissues. These organs are part of a larger system, the organ systems. For example, your brain works together with your spinal cord and other nerves to form the nervous system. This organ system must be organized with other organ systems, such as the circulatory system and the digestive system, for your body to work. Organ systems work together to form the entire organism. There are many levels of organization in living things ( Figure 1.4). Levels of organization, from the atom (smallest) to the organism (largest). Notice that organelles are inside a cell, and organs are inside an organ- ism. | text | null |
L_0476 | cell cycle | T_2806 | The process of cell division in eukaryotic cells is carefully controlled. The cell cycle ( Figure 1.1) is the life cycle of an eukaryotic cell, with cell division at the end of the cycle. Like a human life cycle, which is made up of different phases, like childhood, adolescence, and adulthood, the cell cycle also occurs in a series of phases. The first cell cycle begins with the formation of a zygote from the fusion of a male and female sex cell ( gamete). The steps of the cell cycle can be divided into two main components: interphase and the mitotic phase. Interphase is the stage when the cell mostly performs its everyday functions. For example, it is when a kidney cell does what a kidney cell is supposed to do. The cell also gets ready to divide during this time. The cell divides during the mitotic phase, which consists of mitosis and cytokinesis. Most of the cell cycle consists of interphase, the time between cell divisions. Interphase can be divided into three stages: 1. The first growth phase (G1): During the G1 stage, the cell doubles in size and doubles the number of organelles. 2. The synthesis phase (S): The DNA is replicated during this phase. In other words, an identical copy of all the cells DNA is made. This ensures that each new cell has a set of genetic material identical to that of the parental cell. This process is called DNA replication. 3. The second growth phase (G2): Proteins are synthesized that will help the cell divide. At the end of interphase, the cell is ready to enter mitosis. Shown is the cell cycle. Notice that most of the cell cycle is spent in Inter- phase (G1, S, and G2). Mitosis and cy- tokinesis occur during the Mitotic phase. Some cells may enter a resting phase dur- ing which progression through the cycle stops. During mitosis, the nucleus divides as the chromosomes are equally separated. One nucleus becomes two nuclei, each with an identical set of chromosomes. Mitosis is followed by cytokinesis, when the cytoplasm divides, | text | null |
L_0477 | cell division | T_2807 | Imagine the first stages of life. In humans and other animals, a sperm fertilizes an egg, forming the first cell. But humans are made up of trillions of cells, so where do the new cells come from? Remember that according to the cell theory, all cells come from existing cells. Once a sperm and egg cell unite and the first cell, called a zygote, forms, an entire baby will develop. And each cell in that baby will be genetically identical, meaning that each cell will have exactly the same DNA. How does a new life go from one cell to so many? The cell divides in half, creating two cells. Then those two cells divide, for a total of four cells. The new cells continue to divide and divide. One cell becomes two, then four, then eight, and so on ( Figure 1.1). This continual process of a cell dividing and creating two new cells is known as cell division. Cell division is part of a cycle of cellular growth and division known as the cell cyclecells must grow before they divide. The cell cycle describes the "life" of a eukayrotic cell. In addition to cell division, the cell cycle includes the division of the nucleus and the cytoplasm. Most cell division produces genetically identical cells, meaning they have the same DNA. The process of mitosis, which specifically is the division of the nucleus, ensures that each cell has the same DNA. During mitosis, the chromosomes equally separate, thus making sure each nucleus in each resulting cell after cell division is genetically identical. A special form of cell division, called meiosis, produces cells with half as much DNA as the parent cell. These cells are used for reproduction. In prokaryotic organisms, cell division is how those organisms reproduce. Besides the development of a baby, there are many other reasons that cell division is necessary for life: Cells divide repeatedly to produce an em- bryo. Previously the one-celled zygote (the first cell of a new organism) divided to make two cells (a). Each of the two cells divides to yield four cells (b), then the four cells divide to make eight cells (c), and so on. Through cell division, an entire embryo forms from one initial cell. 1. To grow and develop, you must form new cells. Imagine how often your cells must divide during a growth spurt. Growing just an inch requires countless cell divisions. Your body must produce new bone cells, new skin cells, new cells in your blood vessels and so on. 2. Cell division is also necessary to repair damaged cells. Imagine you cut your finger. After the scab forms, it will eventually disappear and new skin cells will grow to repair the wound. Where do these cells come from? Some of your existing skin cells divide and produce new cells. 3. Your cells can also simply wear out. Over time you must replace old and worn-out cells. Cell division is essential to this process. | text | null |
L_0478 | cell membrane | T_2808 | If the outside environment of a cell is water-based, and the inside of the cell is also mostly water, something has to make sure the cell stays intact in this environment. What would happen if a cell dissolved in water, like sugar does? Obviously, the cell could not survive in such an environment. So something must protect the cell and allow it to survive in its water-based environment. All cells have a barrier around them that separates them from the environment and from other cells. This barrier is called the plasma membrane, or cell membrane. | text | null |
L_0478 | cell membrane | T_2809 | The plasma membrane ( Figure 1.1) is made of a double layer of special lipids, known as phospholipids. The phospholipid is a lipid molecule with a hydrophilic ("water-loving") head and two hydrophobic ("water-hating") tails. Because of the hydrophilic and hydrophobic nature of the phospholipid, the molecule must be arranged in a specific pattern as only certain parts of the molecule can physically be in contact with water. Remember that there is water outside the cell, and the cytoplasm inside the cell is mostly water as well. So the phospholipids are arranged in a double layer (a bilayer) to keep the cell separate from its environment. Lipids do not mix with water (recall that oil is a lipid), so the phospholipid bilayer of the cell membrane acts as a barrier, keeping water out of the cell, and keeping the cytoplasm inside the cell. The cell membrane allows the cell to stay structurally intact in its water-based environment. The function of the plasma membrane is to control what goes in and out of the cell. Some molecules can go through the cell membrane to enter and leave the cell, but some cannot. The cell is therefore not completely permeable. "Permeable" means that anything can cross a barrier. An open door is completely permeable to anything that wants to enter or exit through the door. The plasma membrane is semipermeable, meaning that some things can enter the cell, and some things cannot. | text | null |
L_0478 | cell membrane | T_2810 | The inside of all cells also contain a jelly-like substance called cytosol. Cytosol is composed of water and other molecules, including enzymes, which are proteins that speed up the cells chemical reactions. Everything in the cell sits in the cytosol, like fruit in a jello mold. The term cytoplasm refers to the cytosol and all of the organelles, the specialized compartments of the cell. The cytoplasm does not include the nucleus. As a prokaryotic cell does not have a nucleus, the DNA is in the cytoplasm. | text | null |
L_0479 | cell nucleus | T_2811 | The nucleus is only found in eukaryotic cells. It contains most of the genetic material (the DNA) of the cell. The genetic material of the nucleus is like a set of instructions. These instructions tell the cell how to build molecules needed for the cell to function properly. That is, the DNA tells the cell how to build molecules needed for life. The nucleus is surrounded by the nuclear envelope, a double membrane (two phospholipid bilayers) that controls what goes in and out of the nucleus. The nucleus also has holes embedded in the nuclear envelope. These holes are known as nuclear pores, and they allow things to flow in and out of the nucleus. | text | null |
L_0479 | cell nucleus | T_2812 | Inside of the nucleus, you will find the chromosomes. Chromosomes are strands of DNA wrapped around proteins. They contain genes, or small units of genetic material (DNA) that contains the code for the creation of a protein. Human cells have 46 chromosomes (23 pairs). There are hundreds to thousands of genes on each chromosome. | text | null |
L_0479 | cell nucleus | T_2813 | The nucleus of many cells also contains a central region called the nucleolus. The job of the nucleolus is to build ribosomes. These ribosomes flow out the nuclear pores into the cytoplasm. Ribosomes are organelles that make proteins in the cytoplasm. See the composition of the nucleus pictured below ( Figure 1.1). | text | null |
L_0480 | cell transport | T_2814 | Cells are found in all different types of environments, and these environments are constantly changing. For example, one-celled organisms, like bacteria, can be found on your skin, in the ground, or in all different types of water. Therefore, cells need a way to protect themselves. This job is done by the cell membrane, which is also known as the plasma membrane. | text | null |
L_0480 | cell transport | T_2815 | The cell membrane is semipermeable, or selectively permeable, which means that only some molecules can pass through the membrane. If the cell membrane were completely permeable, the inside of the cell would be the same as the outside of the cell. It would be impossible for the cell to maintain homeostasis. Homeostasis means maintaining a stable internal environment. For example, if your body cells have a temperature of 98.6 F, and it is freezing outside, your cells will maintain homeostasis if the temperature of the cells stays the same and does not drop with the outside temperature. How does the cell ensure it is semipermeable? How does the cell control what molecules enter and leave the cell? The composition of the cell membrane helps to control what can pass through it. | text | null |
L_0480 | cell transport | T_2816 | Molecules in the cell membrane allow it to be semipermeable. The membrane is made of a double layer of phospholipids (a "bilayer") and proteins ( Figure below). Recall that phospholipids, being lipids, do not mix with water. It is this quality that allows them to form the outside barrier of the cell. A single phospholipid molecule has two parts: 1. A polar head that is hydrophilic, or water-loving. 2. A fatty acid tail that is hydrophobic, or water-fearing. The cell membrane is made up of a phos- pholipid bilayer, two layers of phospho- lipid molecules. Notice the polar head group of the phospholipid is attached to the phosphate, and the tails are two fatty acid chains. The head group and tails are attached by a glycerol backbone. There is water found on both the inside and the outside of cells. Since hydrophilic means water-loving, and they want to be near water, the heads face the inside and outside of the cell where water is found. The water-fearing, hydrophobic tails face each other in the middle of the cell membrane, because water is not found in this space. The phospholipid bilayer allows the cell to stay intact in a water-based environment. An interesting quality of the plasma membrane is that it is very "fluid" and constantly moving, like a soap bubble. This fluid nature of the membrane is important in maintaining homeostasis. It allows the proteins in the membrane to float to areas where they are needed. Due to the composition of the cell membrane, small molecules such as oxygen and carbon dioxide can pass freely through the membrane, but other molecules, especially large molecules, cannot easily pass through the plasma membrane. These molecules need assistance to get across the membrane. That assistance will come in the form of transport proteins. | text | null |
