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L_0403 | first two lines of defense | T_2312 | The skin is a very important barrier to pathogens. It is the bodys largest organ and the most important defense against disease. It forms a physical barrier between the body and the outside environment. The outer layer of the skin, called the epidermis, consists of dead cells filled with the protein keratin. These cells form a tough, waterproof covering on the body. It is very difficult for pathogens to get through the epidermis. The inside of the mouth and nose are lined with mucous membranes. Other organs that are exposed to substances from the environment are also lined with mucous membranes. These include the respiratory and digestive organs. Mucous membranes arent tough like skin, but they have other ways of keeping out pathogens. One way mucous membranes protect the body is by producing mucus. Mucus is a sticky, moist secretion that covers mucous membranes. The mucus traps pathogens and particles so they cant enter the body. Many mucous membranes are also covered with cilia. These are tiny, hair-like projections. Cilia move in waves and sweep mucus and trapped pathogens toward body openings. You can see this in the diagram in Figure 21.10. When you clear your throat or blow your nose, you remove mucus and pathogens from your body. | text | null |
L_0403 | first two lines of defense | T_2313 | In addition to mucus, your body releases a variety of fluids, including tears, saliva, and sweat. These fluids contain enzymes called lysozymes. Lysozymes break down the cell walls of bacteria and kill them. Your stomach contains a very strong acid, called hydrochloric acid. This acid kills most pathogens that enter the stomach in food or water. Urine is also acidic, so few pathogens are able to grow in it. | text | null |
L_0403 | first two lines of defense | T_2314 | Your skin is covered by millions of bacteria. Millions more live inside your body, mainly in your gastrointestinal tract. Most of these bacteria are helpful. For one thing, they help defend your body from pathogens. They do it by competing with harmful bacteria for food and space. They prevent the harmful bacteria from multiplying and making you sick. | text | null |
L_0403 | first two lines of defense | T_2315 | Did you ever get a splinter in your skin, like the one in Figure 21.11? It doesnt look like a serious injury, but even a tiny break in the skin may let pathogens enter the body. If bacteria enter through the break, for example, they could cause an infection. These bacteria would then face the bodys second line of defense. | text | null |
L_0403 | first two lines of defense | T_2316 | If bacteria enter the skin through a splinter or other wound, the area may become red, warm, and painful. These are signs of inflammation. Inflammation is one way the body reacts to infections or injuries. It occurs due to chemicals that are released when tissue is damaged. The chemicals cause nearby blood vessels to dilate, increasing blood flow to the area. The chemicals also attract white blood cells to the area. The white blood cells leak out of the blood vessels and into the damaged tissue. You can see an animation of the inflammatory response by watching this video: MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0403 | first two lines of defense | T_2317 | The white blood cells that go to a site of inflammation and leak into damaged tissue are called phagocytes. They start eating pathogens and dead cells by engulfing and destroying them. This process is called phagocytosis. You can see how it happens in Figure ??. You can see it in action in the animation at this link: http://commons.wikim | text | null |
L_0403 | first two lines of defense | T_2318 | Phagocytes also release 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 quickly at this temperature. When the temperature rises higher, the pathogens cant reproduce as quickly. Therefore, a fever helps to limit the infection. A fever also causes the immune system to make more white blood cells to fight the infection. | text | null |
L_0404 | immune system defenses | T_2319 | The immune system is the body system that fights to protect the body from specific pathogens. It has a special response for each type of pathogen. The immune systems specific reaction to a pathogen is called an immune response. The immune system is shown in Figure 21.13. It includes several organs and a network of vessels that carry lymph. Lymph is a yellowish liquid that normally leaks out of tiny blood vessels into spaces between cells in tissues. When inflammation occurs, more lymph leaks into tissues, and the lymph is likely to contain pathogens. | text | null |
L_0404 | immune system defenses | T_2320 | Immune system organs include bone marrow, the thymus gland, the spleen, and the tonsils. Each organ has a different job in the immune system. Bone marrow is found inside many bones. Its role in the immune system is to produce white blood cells called lymphocytes. The thymus gland is in the chest behind the breast bone. It stores some types of lymphocytes while they mature. The spleen is in the abdomen below the lungs. Its job is to filter pathogens out of the blood. The two tonsils are located on either side of the throat. They trap pathogens that enter the body through the mouth or nose. | text | null |
L_0404 | immune system defenses | T_2321 | Lymph vessels make up a circulatory system that is similar to the blood vessels of the cardiovascular system. However, lymph vessels circulate lymph instead of blood, and the heart does not pump lymph through the vessels. Lymph that collects in tissues slowly passes into tiny lymph vessels. Lymph then travels from smaller to larger lymph vessels. Muscles around the lymph vessels contract and squeeze the lymph through the vessels. The lymph vessels also contract to help move the lymph along. Eventually, lymph reaches the main lymph vessels, which are located in the chest. From these vessels, lymph drains into two large veins of the cardiovascular system. This is how lymph returns to the blood. Before lymph reaches the bloodstream, it passes through small oval structures called lymph nodes, which are located along the lymph vessels. Figure 21.14 shows where some of the bodys many lymph nodes are concentrated. Lymph nodes act like filters and remove pathogens from lymph. | text | null |
L_0404 | immune system defenses | T_2322 | A lymphocyte is the type of white blood cell involved in an immune system response. You can see what a lymphocyte looks like, greatly magnified, in Figure 21.15. Lymphocytes make up about one quarter of all white blood cells, but there are trillions of them in the human body. Usually, fewer than half of the bodys lymphocytes are in the blood. The majority are in the lymph, lymph nodes, and lymph organs. There are two main types of lymphocytes, called B cells and T cells. Both types of lymphocytes are produced in bone marrow. They are named for the sites where they grow and mature. The B in B cells stands for bone marrow, where B cells mature. The T in T cells stands for thymus gland, where T cells mature. Both B cells and T cells must be switched on in order to fight a specific pathogen. Once this happens, they produce an army of cells that are ready to fight that particular pathogen. How can B and T cells recognize specific pathogens? Pathogens have unique antigens, often located on their cell surface. Antigens are proteins that the body recognizes either as self or nonself. Self antigens include those found on red blood cells that determine a persons blood type. Generally, the immune system doesnt respond to self antigens. Nonself antigens include those found on bacteria, viruses, and other pathogens. Nonself antigens are also found on other cells, such as pollen cells and cancer cells. It is these antigens that trigger an immune response. | text | null |
