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L_0439 | protists | T_2669 | Despite the diversity of protists, they do share some traits. The cells of all protists have a nucleus. They also have other membrane-bound organelles. For example, all of them have mitochondria, and some of them have chloroplasts. Most protists consist of a single cell. Some are multicellular but they lack specialized cells. Most protists live in wet places. They are found in oceans, lakes, swamps, or damp soils. Many protists can move. Most protists also have a complex life cycle. The life cycle of an organism is the cycle of phases it goes through until it returns to the starting phase. The protist life cycle includes both sexual and asexual reproduction. Why reproduce both ways? Each way has benefits. Asexual reproduction is fast. It allows rapid population growth when conditions are stable. Sexual reproduction increases genetic variation. This helps ensure that some organisms will survive if conditions change. | text | null |
L_0439 | protists | T_2670 | Protists are classified based on traits they share with other eukaryotes. There are animal-like, plant-like, and fungus- like protists. The three groups differ mainly in how they get carbon and energy. | text | null |
L_0439 | protists | T_2671 | Animal-like protists are called protozoa (protozoan, singular). Most protozoa consist of a single cell. Protozoa are probably ancestors of animals. Protozoa are like animals in two ways: 1. Protozoa are heterotrophs. Heterotrophs get food by eating other organisms. Some protozoa prey on bacteria. Some are parasites of animals. Others graze on algae. Still others are decomposers that break down dead organic matter. 2. Almost all protozoa can move. They have special appendages for this purpose. You can see different types in Figure 9.3. Cilia (cilium, singular) are short, hair-like projections. Pseudopods are temporary extensions of the cytoplasm. Flagella are long, whip-like structures. Flagella are also found in most prokaryotes. | text | null |
L_0439 | protists | T_2672 | Plant-like protists are commonly called algae (alga, singular). Some algae consist of single cells. They are called diatoms. Other algae are multicellular. An example is seaweed. Seaweed called kelp can grow as large as trees. You can see both a diatom and kelp in Figure 9.4. Algae are probably ancestors of plants. Algae are like plants mainly because they contain chloroplasts. This allows them to make food by photosynthesis. Algae are important producers in water-based ecosystems such as the ocean. On the other hand, algae lack other plant structures. For example, they dont have roots, stems, or leaves. Also unlike plants, some algae can move. They may move with pseudopods or flagella. | text | null |
L_0439 | protists | T_2673 | Fungus-like protists include slime molds and water molds, both shown in Figure 9.5. They exist as individual cells or as many cells that form a blob-like colony. They are probably ancestors of fungi. Like fungi, many fungus-like protists are decomposers. They absorb nutrients from dead logs, compost, and other organic remains Slime molds are commonly found on rotting organic matter such as compost. Swarms of cells move very slowly over the surface. They digest and absorb nutrients as they go. Water molds are commonly found in moist soil and surface water. Many water molds are plant pathogens or fish parasites. | text | null |
L_0439 | protists | T_2674 | Many human diseases are caused by protists. Most of them are caused by protozoa. They are parasites that invade and live in the human body. The parasites get a place to live and nutrients from the human host. In return, they make the host sick. Examples of human diseases caused by protozoa include giardiasis and malaria. Protozoa that cause giardiasis are spread by contaminated food or water. They live inside the intestine. They may cause abdominal pain, fever, and diarrhea. Protozoa that cause malaria are spread by a vector. They enter the blood through the bite of an infected mosquito. They live inside red blood cells. They cause overall body pain, fever, and fatigue. Malaria kills several million people each year. Most of the deaths occur in children. | text | null |
L_0440 | fungi | T_2675 | Fungi (fungus, singular) are relatively simple eukaryotic organisms. They are placed in their own kingdom, the Fungus Kingdom. Most fungi are multicellular organisms. These fungi are called molds. However, some fungi exist as single cells. These fungi are called yeasts. You can see examples of different types of fungi in Figure 9.7. For a funny, fast-paced overview of fungi, watch this video: . MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0440 | fungi | T_2676 | For a long time, scientists classified fungi as members of the Plant Kingdom. Fungi share several obvious traits with plants. For example, both fungi and plants lack the ability to move. Both grow in soil, and both have cell walls. Some fungi even look like plants. | text | null |
L_0440 | fungi | T_2677 | Today, fungi are no longer classified as plants. We now know that they have important traits that set them apart from plants. Thats why they are placed in their own kingdom. How do fungi differ from plants? The cell walls of fungi are made of chitin. Chitin is a tough carbohydrate that also makes up the outer skeleton of insects. The cell walls of plants are made of cellulose. Fungi are heterotrophs that absorb food from other organisms. Plants are autotrophs that make their own food. The Fungus Kingdom is large and diverse. It may contain more than a million species. However, fewer than 100,000 species of fungi have been identified. | text | null |