L_0484 | characteristics of life | T_2828 | How do you define a living thing? What do mushrooms, daisies, cats, and bacteria have in common? All of these are living things, or organisms. It might seem hard to think of similarities among such different organisms, but they actually have many properties in common. Living organisms are similar to each other because all organisms evolved from the same common ancestor that lived billions of years ago. All living organisms: 1. Need energy to carry out life processes. 2. Are composed of one or more cells. 3. Respond to their environment. 4. Grow and reproduce. 5. Maintain a stable internal environment. | text | null |
L_0484 | characteristics of life | T_2829 | Why do you eat everyday? To get energy. Energy is the ability to do work. Without energy, you could not do any "work." Though not doing any "work" may sound nice, the "work" fueled by energy includes everyday activities, such as walking, writing, and thinking. But you are not the only one who needs energy. In order to grow and reproduce and carry out the other process of life, all living organisms need energy. But where does this energy come from? The source of energy differs for each type of living thing. In your body, the source of energy is the food you eat. Here is how animals, plants, and fungi obtain their energy: All animals must eat in order to obtain energy. Animals also eat to obtain building materials. Animals eat plants and other animals. Plants dont eat. Instead, they use energy from the sun to make their "food" through the process of photosyn- thesis. Mushrooms and other fungi obtain energy from other organisms. Thats why you often see fungi growing on a fallen tree; the rotting tree is their source of energy ( Figure 1.1). Since plants harvest energy from the sun and other organisms get their energy from plants, nearly all the energy of living things initially comes from the sun. Orange bracket fungi on a rotting log in the Oak Openings Preserve in Ohio. Fungi obtain energy from breaking down dead organisms, such as this rotting log. | text | null |
L_0484 | characteristics of life | T_2830 | If you zoom in very close on a leaf of a plant, or on the skin on your hand, or a drop of blood, you will find cells, you will find cells ( Figure 1.2). Cells are the smallest structural and functional unit of all living organisms. Most cells are so small that they are usually visible only through a microscope. Some organisms, like bacteria, plankton that live in the ocean, or the Paramecium, shown in Figure 1.3, are unicellular, made of just one cell. Other organisms have millions, billions, or trillions of cells. All cells have at least some structures in common, such as ribosomes, which are the sites where proteins are made. All cells also have DNA and proteins. The nucleus is clearly visible in the blood cells ( Figure 1.2). The nucleus can be described as the "information center," containing the instructions (DNA) for making all the proteins in a cell, as well as how much of each protein to make. The nucleus is also the main distinguishing feature between the two general categories of cell, with cells known as prokaryotic lacking a nucleus. Although the cells of different organisms are built differently, they all have certain general functions. Every cell must get energy from food, be able to grow and divide, and respond to its environment. More about cell structure and function will be discussed in additional concepts. | text | null |
L_0484 | characteristics of life | T_2830 | If you zoom in very close on a leaf of a plant, or on the skin on your hand, or a drop of blood, you will find cells, you will find cells ( Figure 1.2). Cells are the smallest structural and functional unit of all living organisms. Most cells are so small that they are usually visible only through a microscope. Some organisms, like bacteria, plankton that live in the ocean, or the Paramecium, shown in Figure 1.3, are unicellular, made of just one cell. Other organisms have millions, billions, or trillions of cells. All cells have at least some structures in common, such as ribosomes, which are the sites where proteins are made. All cells also have DNA and proteins. The nucleus is clearly visible in the blood cells ( Figure 1.2). The nucleus can be described as the "information center," containing the instructions (DNA) for making all the proteins in a cell, as well as how much of each protein to make. The nucleus is also the main distinguishing feature between the two general categories of cell, with cells known as prokaryotic lacking a nucleus. Although the cells of different organisms are built differently, they all have certain general functions. Every cell must get energy from food, be able to grow and divide, and respond to its environment. More about cell structure and function will be discussed in additional concepts. | text | null |
L_0484 | characteristics of life | T_2831 | All living organisms are able to react to something important or interesting in their external environment. For example, living organisms constantly respond to their environment. They respond to changes in light, heat, sound, and chemical and mechanical contact. Organisms have means for receiving information, such as eyes, ears, taste buds, or other structures. | text | null |
L_0484 | characteristics of life | T_2832 | All living things reproduce to make the next generation. Organisms that do not reproduce will go extinct. As a result, there are no species that do not reproduce ( Figure 1.4). Some organisms reproduce asexually ( asexual reproduction), especially single-celled organisms, and make identical copies (or clones) of themselves. Other organisms reproduce sexually ( sexual reproduction), combining genetic information from two parents to make genetically unique offspring. | text | null |
L_0484 | characteristics of life | T_2833 | When you are cold, what does your body do to keep warm? You shiver to warm up your body. When you are too warm, you sweat to release heat. When any living organism gets thrown off balance, its body or cells help it return to normal. In other words, living organisms have the ability to keep a stable internal environment. Maintaining a balance inside the body or cells of organisms is known as homeostasis. Like us, many animals have evolved behaviors that control their internal temperature. A lizard may stretch out on a sunny rock to increase its internal temperature, and a bird may fluff its feathers to stay warm ( Figure 1.5). A bird fluffs its feathers to stay warm and to maintain homeostasis. | text | null |
L_0490 | cloning | T_2854 | Cloning is the process of creating an exact genetic replica of an organism. The clones DNA is exactly the same as the parents DNA. Bacteria and other single-celled organisms have long been able to clone themselves through asexual reproduction. Plants can also reproduce asexually. In animals, however, cloning does not happen naturally. In 1997, that all changed when a sheep named Dolly was the first large mammal ever to be successfully cloned. Other animals can now also be cloned in a laboratory. The process of producing an animal like Dolly starts with a single cell from the animal that is going to be cloned. Below are the steps involved in the process of cloning: 1. In the case of Dolly, cells from the mammary glands were taken from the adult that was to be cloned. But other somatic cells can be used. Somatic cells come from the body and are not gametes like sperm or egg. 2. The nucleus is removed from this cell. 3. The nucleus is placed in a donor egg that has had its nucleus removed. The nucleus must be removed from the donor egg to maintain the appropriate chromosome number. 4. The new cell is stimulated with an electric shock and embryo development begins, as if it were a normal zygote. The zygote is the first cell of a new organism. 5. The resulting embryo is implanted into a mother sheep, where it continue its development ( Figure 1.1). To clone an animal, a nucleus from the animals cells are fused with an egg cell (from which the nucleus has been re- moved) from a donor, creating a new zy- gote. | text | null |
L_0490 | cloning | T_2855 | Cloning is not always successful. Most of the time, this cloning process does not result in a healthy adult animal. The process has to be repeated many times until it works. In fact, 277 tries were needed to produce Dolly. This high failure rate is one reason that human cloning is banned in the United States. In order to produce a cloned human, many attempts would result in the surrogate mothers experiencing miscarriages, stillbirths, or deformities in the infant. There are also many additional ethical considerations related to human cloning. Can you think of reasons why people are for or against cloning? | text | null |
L_0493 | components of blood | T_2860 | Did you know that blood is a tissue? Blood is a fluid connective tissue that is made up of red blood cells, white blood cells, platelets, and plasma. The cells that make up blood are pictured below ( Figure 1.1). The different parts of blood have different roles. A scanning electron microscope (SEM) image of human blood cells. Red blood cells are the flat, bowl-shaped cells, the tiny disc-shaped pieces are platelets, and white blood cells are the round cells shown in the center. | text | null |
L_0493 | components of blood | T_2861 | If you were to filter out all the cells in blood, a golden-yellow liquid would be left behind. Plasma is this fluid part of the blood. Plasma is about 90% water and about 10% dissolved proteins, glucose, ions, hormones, and gases. Blood is made up mostly of plasma. | text | null |
L_0493 | components of blood | T_2862 | Red blood cells (RBCs) are flattened, disk-shaped cells that carry oxygen. They are the most common blood cell in the blood. There are about 4 to 6 million RBCs per cubic millimeter of blood. Each RBC has about 200 million molecules of hemoglobin. Hemoglobin is the protein that carries oxygen. Hemoglobin also gives the red blood cells their red color. Red blood cells ( Figure 1.2) are made in the red marrow of long bones, rib bones, the skull, and vertebrae. Each red blood cell lives for only 120 days (about four months). After this time, they are destroyed in the liver and spleen. Mature red blood cells do not have a nucleus or other organelles. Lacking these components allows the cells to have more hemoglobin and carry more oxygen. The flattened shape of red blood cells helps them carry more oxygen than if they were rounded. | text | null |
L_0493 | components of blood | T_2863 | White blood cells (WBCs) are usually larger than red blood cells. They do not have hemoglobin and do not carry oxygen. White blood cells make up less than one percent of the bloods volume. Most WBCs are made in the bone marrow, and some mature in the lymphatic system. There are different WBCs with different jobs. WBCs defend the body against infection by bacteria, viruses, and other pathogens. WBCs do have a nucleus and other organelles. Neutrophils are WBCs that can squeeze through capillary walls and swallow particles such as bacteria and parasites. Macrophages are large WBCs that can also swallow and destroy old and dying cells, bacteria, or viruses. Below, a macrophage is attacking and swallowing two particles, possibly disease-causing pathogens ( Figure Lymphocytes are WBCs that fight infections caused by viruses and bacteria. Some lymphocytes attack and kill cancer cells. Lymphocytes called B-cells make antibodies. A type of white blood cell, called a macrophage, is attacking a cancer cell. | text | null |
L_0493 | components of blood | T_2864 | Platelets ( Figure 1.4) are very small, but they are very important in blood clotting. Platelets are not cells. They are sticky little pieces of larger cells. Platelets bud off large cells that stay in the bone marrow. When a blood vessel gets cut, platelets stick to the injured areas. They release chemicals called clotting factors, which cause proteins to form over the wound. This web of proteins catches red blood cells and forms a clot. This clot stops more blood from leaving the body through the cut blood vessel. The clot also stops bacteria from entering the body. Platelets survive in the blood for ten days before they are removed by the liver and spleen. | text | null |