L_0404 | immune system defenses | T_2323 | There are two different types of immune responses. Both types involve lymphocytes. However, one type of response involves B cells. The other type involves T cells. | text | null |
L_0404 | immune system defenses | T_2323 | There are two different types of immune responses. Both types involve lymphocytes. However, one type of response involves B cells. The other type involves T cells. | text | null |
L_0404 | immune system defenses | T_2324 | B cells respond to pathogens in the blood and lymph. Most B cells fight infections by making antibodies. An antibody is a large, Y-shaped molecule that binds to an antigen. Each antibody can bind with just one specific type of antigen. The antibody and antigen fit together like a lock and key. You can see how this works in Figure 21.16. The antibody in the figure can bind only with the type of antigen that is colored yellow. Once the antibody binds with the antigen, it signals a phagocyte to engulf and destroy them, along with the pathogen that carries the antigen on its surface. You can watch an animation of the antibody-antigen binding process at this link: MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0404 | immune system defenses | T_2325 | There are different types of T cells, including killer T cells and helper T cells. Killer T cells destroy infected, damaged, or cancerous body cells. Figure 21.17 shows how a killer T cells destroys an infected cell. When the killer T cell comes into contact with the infected cell, it releases toxins. The toxins make tiny holes in the infected cells membrane. This causes the cell to burst open. Both the infected cell and the pathogens inside it are destroyed. Helper T cells do not destroy infected, damaged, or cancerous body cells. However, they are still needed for an immune response. They help by releasing chemicals that control other lymphocytes. The chemicals released by helper T cells switch on B cells and killer T cells so they can recognize and fight specific pathogens. | text | null |
L_0404 | immune system defenses | T_2326 | Most B cells 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 Memory cells allow the immune system to remember a pathogen after the infection is over. If the pathogen invades the body again, the memory cells will start dividing in order to fight it. They 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 fight off and resist a pathogen in this way is called immunity. You dont have to suffer through an infection to gain immunity to some diseases. Immunity can also come about by vaccination. Vaccination is the process of exposing a person to pathogens on purpose so the person will develop immunity to them. In vaccination, the pathogens are usually injected under the skin. Only part of the pathogens are injected, or else weakened or dead pathogens are used. This causes an immune response without causing the disease. Diseases you are likely to have been vaccinated against include measles, mumps, and chicken pox. | text | null |
L_0421 | lifes building blocks | T_2455 | Cells were first discovered in the mid-1600s. The cell theory came about some 200 years later. You can see a re- enactment of some of the discoveries that led to the cell theory in this video: MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0421 | lifes building blocks | T_2456 | British scientist Robert Hooke first discovered cells in 1665. He was one of the earliest scientists to study living things under a microscope. He saw that cork was divided into many tiny compartments, like little rooms. (Do the cells in Figure 3.1 look like little rooms to you too?) Hooke called these little rooms cells. Cork comes from trees, so what Hooke observed was dead plant cells. In the late 1600s, Dutch scientist Anton van Leeuwenhoek made more powerful microscopes. He used them to observe cells of other organisms. For example, he saw human blood cells and bacterial cells. Over the next century, microscopes were improved and more cells were observed. | text | null |
L_0421 | lifes building blocks | T_2457 | By the early 1800s, scientists had seen cells in many different types of organisms. Every organism that was examined was found to consist of cells. From all these observations, German scientists Theodor Schwann and Matthias Schleiden drew two major conclusions about cells. They concluded that: cells are alive. all living things are made of cells. Around 1850, a German doctor named Rudolf Virchow was observing living cells under a microscope. As he was watching, one of the cells happened to divide. Figure 3.2 shows a cell dividing, like the cell observed by Virchow. This was an aha moment for Virchow. He realized that living cells produce new cells by dividing. This was evidence that cells arise from other cells. The work of Schwann, Schleiden, and Virchow led to the cell theory. This is one of the most important theories in life science. The cell theory can be summed up as follows: All organisms consist of one or more cells. Cells are alive and the site of all life processes. All cells come from pre-existing cells. | text | null |
L_0421 | lifes building blocks | T_2458 | All cells have certain parts in common. These parts include the cell membrane, cytoplasm, DNA, and ribosomes. The cell membrane is a thin coat of phospholipids that surrounds the cell. Its like the skin of the cell. It forms a physical boundary between the contents of the cell and the environment outside the cell. It also controls what enters and leaves the cell. The cell membrane is sometimes called the plasma membrane. Cytoplasm is the material inside the cell membrane. It includes a watery substance called cytosol. Besides water, cytosol contains enzymes and other substances. Cytoplasm also includes other cell structures suspended in the cytosol. DNA is a nucleic acid found in cells. It contains genetic instructions that cells need to make proteins. Ribosomes are structures in the cytoplasm where proteins are made. They consist of RNA and proteins. These four components are found in all cells. They are found in the cells of organisms as different as bacteria and people. How did all known organisms come to have such similar cells? The answer is evolution. The similarities show that all life on Earth evolved from a common ancestor. | text | null |
L_0421 | lifes building blocks | T_2459 | Besides the four parts listed above, many cells also have a nucleus. The nucleus of a cell is a structure enclosed by a membrane that contains most of the cells DNA. Cells are classified in two major groups based on whether or not they have a nucleus. The two groups are prokaryotic cells and eukaryotic cells. | text | null |
L_0421 | lifes building blocks | T_2460 | Prokaryotic cells are cells that lack a nucleus. The DNA in prokaryotic cells is in the cytoplasm, rather than enclosed within a nuclear membrane. All the organisms in the Bacteria and Archaea Domains have prokaryotic cells. No other organisms have this type of cell. Organisms with prokaryotic cells are called prokaryotes. They are all single-celled organisms. They were the first type of organisms to evolve. They are still the most numerous organisms today. You can see a model of a prokaryotic cell in Figure 3.3. The cell in the figure is a bacterium. Notice how it contains a cell membrane, cytoplasm, ribosomes, and several other structures. However, the cell lacks a nucleus. The cells DNA is circular. It coils up in a mass called a nucleoid that floats in the cytoplasm. | text | null |