L_0440 | fungi | T_2678 | The earliest fungi evolved about 600 million years ago. They lived in the water. Fungi colonized the land around the same time as plants. That was probably between 400 and 500 million years ago. After that, fungi became very abundant on land. By 250 million years ago, they may have been the dominant life forms on land. | text | null |
L_0440 | fungi | T_2679 | Yeasts grow as single cells. Other fungi grow into multicellular, thread-like structures. These structures are called hyphae (hypha, singular). You can see a photo of hyphae in Figure 9.8. They resemble plant roots. Each hypha consists of a group of cells surrounded by a tubular cell wall. A mass of hyphae make up the body of a fungus. The body is called the mycelium (mycelia, plural). A mycelium may range in size from microscopic to very large. In fact, the largest living thing on Earth is the mycelium of a single fungus. Nicknamed the humongous fungus, it grows in a forest in Oregon. A small part of the fungus is pictured in Figure 9.9. The giant fungus covers an area of 2384 acres. Thats about the size of 1,665 football fields! The fungus is estimated to be at least 2400 years old, but it could be much older. | text | null |
L_0440 | fungi | T_2680 | Most fungi reproduce both asexually and sexually. In both types of reproduction, they produce spores. A spore is a special reproductive cell. When fungi reproduce asexually, they can spread quickly. This is good when conditions are stable. They can increase their genetic variation by sexual reproduction. This is beneficial when conditions are changing. Variation helps ensure that at least some organisms survive the changing conditions. Figure 9.10 shows how asexual and sexual reproduction occur in fungi. Refer to the figure as you read about each of them below. | text | null |
L_0440 | fungi | T_2680 | Most fungi reproduce both asexually and sexually. In both types of reproduction, they produce spores. A spore is a special reproductive cell. When fungi reproduce asexually, they can spread quickly. This is good when conditions are stable. They can increase their genetic variation by sexual reproduction. This is beneficial when conditions are changing. Variation helps ensure that at least some organisms survive the changing conditions. Figure 9.10 shows how asexual and sexual reproduction occur in fungi. Refer to the figure as you read about each of them below. | text | null |
L_0440 | fungi | T_2681 | During asexual reproduction, fungi produce haploid spores by mitosis of a haploid parent cell. A haploid cell has just one of each pair of chromosomes. The haploid spores are genetically identical to the parent cell. Spores may be spread by moving water, wind, or other organisms. Wherever the spores land, they will develop into new hyphae only when conditions are suitable for growth. Yeasts are an exception. They reproduce asexually by budding instead of by producing spores. An offspring cell forms on a parent cell. After it grows and develops, it buds off to form a new cell. The offspring cell is genetically identical to the parent cell. You can see yeast cells budding in Figure 9.11. | text | null |
L_0440 | fungi | T_2681 | During asexual reproduction, fungi produce haploid spores by mitosis of a haploid parent cell. A haploid cell has just one of each pair of chromosomes. The haploid spores are genetically identical to the parent cell. Spores may be spread by moving water, wind, or other organisms. Wherever the spores land, they will develop into new hyphae only when conditions are suitable for growth. Yeasts are an exception. They reproduce asexually by budding instead of by producing spores. An offspring cell forms on a parent cell. After it grows and develops, it buds off to form a new cell. The offspring cell is genetically identical to the parent cell. You can see yeast cells budding in Figure 9.11. | text | null |
L_0440 | fungi | T_2682 | Sexual reproduction also occurs in most fungi. It happens when two haploid hyphae mate. During mating, two haploid parent cells fuse. The single fused cell that results is a diploid spore. It is genetically different from both parents. The spore undergoes meiosis to form haploid daughter cells. These haploid cells develop into new hyphae. | text | null |
L_0440 | fungi | T_2683 | Most fungi grow on moist soil or rotting vegetation such as dead logs. Some fungi live in water. Others live in or on other organisms. Fungi get their nutrition by absorbing organic compounds from other organisms. The other organisms may be dead or alive, depending on the fungus. | text | null |
L_0440 | fungi | T_2684 | Most fungi get organic compounds from dead organisms. Fungi use their hyphae to penetrate deep into decaying organic matter. They produce enzymes at the tips of their hyphae. The enzymes digest the organic matter so the fungal cells can absorb it. Fungi are the main decomposers in forests. They are the only decomposers that can break down cellulose and wood. They have special enzymes for this purpose. | text | null |
L_0440 | fungi | T_2685 | Many fungi get organic compounds from living organisms. They have close relationships with other species. A close relationship between two species is called a symbiotic relationship. Two symbiotic relationships in fungi are mycorrhiza and lichen. These relationships are beneficial for both species. Mycorrhiza is a relationship between a fungus and a plant. The fungus grows in or on the plants roots. The fungus benefits from easy access to food made by the plant. The plant benefits because the fungal hyphae absorb water and nutrients from the soil that the plant needs. Lichen is a relationship between a fungus and cyanobacteria or green algae. The fungus grows around the bacterial or algal cells. The fungus benefits by getting some of the food made by the photosynthetic cells. The bacteria or algae benefit by getting some of the water and nutrients absorbed by the fungus. You can see a picture of lichen in Figure 9.12. Some fungi have a different kind of relationship with plants. They are plant parasites. They get food from the plants and cause harm to the plants in return. Fungi are the major causes of disease in agricultural crops. They may eventually kill their plant hosts. Some fungi are animal parasites. The wasp in Figure 9.13 is infected with a fungus. The fungus is the white fuzzy matter on the dark brown moth. | text | null |