L_0500 | diffusion | T_2884 | Small molecules can pass through the plasma membrane through a process called diffusion. Diffusion is the movement of molecules from an area where there is a higher concentration (larger amount) of the substance to an area where there is a lower concentration (lower amount) of the substance ( Figure 1.1). The amount of a substance in relation to the total volume is the concentration. During diffusion, molecules are said to flow down their concentration gradient, flowing from an area of high concentration to an area of low concentration. Molecules flowing down a concentration gradient is a natural process and does not require energy. Diffusion can occur across a semipermeable membrane, such as the cell membrane, as long as a concentration gradient exists. Molecules will continue to flow in this manner until equilibrium is reached. At equilibrium, there is no longer an area of high concentration or low concentration, and molecules flow equally in both directions across the semipermeable membrane. At equilibrium, equal amounts of a molecule are entering and leaving a cell. Diffusion is the movement of a substance from an area of a higher amount toward an area of lower amount. A concentra- tion gradient initially exists across the cell membrane. Equilibrium is reached when there is an equal amount of the substance on both sides of the membrane. | text | null |
L_0500 | diffusion | T_2885 | The diffusion of water across a membrane because of a difference in concentration is called osmosis. Lets explore three different situations and analyze the flow of water. 1. A hypotonic solution means the environment outside of the cell has a lower concentration of dissolved material than the inside of the cell. If a cell is placed in a hypotonic solution, water will move into the cell. This causes the cell to swell, and it may even burst. 2. A hypertonic solution means the environment outside of the cell has more dissolved material than inside of the cell. If a cell is placed in a hypertonic solution, water will leave the cell. This can cause a cell to shrink and shrivel. 3. An isotonic solution is a solution in which the amount of dissolved material is equal both inside and outside of the cell. Water still flows in both directions, but an equal amount enters and leaves the cell. | text | null |
L_0500 | diffusion | T_2886 | How do marine animals keep their cells from shrinking? How do you keep your blood cells from bursting? Both of these questions have to do with the cell membrane and osmosis. Marine animals live in salt water, which is a hypertonic environment; there is more salt in the water than in their cells. To prevent losing too much water from their bodies, these animals intake large quantities of salt water and then secrete the excess salt. Red blood cells can be kept from bursting or shriveling if put in a solution that is isotonic to the blood cells. If the blood cells were put in pure water, the solution would be hypotonic to the blood cells, so water would enter the blood cells, and they would swell and burst ( Figure 1.2). Osmosis causes these red blood cells to change shape by losing or gaining water. | text | null |
L_0504 | dna structure and replication | T_2900 | DNA must replicate (copy) itself so that each resulting cell after mitosis and cell division has the same DNA as the parent cell. All these cells, the parent cell and the two new daughter cells, are genetically identical. DNA replication occurs during the S phase (the Synthesis phase) of the cell cycle, before mitosis and cell division. The base pairing rules are crucial for the process of replication. DNA replication occurs when DNA is copied to form an identical molecule of DNA. The general steps involved in DNA replication are as follows: 1. The DNA helix unwinds like a zipper as the bonds between the base pairs are broken. The enzyme DNA Helicase is involved in breaking these bonds. 2. The two single strands of DNA then each serve as a template for a new stand to be created. Using DNA as a template means that on the new strand, the bases are placed in the correct order because of the base pairing rules. Recall that A and T are complementary bases, as are G and C. As a template strand is read, the new strand is created. If ATGCCA is on the "template strand," then TACGGT will be on the new DNA strand. The enzyme DNA Polymerase reads the template and builds the new strand of DNA. 3. The new set of nucleotides then join together to form a new strand of DNA. The process results in two DNA molecules, each with one old strand and one new strand of DNA. This process is known as semiconservative replication because one strand is conserved (kept the same) in each new DNA molecule ( Figure 1.1). DNA replication occurs when the DNA strands unzip, and the original strands of DNA serve as a template for new nucleotides to join and form a new strand. | text | null |
L_0506 | domains of life | T_2905 | Lets explore the domain, the least specific category of classification. All of life can be divided into three domains, based on the type of cell of the organism: 1. Bacteria: cells do not contain a nucleus. 2. Archaea: cells do not contain a nucleus; they have a different cell wall from bacteria. 3. Eukarya: cells do contain a nucleus. | text | null |
L_0506 | domains of life | T_2906 | The Archaea and Bacteria domains ( Figure 1.1) are both entirely composed of small, single-celled organisms and seem very similar, but they also have significant differences. Both are composed of prokaryotic cells, which are cells without a nucleus. In addition, both domains are composed of species that reproduce asexually ( asexual reproduction) by dividing in two. Both domains also have species with cells surrounded by a cell wall, however, the cell walls are made of different materials. Bacterial cell walls contain the polysaccharide peptidoglycan. Lastly, Archaea often live in extreme environments including hot springs, geysers, and salt flats. Bacteria do not live in these environments. The Group A Streptococcus organism (left) is in the domain Bacteria, one of the three domains of life. The Halobacterium (right) is in the domain Archaea, another one of the three domains. | text | null |
L_0506 | domains of life | T_2907 | All of the cells in the domain Eukarya keep their genetic material, or DNA, inside the nucleus. The domain Eukarya is made up of four kingdoms: 1. Plantae: Plants, such as trees and grasses, survive by capturing energy from the sun, a process called photo- synthesis. 2. Fungi: Fungi, such as mushrooms and molds, survive by "eating" other organisms or the remains of other organisms. These organisms absorb their nutrients from other organisms. 3. Animalia: Animals also survive by eating other organisms or the remains of other organisms. Animals range from tiny ants to the largest whales, and include arthropods, fish, amphibians, reptiles, and mammals ( Figure 4. Protista: Protists are not all descended from a single common ancestor in the way that plants, animals, and fungi are. Protists are all the eukaryotic organisms that do not fit into one of the other three kingdoms. They include many kinds of microscopic one-celled (unicellular) organisms, such as algae and plankton, but also giant seaweeds that can grow to be 200 feet long. Plants, animals, fungi, and protists might seem very different, but remember that if you look through a microscope, you will find similar cells with a membrane-bound nucleus in all of them. These are eukaryotic cells. These cells also have membrane-bound organelles, which prokaryotic cells lack. The main characteristics of the three domains of life are summarized in Table 1.1. Multicelluar Cell wall Nucleus (Membrane- Enclosed DNA) Membrane-Bound Organelles Archaea No Yes, without peptidogly- can Bacteria No Yes, with peptidoglycan No No Eukarya Yes Varies. Plants and fungi have a cell wall; animals do not. Yes No No Yes Diversity of Animals. These photos give just an inkling of the diversity of organisms that belong to the animal kingdom. (A) Sponge, (B) Flatworm, (C) Flying Insect, (D) Frog, (E) Tiger, (F) Gorilla. | text | null |
L_0519 | fertilization | T_2933 | The sperm and egg dont look anything like a human baby ( Figure 1.1). After they come together, they will develop into a human being. How does a single cell become a complex organism made up of billions of cells? Keep reading to find out. Sexual reproduction happens when a sperm and an egg cell combine together. This is called fertilization. Sperm are released into the vagina during sexual intercourse. They swim through the uterus and enter a fallopian tube. This is where fertilization normally takes place. A sperm that is about to enter an egg is pictured below ( Figure 1.1). If the sperm breaks through the eggs membrane, it will immediately cause changes in the egg that keep other sperm out. This ensures that only a single sperm can penetrate an egg. It will also cause the egg to go through meiosis. Recall that meiosis, cell division that creates the egg, begins long before an egg is released from an ovary. In fact, it begins prior to birth. The sperm and egg each have only half the number of chromosomes as other cells in the body. These cells are haploid, with a single set of chromosomes. This is because when they combine together, they form a cell with the full number of chromosomes. The cell they form is called a zygote. The zygote is diploid, with two sets of chromosomes, one from each parent. A human zygote has two sets of 23 chromosomes, for a total of 46 chromosomes (23 pairs). The zygote slowly travels down the fallopian tube to the uterus. As it travels, it divides by mitosis many times. It forms a hollow ball of cells. After the ball of cells reaches the uterus, it fixes itself to the side of the uterus. This is called implantation. It usually happens about a week after fertilization. Now the implanted ball of cells is ready to continue its development into a baby boy or girl. | text | null |
L_0530 | fungi structure | T_2962 | The most important body parts of fungi include: 1. Cell wall: A layer around the cell membrane of fungi cells made largely of chitin and other polysaccharides. It is similar to that found in plant cells, though the plant cell wall contains the polysaccharide cellulose. 2. Hyphae: These are thread-like strands which interconnect and bunch up into a mycelium ( Figure 1.1). Ever see mold on a damp wall or on old bread? The things that you are seeing are really mycelia. The hyphae and mycelia help the fungi absorb nutrients from other organisms. Most of the mycelium is hidden from view deep within the fungal food source, such as rotting matter in the soil, leaf litter, rotting wood, or dead animals. Fungi produce enzymes to digest cellulose and various other materials found in rotting matter, helping with the decaying process. 3. Specialized structures for reproduction: One example is a fruiting body. Just like a fruit is involved in the reproduction of a fruiting plant, a fruiting body is involved in the reproduction of a fungus. A mushroom is a fruiting body, which is the part of the fungus that produces spores ( Figure 1.2). The spores are the basic reproductive units of fungi. The mycelium remains hidden until it develops one or more fruiting bodies. The fruiting bodies are usually produced at the surface of the food source, rather than hidden within it. This allows the reproductive spores to be easily shed and carried away by the wind, water, or animals. The fruiting bodies are usually the only indication that a fungus is present. Like icebergs, the fruiting bodies represent only a tiny fraction of the whole fungus, with most of the fungus hidden from view. Hyphae of a Penicillium mold. The little trees are specialized hyphae on which spores are produced. A mushroom is a fruiting body. | text | null |
L_0530 | fungi structure | T_2962 | The most important body parts of fungi include: 1. Cell wall: A layer around the cell membrane of fungi cells made largely of chitin and other polysaccharides. It is similar to that found in plant cells, though the plant cell wall contains the polysaccharide cellulose. 2. Hyphae: These are thread-like strands which interconnect and bunch up into a mycelium ( Figure 1.1). Ever see mold on a damp wall or on old bread? The things that you are seeing are really mycelia. The hyphae and mycelia help the fungi absorb nutrients from other organisms. Most of the mycelium is hidden from view deep within the fungal food source, such as rotting matter in the soil, leaf litter, rotting wood, or dead animals. Fungi produce enzymes to digest cellulose and various other materials found in rotting matter, helping with the decaying process. 3. Specialized structures for reproduction: One example is a fruiting body. Just like a fruit is involved in the reproduction of a fruiting plant, a fruiting body is involved in the reproduction of a fungus. A mushroom is a fruiting body, which is the part of the fungus that produces spores ( Figure 1.2). The spores are the basic reproductive units of fungi. The mycelium remains hidden until it develops one or more fruiting bodies. The fruiting bodies are usually produced at the surface of the food source, rather than hidden within it. This allows the reproductive spores to be easily shed and carried away by the wind, water, or animals. The fruiting bodies are usually the only indication that a fungus is present. Like icebergs, the fruiting bodies represent only a tiny fraction of the whole fungus, with most of the fungus hidden from view. Hyphae of a Penicillium mold. The little trees are specialized hyphae on which spores are produced. A mushroom is a fruiting body. | text | null |