L_0421 | lifes building blocks | T_2461 | Eukaryotic cells are cells that contain a nucleus. They are larger than prokaryotic cells. They are also more complex. Living things with eukaryotic cells are called eukaryotes. All of them belong to the Eukarya Domain. This domain includes protists, fungi, plants, and animals. Many protists consist of a single cell. However, most eukaryotes have more than one cell. You can see a model of a eukaryotic cell in Figure 3.4. The cell in the figure is an animal cell. The nucleus is an example of an organelle. An organelle is any structure inside a cell that is enclosed by a membrane. Eukaryotic cells may contain many different organelles. Each does a special job. For example, the mitochondrion is an organelle that provides energy to the cell. You can see a mitochondrion and several other organelles in the animal cell in Figure 3.4. Organelles allow eukaryotic cells to carry out more functions than prokaryotic cells can. | text | null |
L_0421 | lifes building blocks | T_2462 | All living cells have certain things in common. Besides having the basic parts described above, all cells can perform the same basic functions. For example, all cells can use energy, respond to their environment, and reproduce. However, cells may also have special functions. Multicellular organisms such as you have many different types of specialized cells. Each specialized cell has a particular job. Cells with special functions generally have a shape that suits them for that job. Figure 3.5 shows four examples of specialized cells. Each type of cell in the figure has a different function. It also has a shape that helps it perform that function. The function of a nerve cell is to carry messages to other cells. It has many long arms that extend outward from the cell. The "arms" let the cell pass messages to many other cells at once. The function of a red blood cell is to carry oxygen to other cells. A red blood cell is small and smooth. This helps it slip through small blood vessels. A red blood cell is also shaped like a fattened disc. This gives it a lot of surface area for transferring oxygen. The function of a sperm cell is to swim through fluid to an egg cell. A sperm cell has a long tail that helps it swim. The function of a pollen cell is to pollinate flowers. The pollen cells in the figure have tiny spikes that help them stick to insects such as bees. The bees then carry the pollen cells to other flowers for pollination. | text | null |
L_0421 | lifes building blocks | T_2463 | Cells and organelles are made of biochemical molecules. These include nucleic acids and proteins. Molecules, in turn, are made of atoms. Figure 3.6 shows these different levels of organization in living things. As you can see in Figure 3.6, living things also have levels of organization higher than the cell. These higher levels are found only in multicellular organisms with specialized cells. Specialized cells may be organized into tissues. A tissue is a group of cells of the same kind that performs the same function. For example, muscle cells are organized into muscle tissue. The function of muscle tissue is to contract in order to move the body or its parts. Tissues may be organized into organs. An organ is a structure composed of two or more types of tissue that work together to do a specific task. For example, the heart is an organ. It consists of muscle, nerve, and other types of tissues. Its task is to pump blood. Organs may be organized into organ systems. An organ system is a group of organs that work together to do the same job. For example, the heart is part of the cardiovascular system. This system also includes blood vessels and blood. The job of the cardiovascular system is to transport substances in blood to and from cells throughout the body. Organ systems are organized into the organism. The different organ systems work together to carry out all the life functions of the individual. For example, cardiovascular and respiratory systems work together to provide the individual with oxygen and rid it of carbon dioxide. | text | null |
L_0421 | lifes building blocks | T_2464 | Cells with different functions often vary in shape. They may also vary in size. However, all cells are very small. Even the largest organisms have microscopic cells. Cells are so small that their diameter is measured in micrometers. A micrometer is just one-millionth of a meter. Use the sliding scale at the following link to see how small cells and cell parts are compared with other objects. Why are cells so small? The answer has to do with the surface area and volume of cells. To carry out life processes, a cell must be able to pass substances into and out of the cell. For example, it must be able to pass nutrients into the cell and waste products out of the cell. Anything that enters or leaves a cell has to go through the cell membrane on the surface of the cell. A bigger cell needs more nutrients and creates more wastes. As the size of a cell increases, its volume increases more quickly that its surface area. If the volume of a cell becomes too great, it wont have enough surface area to transfer all of its nutrients and wastes. | text | null |
L_0422 | cell structures | T_2465 | The cell membrane is like the bag holding the Jell-O. It encloses the cytoplasm of the cell. It forms a barrier between the cytoplasm and the environment outside the cell. The function of the cell membrane is to protect and support the cell. It also controls what enters or leaves the cell. It allows only certain substances to pass through. It keeps other substances inside or outside the cell. | text | null |
L_0422 | cell structures | T_2466 | The structure of the cell membrane explains how it can control what enters and leaves the cell. The membrane is composed mainly of two layers of phospholipids. Figure 3.8 shows how the phospholipids are arranged in the cell membrane. Each phospholipid molecule has a head and two tails. The heads are water loving (hydrophilic), and the tails are water fearing (hydrophobic). The water-loving heads are on the outer surfaces of the cell membrane. They point toward the watery cytoplasm within the cell or the watery fluid that surrounds the cell. The water-fearing tails are in the middle of the cell membrane. | text | null |
L_0422 | cell structures | T_2467 | Hydrophobic molecules like to be near other hydrophobic molecules. They fear being near hydrophilic molecules. The opposite is true of hydrophilic molecules. They like to be near other hydrophilic molecules. They fear being near hydrophobic molecules. These likes and fears explain why some molecules can pass through the cell membrane while others cannot. Hydrophobic molecules can pass through the cell membrane. Thats because they like the hydrophobic interior of the membrane and fear the hydrophilic exterior of the membrane. Hydrophilic molecules cant pass through the cell membrane. Thats because they like the hydrophilic exterior of the membrane and fear the hydrophobic interior of the membrane. You can see how this works in the video at this link: . MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0422 | cell structures | T_2468 | Cytoplasm is everything inside the cell membrane (except the nucleus if there is one). It includes the watery, gel-like cytosol. It also includes other structures. The water in the cytoplasm makes up about two-thirds of the cells weight. It gives the cell many of its properties. | text | null |
L_0422 | cell structures | T_2469 | Why does a cell have cytoplasm? Cytoplasm has several important functions. These include: suspending cell organelles. pushing against the cell membrane to help the cell keep its shape. providing a site for many of the biochemical reactions of the cell. | text | null |