L_0440 | fungi | T_2685 | Many fungi get organic compounds from living organisms. They have close relationships with other species. A close relationship between two species is called a symbiotic relationship. Two symbiotic relationships in fungi are mycorrhiza and lichen. These relationships are beneficial for both species. Mycorrhiza is a relationship between a fungus and a plant. The fungus grows in or on the plants roots. The fungus benefits from easy access to food made by the plant. The plant benefits because the fungal hyphae absorb water and nutrients from the soil that the plant needs. Lichen is a relationship between a fungus and cyanobacteria or green algae. The fungus grows around the bacterial or algal cells. The fungus benefits by getting some of the food made by the photosynthetic cells. The bacteria or algae benefit by getting some of the water and nutrients absorbed by the fungus. You can see a picture of lichen in Figure 9.12. Some fungi have a different kind of relationship with plants. They are plant parasites. They get food from the plants and cause harm to the plants in return. Fungi are the major causes of disease in agricultural crops. They may eventually kill their plant hosts. Some fungi are animal parasites. The wasp in Figure 9.13 is infected with a fungus. The fungus is the white fuzzy matter on the dark brown moth. | text | null |
L_0440 | fungi | T_2686 | Fungi may cause disease in people as well as other organisms. On the other hand, people have been using fungi for thousands of years. | text | null |
L_0440 | fungi | T_2687 | One way we use fungi is by eating them. Many species of mushrooms are edible. Yeasts are used for break making. Other fungi are used to ferment foods, such as soy sauce and cheeses. You can see the fungus growing through the blue cheese in Figure 9.14. The fungus gives the cheese its distinctive appearance and taste. People also use fungi: to produce antibiotics. to produce human hormones such as insulin. as natural pesticides. as model research organisms. | text | null |
L_0440 | fungi | T_2688 | Several common human diseases are caused by fungi. They include ringworm and athletes foot, both shown in Figure 9.15. Ringworm isnt caused by a worm. Its a skin infection by a fungus that leads to a ring-shaped rash. The rash may occur on the head, neck, trunk, arms, or legs. Athletes foot is caused by the same fungus as ringworm. But in athletes foot, the fungus infects the skin between the toes. Athletes foot is the second most common skin disease in the U.S. | text | null |
L_0441 | active transport | T_2689 | During active transport, molecules move from an area of low concentration to an area of high concentration. This is the opposite of diffusion, and these molecules are said to flow against their concentration gradient. Active transport is called "active" because this type of transport requires energy to move molecules. ATP is the most common source of energy for active transport. As molecules are moving against their concentration gradients, active transport cannot occur without assistance. A carrier protein is always required in this process. Like facilitated diffusion, a protein in the membrane carries the molecules across the membrane, except this protein moves the molecules from a low concentration to a high concentration. These proteins are often called "pumps" because they use energy to pump the molecules across the membrane. There are many cells in your body that use pumps to move molecules. For example, your nerve cells (neurons) would not send messages to your brain unless you had protein pumps moving molecules by active transport. The sodium-potassium pump ( Figure 1.1) is an example of an active transport pump. The sodium-potassium pump uses ATP to move three sodium (Na+ ) ions and two potassium (K+ ) ions to where they are already highly concentrated. Sodium ions move out of the cell, and potassium ions move into the cell. How do these ions then return to their original positions? As the ions now can flow down their concentration gradients, facilitated diffusion returns the ions to their original positions either inside or outside the cell. | text | null |
L_0451 | archaea | T_2724 | For many years, archaea were classified as bacteria. Like the bacteria, archaea lacked a nucleus and membrane- bound organelles and, therefore, were prokaryotic cells. However, when scientists compared the DNA of the two prokaryotes, they found that there were distinct differences. They concluded that there must be two distinct types of prokaryotes, which they named archaea and bacteria. Even though the two groups might seem similar, archaea have many features that distinguish them from bacteria: 1. The cell walls of archaea are distinct from those of bacteria. While bacteria have cell walls made up of the polymer peptidoglycan, most archaea do not have peptidoglycan in their cell walls. 2. The plasma membranes of the archaea are also made up of lipids that are distinct from those in bacteria. 3. The ribosomal proteins of the archaea are similar to those in eukaryotic cells, not those in bacteria. Although archaea and bacteria share some fundamental differences, they are still similar in many ways: 1. They both are single-celled, microscopic organisms that can come in a variety of shapes ( Figure 1.1). 2. Both archaea and bacteria have a single circular chromosome of DNA and lack membrane-bound organelles. 3. Like bacteria, archaea can have flagella to assist with movement. Archaea shapes can vary widely, but some are bacilli (rod-shaped). | text | null |
