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L_0411 | communities | T_2377 | Some predator species play a special role in their community. They are called keystone species. When the population size of a keystone species changes, the populations of many other species are affected. Prairie dogs, pictured in Figure 23.10, are an example of a keystone species. Their numbers affect most of the other species in their community. Prairie dog actions improve the quality of soil and water for plants, upon which most other species in the community depend. | text | null |
L_0411 | communities | T_2378 | Both predators and prey have adaptations to predation that evolve through natural selection. Predator adaptations help them capture prey. Prey adaptations help them avoid predators. A common adaptation in both predator and prey species is camouflage. You can see an example in Figure 23.11. You can also see some amazing examples in this video: MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0411 | communities | T_2378 | Both predators and prey have adaptations to predation that evolve through natural selection. Predator adaptations help them capture prey. Prey adaptations help them avoid predators. A common adaptation in both predator and prey species is camouflage. You can see an example in Figure 23.11. You can also see some amazing examples in this video: MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0411 | communities | T_2379 | Competition is a relationship between organisms that depend on the same resources. The resources might be food, water, or space. Competition can occur between organisms of the same species or between organisms of different species. Competition within a species is called intraspecific competition. It leads to natural selection within the species, so the species becomes better adapted to its environment. Competition between different species is called interspecific competition. It might lead to the less well-adapted species going extinct. Or it might lead to one or both species evolving specialized adaptations. For example, competing species might evolve adaptations that allow them to use different food sources. You can see an example in Figure 23.12. | text | null |
L_0411 | communities | T_2380 | Symbiosis is a close relationship between two species in which at least one species benefits. For the other species, the relationship may be beneficial, harmful, or neutral. There are three types of symbiosis: mutualism, parasitism, and commensalism. | text | null |
L_0411 | communities | T_2381 | Mutualism is a symbiotic relationship in which both species benefit. An example of mutualism is pictured in Figure can inject poison in the anemones prey. The clownfish is protected from the stingers by mucus that covers its body. How do the two species benefit from their close relationship? The anemone provides the clownfish with a safe place to live by keeping away predatory fish. The clownfish also feeds on the remains of the anemones prey. In return, the clownfish helps the anemone catch food by attracting prey with its bright colors. Its feces also provide nutrients to the anemone. | text | null |
L_0411 | communities | T_2382 | Parasitism is a symbiotic relationship in which one species benefits and the other species is harmed. The species that benefits is called the parasite. The species that is harmed is called the host. Many species of animals are parasites, at least during some stage of their life cycle. Most animal species are also hosts to one or more parasites. A parasite generally lives in or on its host. An example of a parasite that lives in its host is the hookworm. Figure from their host, which is harmed by the loss of nutrients and blood. Some parasites kill their host, but most do not. Its easy to see why. If a parasite kills its host, the parasite may also die. Instead, parasites usually cause relatively minor damage to their host. | text | null |
L_0411 | communities | T_2383 | Commensalism is a symbiotic relationship in which one species benefits while the other species is not affected. An example is the relationship between birds called cattle egrets and cattle (see Figure 23.15). Cattle egrets feed on insects. They follow cattle herds around to take advantage of the insects stirred up by the feet of the cattle. The egrets get ready access to food from the relationship, whereas the cattle are not affected. | text | null |
L_0412 | ecosystems | T_2384 | Ecosystems need a constant input of energy to supply the needs of their organisms. Most ecosystems get energy from sunlight. A few ecosystems get energy from chemical compounds. Unlike energy, matter doesnt need to be constantly added to ecosystems. Instead, matter is recycled through ecosystems. Water and elements such as carbon and nitrogen that living things need are used over and over again. | text | null |
L_0412 | ecosystems | T_2385 | Two important concepts associated with the ecosystem are niche and habitat. | text | null |
L_0412 | ecosystems | T_2386 | Niche is the role that a particular species plays in its ecosystem. This role includes all the ways that the species interacts with the biotic and abiotic factors in the ecosystem. A major aspect of any niche is how the species obtains energy and matter. Look at Figure 23.16. The grass in the figure obtains energy from sunlight and uses it to convert carbon dioxide and water to sugar by photosynthesis. The deer in the figure gets matter and energy by consuming and digesting the grass. Each species has a different and distinctive niche. | text | null |
L_0412 | ecosystems | T_2387 | Another important aspect of a species niche is its habitat. Habitat is the physical environment in which a species lives and to which it has adapted. Features of a habitat depend mainly on abiotic factors, such as temperature and rainfall. These factors influence the traits of the organisms that live there. | text | null |