L_0531 | fungus like protists | T_2963 | Fungus-like protists share many features with fungi. Like fungi, they are heterotrophs, meaning they must obtain food outside themselves. They also have cell walls and reproduce by forming spores, just like fungi. Fungus-like protists usually do not move, but a few develop movement at some point in their lives. Two major types of fungus- like protists are slime molds and water molds. | text | null |
L_0531 | fungus like protists | T_2964 | Slime molds usually measure about one or two centimeters, but a few slime molds are as big as several meters. They often have bright colors, such as a vibrant yellow ( Figure 1.1). Others are brown or white. Stemonitis is a kind of slime mold which forms small brown bunches on the outside of rotting logs. Physarum polycephalum lives inside rotting logs and is a gooey mesh of yellow "threads" that are several centimeters long. Fuligo, sometimes called vomit mold, is a yellow slime mold found in decaying wood. An example of a slime mold. | text | null |
L_0531 | fungus like protists | T_2965 | Water molds mostly live in water or moist soil. They can be parasites of plants and animals, getting their nutrients from these organisms and also from decaying organisms. They are a common problem for farmers since they cause a variety of plant diseases. One of the most famous of these diseases was the fungus that caused the Irish potato famine in the 1800s. At this time, potatoes were the main source of food for many of the Irish people. The failure of the potato crop meant that many people in Ireland died of starvation or migrated to other countries. | text | null |
L_0532 | gene therapy | T_2966 | Gene therapy is the insertion of genes into a persons cells to cure a genetic disorder. Could gene therapy be the cure for AIDS? No, AIDS is caused by a virus. Gene therapy only works to fix disorders caused by a faulty gene. The patient would have had this disorder from birth. Though gene therapy is still in experimental stages, the common use of this therapy may occur during your lifetime. There are two main types of gene therapy: 1. One done inside the body ( in vivo). 2. One done outside the body ( ex vivo). Both types of gene therapy use a vector, or carrier molecule for the gene. The vector helps incorporate the desired gene into the patients DNA. Usually this vector is modified viral DNA in which the viral genes have been removed. Dont worry, the virus used in gene therapy has been deactivated. | text | null |
L_0532 | gene therapy | T_2967 | During in vivo gene therapy, done inside the body, the vector with the gene of interest is introduced directly into the patient and taken up by the patients cells ( Figure 1.1). For example, cystic fibrosis gene therapy is targeted at the respiratory system, so a solution with the vector can be sprayed into the patients nose. Recently, in vivo gene therapy was also used to partially restore the vision of three young adults with a rare type of eye disease. In ex vivo gene therapy, done outside the body, cells are removed from the patient and the proper gene is inserted using a virus as a vector. The modified cells are placed back into the patient. One of the first uses of this type of gene therapy was in the treatment of a young girl with a rare genetic disease, adenosine deaminase deficiency, or ADA deficiency. People with this disorder are missing the ADA enzyme, which breaks down a toxin called deoxyadenosine. If the toxin is not broken down, it accumulates and destroys immune cells. As a result, individuals with ADA deficiency do not have a healthy immune system to fight off infections. In the gene therapy treatment for this disorder, bone marrow stem cells were taken from the girls body, and the missing gene was inserted into these cells outside the body. Then the modified cells were put back into her bloodstream. This treatment successfully restored the function of her immune system, but only with repeated treatments. | text | null |
L_0549 | human egg cells | T_3022 | When a baby girl is born, her ovaries contain all of the eggs they will ever produce. But these eggs are not fully developed. They develop only after she starts having menstrual periods at about age 12 or 13. Just one egg develops each month. A woman will release an egg once each month until she is in her 40s. A girl is born with over a million eggs. They die off and by puberty about 40,000 remain. | text | null |
L_0549 | human egg cells | T_3023 | Eggs are very big cells. In fact, they are the biggest cells in the human female body. (How many egg cells are in the human male body?) An egg is about 30 times as wide as a sperm cell! You can even see an egg cell without a microscope. Like a sperm cell, the egg contains a nucleus with half the number of chromosomes as other body cells. Unlike a sperm cell, the egg contains a lot of cytoplasm, the contents of the cell, which is why it is so big. The egg also does not have a tail. | text | null |
L_0549 | human egg cells | T_3024 | Egg production takes place in the ovaries. It takes several steps to make an egg: 1. Before birth, special cells in the ovaries go through mitosis (cell division), producing identical cells. 2. The daughter cells then start to divide by meiosis. But they only go through the first of the two cell divisions of meiosis at that time. They go through the second stage of cell division after the female goes through puberty. 3. In a mature female, an egg develops in an ovary about once a month. The drawing below shows how this happens ( Figure 1.1). As you can see from the figure, the egg rests in a nest of cells called a follicle. The follicle and egg grow larger and go through other changes. The follicle protects the egg as it matures in the ovary. After a couple of weeks, the egg bursts out of the follicle and through the wall of the ovary. This is called ovulation, which usually occurs at the midpoint of a monthly cycle. In a 28 day cycle, ovulation would occur around day 14. The moving fingers of the nearby fallopian tube then sweep the egg into the tube. At this time, if sperm are present the egg can be fertilized. Fertilization occurs if a sperm enters the egg while it is passing through the fallopian tube. When this happens, the egg finally completes meiosis. This results in two daughter cells that are different in size. The smaller cell is called a polar body. It contains very little cytoplasm. It soon breaks down and disappears. The larger cell is the egg. It contains most of the cytoplasm. This will develop into a child. | text | null |
L_0553 | human sperm | T_3033 | Sperm ( Figure 1.1), the male reproductive cells, are tiny. In fact, they are the smallest cells in the human body. What do you think a sperm cell looks like? Some people think that it looks like a tadpole. Do you agree? | text | null |
L_0553 | human sperm | T_3034 | A sperm has three main parts: 1. The head of the sperm contains the nucleus. The nucleus holds the DNA of the cell. The head also contains enzymes that help the sperm break through the cell membrane of an egg. 2. The midpiece of the sperm is packed with mitochondria. Mitochondria are organelles in cells that produce energy. Sperm use the energy in the midpiece to move. 3. The tail of the sperm moves like a propeller, around and around. This tail is a long flagella that pushes the sperm forward. A sperm can travel about 30 inches per hour. This may not sound very fast, but dont forget how small a sperm is. For its size, a sperm moves about as fast as you do when you walk briskly. This drawing of a sperm shows its main parts. What is the role of each part? How do you think the shape of the sperm might help it swim? | text | null |
L_0553 | human sperm | T_3035 | To make sperm, cells start in the testes and end in the epididymis. It takes up to two months to make sperm. The steps are explained below: 1. Special cells in the testes go through mitosis (cell division) to make identical copies of themselves. 2. The copies of the original cells divide by meiosis, producing cells called spermatids. The spermatids have half the number of chromosomes as the original cell. The spermatids are immature and cannot move on their own. 3. The spermatids are transported from the testes to the epididymis. Involuntary muscular contraction moves the spermatids along. 4. In the epididymis, spermatids slowly grow older and mature. They grow a tail. They also lose some of the cytoplasm from the head. It is here that the spermatids mature, becoming sperm cells. 5. When sperm are mature, they can swim. The mature sperm are stored in the epididymis until it is time for them to leave the body. Sperm leave the epididymis through the vas deferens. As they travel through the vas deferens, they pass by the prostate and other glands. The sperm mix with liquids from these glands, forming semen. The semen travels through the urethra and leaves the body through the penis. A teaspoon of semen may contain as many as 500 million sperm! | text | null |
L_0557 | immunity | T_3047 | In previous concepts, you learned about B and T cells, special types of white blood cells that help your body to fight off a specific pathogen. They are necessary when the body is fighting off an infection. But what happens to them after the pathogen has been destroyed? Most B and T cells die after an infection has been brought under control. But some of them survive for many years. They may even survive for a persons lifetime. These long-lasting B and T cells are called memory cells. They allow the immune system to remember the pathogen after the infection is over. If the pathogen invades the body again, the memory cells will start dividing in order to fight the pathogen or disease. These dividing cells will quickly produce a new army of B or T cells to fight the pathogen. They will begin a faster, stronger attack than the first time the pathogen invaded the body. As a result, the immune system will be able to destroy the pathogen before it can cause an infection. Being able to attack the pathogen in this way is called immunity. Immunity can also be caused by vaccination. Vaccination is the process of exposing a person to a pathogen on purpose in order to develop immunity. In vaccination, a modified pathogen is usually injected under the skin by a shot. Only part of the pathogen is injected, or a weak or dead pathogen is used. It sounds dangerous, but the shot prepares your body for fighting the pathogen without causing the actual illness. Vaccination triggers an immune response against the injected antigen. The body prepares "memory" cells for use at a later time, in case the antigen is ever encountered again. Essentially, a vaccine imitates an infection, triggering an immune response, without making a person sick. In many countries, children receive their first vaccination at birth with the Hepatitis B shot, which protects infants from Hepatitis B, a serious liver disease. Before vaccines, many children died from diseases that vaccines now prevent, such as whooping cough, measles, and polio. Those same germs exist today, but because babies are now protected by vaccines, we do not see these diseases nearly as often. Diseases you have probably been vaccinated against include measles, mumps, and chicken pox. How does a vaccine work? See How a Vaccine Works at and The History of Vaccines at . Click image to the left or use the URL below. URL: | text | null |
L_0570 | inflammatory response | T_3090 | The little girl pictured below ( Figure 1.1) has a scraped knee. A scrape is a break in the skin that may let pathogens enter the body. If bacteria enter through the scrape, they could cause an infection. These bacteria would then face the bodys second line of defense. The second line of defense is also nonspecific, fighting many types of pathogens. | text | null |