L_0451 | archaea | T_2725 | Most archaea are chemotrophs and derive their energy and nutrients from breaking down molecules in their envi- ronment. A few species of archaea are photosynthetic and capture the energy of sunlight. Unlike bacteria, which can be parasites and are known to cause a variety of diseases, there are no known archaea that act as parasites. Some archaea do live within other organisms. But these archea form mutualistic relationships with their host, where both the archaea and the host benefit. In other words, they assist the host in some way, for example by helping to digest food. | text | null |
L_0451 | archaea | T_2726 | Like bacteria, reproduction in archaea is asexual. Archaea can reproduce through binary fission, where a parent cell divides into two genetically identical daughter cells. Archaea can also reproduce asexually through budding and fragmentation, where pieces of the cell break off and form a new cell, also producing genetically identical organisms. | text | null |
L_0453 | asexual vs. sexual reproduction | T_2730 | Animals and other organisms cannot live forever. They must reproduce if their species is to survive. But what does it mean to reproduce? Reproduction is the ability to make the next generation, and it is one of the basic characteristics of life. Two methods of reproduction are: 1. Asexual reproduction, the process of forming a new individual from a single parent. 2. Sexual reproduction, the process of forming a new individual from two parents. There are advantages and disadvantages to each method, but the result is always the same: a new life begins. | text | null |
L_0453 | asexual vs. sexual reproduction | T_2731 | When humans reproduce, there are two parents involved. DNA must be passed from both the mother and father to the child. Humans cannot reproduce with just one parent; humans can only reproduce sexually. But having just one parent is possible in other eukaryotic organisms, including some insects, fish, and reptiles. These organisms can reproduce asexually, meaning the offspring ("children") have a single parent and share the exact same genetic material as the parent. This is very different from reproduction in humans. Bacteria, being a prokaryotic, single- celled organism, must reproduce asexually. The advantage of asexual reproduction is that it can be very quick and does not require the meeting of a male and female organism. The disadvantage of asexual reproduction is that organisms do not receive a mix of traits from both parents. An organism that is born through asexual reproduction only has the DNA from the one parent. In fact, the offspring is genetically an exact copy of the parent. This can cause problems for the individual. For example, if the parent has a gene that causes a particular disease, the offspring will also have the gene that causes that disease. Organisms produced sexually may or may not inherit the disease gene because they receive a mix of their parents genes. Types of organisms that reproduce asexually include: 1. Prokaryotic organisms, like bacteria. Bacteria reproduce through binary fission, where they grow and divide in half ( Figure 1.1). First, their chromosome replicates and the cell enlarges. The cell then divides into two cells as new membranes form to separate the two cells. After cell division, the two new cells each have one identical chromosome. This simple process allows bacteria to reproduce very rapidly. 2. Flatworms, an invertebrate animal species. Flatworms divide in two, then each half regenerates into a new flatworm identical to the original, a process called fragmentation. 3. Different types of insects, fish, and lizards. These organisms can reproduce asexually through a process called parthenogenesis. Parthenogenesis happens when an unfertilized egg cell grows into a new organism. The resulting organism has half the amount of genetic material of the parent. Parthenogenesis is common in honeybees. In a hive, the sexually produced eggs become workers, while the asexually produced eggs become drones. | text | null |
L_0453 | asexual vs. sexual reproduction | T_2732 | During sexual reproduction, two parents are involved. Most animals are dioecious, meaning there is a separate male and female sex, with the male producing sperm and the female producing eggs. When a sperm and egg meet during fertilization, a zygote, the first cell of a new organism, is formed ( Figure 1.2). This process combines the genetic material from both parents. The resulting organism will be genetically unique. The zygote will divide by mitosis and grow into the embryo. Lets explore how animals, plants, and fungi reproduce sexually: Animals often have gonads, organs that produce eggs or sperm. The male gonads are the testes, and the female gonads are the ovaries. Testes produce sperm; ovaries produce eggs. Sperm and egg, the two sex cells, are known as gametes, and can combine two different ways, both of which combine the genetic material from the two parents. Gametes have half the amount of the genetic material of a regular body cell; they are haploid cells. In humans, gametes have one set of 23 chromosomes. Gametes are produced through a special type of cell