L_0412 | ecosystems | T_2388 | A given habitat may contain many different species. However, each species in the same habitat must have a different niche. Two different species cannot occupy the same niche in the same habitat at the same time. This is called the competitive exclusion principle. What do you think would happen if two species were to occupy the same niche in the same habitat? The two species would compete for everything they needed in the environment. One species might outcompete and replace the other. Or, both species might evolve different specializations so they can fill slightly different niches. | text | null |
L_0414 | flow of energy | T_2397 | Living things can be classified based on how they obtain energy. Some use the energy in sunlight or chemical compounds directly to make food. Some get energy indirectly by consuming other organisms, either living or dead. | text | null |
L_0414 | flow of energy | T_2398 | Producers are living things that produce food for themselves and other organisms. They use energy and simple inorganic molecules to make organic compounds. Producers are vital to all ecosystems because all organisms need organic compounds for energy. Producers are also called autotrophs. There are two basic types of autotrophs: photoautotrophs and chemoautotrophs. Photoautotrophs use energy in sunlight to make organic compounds by photosynthesis. They include plants, algae, and some bacteria (see Figure 24.1). Chemoautotrophs use energy in chemical compounds to make organic compounds. This process is called chemosynthesis. Chemoautotrophs include certain bacteria and archaea. | text | null |
L_0414 | flow of energy | T_2399 | Consumers are organisms that depend on other living things for food. They take in organic compounds by eating or absorbing other living things. Consumers include all animals and fungi. They also include some bacteria and protists. Consumers are also called heterotrophs. There are several different types of heterotrophs depending on exactly what they consume. They may be herbivores, carnivores, or omnivores. Herbivores are heterotrophs that consume producers such as plants or algae. Examples include rabbits and snails. Carnivores are heterotrophs that consume animals. Examples include lions and frogs. Omnivores are heterotrophs that consume both plants and animals. They include crows and human beings. The grizzly bears pictured in Figure 24.2 are also omnivores. | text | null |
L_0414 | flow of energy | T_2400 | Decomposers are heterotrophs that break down the wastes of other organisms or the remains of dead organisms. When they do, they release simple inorganic molecules back into the environment. Producers can then use the inorganic molecules to make new organic compounds. For this reason, decomposers are essential to every ecosystem. Imagine what would happen if there were no decomposers. Organic wastes and dead organisms would pile up everywhere, and their nutrients would no longer be recycled. Decomposers are classified by the type of organic matter they break down. They may be scavengers, detritivores, or saprotrophs. Scavengers are decomposers that consume the soft tissues of dead animals. Examples of scavengers include hyenas and cockroaches. Detritivores are decomposers that consume dead leaves, animal feces, and other organic debris that collects on the ground or at the bottom of a body of water. Examples of detritivores include earthworms and catfish. You can see another example in Figure 24.3. Saprotrophs are decomposers that feed on any remaining organic matter that is left after other decomposers do their work. Examples of saprotrophs include fungi and protozoa. | text | null |
L_0414 | flow of energy | T_2400 | Decomposers are heterotrophs that break down the wastes of other organisms or the remains of dead organisms. When they do, they release simple inorganic molecules back into the environment. Producers can then use the inorganic molecules to make new organic compounds. For this reason, decomposers are essential to every ecosystem. Imagine what would happen if there were no decomposers. Organic wastes and dead organisms would pile up everywhere, and their nutrients would no longer be recycled. Decomposers are classified by the type of organic matter they break down. They may be scavengers, detritivores, or saprotrophs. Scavengers are decomposers that consume the soft tissues of dead animals. Examples of scavengers include hyenas and cockroaches. Detritivores are decomposers that consume dead leaves, animal feces, and other organic debris that collects on the ground or at the bottom of a body of water. Examples of detritivores include earthworms and catfish. You can see another example in Figure 24.3. Saprotrophs are decomposers that feed on any remaining organic matter that is left after other decomposers do their work. Examples of saprotrophs include fungi and protozoa. | text | null |
L_0414 | flow of energy | T_2401 | Energy flows through ecosystems from producers, to consumers, to decomposers. Food chains and food webs are diagrams that model this flow of energy. They represent feeding relationships by showing who eats whom. | text | null |
L_0414 | flow of energy | T_2402 | A food chain is a diagram that represents a single pathway through which energy flows through an ecosystem. Food chains are generally simpler than what really happens in nature. Thats because most organisms consume and are consumed by more than one species. You can see examples of terrestrial and aquatic food chains in Figure 24.4. See if you can construct a food chain of each type by playing the animation at this link: | text | null |