L_0570 | inflammatory response | T_3091 | The bodys second line of defense against pathogens includes the inflammatory response. If bacteria enter the skin through a scrape, the area may become red, warm, and painful. These are signs of inflammation. Inflammation is one way the body reacts to infections or injuries. Inflammation is caused by chemicals that are released when skin or other tissues are damaged. The chemicals cause nearby blood vessels to dilate, or expand. This increases blood flow to the damaged area, which makes the area red and slightly warm. The chemicals also attract white blood cells called neutrophils to the wound and cause them to leak out of blood vessels into the damaged tissue. This little girl just got her first scraped knee. It doesnt seem to hurt, but the break in her skin could let pathogens enter her body. Thats why scrapes should be kept clean and protected until they heal. | text | null |
L_0570 | inflammatory response | T_3092 | What do these white blood cells do at the site of inflammation? The main role of white blood cells is to fight pathogens in the body. There are actually several different kinds of white blood cells. Some white blood cells have very specific functions. They attack only certain pathogens. Other white blood cells attack any pathogen they find. These white blood cells travel to areas of the body that are inflamed. They are called phagocytes, which means eating cells. Neutrophils are a type of phagocyte. In addition to pathogens, phagocytes eat dead cells. They surround the pathogens and destroy them. Sometimes it is said that the phagocyte engulfs the pathogen, and then destroys it. This process is called phagocytosis. White blood cells also make chemicals that cause a fever. A fever is a higher-than-normal body temperature. Normal human body temperature is 98.6 F (37 C). Most bacteria and viruses that infect people reproduce fastest at this temperature. When the temperature is higher, the pathogens cannot reproduce as fast, so the body raises the temperature to kill them. A fever also causes the immune system to make more white blood cells. In these ways, a fever helps the body fight infection. | text | null |
L_0589 | lymphatic system | T_3152 | If pathogens get through the bodys first two lines of defense, a third line of defense takes over. This third line of defense involves the immune system. It is called an immune response, and is a specific type of response. The immune system has a special response for each type of pathogen. The immune system ( Figure 1.1) is also part of the lymphatic systemnamed for lymphocytes, which are the type of white blood cells involved in an immune response. They include several lymph organs, lymph vessels, lymph, and lymph nodes. This diagram shows the parts of the im- mune system. The immune system in- cludes several organs and a system of vessels that carry lymph. Lymph nodes are located along the lymph vessels. | text | null |
L_0589 | lymphatic system | T_3153 | The lymph organs are the red bone marrow, tonsils, spleen, and thymus gland. They are described below ( Figure Each lymph organ has a different job in the immune system. | text | null |
L_0589 | lymphatic system | T_3154 | Lymph vessels make up a circulatory system that is similar to the cardiovascular system, which you can read about in a previous concept. Lymph vessels are like blood vessels, except they move lymph instead of blood. Lymph is a yellowish liquid that leaks out of tiny blood vessels into spaces between cells in tissues. Where there is more inflammation, there is usually more lymph in tissues. This lymph may contain many pathogens. The lymph that collects in tissues slowly passes into tiny lymph vessels. It then travels from smaller to larger lymph vessels. Lymph is not pumped through lymph vessels like blood is pumped through blood vessels by the heart. Instead, muscles around the lymph vessels contract and squeeze the lymph through the vessels. The lymph vessels also contract to help move the lymph along. The lymph finally reaches the main lymph vessels in the chest. Here, the lymph drains into two large veins. This is how the lymph returns to the bloodstream. Before lymph reaches the bloodstream, pathogens are removed from it at lymph nodes. Lymph nodes are small, oval structures located along the lymph vessels. They act like filters. Any pathogens filtered out of the lymph at lymph nodes are destroyed by lymphocytes in the nodes. | text | null |
L_0589 | lymphatic system | T_3155 | Lymphocytes ( Figure 1.3), a type of white blood cell, are the key cells of an immune response. There are trillions of lymphocytes in the human body. They make up about one quarter of all white blood cells. Usually, fewer than half of the bodys lymphocytes are in the blood. The rest are in the lymph, lymph nodes, and lymph organs. There are two main types of lymphocytes: 1. B cells. This image of a lymphocyte was made with an electron microscope. The lym- phocyte is shown 10,000 times its actual size. 2. T cells. Both types of lymphocytes are produced in the red bone marrow. They are named for the sites where they grow larger. The "B" in B cells stands for bone. B cells grow larger in red bone marrow. The "T" in T cells stands for thymus. T cells mature in the thymus gland. B and T cells must be switched on in order to fight a specific pathogen. Once this happens, they produce an army of cells ready to fight that particular pathogen. How can B and T cells recognize specific pathogens? Pathogens have proteins, often located on their cell surface. These proteins are called antigens. An antigen is any protein that causes an immune response, because it is unlike any protein that the body makes. Antigens are found on bacteria, viruses, and other pathogens. Your body sees these as foreign, meaning they do not belong in your body. | text | null |
L_0596 | meiosis | T_3162 | Sexual reproduction combines gametes from two parents. Gametes are reproductive cells, such as sperm and egg. As gametes are produced, the number of chromosomes must be reduced by half. Why? The zygote must contain genetic information from the mother and from the father, so the gametes must contain half of the chromosomes found in normal body cells. When two gametes come together at fertilization, the normal amount of chromosomes results. Gametes are produced by a special type of cell division known as meiosis. Meiosis contains two rounds of cell division without DNA replication in between. This process reduces the number of chromosomes by half. Human cells have 23 pairs of chromosomes, and each chromosome within a pair is called a homologous chromo- some. For each of the 23 chromosome pairs, you received one chromosome from your father and one chromosome from your mother. Alleles are alternate forms of genes found on chromosomes. Homologous chromosomes have the same genes, though they may have different alleles. So, though homologous chromosomes are very similar, they are not identical. The homologous chromosomes are separated when gametes are formed. Therefore, gametes have only 23 chromosomes, not 23 pairs. | text | null |
L_0596 | meiosis | T_3163 | A cell with two sets of chromosomes is diploid, referred to as 2n, where n is the number of sets of chromosomes. Most of the cells in a human body are diploid. A cell with one set of chromosomes, such as a gamete, is haploid, referred to as n. Sex cells are haploid. When a haploid sperm (n) and a haploid egg (n) combine, a diploid zygote will be formed (2n). In short, when a diploid zygote is formed, half of the DNA comes from each parent. | text | null |
L_0596 | meiosis | T_3164 | Before meiosis begins, DNA replication occurs, so each chromosome contains two sister chromatids that are identical to the original chromosome. Meiosis ( Figure 1.1) is divided into two divisions: Meiosis I and Meiosis II. Each division can be divided into the same phases: prophase, metaphase, anaphase, and telophase. Cytokinesis follows telophase each time. Between the two cell divisions, DNA replication does not occur. Through this process, one diploid cell will divide into four haploid cells. Overview of Meiosis. During meiosis, four haploid cells are created from one diploid parent cell. | text | null |
L_0596 | meiosis | T_3165 | During meiosis I, the pairs of homologous chromosomes are separated from each other. This requires that they line up in their homologous paris during metaphase I. The steps are outlined below: 1. Prophase I: The homologous chromosomes line up together. During this time, a process that only happens in meiosis can occur. This process is called crossing-over ( Figure 1.2), which is the exchange of DNA between homologous chromosomes. Crossing-over forms new combinations of alleles on the resulting chromosome. Without crossing-over, the offspring would always inherit all of the alleles on one of the homologous chromo- somes. Also during prophase I, the spindle forms, the chromosomes condense as they coil up tightly, and the nuclear envelope disappears. 2. Metaphase I: The homologous chromosomes line up in their pairs in the middle of the cell. Chromosomes from the mother or from the father can each attach to either side of the spindle. Their attachment is random, so all of the chromosomes from the mother or father do not end up in the same gamete. The gamete will contain some chromosomes from the mother and some chromosomes from the father. 3. Anaphase I: The homologous chromosomes are separated as the spindle shortens, and begin to move to opposite sides (opposite poles) of the cell. 4. Telophase I: The spindle fibers dissolves, but a new nuclear envelope does not need to form. This is because, after cytokinesis, the nucleus will immediately begin to divide again. No DNA replication occurs between meiosis I and meiosis II because the chromosomes are already duplicated. After cytokinesis, two haploid cells result, each with chromosomes made of sister chromatids. Since the separation of chromosomes into gametes is random during meiosis I, this process results in different combinations of chromosomes (and alleles) in each gamete. With 23 pairs of chromosomes, there is a possibility of over 8 million different combinations of chromosomes (223 ) in a human gamete. During crossing-over, segments of DNA are exchanged between non-sister chro- matids of homologous chromosomes. Notice how this can result in an allele (A) on one chromatid being moved onto the other non-sister chromatid. | text | null |
L_0596 | meiosis | T_3166 | During meiosis II, the sister chromatids are separated and the gametes are generated. This cell division is similar to that of mitosis, but results in four genetically unique haploid cells. The steps are outlined below: 1. Prophase II: The chromosomes condense. 2. Metaphase II: The chromosomes line up one on top of each other along the middle of the cell, similar to how they line up in mitosis. The spindle is attached to the centromere of each chromosome. 3. Anaphase II: The sister chromatids separate as the spindle shortens and move to opposite ends of the cell. 4. Telophase II: A nuclear envelope forms around the chromosomes in all four cells. This is followed by cytokinesis. After cytokinesis, each cell has divided again. Therefore, meiosis results in four haploid genetically unique daughter cells, each with half the DNA of the parent cell ( Figure 1.3). In human cells, the parent cell has 46 chromosomes (23 pairs), so the cells produced by meiosis have 23 chromosomes. These cells will become gametes. | text | null |
L_0602 | mitosis and cytokinesis | T_3180 | The genetic information of the cell, or DNA, is stored in the nucleus. During mitosis, two nuclei (plural for nucleus) must form, so that one nucleus can be in each of the new cells after the cell divides. In order to create two genetically identical nuclei, DNA inside of the nucleus must be copied or replicated. This occurs during the S phase of the cell cycle. During mitosis, the copied DNA is divided into two complete sets, so that after cytokinesis, each cell has a complete set of genetic instructions. | text | null |
L_0602 | mitosis and cytokinesis | T_3181 | To begin mitosis, the DNA in the nucleus wraps around proteins to form chromosomes. Each organism has a unique number of chromosomes. In human cells, our DNA is divided up into 23 pairs of chromosomes. Replicated DNA forms a chromosome made from two identical sister chromatids, forming an "X" shaped molecule ( Figure 1.1). The two chromatids are held together on the chromosome by the centromere. The centromere is also where spindle fiber microtubules attach during mitosis. The spindles separate sister chromatids from each other. | text | null |