L_0422 | cell structures | T_2470 | Crisscrossing the cytoplasm is a structure called the cytoskeleton. It consists of thread-like filaments and tubules. The cytoskeleton is like a cellular skeleton. It helps the cell keep its shape. It also holds cell organelles in place within the cytoplasm. Figure 3.9 shows several cells. In the figure, the filaments of their cytoskeletons are colored green. The tubules are colored red. The blue dots are the cell nuclei. | text | null |
L_0422 | cell structures | T_2471 | Eukaryotic cells contain a nucleus and several other types of organelles. These structures carry out many vital cell functions. | text | null |
L_0422 | cell structures | T_2472 | The nucleus is the largest organelle in a eukaryotic cell. It contains most of the cells DNA. DNA, in turn, contains the genetic code. This code tells the cell which proteins to make and when to make them. You can see a diagram of a cell nucleus in Figure 3.10. Besides DNA, the nucleus contains a structure called a nucleolus. Its function is to form ribosomes. The membrane enclosing the nucleus is called the nuclear envelope. The envelope has tiny holes, or pores, in it. The pores allow substances to move into and out of the nucleus. | text | null |
L_0422 | cell structures | T_2473 | The mitochondrion (mitochondria, plural) is an organelle that makes energy available to the cell. Its like the power plant of a cell. It uses energy in glucose to make smaller molecules called ATP (adenosine triphosphate). ATP packages energy in smaller amounts that cells can use. Think about buying a bottle of water from a vending machine. The machine takes only quarters, and you have only dollar bills. The dollar bills wont work in the vending machine. Glucose is like a dollar bill. It contains too much energy for cells to use. ATP is like a quarter. It contains just the right amount of energy for use by cells. | text | null |
L_0422 | cell structures | T_2474 | A ribosome is a small organelle where proteins are made. Its like a factory in the cell. It gathers amino acids and joins them together into proteins. Unlike other organelles, the ribosome is not surrounded by a membrane. As a result, some scientists do not classify it as an organelle. Ribosomes may be found floating in the cytoplasm. Some ribosomes are located on the surface of another organelle, the endoplasmic reticulum. | text | null |
L_0422 | cell structures | T_2475 | The endoplasmic reticulum (ER) is an organelle that helps make and transport proteins and lipids. Its made of folded membranes. Bits of membrane can pinch off to form tiny sacs called vesicles. The vesicles carry proteins or lipids away from the ER. There are two types of endoplasmic reticulum. They are called rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER). Both types are shown in Figure 3.11. NOTE: Crop to include only part a of the original image.] | text | null |
L_0422 | cell structures | T_2475 | The endoplasmic reticulum (ER) is an organelle that helps make and transport proteins and lipids. Its made of folded membranes. Bits of membrane can pinch off to form tiny sacs called vesicles. The vesicles carry proteins or lipids away from the ER. There are two types of endoplasmic reticulum. They are called rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER). Both types are shown in Figure 3.11. NOTE: Crop to include only part a of the original image.] | text | null |
L_0422 | cell structures | T_2476 | The Golgi apparatus is a large organelle that sends proteins and lipids where they need to go. Its like a post office. It receives molecules from the endoplasmic reticulum. It packages and labels the molecules. Then it sends them where they are needed. Some molecules are sent to different parts of the cell. Others are sent to the cell membrane for transport out of the cell. Small bits of membrane pinch off the Golgi apparatus to enclose and transport the proteins and lipids. You can see a Golgi apparatus at work in this animation: | text | null |
L_0422 | cell structures | T_2477 | Both vesicles and vacuoles are sac-like organelles. They store and transport materials in the cell. They are like movable storage containers. Some vacuoles are used to isolate materials that are harmful to the cell. Other vacuoles are used to store needed substances such as water. Vesicles are much smaller than vacuoles and have a variety of functions. Some vesicles pinch off from the membranes of the endoplasmic reticulum and Golgi apparatus. These vesicles store and transport proteins and lipids. Other vesicles are used as chambers for biochemical reactions. | text | null |
L_0422 | cell structures | T_2478 | A lysosome is an organelle that recycles unneeded molecules. It uses enzymes to break down the molecules into their components. Then the components can be reused to make new molecules. Lysosomes are like recycling centers. | text | null |
L_0422 | cell structures | T_2479 | Centrioles are organelles that are found only in animal cells. They are located near the nucleus. They help organize the DNA in the nucleus before cell division takes place. They ensure that the DNA divides correctly when the cell divides. | text | null |
L_0422 | cell structures | T_2480 | All but one of the structures described above are found in plant cells as well as animal cells. The only exception is centrioles, which are not found in plant cells. Plant cells have three additional structures that are not found in animals cells. These include a cell wall, large central vacuole, and organelles called plastids. You can see these structures in the model of a plant cell in Figure 3.12. You can also see them in the interactive plant cell at this link: | text | null |
L_0422 | cell structures | T_2481 | The cell wall is a rigid layer that surrounds the cell membrane of a plant cell. Its made mainly of the complex carbohydrate called cellulose. The cell wall supports and protects the cell. The cell wall isnt solid like a brick wall. It has tiny holes in it called pores. The pores let water, nutrients, and other substances move into and out of the cell. | text | null |
L_0422 | cell structures | T_2482 | Most plant cells have a large central vacuole. It can make up as much as 90 percent of a plant cells total volume. The central vacuole is like a large storage container. It may store substances such as water, enzymes, and salts. It may have other roles as well. For example, the central vacuole helps stems and leaves hold their shape. It may also contain pigments that give flowers their colors. | text | null |
L_0422 | cell structures | T_2483 | Plastids are organelles in plant cells that may have various jobs. The main types of plastids are chloroplasts, chromoplasts, and leucoplasts. Chloroplasts are plastids that contain chlorophyll. Chlorophyll is a green pigment. It gives plants their green color. Photosynthesis takes place in chloroplasts. They capture sunlight and use its energy to make glucose. Chromoplasts are plastids that contain other pigments. These other pigments give flowers and fruits their colors. Leucoplasts are plastids that make or store other molecules. For example, some leucoplasts make amino acids. Other leucoplasts store starch or oil. | text | null |