division known as meiosis. Normal human cells have 46 chromosomes. They are diploid cells, with two sets of 23 chromosomes (23 pairs). Bacteria reproduce by binary fission. Shown is one bacterium reproducing and becoming two bacteria. During sexual reproduction, a sperm fer- tilizes an egg. Fish and other aquatic animals release their gametes in the water, which is called external fertilization ( Figure by internal fertilization. Typically males have a penis that deposits sperm into the vagina of the female. Birds do not have penises, but they do have a chamber called the cloaca that they place close to another birds cloaca to deposit sperm. Amphibians must live close to water as they must lay their eggs in a moist or wet environment prior to external fertilization. This fish guards her eggs, which will be fertilized externally. Plants can also reproduce sexually, but their reproductive organs are different from animals gonads. Plants that have flowers have their reproductive parts in the flower. The sperm is contained in the pollen, while the egg is contained in the ovary, deep within the flower. The sperm can reach the egg two different ways: 1. In self-pollination, the egg is fertilized by the pollen of the same flower. 2. In cross-pollination, sperm from the pollen of one flower fertilizes the egg of another flower. Like other types of sexual reproduction, cross-pollination allows new combinations of traits. Cross-pollination occurs when pollen is carried by the wind to another flower. It can also occur when animal pollinators, like honeybees or butterflies ( Figure 1.4), carry the pollen from flower to flower. Butterflies receive nectar when they de- posit pollen into flowers, resulting in cross-pollination. | text | null |
L_0453 | asexual vs. sexual reproduction | T_2732 | During sexual reproduction, two parents are involved. Most animals are dioecious, meaning there is a separate male and female sex, with the male producing sperm and the female producing eggs. When a sperm and egg meet during fertilization, a zygote, the first cell of a new organism, is formed ( Figure 1.2). This process combines the genetic material from both parents. The resulting organism will be genetically unique. The zygote will divide by mitosis and grow into the embryo. Lets explore how animals, plants, and fungi reproduce sexually: Animals often have gonads, organs that produce eggs or sperm. The male gonads are the testes, and the female gonads are the ovaries. Testes produce sperm; ovaries produce eggs. Sperm and egg, the two sex cells, are known as gametes, and can combine two different ways, both of which combine the genetic material from the two parents. Gametes have half the amount of the genetic material of a regular body cell; they are haploid cells. In humans, gametes have one set of 23 chromosomes. Gametes are produced through a special type of cell division known as meiosis. Normal human cells have 46 chromosomes. They are diploid cells, with two sets of 23 chromosomes (23 pairs). Bacteria reproduce by binary fission. Shown is one bacterium reproducing and becoming two bacteria. During sexual reproduction, a sperm fer- tilizes an egg. Fish and other aquatic animals release their gametes in the water, which is called external fertilization ( Figure by internal fertilization. Typically males have a penis that deposits sperm into the vagina of the female. Birds do not have penises, but they do have a chamber called the cloaca that they place close to another birds cloaca to deposit sperm. Amphibians must live close to water as they must lay their eggs in a moist or wet environment prior to external fertilization. This fish guards her eggs, which will be fertilized externally. Plants can also reproduce sexually, but their reproductive organs are different from animals gonads. Plants that have flowers have their reproductive parts in the flower. The sperm is contained in the pollen, while the egg is contained in the ovary, deep within the flower. The sperm can reach the egg two different ways: 1. In self-pollination, the egg is fertilized by the pollen of the same flower. 2. In cross-pollination, sperm from the pollen of one flower fertilizes the egg of another flower. Like other types of sexual reproduction, cross-pollination allows new combinations of traits. Cross-pollination occurs when pollen is carried by the wind to another flower. It can also occur when animal pollinators, like honeybees or butterflies ( Figure 1.4), carry the pollen from flower to flower. Butterflies receive nectar when they de- posit pollen into flowers, resulting in cross-pollination. | text | null |
L_0453 | asexual vs. sexual reproduction | T_2732 | During sexual reproduction, two parents are involved. Most animals are dioecious, meaning there is a separate male and female sex, with the male producing sperm and the female producing eggs. When a sperm and egg meet during fertilization, a zygote, the first cell of a new organism, is formed ( Figure 1.2). This process combines the genetic material from both parents. The resulting organism will be genetically unique. The zygote will divide by mitosis and grow into the embryo. Lets explore how animals, plants, and fungi reproduce sexually: Animals often have gonads, organs that produce eggs or sperm. The male gonads are the testes, and the female gonads are the ovaries. Testes produce sperm; ovaries produce eggs. Sperm and egg, the two sex cells, are known as gametes, and can combine two different ways, both of which combine the genetic material from the two parents. Gametes have half the amount of the genetic material of a regular body cell; they are haploid cells. In humans, gametes have one set of 23 chromosomes. Gametes are produced