L_0602 | mitosis and cytokinesis | T_3182 | During mitosis, the two sister chromatids must be divided. This is a precise process that has four individual phases to it. After the sister chromatids separate, each separate chromatid is now known as a chromosome. Each resulting chromosome is made of DNA from just one chromatid. So, each chromosome after this separation is made of "1/2 of the X." Through this process, each daughter cell receives one copy of each chromosome. The four phases of mitosis are prophase, metaphase, anaphase and telophase ( Figure 1.2). 1. Prophase: The chromatin, which is unwound DNA, condenses forming chromosomes. The DNA becomes so tightly wound that you can see them under a microscope. The membrane around the nucleus, called the nuclear envelope, disappears. Spindles also form and attach to chromosomes to help them move. 2. Metaphase: The chromosomes line up in the center, or the equator, of the cell. The chromosomes line up in a row, one on top of the next. 3. Anaphase: The two sister chromatids of each chromosome separate as the spindles pull the chromatids apart, resulting in two sets of identical chromosomes. 4. Telophase: The spindle dissolves and nuclear envelopes form around the chromosomes in both cells. An overview of the cell cycle and mito- sis: during prophase the chromosomes condense, during metaphase the chromo- somes line up, during anaphase the sister chromatids are pulled to opposite sides of the cell, and during telophase the nuclear envelope forms. This is a representation of dividing plant cells. Cell division in plant cells differs slightly from animal cells as a cell wall must form. Note that most of the cells are in interphase. Can you find examples of the different stages of mitosis? | text | null |
L_0602 | mitosis and cytokinesis | T_3182 | During mitosis, the two sister chromatids must be divided. This is a precise process that has four individual phases to it. After the sister chromatids separate, each separate chromatid is now known as a chromosome. Each resulting chromosome is made of DNA from just one chromatid. So, each chromosome after this separation is made of "1/2 of the X." Through this process, each daughter cell receives one copy of each chromosome. The four phases of mitosis are prophase, metaphase, anaphase and telophase ( Figure 1.2). 1. Prophase: The chromatin, which is unwound DNA, condenses forming chromosomes. The DNA becomes so tightly wound that you can see them under a microscope. The membrane around the nucleus, called the nuclear envelope, disappears. Spindles also form and attach to chromosomes to help them move. 2. Metaphase: The chromosomes line up in the center, or the equator, of the cell. The chromosomes line up in a row, one on top of the next. 3. Anaphase: The two sister chromatids of each chromosome separate as the spindles pull the chromatids apart, resulting in two sets of identical chromosomes. 4. Telophase: The spindle dissolves and nuclear envelopes form around the chromosomes in both cells. An overview of the cell cycle and mito- sis: during prophase the chromosomes condense, during metaphase the chromo- somes line up, during anaphase the sister chromatids are pulled to opposite sides of the cell, and during telophase the nuclear envelope forms. This is a representation of dividing plant cells. Cell division in plant cells differs slightly from animal cells as a cell wall must form. Note that most of the cells are in interphase. Can you find examples of the different stages of mitosis? | text | null |
L_0603 | mitosis vs. meiosis | T_3183 | Mitosis, meiosis, and sexual reproduction are discussed at . Click image to the left or use the URL below. URL: Both mitosis and meiosis result in eukaryotic cells dividing. So what is the difference between mitosis and meiosis? The primary difference is the differing goals of each process. The goal of mitosis is to produce two daughter cells that are genetically identical to the parent cell, meaning the new cells have exactly the same DNA as the parent cell. Mitosis happens when you want to grow, for example. You want all your new cells to have the same DNA as the previous cells. The goal of meiosis, however, is to produce sperm or eggs, also known as gametes. The resulting gametes are not genetically identical to the parent cell. Gametes are haploid cells, with only half the DNA present in the diploid parent cell. This is necessary so that when a sperm and an egg combine at fertilization, the resulting zygote has the correct amount of DNAnot twice as much as the parents. The zygote then begins to divide through mitosis. Pictured below is a comparison between binary fission (Figure 1.1), which is cell division of prokaryotic organisms, mitosis, and meiosis. Mitosis and meiosis are also compared in the table that follows (Table 1.1). A comparison between binary fission, mi- tosis, and meiosis. Purpose Number of Cells Produced Rounds of Cell Division Haploid or Diploid Daughter cells identical to parent cells? Daughter cells identical to each other? Mitosis To produce new cells 2 1 Diploid Yes Meiosis To produce gametes 4 2 Haploid No Yes No | text | null |
L_0612 | nerve cells and nerve impulses | T_3206 | The nervous system is made up of nerves. A nerve is a bundle of nerve cells. A nerve cell that carries messages is called a neuron ( Figure 1.1). The messages carried by neurons are called nerve impulses. Nerve impulses can travel very quickly because they are electrical impulses. Think about flipping on a light switch when you enter a room. When you flip the switch, the electricity flows to the light through wires inside the walls. The electricity may have to travel many meters to reach the light, but the light still comes on as soon as you flip the switch. Nerve impulses travel just as fast through the network of nerves inside the body. The axons of many neurons, like the one shown here, are covered with a fatty layer called myelin sheath. The sheath covers the axon, like the plastic covering on an electrical wire, and allows nerve impulses to travel faster along the axon. The node of Ranvier, shown in this diagram, is any gap in the myelin sheath; it allows faster transmission of a signal. | text | null |
L_0612 | nerve cells and nerve impulses | T_3207 | A neuron has a special shape that lets it pass signals from one cell to another. A neuron has three main parts ( Figure 1. The cell body. 2. Many dendrites. 3. One axon. The cell body contains the nucleus and other organelles. Dendrites and axons connect to the cell body, similar to rays coming off of the sun. Dendrites receive nerve impulses from other cells. Axons pass the nerve impulses on to other cells. A single neuron may have thousands of dendrites, so it can communicate with thousands of other cells but only one axon. The axon is covered with a myelin sheath, a fatty layer that insulates the axon and allows the electrical signal to travel much more quickly. The node of Ranvier is any gap within the myelin sheath exposing the axon, and it allows even faster transmission of a signal. | text | null |
L_0612 | nerve cells and nerve impulses | T_3208 | Neurons are usually classified based on the role they play in the body. Two main types of neurons are sensory neurons and motor neurons. Sensory neurons carry nerve impulses from sense organs and internal organs to the central nervous system. Motor neurons carry nerve impulses from the central nervous system to organs, glands, and musclesthe opposite direction. Both types of neurons work together. Sensory neurons carry information about the environment found inside or outside of the body to the central nervous system. The central nervous system uses the information to send messages through motor neurons to tell the body how to respond to the information. | text | null |
L_0612 | nerve cells and nerve impulses | T_3209 | The place where the axon of one neuron meets the dendrite of another is called a synapse. Synapses are also found between neurons and other types of cells, such as muscle cells. The axon of the sending neuron does not actually touch the dendrite of the receiving neuron. There is a tiny gap between them, the synaptic cleft ( Figure 1.2). The following steps describe what happens when a nerve impulse reaches the end of an axon. 1. 2. 3. 4. When a nerve impulse reaches the end of an axon, the axon releases chemicals called neurotransmitters. Neurotransmitters travel across the synapse between the axon and the dendrite of the next neuron. Neurotransmitters bind to the membrane of the dendrite. The binding allows the nerve impulse to travel through the receiving neuron. Did you ever watch a relay race? After the first runner races, he or she passes the baton to the next runner, who takes over. Neurons are a little like relay runners. Instead of a baton, they pass neurotransmitters to the next neuron. Examples of neurotransmitters are chemicals such as serotonin, dopamine, and adrenaline. You can watch an animation of nerve impulses and neurotransmitters at Some people have low levels of the neurotransmitter called serotonin in their brain. Scientists think that this is one cause of depression. Medications called antidepressants help bring serotonin levels back to normal. For many people with depression, antidepressants control the symptoms of their depression and help them lead happy, productive lives. | text | null |
L_0615 | non mendelian inheritance | T_3214 | In all of Mendels experiments, he worked with traits where a single gene controlled the trait. Each also had one allele that was always dominant over the recessive allele. But this is not always true. There are exceptions to Mendels rules, and these exceptions usually have something to do with the dominant allele. If you cross a homozygous red flower with a homozygous white flower, according to Mendels laws, what color flower should result from the cross? Either a completely red or completely white flower, depending on which allele is dominant. But since Mendels time, scientists have discovered this is not always the case. | text | null |
L_0615 | non mendelian inheritance | T_3215 | One allele is NOT always completely dominant over another allele. Sometimes an individual has a phenotype between the two parents because one allele is not dominant over another. This pattern of inheritance is called incomplete dominance. For example, snapdragon flowers show incomplete dominance. One of the genes for flower color in snapdragons has two alleles, one for red flowers and one for white flowers. A plant that is homozygous for the red allele (RR) will have red flowers, while a plant that is homozygous for the white allele will have white flowers (WW). But the heterozygote will have pink flowers (RW) ( Figure 1.1) as both alleles are expressed. Neither the red nor the white allele is dominant, so the phenotype of the offspring is a blend of the two parents. Pink snapdragons are an example of in- complete dominance. Another example of incomplete dominance is with sickle cell anemia, a disease in which a blood protein called hemoglobin is produced incorrectly. This causes the red blood cells to have a sickle shape, making it difficult for these misshapen cells to pass through the smallest blood vessels. A person that is homozygous recessive (ss) for the sickle cell trait will have red blood cells that all have the incorrect hemoglobin. A person who is homozygous dominant (SS) will have normal red blood cells. What type of blood cells do you think a person who is heterozygous (Ss) for the trait will have? They will have some misshapen cells and some normal cells ( Figure 1.2). Both the dominant and recessive alleles are expressed, so the result is a phenotype that is a combination of the recessive and dominant traits. Sickle cell anemia causes red blood cells to become misshapen and curved unlike normal, rounded red blood cells. | text | null |
L_0615 | non mendelian inheritance | T_3215 | One allele is NOT always completely dominant over another allele. Sometimes an individual has a phenotype between the two parents because one allele is not dominant over another. This pattern of inheritance is called incomplete dominance. For example, snapdragon flowers show incomplete dominance. One of the genes for flower color in snapdragons has two alleles, one for red flowers and one for white flowers. A plant that is homozygous for the red allele (RR) will have red flowers, while a plant that is homozygous for the white allele will have white flowers (WW). But the heterozygote will have pink flowers (RW) ( Figure 1.1) as both alleles are expressed. Neither the red nor the white allele is dominant, so the phenotype of the offspring is a blend of the two parents. Pink snapdragons are an example of in- complete dominance. Another example of incomplete dominance is with sickle cell anemia, a disease in which a blood protein called hemoglobin is produced incorrectly. This causes the red blood cells to have a sickle shape, making it difficult for these misshapen cells to pass through the smallest blood vessels. A person that is homozygous recessive (ss) for the sickle cell trait will have red blood cells that all have the incorrect hemoglobin. A person who is homozygous dominant (SS) will have normal red blood cells. What type of blood cells do you think a person who is heterozygous (Ss) for the trait will have? They will have some misshapen cells and some normal cells ( Figure 1.2). Both the dominant and recessive alleles are expressed, so the result is a phenotype that is a combination of the recessive and dominant traits. Sickle cell anemia causes red blood cells to become misshapen and curved unlike normal, rounded red blood cells. | text | null |