L_0423 | transport | T_2484 | Youve probably blown soap bubbles like the child in Figure 4.1. In some ways, the thin film of soap molecules that forms a bubble resembles the cell membrane. Like the soap film, the cell membrane consists of a thin skin of molecules. You can see a model of the cell membrane in Figure below. The molecules that make up the cell membrane are mainly phospholipids. There are two layers of phospholipids. They are arranged so the lipid tails are on the inside of the membrane. They make the interior of the membrane hydrophobic, or "water fearing". The lipid heads point toward the outside of the membrane. The make the outer surfaces of the membrane hydrophilic, or "water loving". Different types of proteins are embedded in the lipid layers. The proteins are needed to help transport many substances across the membrane. The passage of a substance through a cell membrane is called transport. There are two basic ways that transport can occur: passive transport and active transport. For a good video introduction to passive and active transport, click on this link: . MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0423 | transport | T_2485 | Passive transport occurs when a substance passes through the cell membrane without needing any energy to pass through. This happens when a substance moves from an area where it is more concentrated to an area where it is less concentrated. Concentration is the number of particles of a substance in a given volume. Lets say you dissolve a teaspoon of salt in a cup of water. Then you dissolve two teaspoons of salt in another cup of water. The second solution will have a higher concentration of salt. Why does passive transport require no energy? A substance naturally moves from an area of higher to lower concentration. This is known as moving down the concentration gradient. The process is called diffusion. Its a little like a ball rolling down a hill. The ball naturally rolls from a higher to lower position without any added energy. You can see diffusion if you place a few drops of food coloring in a pan of water. Even without shaking or stirring, the food coloring gradually spreads throughout the water in the pan. Some substances can also diffuse through a cell membrane. This can occur in two ways: simple diffusion or facilitated diffusion. | text | null |
L_0423 | transport | T_2486 | Simple diffusion occurs when a substance diffuses through a cell membrane without any help from other molecules. The substance simply passes through tiny spaces in the membrane. It moves from the side of the membrane where it is more concentrated to the side where it is less concentrated. You can see how this happens in Figure 4.2. Substances that cross cell membranes by simple diffusion must squeeze between the lipid molecules in the mem- brane. As a result, the diffusing molecules must be very small. Oxygen (O2 ) and carbon dioxide (CO2 ) are examples of molecules that can cross cell membranes this way. When you breathe in, oxygen is more concentrated in the air in your lungs than it is in your blood. So oxygen diffuses from your lungs to your blood. The reverse happens with carbon dioxide. Carbon dioxide is more concentrated in your blood than it is in the air in your lungs. So carbon dioxide diffuses out of your blood to your lungs. | text | null |
L_0423 | transport | T_2487 | Hydrophilic molecules and very large molecules cant pass through the cell membrane by simple diffusion. They need help to pass through the membrane. The help is provided by proteins called transport proteins. This process is known as facilitated diffusion. There are two types of transport proteins: channel proteins and carrier proteins. They work in different ways. You can see how they work in Figure 4.3. A channel protein forms a tiny hole called a pore in the cell membrane. This allows water or hydrophilic molecules to bypass the hydrophobic interior of the membrane. A carrier protein binds with a diffusing molecule. This causes the carrier protein to change shape. As it does, it carries the molecule across the membrane. This allows large molecules to pass through the cell membrane. | text | null |
L_0423 | transport | T_2488 | Osmosis is the special case of the diffusion of water. Its an important means of transport in cells because the fluid inside and outside cells is mostly water. Water can pass through the cell membrane by simple diffusion, but it can happen more quickly with the help of channel proteins. Water moves in or out of a cell by osmosis until its concentration is the same on both sides of the cell membrane. | text | null |
L_0423 | transport | T_2489 | Active transport occurs when a substance passes through the cell membrane with the help of extra energy. This happens when a substance moves from an area where it is less concentrated to an area where it is more concentrated. This is the opposite of diffusion. The substance moves up, instead of down, the concentration gradient. Like rolling a ball uphill, this requires an input of energy. The energy comes from the molecule named ATP (adenosine triphosphate). The energy allows special transport proteins called pumps to move substances to areas of higher concentration. An example is the sodium-potassium pump. | text | null |
L_0423 | transport | T_2490 | Sodium and potassium are two of the most important elements in living things. They are present mainly as positively charged ions dissolved in water. The sodium-potassium pump moves sodium ions (Na+ ) out of the cell and potassium ions (K+ ) into the cell. In both cases, the ions are moving from an area of lower to higher concentration. Energy in ATP is needed for this "uphill" process. Figure 4.4 shows how this pump works. Trace these steps from left to right in the figure: 1. Three sodium ions inside the cell bind with a carrier protein in the cell membrane. 2. The carrier protein receives a phosphate from ATP. This forms ADP (adenosine diphosphate) and releases energy. 3. The energy causes the carrier protein to change shape. As it does, it pumps the three sodium ions out of the cell. 4. Two potassium ions outside the cell next bind with the carrier protein. Then the process reverses, and the two potassium ions are pumped into the cell. | text | null |
L_0423 | transport | T_2491 | Some substances are too big to be pumped across the cell membrane. They may enter or leave the cell by vesicle transport. This takes energy, so its another form of active transport. You can see how vesicle transport occurs in Figure 4.5. Vesicle transport out of the cell is called exocytosis. A vesicle containing the substance moves through the cytoplasm to the cell membrane. Then the vesicle fuses with the cell membrane and releases the substance outside the cell. You can watch this happening in this very short animation: MEDIA Click image to the left or use the URL below. URL: Vesicle transport into the cell is called endocytosis. The cell membrane engulfs the substance. Then a vesicle pinches off from the membrane and carries the substance into the cell. | text | null |
L_0426 | cell division | T_2513 | DNA stands for deoxyribonucleic acid. It is a very large molecule. It consists of two strands of smaller molecules called nucleotides. Before learning how DNA is copied, its a good idea to review its structure. | text | null |
L_0426 | cell division | T_2514 | As you can see in Figure 5.1, each nucleotide includes a sugar, a phosphate, and a nitrogen base. The sugar in DNA is called deoxyribose. There are four different nitrogen bases in DNA: adenine (A), thymine (T), cytosine (C), and guanine (G). Chemical bonds between the bases hold the two strands of DNA together. Adenine always bonds with thymine, and cytosine always bonds with guanine. These pairs of bases are called complementary base pairs. | text | null |
L_0426 | cell division | T_2515 | As a cell prepares to divide, its DNA first forms one or more structures called chromosomes. A chromosome consists of DNA and protein molecules coiled into a definite shape. Chromosomes are circular in prokaryotes and rodlike in eukaryotes. You can see an example of a human chromosome in Figure below. The rest of the time, DNA looks like a tangled mass of strings. In this form, it would be very difficult to copy and divide. | text | null |