through a special type of cell division known as meiosis. Normal human cells have 46 chromosomes. They are diploid cells, with two sets of 23 chromosomes (23 pairs). Bacteria reproduce by binary fission. Shown is one bacterium reproducing and becoming two bacteria. During sexual reproduction, a sperm fer- tilizes an egg. Fish and other aquatic animals release their gametes in the water, which is called external fertilization ( Figure by internal fertilization. Typically males have a penis that deposits sperm into the vagina of the female. Birds do not have penises, but they do have a chamber called the cloaca that they place close to another birds cloaca to deposit sperm. Amphibians must live close to water as they must lay their eggs in a moist or wet environment prior to external fertilization. This fish guards her eggs, which will be fertilized externally. Plants can also reproduce sexually, but their reproductive organs are different from animals gonads. Plants that have flowers have their reproductive parts in the flower. The sperm is contained in the pollen, while the egg is contained in the ovary, deep within the flower. The sperm can reach the egg two different ways: 1. In self-pollination, the egg is fertilized by the pollen of the same flower. 2. In cross-pollination, sperm from the pollen of one flower fertilizes the egg of another flower. Like other types of sexual reproduction, cross-pollination allows new combinations of traits. Cross-pollination occurs when pollen is carried by the wind to another flower. It can also occur when animal pollinators, like honeybees or butterflies ( Figure 1.4), carry the pollen from flower to flower. Butterflies receive nectar when they de- posit pollen into flowers, resulting in cross-pollination. | text | null |
L_0453 | asexual vs. sexual reproduction | T_2732 | During sexual reproduction, two parents are involved. Most animals are dioecious, meaning there is a separate male and female sex, with the male producing sperm and the female producing eggs. When a sperm and egg meet during fertilization, a zygote, the first cell of a new organism, is formed ( Figure 1.2). This process combines the genetic material from both parents. The resulting organism will be genetically unique. The zygote will divide by mitosis and grow into the embryo. Lets explore how animals, plants, and fungi reproduce sexually: Animals often have gonads, organs that produce eggs or sperm. The male gonads are the testes, and the female gonads are the ovaries. Testes produce sperm; ovaries produce eggs. Sperm and egg, the two sex cells, are known as gametes, and can combine two different ways, both of which combine the genetic material from the two parents. Gametes have half the amount of the genetic material of a regular body cell; they are haploid cells. In humans, gametes have one set of 23 chromosomes. Gametes are produced through a special type of cell division known as meiosis. Normal human cells have 46 chromosomes. They are diploid cells, with two sets of 23 chromosomes (23 pairs). Bacteria reproduce by binary fission. Shown is one bacterium reproducing and becoming two bacteria. During sexual reproduction, a sperm fer- tilizes an egg. Fish and other aquatic animals release their gametes in the water, which is called external fertilization ( Figure by internal fertilization. Typically males have a penis that deposits sperm into the vagina of the female. Birds do not have penises, but they do have a chamber called the cloaca that they place close to another birds cloaca to deposit sperm. Amphibians must live close to water as they must lay their eggs in a moist or wet environment prior to external fertilization. This fish guards her eggs, which will be fertilized externally. Plants can also reproduce sexually, but their reproductive organs are different from animals gonads. Plants that have flowers have their reproductive parts in the flower. The sperm is contained in the pollen, while the egg is contained in the ovary, deep within the flower. The sperm can reach the egg two different ways: 1. In self-pollination, the egg is fertilized by the pollen of the same flower. 2. In cross-pollination, sperm from the pollen of one flower fertilizes the egg of another flower. Like other types of sexual reproduction, cross-pollination allows new combinations of traits. Cross-pollination occurs when pollen is carried by the wind to another flower. It can also occur when animal pollinators, like honeybees or butterflies ( Figure 1.4), carry the pollen from flower to flower. Butterflies receive nectar when they de- posit pollen into flowers, resulting in cross-pollination. | text | null |
L_0455 | b and t cell response | T_2736 | Some defenses, like your skin and mucous membranes, are not designed to ward off a specific pathogen. They are just general defenders against disease. Your body also has defenses that are more specialized. Through the help of your immune system, your body can generate an army of cells to kill that one specific pathogen. There are two different types of specific immune responses. One type involves B cells. The other type involves T cells. Recall that B cells and T cells are types of white blood cells that are key in the immune response. Whereas the immune systems first and second line of defense are more generalized or non-specific, the immune response is specific. It can be described as a specific response to a specific pathogen, meaning it uses methods to target just one pathogen at a time. These methods involve B and T cells. | text | null |