L_0615 | non mendelian inheritance | T_3216 | Another exception to Mendels laws is a phenomenon called codominance. For example, our blood type shows codominance. Do you know what your blood type is? Are you A? O? AB? Those letters actually represent alleles. Unlike other traits, your blood type has three alleles, instead of two! The ABO blood types ( Figure 1.3) are named for the protein attached to the outside of the blood cell. In this case, two alleles are dominant and completely expressed (IA and IB ), while one allele is recessive (i). The IA allele encodes for red blood cells with the A antigen, while the IB allele encodes for red blood cells with the B antigen. The recessive allele (i) does not encode for any proteins. Therefore a person with two recessive alleles (ii) has type O blood. As no dominant (IA and IB ) allele is present, the person cannot have type A or type B blood. What are the genotypes of a person with type A or type B blood? An example of codominant inheritance is ABO blood types. There are two possible genotypes for type A blood, homozygous (IA IA ) and heterozygous (IA i), and two possible genotypes for type B blood, (IB IB and IB i). If a person is heterozygous for both the IA and IB alleles, they will express both and have type AB blood with both proteins on each red blood cell. This pattern of inheritance is significantly different than Mendels rules for inheritance, because both alleles are expressed completely, and one does not mask the other. | text | null |
L_0618 | organelles | T_3222 | Eukaryotic cells have many specific functions, so it can be said that a cell is like a factory. A factory has many machines and people, and each has a specific role. Just like a factory, the cell is made up of many different parts. Each part has a special role. The different parts of the cell are called organelles, which means "small organs." All organelles are found in eukaryotic cells. Prokaryotic cells are "simpler" than eukaryotic cells. Though prokaryotic cells still have many functions, they are not as specialized as eukaryotic cells, lacking membrane-bound organelles. Thus, most organelles are not found in prokaryotic cells. Below are the main organelles found in eukaryotic cells ( Figure 1.1): 1. The nucleus of a cell is like a safe containing the factorys trade secrets. The nucleus contains the genetic material (DNA), the information needed to build thousands of proteins. 2. The mitochondria are the powerhouses of the cell. Mitochondria are the organelles where cellular energy is produced, providing the energy needed to power chemical reactions. This process, known as cellular respiration, produces energy is in the form of ATP (adenosine triphosphate). Cells that use a lot of energy may have thousands of mitochondria. 3. Vesicles are small membrane bound sacs that transport materials around the cell and to the cell membrane. 4. The vacuoles are like storage centers. Plant cells have larger vacuoles than animal cells. Plants store water and nutrients in their large central vacuoles. 5. Lysosomes are like the recycling trucks that carry waste away from the factory. Lysosomes have digestive enzymes that break down old molecules into parts that can be recycled. 6. In both eukaryotes and prokaryotes, ribosomes are the non-membrane bound organelles where proteins are made. Ribosomes are like the machines in the factory that produce the factorys main product. Proteins are the main product of the cell. 7. Some ribosomes can be found on folded membranes called the endoplasmic reticulum (ER), others float freely in the cytoplasm. If the ER is covered with ribosomes, it looks bumpy like sandpaper, and is called the rough endoplasmic reticulum. If the ER does not contain ribosomes, it is smooth and called the smooth endoplasmic reticulum. Many proteins are made on the ribosomes on the rough ER. These proteins immedi- ately enter the ER, where they are modified, packaged into vesicles and sent to the Golgi apparatus. Lipids are made in the smooth ER. 8. The Golgi apparatus works like a mail room. The Golgi apparatus receives proteins from the rough ER and puts "shipping addresses" on them. The Golgi then packages the proteins into vesicles and sends them to the right place in the cell or to the cell membrane. Some of these proteins are secreted from the cell (they exit the cell); others are placed into the cell membrane. Also, the cytoskeleton gives the cell its shape, and the flagella helps the cell to move. Prokaryotic cells may also have flagella. | text | null |
L_0625 | passive transport | T_3247 | Recall that the cell membrane is semipermeable. It does not allow everything to pass through. Some molecules can pass easily through your cell membranes, while others have more difficulty. Sometimes molecules need the help of special transport proteins to move across the cell membrane. Some molecules even need an input of energy to help get them across the cell membrane. The movement of molecules across a membrane without the input of energy is known as passive transport. When energy (ATP) is needed, the movement is known as active transport. Active transport moves molecules against their concentration gradient, from an area of low concentration to an area of high concentration. | text | null |
L_0625 | passive transport | T_3248 | One example of passive transport is diffusion, when molecules move from an area of high concentration (large amount) to an area of low concentration (low amount). Molecules are said to naturally flow down their concentration gradient. This type of diffusion proceeds without an input of energy. In simple diffusion, molecules that are small and uncharged can freely diffuse across a cell membrane. They simply flow through the cell membrane. Simple diffusion does not require energy or need the assistance of a transport protein. Other larger or charged molecules that diffuse across a membrane may need assistance from a protein. Oxygen is a molecule that can freely diffuse across a cell membrane. For example, oxygen diffuses out of the air sacs in your lungs into your bloodstream because oxygen is more concentrated in your lungs than in your blood. Oxygen moves from the high concentration of oxygen in your lungs to the low concentration of oxygen in your bloodstream. Carbon dioxide, which is exhaled, moves in the opposite direction - from a high concentration in your bloodstream to a low concentration in your lungs. | text | null |
L_0625 | passive transport | T_3249 | Sometimes, molecules cannot move through the cell membrane on their own. These molecules need special transport proteins to help them move across the membrane, a process known as facilitative diffusion. These special proteins are called channel proteins or carrier proteins ( Figure 1.1), and they are attached to the cell membrane. In fact, they go through the cell membrane, from the inside of the cell to the outside. Channel proteins provide an open channel or passageway through the cell membrane for molecules to move across. Many channel proteins allow the diffusion of ions. Ions are charged atoms. The charge makes it difficult to cross the cell membrane without assistance. Channel proteins are specific for the molecule they transport. For example a sodium ion crosses the membrane through a channel protein specific for sodium ions. Carrier proteins bind and carry the molecules across the cell membrane. These proteins bind a molecule on one side of the membrane, change shape as they carry the molecule across the membrane, and deposit the molecule on the other side of the membrane. Even though a protein is involved in both these methods of transport, neither method requires energy. Therefore these are still types of passive transport. | text | null |
L_0630 | plant cell structures | T_3261 | Even though plants and animals are both eukaryotes, plant cells differ in some ways from animal cells ( Figure organelles of photosynthesis. Photosynthesis converts the suns solar energy into chemical energy. This chemical energy, which is the carbohydrate glucose, serves as "food" for the plant. | text | null |
L_0630 | plant cell structures | T_3262 | First, plant cells have a large central vacuole that holds a mixture of water, nutrients, and wastes. A plant cells vacuole can make up 90% of the cells volume. The large central vacuole essentially stores water. In animal cells, vacuoles are much smaller. A plant cell has several features that make it different from an animal cell, including a cell wall, huge vacuoles, and chloroplasts, which photosynthesize. | text | null |
L_0630 | plant cell structures | T_3263 | Second, plant cells have a cell wall, while animal cells do not ( Figure 1.2). The cell wall surrounds the plasma membrane but does not keep substances from entering or leaving the cell. A cell wall gives the plant cell strength and protection. In this photo of plant cells taken with a light microscope, you can see green chloroplasts, as well as a cell wall around each cell. | text | null |
L_0630 | plant cell structures | T_3264 | A third difference between plant and animal cells is that plants have several kinds of organelles called plastids. And there are several different kinds of plastids in plant cells. For example, Chloroplasts are needed for photosynthesis, leucoplasts can store starch or oil, and brightly colored chromoplasts give some flowers and fruits their yellow, orange, or red color. It is the presence of chloroplasts and the ability to photosynthesize, that is one of the defining features of a plant. No animal or fungi can photosynthesize, and only some protists are able to. The photosynthetic protists are the plant-like protists, represented mainly by the unicellular algae. | text | null |
L_0635 | plant reproduction and life cycle | T_3275 | The life cycle of a plant is very different from the life cycle of an animal. Humans are made entirely of diploid cells (cells with two sets of chromosomes, referred to as 2n). Our only cells that are haploid cells (cells with one set of chromosomes, n) are sperm and egg cells. Plants, however, can live when they are are at the stage of having haploid cells or diploid cells. If a plant has a haploid chromosome number of 20, what is the diploid chromosome number? If the diploid chromosome number is 20, what is the haploid number? Plants alternate between diploid-cell plants and haploid-cell plants. This is called alternation of generations, because the plant type alternates from generation to generation. In alternation of generations, the plant alternates between a sporophyte that has diploid cells and a gametophyte that has haploid cells. Alternation of generations can be summarized in the following four steps: follow along in the Figure 1.1 as you read through the steps. 1. The haploid gametophyte produces the gametes, or sperm and egg, by mitosis. Remember, gametes are haploid, having one set of chromosomes. 2. Then, the sperm fertilizes the egg, producing a diploid zygote that develops into the sporophyte, which of course, is diploid. 3. The diploid sporophyte produces haploid spores by meiosis. 4. The haploid spores go through mitosis, developing into the haploid gametophyte. As we will see in additional Plants concepts, the generation in which the plant spends most of its life cycle is different between various plants. In the plants that first evolved, the gametophyte takes up the majority of the life cycle of the plant. During the course of evolution, the sporophyte became the major stage of the life cycle of the plant. In ferns, the sporophyte is dominant and produces spores that germinate into a heart-shaped gametophyte. | text | null |