L_0426 | cell division | T_2516 | The process in which DNA is copied is called DNA replication. You can see how it happens in Figure 5.3. An enzyme breaks the bonds between the two DNA strands. Another enzyme pairs new, complementary nucleotides with those in the original chains. Two daughter DNA molecules form. Each contains one new chain and one original chain. | text | null |
L_0426 | cell division | T_2516 | The process in which DNA is copied is called DNA replication. You can see how it happens in Figure 5.3. An enzyme breaks the bonds between the two DNA strands. Another enzyme pairs new, complementary nucleotides with those in the original chains. Two daughter DNA molecules form. Each contains one new chain and one original chain. | text | null |
L_0426 | cell division | T_2517 | How cell division proceeds depends on whether a cell has a nucleus. Prokaryotic cells lack a nucleus. Their DNA is in the cytoplasm. It forms just one circular chromosome. Eukaryotic cells have a nucleus holding their DNA. Their DNA forms multiple rodlike chromosomes, like the one in Figure 5.2. Eukaryotic cells also have other organelles. For these reasons, cell division is more complex in eukaryotic cells. | text | null |
L_0426 | cell division | T_2518 | You can see how a prokaryotic cell divides in Figure 5.4. This type of cell division is called binary fission. The cell simply splits into two equal halves. Binary fission occurs in bacteria and other prokaryotes. It takes place in three continuous steps: 1. The cells chromosome is copied to form two identical chromosomes. This is DNA replication. 2. The copies of the chromosome separate from each other. They move to opposite poles, or ends, of the cell. This is called chromosome segregation. 3. The cell wall grows toward the center of the cell. The cytoplasm splits apart, and the cell pinches in two. This is called cytokinesis. | text | null |
L_0426 | cell division | T_2519 | Before a eukaryotic cell divides, the nucleus and other organelles must be copied. Only then will each daughter cell have all the needed structures. 1. The first step in eukaryotic cell division, as it is in prokaryotic cell division, is DNA replication. As you can see in Figure 5.5, each chromosome then consists of two identical copies. The two copies are called sister chromatids. They are attached to each other at a point called the centromere. 2. The second step in eukaryotic cell division is division of the cells nucleus. This includes division of the chromosomes. This step is called mitosis. It is a complex process that occurs in four phases. The phases of mitosis are described below. 3. The third step is the division of the rest of the cell. This is called cytokinesis, as it is in a prokaryotic cell. During this step, the cytoplasm divides, and two daughter cells form. These three steps are shown in Figure 5.6. | text | null |
L_0426 | cell division | T_2520 | Mitosis, or division of the nucleus, occurs only in eukaryotic cells. By the time mitosis occurs, the cells DNA has already replicated. Mitosis occurs in four phases, called prophase, metaphase, anaphase, and telophase. You can see what happens in each phase in Figure below. The phases are described below. You can also learn more about the phases of mitosis by watching this video: . MEDIA Click image to the left or use the URL below. URL: 1. Prophase: Chromosomes form, and the nuclear membrane breaks down. In animal cells, the centrioles near the nucleus move to opposite poles of the cell. Fibers called spindles form between the centrioles. 2. Metaphase: Spindle fibers attach to the centromeres of the sister chromatids. The sister chromatids line up at the center of the cell. 3. Anaphase: Spindle fibers shorten, pulling the sister chromatids toward the opposite poles of the cell. This gives each pole a complete set of chromosomes. 4. Telophase: The chromosomes uncoil, and the spindle fibers break down. New nuclear membranes form. | text | null |
L_0426 | cell division | T_2520 | Mitosis, or division of the nucleus, occurs only in eukaryotic cells. By the time mitosis occurs, the cells DNA has already replicated. Mitosis occurs in four phases, called prophase, metaphase, anaphase, and telophase. You can see what happens in each phase in Figure below. The phases are described below. You can also learn more about the phases of mitosis by watching this video: . MEDIA Click image to the left or use the URL below. URL: 1. Prophase: Chromosomes form, and the nuclear membrane breaks down. In animal cells, the centrioles near the nucleus move to opposite poles of the cell. Fibers called spindles form between the centrioles. 2. Metaphase: Spindle fibers attach to the centromeres of the sister chromatids. The sister chromatids line up at the center of the cell. 3. Anaphase: Spindle fibers shorten, pulling the sister chromatids toward the opposite poles of the cell. This gives each pole a complete set of chromosomes. 4. Telophase: The chromosomes uncoil, and the spindle fibers break down. New nuclear membranes form. | text | null |
L_0426 | cell division | T_2521 | Cell division is just one of the stages that a cell goes through during its lifetime. All of the stages that a cell goes through make up the cell cycle. | text | null |
L_0426 | cell division | T_2522 | The cell cycle of a prokaryotic cell is simple. The cell grows in size, its DNA replicates, and the cell divides. | text | null |
L_0426 | cell division | T_2523 | In eukaryotes, the cell cycle is more complicated. The diagram in Figure 5.7 shows the stages that a eukaryotic cell goes through in its lifetime. There are two main stages: interphase and mitotic phase. They are described below. You can watch a eukaryotic cell going through the phases of the cell cycle at this link: Interphase is longer than mitotic phase. Interphase, in turn, is divided into three phases: Mitotic phase is when the cell divides. It includes mitosis (M) and cytokinesis (C). | text | null |
L_0427 | reproduction | T_2524 | Asexual reproduction is simpler than sexual reproduction. It involves just one parent. The offspring are genetically identical to each other and to the parent. All prokaryotes and some eukaryotes reproduce this way. There are several different methods of asexual reproduction. They include binary fission, fragmentation, and budding. | text | null |
L_0427 | reproduction | T_2525 | Binary fission occurs when a parent cell simply splits into two daughter cells. This method is described in detail in the lesson "Cell Division." Bacteria reproduce this way. You can see a bacterial cell reproducing by binary fission in Figure 5.9. | text | null |
L_0427 | reproduction | T_2526 | Fragmentation occurs when a piece breaks off from a parent organism. Then the piece develops into a new organism. Sea stars, like the one in Figure 5.10, can reproduce this way. In fact, a new sea star can form from a single arm. | text | null |
L_0427 | reproduction | T_2527 | Budding occurs when a parent cell forms a bubble-like bud. The bud stays attached to the parent while it grows and develops. It breaks away from the parent only after it is fully formed. Yeasts can reproduce this way. You can see two yeast cells budding in Figure 5.11. | text | null |
L_0427 | reproduction | T_2528 | Sexual reproduction is more complicated. It involves two parents. Special cells called gametes are produced by the parents. A gamete produced by a female parent is generally called an egg. A gamete produced by a male parent is usually called a sperm. An offspring forms when two gametes unite. The union of the two gametes is called fertilization. You can see a human sperm and egg uniting in Figure 5.12. The initial cell that forms when two gametes unite is called a zygote. | text | null |