L_0455 | b and t cell response | T_2737 | B cells respond to pathogens and other cells from outside the body in the blood and lymph. Most B cells fight infections by making antibodies. An antibody is a large, Y-shaped protein that binds to an antigen, a protein that is recognized as foreign. Antigens are found on the outside of bacteria, viruses and other foreign microorganisms. Each antibody can bind with just one specific type of antigen ( Figure 1.1). They fit together like a lock and key. Once an antigen and antibody bind together, they signal for a phagocyte to destroy them. Phagocytes are white blood cells that engulf targeted antigens by phagocytosis. As the antigen is on the outside of a pathogen, the pathogen is destroyed by this process. At any one time the average human body contains antibodies that can react with about 100,000,000 different antigens. This means that there can be 100,000,000 different antibody proteins in the body. | text | null |
L_0455 | b and t cell response | T_2738 | 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 1.2). When the killer T cell comes into contact with the infected cell, it releases poisons. The poisons make tiny holes in the cell membrane of the infected cell. 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 or damaged body cells. But they are still necessary for an immune response. They help by releasing chemicals that control other lymphocytes. The chemicals released by helper T cells switch on both B cells and killer T cells so they can recognize and fight specific pathogens. | text | null |
L_0456 | bacteria characteristics | T_2739 | Bacteria are the most successful organisms on the planet. They lived on this planet for two billion years before the first eukaryotes and, during that time, evolved into millions of different species. | text | null |
L_0456 | bacteria characteristics | T_2740 | Bacteria are so small that they can only be seen with a microscope. When viewed under the microscope, they have three distinct shapes ( Figure 1.1). Bacteria can be identified and classified by their shape: 1. Bacilli are rod-shaped. 2. Cocci are sphere-shaped. 3. Spirilli are spiral-shaped. Bacteria come in many different shapes. Some of the most common shapes are bacilli (rods), cocci (spheres), and spirilli (spirals). Bacteria can be identified and classified by their shape. | text | null |
L_0456 | bacteria characteristics | T_2741 | Like eukaryotic cells, bacterial cells have: 1. 2. 3. 4. Cytoplasm, the fluid inside the cell. A plasma or cell membrane, which acts as a barrier around the cell. Ribosomes, in which proteins are put together. DNA. By contrast though, bacterial DNA is contained in a large, circular strand. This single chromosome is located in a region of the cell called the nucleoid. The nucleoid is not an organelle, but a region within the cytoplasm. Many bacteria also have additional small rings of DNA known as plasmids. See bacterial cell pictured below ( Figure 1.2). The structure of a bacterial cell is dis- tinctive from a eukaryotic cell because of features such as an outer cell wall, the circular DNA of the nucleoid, and the lack of membrane-bound organelles. | text | null |
L_0456 | bacteria characteristics | T_2742 | Bacteria lack many of the structures that eukaryotic cells contain. For example, they dont have a nucleus. They also lack membrane-bound organelles, such as mitochondria or chloroplasts. The DNA of a bacterial cell is also different from a eukaryotic cell. Bacterial DNA is contained in one circular chromosome, located in the cytoplasm. Eukaryotes have several linear chromosomes. Bacteria also have two additional unique features: a cell wall and flagella. Some bacteria also have a capsule outside the cell wall. | text | null |
L_0456 | bacteria characteristics | T_2743 | Bacteria are surrounded by a cell wall consisting of peptidoglycan. This complex molecule consists of sugars and amino acids. The cell wall is important for protecting bacteria. The cell wall is so important that some antibiotics, such as penicillin, kill bacteria by preventing the cell wall from forming. Some bacteria depend on a host organism for energy and nutrients. These bacteria are known as parasites. If the host starts attacking the parasitic bacteria, the bacteria release a layer of slime that surrounds the cell wall. This slime offers an extra layer of protection. | text | null |
L_0456 | bacteria characteristics | T_2744 | Some bacteria also have tail-like structures called flagella ( Figure 1.3). Flagella help bacteria move. As the flagella rotate, they spin the bacteria and propel them forward. It is often said the flagella looks like a tiny whip, propelling the bacteria forward. Though some eukaryotic cells do have a flagella, a flagella in eukaryotes is rare. The flagella facilitate movement in bacte- ria. Bacteria may have one, two, or many flagellaor none at all. | text | null |
L_0459 | bacteria reproduction | T_2752 | Bacteria, being single-celled prokaryotic organisms, do not have a male or female version. Bacteria reproduce asexually. In asexual reproduction, the "parent" produces a genetically identical copy of itself. | text | null |
L_0459 | bacteria reproduction | T_2753 | Bacteria reproduce through a process called binary fission. During binary fission, the chromosome copies itself, forming two genetically identical copies. Then, the cell enlarges and divides into two new daughter cells. The two daughter cells are identical to the parent cell. Binary fission can happen very rapidly. Some species of bacteria can double their population in less than ten minutes! This process makes it possible for a tremendous bacterial colony to start from a single cell. | text | null |