L_0648 | prokaryotic and eukaryotic cells | T_3307 | There are two basic types of cells, prokaryotic cells and eukaryotic cells. The main difference between eukaryotic and prokaryotic cells is that eukaryotic cells have a nucleus. The nucleus is where cells store their DNA, which is the genetic material. The nucleus is surrounded by a membrane. Prokaryotic cells do not have a nucleus. Instead, their DNA floats around inside the cell. Organisms with prokaryotic cells are called prokaryotes. All prokaryotes are single-celled (unicellular) organisms. Bacteria and Archaea are the only prokaryotes. Organisms with eukaryotic cells are called eukaryotes. Animals, plants, fungi, and protists are eukaryotes. All multicellular organisms are eukaryotes. Eukaryotes may also be single-celled. Both prokaryotic and eukaryotic cells have structures in common. All cells have a plasma membrane, ribosomes, cytoplasm, and DNA. The plasma membrane, or cell membrane, is the phospholipid layer that surrounds the cell and protects it from the outside environment. Ribosomes are the non-membrane bound organelles where proteins are made, a process called protein synthesis. The cytoplasm is all the contents of the cell inside the cell membrane, not including the nucleus. | text | null |
L_0648 | prokaryotic and eukaryotic cells | T_3308 | Eukaryotic cells usually have multiple chromosomes, composed of DNA and protein. Some eukaryotic species have just a few chromosomes, others have close to 100 or more. These chromosomes are protected within the nucleus. In addition to a nucleus, eukaryotic cells include other membrane-bound structures called organelles. Organelles allow eukaryotic cells to be more specialized than prokaryotic cells. Pictured below are the organelles of eukaryotic cells ( Figure 1.1), including the mitochondria, endoplasmic reticulum, and Golgi apparatus. These will be discussed in additional concepts. DNA (chromatin) is stored. Organelles give eukaryotic cells more functions than prokaryotic cells. | text | null |
L_0648 | prokaryotic and eukaryotic cells | T_3309 | Prokaryotic cells ( Figure 1.2) are usually smaller and simpler than eukaryotic cells. They do not have a nucleus or other membrane-bound organelles. In prokaryotic cells, the DNA, or genetic material, forms a single large circle that coils up on itself. The DNA is located in the main part of the cell. Nucleus DNA Membrane-Bound Organelles Examples Prokaryotic Cells No Single circular piece of DNA No Bacteria Eukaryotic Cells Yes Multiple chromosomes Yes Plants, animals, fungi | text | null |
L_0649 | protein synthesis and gene expression | T_3310 | A monomer is a molecule that can bind to other monomers to form a polymer. Amino acids are the monomers of a protein. The DNA sequence contains the instructions to place amino acids into a specific order. When the amino acid monomers are assembled in that specific order, proteins are made, a process called protein synthesis. In short, DNA contains the instructions to create proteins. But DNA does not directly make the proteins. Proteins are made on the ribosomes in the cytoplasm, and DNA (in an eukaryotic cell) is in the nucleus. So the cell uses an RNA intermediate to produce proteins. Each strand of DNA has many separate sequences that code for a specific protein. Insulin is an example of a protein made by your cells ( Figure 1.1). Units of DNA that contain code for the creation of a protein are called genes. | text | null |
L_0649 | protein synthesis and gene expression | T_3311 | There are about 22,000 genes in every human cell. Does every human cell have the same genes? Yes. Does every human cell make the same proteins? No. In a multicellular organism, such as us, cells have specific functions because they have different proteins. They have different proteins because different genes are expressed in different cell types (which is known as gene expression). Imagine that all of your genes are "turned off." Each cell type only "turns on" (or expresses) the genes that have the code for the proteins it needs to use. So different cell types "turn on" different genes, allowing different proteins to be made. This gives different cell types different functions. Once a gene is expressed, the protein product of that gene is usually made. For this reason, gene expression and protein synthesis are often considered the same process. | text | null |
L_0683 | sponges | T_3408 | Sponges ( Figure 1.1) are classified in the phylum Porifera, from the Latin words meaning "having pores." These pores allow the movement of water into the sponges sac-like bodies. Sponges must pump water through their bodies in order to eat. Because sponges are sessile, meaning they cannot move, they filter water to obtain their food. They are, therefore, known as filter feeders. Filter feeders must filter the water to separate out the organisms and nutrients they want to eat from those they do not. You might think that sponges dont look like animals at all. They dont have a head or legs. Internally, they do not have brains, stomachs, or other organs. This is because sponges evolved much earlier than other animals. In fact, sponges do not even have true tissues. Instead, their bodies are made up of specialized cells (cell-level organization) that do specific jobs. Other animals, including humans, have tissue-level organization because they have tissues with specific functions. Sponge cells perform a variety of bodily functions and appear to be more independent of each other than are the cells of other animals. For example, some cells control the flow of water, in and out of the sponge, by increasing or decreasing the size of the pores. The sponges often have tube-like bodies with many tiny pores. There are roughly 5,000 sponge species. Sponges are characterized by a feeding system unique among animals. As sponges dont have mouths, they must feed by some other method. Sponges have tiny pores in their outer walls through which water is drawn. Cells in the sponge walls filter food from the water as the water is pumped through the body and out other larger openings. The flow of water through the sponge is unidirectional, driven by the beating of flagella, which line the surface of chambers connected by a series of canals. Sponges reproduce by both asexual and sexual means. Sponges that reproduce asexually produce buds or, more often, structures called gemmules, which are packets of several cells of various types inside a protective covering. Freshwater sponges often produce gemmules prior to winter, which then develop into adult sponges beginning the following spring. Most sponges that reproduce sexually are hermaphroditic and produce eggs and sperm at different times. Sperm are frequently released into the water, where they are captured by sponges of the same species. The sperm are then transported to eggs, fertilization occurs and the zygotes develop into larvae. Some sponges release their larvae, where others retain them for some time. Once the larvae are in the water, they settle and develop into juvenile sponges. | text | null |
L_0708 | viruses | T_3484 | We have all heard of viruses. The flu, the common cold, and many other diseases are caused by viruses. But what is a virus? Do you think viruses are living? Which domain do they belong to? Bacteria? Archaea? Eukarya? | text | null |
L_0708 | viruses | T_3485 | The answer is actually no. A virus is essentially DNA or RNA surrounded by a coat of protein ( Figure 1.1). It is not made of a cell, and cannot maintain a stable internal environment ( homeostasis). Recall that a cell is the basic unit of living organisms. So if a virus is not made of at least one cell, can it be living? Viruses also cannot reproduce on their ownthey need to infect a host cell to reproduce. So a virus is very different from any of the organisms that fall into the three domains of life. Though viruses are not considered living, they share two important traits with living organisms. They have genetic material like all cells do (though they are not made of cells), and they can evolve. The genetic material of a virus can change (mutate), altering the traits of the virus. As the process of evolution has resulted in all life on the planet today, the classification of viruses has been controversial. It calls into question the very definition of life. | text | null |
L_0708 | viruses | T_3486 | Viruses infect a variety of organisms, including plants, animals, and bacteria, injecting its genetic material into a cell of the host organism. Once inside the host cell, they use the cells own ATP (energy), ribosomes, enzymes, and other cellular parts to make copies of themselves. The host cell makes a copy of the viral DNA and produces viral proteins. These are then packaged into new viruses. So viruses cannot replicate or reproduce on their own; they rely on a host cell to make additional viruses. | text | null |
L_0708 | viruses | T_3487 | Viruses cause many human diseases. In addition to the flu and the common cold, viruses cause rabies, diarrheal diseases, AIDS, cold sores, and many other diseases ( Figure 1.2). Viral diseases range from mild to fatal. Cold sores are caused by a herpes virus. | text | null |
L_0714 | introduction to solutions | T_3508 | A solution forms when one substance dissolves in another. The substance that dissolves is called the solute. The substance it dissolves in is called the solvent. For example, ocean water is a solution in which the solute is salt and the solvent is water. In this example, a solid (salt) is dissolved in a liquid (water). However, matter in any state can be the solute or solvent in a solution. Solutions may be gases, liquids, or solids. In Table 10.1 and the video at the URL below, you can learn about solutions involving other states of matter. Solution Gas dissolved in gas Example: Earths atmosphere Gas dissolved in liquid Example: carbonated water Liquid dissolved in gas Example: moist air Solute oxygen (and other gases) Solvent nitrogen carbon dioxide water water air Solution Liquid dissolved in liquid Example: vinegar Solid dissolved in liquid Example: sweet tea Solid dissolved in solid Example: bronze Solute acetic acid Solvent water sugar tea copper tin When a solute dissolves in a solvent, it changes to the same state as the solvent. For example, when solid salt dissolves in liquid water, it becomes part of the liquid solution, salt water. If the solute and solvent are already in the same state, the substance present in greater quantity is considered to be the solvent. For example, nitrogen is the solvent in Earths atmosphere because it makes up 78 percent of air. | text | null |
L_0714 | introduction to solutions | T_3509 | When a solute dissolves, it separates into individual particles that spread evenly throughout the solvent. Exactly how this happens depends on the type of bonds the solute contains. Solutes with ionic bonds, such as table salt (NaCl), separate into individual ions (Na+ and Cl ). Solutes with covalent bonds, such as glucose (H6 C12 O6 ), separate into individual molecules. In either case, the individual ions or molecules spread apart and are surrounded by molecules of the solvent. This is illustrated in Figure 10.1 and in the videos at the URLs below. MEDIA Click image to the left or use the URL below. URL: MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0714 | introduction to solutions | T_3510 | When you add sugar to a cold drink, you may stir it to help the sugar dissolve. If you dont stir, the sugar may eventually dissolve, but it will take much longer. Stirring is one of several factors that affect how fast a solute dissolves in a solvent. Temperature is another factor. A solid solute dissolves faster at a higher temperature. For example, sugar dissolves faster in hot tea than in ice tea. A third factor that affects the rate of dissolving is the surface area of the solute. For example, if you put granulated sugar in a glass of ice tea, it will dissolve more quickly than the same amount of sugar in a cube. Thats because granulated sugar has much more surface area than a cube of sugar. You can see videos of all three factors at these URLs: MEDIA Click image to the left or use the URL below. URL: MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0714 | introduction to solutions | T_3511 | Water is a polar compound. This means it has positively and negatively charged ends. This is why it is so good at dissolving ionic compounds such as salt and polar covalent compounds such as sugar. Solutes that can dissolve in a given solvent, such as water, are said to be soluble in that solvent. So many solutes are soluble in water that water is called the universal solvent. However, there are substances that dont dissolve in water. Did you ever try to clean a paintbrush after painting with an oil-based paint? It doesnt work. Oil-based paint is nonpolar, so it doesnt dissolve in water. In other words, it is insoluble in water. Instead, a nonpolar solvent such as paint thinner must be used to dissolve nonpolar paint. You can see a video about soluble and insoluble solutes at this URL: (1:51). MEDIA Click image to the left or use the URL below. URL: | text | null |
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