L_0427 | reproduction | T_2529 | In species with sexual reproduction, each cell of the body has two copies of each chromosome. For example, human beings have 23 different chromosomes. Each body cell contains two of each chromosome, for a total of 46 chromosomes. You can see the 23 pairs of human chromosomes in Figure 5.13. The number of different types of chromosomes is called the haploid number. In humans, the haploid number is 23. The number of chromosomes in normal body cells is called the diploid number. The diploid number is twice the haploid number. In humans, the diploid number is two times 23, or 46. | text | null |
L_0427 | reproduction | T_2530 | The two members of a given pair of chromosomes are called homologous chromosomes. We get one of each homologous pair, or 23 chromosomes, from our father. We get the other one of each pair, or 23 chromosomes, from our mother. A gamete must have the haploid number of chromosomes. That way, when two gametes unite, the zygote will have the diploid number. How are haploid cells produced? The answer is meiosis. | text | null |
L_0427 | reproduction | T_2531 | Meiosis is a special type of cell division. It produces haploid daughter cells. It occurs when an organism makes gametes. Meiosis is basically mitosis times two. The original diploid cell divides twice. The first time is called meiosis I. The second time is called meiosis II. However, the DNA replicates only once. It replicates before meiosis I but not before meiosis II. This results in four haploid daughter cells. Meiosis I and meiosis II occurs in the same four phases as mitosis. The phases are prophase, metaphase, anaphase, and telophase. However, meiosis I has an important difference. In meiosis I, homologous chromosomes pair up and then separate. As a result, each daughter cell has only one chromosome from each homologous pair. Figure 5.14 is a simple model of meiosis. It shows both meiosis I and II. You can read more about the stages below. You can also learn more about them by watching this video: . MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0427 | reproduction | T_2532 | After DNA replicates during interphase, the nucleus of the cell undergoes the four phases of meiosis I: 1. Prophase I: Chromosomes form, and the nuclear membrane breaks down. Centrioles move to opposite poles of the cell. Spindle fibers form between the centrioles. Heres whats special about meiosis: Homologous chromosomes pair up! You can see this in Figure below. 2. Metaphase I: Spindle fibers attach to the centromeres of the paired homologous chromosomes. The paired chromosomes line up at the center of the cell. 3. Anaphase I: Spindle fibers shorten, pulling apart the chromosome pairs. The chromosomes are pulled toward opposite poles of the cell. One of each pair goes to one pole. The other of each pair goes to the opposite pole. 4. Telophase I: The chromosomes uncoil, and the spindle fibers break down. New nuclear membranes form. Phases of meiosis I Meiosis I is followed by cytokinesis. Thats when the cytoplasm of the cell divides. Two haploid daughter cells result. Both of these cells go on to meiosis II. | text | null |
L_0427 | reproduction | T_2533 | Meiosis II is just like mitosis. 1. Prophase II: Chromosomes form. The nuclear membrane breaks down. Centrioles move to opposite poles. Spindle fibers form. 2. Metaphase II: Spindle fibers attach to the centromeres of sister chromatids. Sister chromatids line up at the center of the cell. 3. Anaphase II: Spindle fibers shorten. They pull the sister chromatids to opposite poles. 4. Telophase II: The chromosomes uncoil. The spindle fibers break down. New nuclear membranes form. Meiosis II is also followed by cytokinesis. This time, four haploid daughter cells result. Thats because both daughter cells from meiosis I have gone through meiosis II. The four daughter cells must continue to develop before they become gametes. For example, in males, the cells must develop tails, among other changes, in order to become sperm. | text | null |
L_0427 | reproduction | T_2534 | Both types of reproduction have certain advantages. | text | null |
L_0427 | reproduction | T_2535 | Asexual reproduction can happen very quickly. It doesnt require two parents to meet and mate. Under ideal conditions, 100 bacteria can divide to produce millions of bacteria in just a few hours! Most bacteria dont live under ideal conditions. Even so, rapid reproduction may allow asexual organisms to be very successful. They may crowd out other species that reproduce more slowly. | text | null |
L_0427 | reproduction | T_2536 | Sexual reproduction is typically slower. However, it also has an advantage. Sexual reproduction results in offspring that are all genetically different. This can be a big plus for a species. The variation may help it adapt to changes in the environment. How does genetic variation arise during sexual reproduction? It happens in three ways: crossing over, independent assortment, and the random union of gametes. Crossing over occurs during meiosis I. It happens when homologous chromosomes pair up during prophase I. The paired chromosomes exchange bits of DNA. This recombines their genetic material. You can see where crossing over has occurred in Figures 5.15 and 5.16. Independent assortment occurs when chromosomes go to opposite poles of the cell in anaphase I. Which chromosomes end up together at each pole is a matter of chance. You can see this in Figures 5.15 and 5.16 as well. In sexual reproduction, two gametes unite to produce an offspring. Which two gametes is a matter of chance. The union of gametes occurs randomly. Due to these sources of variation, each human couple has the potential to produce more than 64 trillion unique offspring. No wonder we are all different! | text | null |
L_0436 | introduction to prokaryotes | T_2634 | Prokaryotes are currently placed in two domains. A domain is the highest taxon in the classification of living things. Its even higher than the kingdom. | text | null |
L_0436 | introduction to prokaryotes | T_2635 | The prokaryote domains are the Bacteria Domain and Archaea Domain, shown in Figure 8.2. All other living things are eukaryotes and placed in the domain Eukarya. (Unlike prokaryotes, eukaryotes have a nucleus in their cells.) | text | null |
L_0436 | introduction to prokaryotes | T_2635 | The prokaryote domains are the Bacteria Domain and Archaea Domain, shown in Figure 8.2. All other living things are eukaryotes and placed in the domain Eukarya. (Unlike prokaryotes, eukaryotes have a nucleus in their cells.) | text | null |
L_0436 | introduction to prokaryotes | T_2636 | Prokaryotes were the first living things to evolve on Earth, probably around 3.8 billion years ago. They were the only living things until the first eukaryotic cells evolved about 2 billion years ago. Prokaryotes are still the most numerous organisms on Earth. Its not certain how the three domains of life are related. Archaea were once thought to be offshoots of Bacteria that were adapted to extreme environments. For their part, Bacteria were considered to be ancestors of Eukarya. Scientists now know that Archaea share several traits with Eukarya that Bacteria do not share. How can this be explained? One hypothesis is that the first Eukarya formed when an archaean cell fused with a bacterial cell. By fusing, the two prokaryotic cells became the nucleus and cytoplasm of a new eukaryotic cell. If this hypothesis is correct, both prokaryotic domains are ancestors of Eukarya. | text | null |
L_0436 | introduction to prokaryotes | T_2637 | All prokaryotes consist of just one cell. They share a number of other traits as well. Watch this entertaining video from the Amoeba Sisters to see how prokaryotes differ in structure from eukaryotes: MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0436 | introduction to prokaryotes | T_2638 | Most prokaryotic cells are much smaller than eukaryotic cells. Prokaryotic cells are typically only 0.2-2.0 microm- eter in diameter. Eukaryotic cells are about 50 times as big. Prokaryotic cells have a variety of different cell shapes. Figure 8.3 shows three of the most common shapes: spirals (helices), spheres, and rods. Bacteria may be classified by their shape. | text | null |