L_0459 | bacteria reproduction | T_2754 | Are there male and female bacteria? Of course the answer is no. So, sexual reproduction does not occur in bacteria. But not all new bacteria are clones. This is because bacteria can acquire new DNA. This process occurs in three different ways: 1. Conjugation: In conjugation, DNA passes through an extension on the surface of one bacterium and travels to another bacterium ( Figure 1.1). Bacteria essential exchange DNA via conjugation. 2. Transformation: In transformation, bacteria pick up pieces of DNA from their environment. 3. Transduction: In transduction, viruses that infect bacteria carry DNA from one bacterium to another. | text | null |
L_0465 | blood diseases | T_2767 | Problems can occur with red blood cells, white blood cells, platelets, and other parts of the blood. Many blood disorders are genetic, meaning they are inherited from a parent. Some blood diseases are caused by not getting enough of a certain nutrient, while others are cancers of the blood. | text | null |
L_0465 | blood diseases | T_2768 | Anemia is a disease that occurs when there is not enough hemoglobin in the blood to carry oxygen to body cells. Hemoglobin is the blood protein that normally carries oxygen from the lungs to the tissues. Anemia leads to a lack of oxygen in organs. Anemia is usually caused by one of the following: A loss of blood from a bleeding wound or a slow leak of blood. The destruction of red blood cells. A lack of red blood cell production. Anemia may not have any symptoms. Some people with anemia feel weak or tired in general or during exercise. They also may have poor concentration. People with more severe anemia often get short of breath during times of activity. Iron-deficiency anemia is the most common type of anemia. It occurs when the body does not receive enough iron. Since there is not enough iron, hemoglobin, which needs iron to bind oxygen, cannot function properly. In the United States, 20% of all women of childbearing age have iron-deficiency anemia, compared with only 2% of adult men. The most common cause of iron-deficiency anemia in young women is blood lost during menstruation. Iron deficiency anemia can be avoided by getting the recommended amount of iron in ones diet. Anemia is often treated or prevented by taking iron supplements. Boys and girls between the ages of 9 and 13 should get 9 mg of iron every day. Girls between the ages of 14 and 18 should get 15 mg of iron every day. Boys between the ages of 14 and 18 should get 11 mg of iron every day. Pregnant women need the most iron27 mg daily. Good sources of iron include shellfish, such as clams and oysters. Red meats, such as beef, are also a good source of iron. Non-animal sources of iron include seeds, nuts, and legumes. Breakfast cereals often have iron added to them in a process called fortification. Some good sources of iron are listed below ( Table 1.1). Eating vitamin C along with iron-containing food increases the amount of iron that the body can absorb. Food Canned clams, drained, 3 oz. Fortified dry cereals, about 1 oz. Roasted pumpkin and squash seeds, 1 oz. Cooked lentils, 12 cup Cooked fresh spinach, 21 cup Cooked ground beef, 3 oz. Cooked sirloin beef, 3 oz. Milligrams (mg) of Iron 23.8 1.8 to 21.1 4.2 3.3 3.2 2.2 2.0 | text | null |
L_0465 | blood diseases | T_2769 | Sickle-cell anemia is a blood disease that is caused by an abnormally shaped hemoglobin protein in red blood cells. Many of the red blood cells of a person with sickle-cell anemia are long and curved (sickle-shaped) ( Figure 1.1). The long, sickle shape of the cells can cause them to get stuck in narrow blood vessels. This clotting means that oxygen cannot reach the cells. People with sickle-cell anemia are most often well but can occasionally have painful attacks. The disease is not curable, but it can be treated with medicines. The red blood cells of a person with sickle-cell anemia (left) are long and pointed, rather than straight, like normal cells (right). The abnormal cells cannot carry oxygen properly and can get stuck in capillaries. | text | null |
L_0465 | blood diseases | T_2770 | Blood cancers affect the production and function of your blood cells. Most of these cancers start in your bone marrow where blood is produced. In most blood cancers, the normal production of blood cells is replaced by uncontrolled growth of an abnormal type of blood cell. These abnormal blood cells are cancerous cells, and prevent your blood from performing many of its functions, like fighting off infections or preventing serious bleeding. Leukemia is a cancer of the blood or bone marrow. It is characterized by an abnormal production of blood cells, usually white blood cells. Lymphoma is a cancer of a type of white blood cell called lymphocytes. There are many types of lymphoma. | text | null |
L_0465 | blood diseases | T_2771 | Hemophilia is the name of a group of hereditary diseases that affect the bodys ability to control blood clotting. Hemophilia is caused by a lack of clotting factors in the blood. Clotting factors are normally released by platelets. Since people with hemophilia cannot produce clots, any cut can put a person at risk of bleeding to death. The risk of internal bleeding is also increased in hemophilia, especially into muscles and joints. This disease affected the royal families of Europe. | text | null |
L_0475 | cell biology | T_2801 | A cell is the smallest structural and functional unit of an organism. Some organisms, like bacteria, consist of only one cell. Big organisms, like humans, consist of trillions of cells. Compare a human to a banana. On the outside, they look very different, but if you look close enough youll see that their cells are actually very similar. | 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_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 |
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