L_0436 | introduction to prokaryotes | T_2639 | Most prokaryotes have one or more long, thin "whips" called flagella (flagellum, plural). You can see flagella in Figure 8.4. Flagella help prokaryotes move toward food or away from toxins. Each flagellum spins around a fixed base. This causes the cell to roll and tumble. | text | null |
L_0436 | introduction to prokaryotes | T_2640 | The cells of prokaryotes have two or three outer layers. Like all other living cells, prokaryotes have a cell membrane. It controls what enters and leaves the cell. Its also the site of many metabolic reactions. For example, cellular respiration takes place in the cell membrane. Most prokaryotes also have a cell wall. It lies just outside the cell membrane. It makes the cell stronger and more rigid. Many prokaryotes have another layer, called a capsule, outside the cell wall. The capsule protects the cell from chemicals and drying out. It also allows the cell to stick to surfaces and to other cells. You can see a model of a prokaryotic cell in Figure 8.5. Find the cell membrane, cell wall, and capsule in the figure. | text | null |
L_0436 | introduction to prokaryotes | T_2641 | Several other prokaryotic cell structures are also shown in Figure 8.5. They include: cytoplasm. Like all other cells, prokaryotic cells are filled with cytoplasm. It includes watery cytosol and other structures. ribosomes. This is the site where proteins are made. cytoskeleton. This is a network of fibers and tubules that crisscrosses the cytoplasm. The cytoskeleton helps the cell keep its shape. pili. These are hair-like projections from the surface of the cell. They help the cell hold on to surfaces or do other jobs for the cell. | text | null |
L_0436 | introduction to prokaryotes | T_2642 | All prokaryotic cells contain DNA, as you can see in Figure 8.6. Most of the DNA is in the form of a single large loop. This DNA coils up in the cytoplasm to form a structure called a nucleoid. There is no membrane surrounding it. Most prokaryotes also have one or more small loops of DNA. They are called plasmids. | text | null |
L_0436 | introduction to prokaryotes | T_2642 | All prokaryotic cells contain DNA, as you can see in Figure 8.6. Most of the DNA is in the form of a single large loop. This DNA coils up in the cytoplasm to form a structure called a nucleoid. There is no membrane surrounding it. Most prokaryotes also have one or more small loops of DNA. They are called plasmids. | text | null |
L_0436 | introduction to prokaryotes | T_2643 | Some prokaryotes form structures consisting of many individual cells, like the cells in Figure 8.7. This is called a biofilm. A biofilm is a colony of prokaryotes that is stuck to a surface. The surface might be a rock or a hosts tissues. The sticky plaque that collects on your teeth between brushings is a biofilm. It consists of millions of prokaryotic cells. | text | null |
L_0436 | introduction to prokaryotes | T_2644 | Like all living things, prokaryotes need energy and carbon. They meet these needs in a variety of ways and in a range of habitats. | text | null |
L_0436 | introduction to prokaryotes | T_2645 | Prokaryotes may have just about any type of metabolism. They may get energy from light or from chemical compounds. They may get carbon from carbon dioxide or from other living things. Most prokaryotes get both energy and carbon from other living things. Many of them are decomposers. They break down wastes and remains of dead organisms. In this way, they help to recycle carbon and nitrogen through ecosystems. Some prokaryotes use energy in sunlight to make food from carbon dioxide. They do this by the process of photosynthesis. They are important producers in aquatic ecosystems. Look at the green streaks on the lake in Figure 8.8. They consist of billions of photosynthetic bacteria called cyanobacteria. | text | null |
L_0436 | introduction to prokaryotes | T_2646 | Prokaryotes live in a wide range of habitats. For example, they may live in habitats with or without oxygen. Prokaryotes that need oxygen are described as aerobic. They use oxygen for cellular respiration. Examples include the prokaryotes that live on your skin. Prokaryotes that dont need oxygen or are poisoned by it are described as anaerobic. They use fermentation or other anaerobic processes rather than cellular respiration. Examples include many of the prokaryotes that live inside your body. Like most other living things, prokaryotes have a temperature range that they "like" best. Thermophiles are prokaryotes that prefer a temperature above 45 C (113 F). They might be found in a compost pile. Mesophiles are prokaryotes that prefer a temperature of about 37 C (98 C). They might be found inside the body of an animal such as you. Psychrophiles are prokaryotes that prefer a temperature below 20 C (68 F). They might be found deep in the ocean. | text | null |
L_0436 | introduction to prokaryotes | T_2647 | Prokaryotes reproduce asexually. This can happen by binary fission or budding. In binary fission, a cell splits in two. First, the large circular chromosome is copied. Then the cell divides to form two new daughter cells. Each has a copy of the parent cells chromosome. In budding, a new cell grows from a bud on the parent cell. It only breaks off to form a new cell when it is fully formed. | text | null |
L_0436 | introduction to prokaryotes | T_2648 | For natural selection to take place, organisms must vary in their traits. Asexual reproduction results in offspring that are all the same. They are also identical to the parent cell. So how can prokaryotes increase genetic variation? They can exchange plasmids. This is called genetic transfer. It may happen by direct contact between cells. Or a "bridge" may form between cells. Genetic transfer mixes the genes of different cells. It creates new combinations of alleles. | text | null |
L_0439 | protists | T_2666 | Protists are placed in the Protist Kingdom. This kingdom is one of four kingdoms in the Eukarya domain. The other three Eukarya kingdoms are the Fungi, Plant, and Animal Kingdoms. | text | null |
L_0439 | protists | T_2667 | The Protist Kingdom is hard to define. It includes many different types of organisms. You can see some examples of protists in Figure 9.1. The Protist Kingdom includes all eukaryotes that dont fit into one of the other three eukaryote kingdoms. For that reason, its sometimes called the trash can kingdom. The number of species in the Protist Kingdom is unknown. It could range from as few as 60,000 to as many as 200,000 species. For a beautiful introduction to the amazing world of protists, watch this video: MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0439 | protists | T_2668 | Scientists think that protists are the oldest eukaryotes. If so, they must have evolved from prokaryotes. How did this happen? How did cells without organelles acquire them? What was the origin of mitochondria, chloroplasts, and other organelles? The most likely way organelles evolved is shown in Figure 9.2. First, smaller prokaryotic cells invaded, or were engulfed by, larger prokaryotic cells. The smaller cells benefited by getting nutrients and a safe place to live. The larger cells benefited by getting some of the organic molecules or energy released by the smaller cells. Eventually, the smaller cells evolved into organelles in the larger cells. After that, neither could live without the other. | text | null |
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