L_0414 | flow of energy | T_2403 | A food web is a diagram that represents many pathways through which energy flows through an ecosystem. It includes a number of intersecting food chains. Food webs are generally more similar to what really happens in nature. They show that most organisms consume and are consumed by multiple species. You can see an example of a food web in Figure 24.5. | text | null |
L_0414 | flow of energy | T_2404 | Each food chain or food web has organisms at different trophic levels. A trophic level is a feeding position in a food chain or web. The trophic levels are identified in the food web in Figure 24.5. All food chains and webs have at least two or three trophic levels, but they rarely have more than four trophic levels. The trophic levels are: 1. 2. 3. 4. Trophic level 1 = producers that make their own food Trophic level 2 = primary consumers that eat producers Trophic level 3 = secondary consumers that eat primary consumers Trophic level 4 = tertiary consumers that eat secondary consumers Many consumers feed at more than one trophic level. For example, the bivalves in Figure 24.5 eat both producers and primary consumers. Therefore, they feed at trophic levels 2 and 3. | text | null |
L_0414 | flow of energy | T_2405 | Energy is passed up a food chain or web from lower to higher trophic levels. However, only about 10 percent of the energy at one level is passed up the next level. This is represented by the ecological pyramid in Figure 24.6. The other 90 percent of energy at each trophic level is used for metabolic processes or given off to the environment as heat. This loss of energy explains why there are rarely more than four trophic levels in a food chain or web. There isnt enough energy left to support additional levels. It also explains why ecosystems need a constant input of energy. You can learn more about ecological pyramids in this video: . MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0414 | flow of energy | T_2406 | Biomass is the total mass of organisms at a trophic level. With less energy at higher trophic levels, there are usually fewer organisms as well. This is also represented in the pyramid in Figure 24.6. Organisms tend to be larger in size at higher trophic levels. However, their smaller numbers result in less biomass. | text | null |
L_0416 | ecosystem change | T_2416 | Primary succession occurs in an area that has never before been colonized by living things. Generally, the area is nothing but bare rock. | text | null |
L_0416 | ecosystem change | T_2417 | Secondary succession occurs in a formerly inhabited area that was disturbed. | text | null |
L_0416 | ecosystem change | T_2418 | This type of environment could come about when: a landslide uncovers bare rock a glacier retreats and leaves behind bare rock lava flows from a volcano and hardens into bare rock (see Figure 24.12) Secondary succession could result from a fire, flood, or human action such as farming. For example, a forest fire might kill all the trees and other plants in a forest, leaving behind only charred wood and soil. | text | null |
L_0416 | ecosystem change | T_2419 | The first few species to colonize a disturbed area are called pioneer species. In primary succession, pioneer species must be organisms that can live on bare rock. They usually include bacteria and lichens (see Figure 24.12). Along with wind and water, the pioneer species help weather the rock and form soil. Once soil begins to form, plants can move in. The first plants are usually grasses and other small plants that can grow in thin, poor soil. As more plants grow and die, organic matter is added to the soil. This improves the soil and helps it hold water. The improved soil allows shrubs and trees to move into the area. Secondary succession is faster than primary succession. The soil is already in place. After a forest fire, for example, the pioneer species are plants such as grasses and fireweed. You can see a forest in this stage of recovery in Figure area. You can see the amazing real-world story of secondary succession on Mount St. Helens by watching this short video: . MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0416 | ecosystem change | T_2419 | The first few species to colonize a disturbed area are called pioneer species. In primary succession, pioneer species must be organisms that can live on bare rock. They usually include bacteria and lichens (see Figure 24.12). Along with wind and water, the pioneer species help weather the rock and form soil. Once soil begins to form, plants can move in. The first plants are usually grasses and other small plants that can grow in thin, poor soil. As more plants grow and die, organic matter is added to the soil. This improves the soil and helps it hold water. The improved soil allows shrubs and trees to move into the area. Secondary succession is faster than primary succession. The soil is already in place. After a forest fire, for example, the pioneer species are plants such as grasses and fireweed. You can see a forest in this stage of recovery in Figure area. You can see the amazing real-world story of secondary succession on Mount St. Helens by watching this short video: . MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0416 | ecosystem change | T_2420 | Does a changing ecosystem ever stop changing? Does its community of organisms ever reach some final, stable state? Scientists used to think that ecological succession always ended at a stable state, called a climax community. Now their thinking has changed. Theoretically, a climax community is possible. But continued change is probably more likely for real-world ecosystems. Most ecosystems are disturbed too often to ever develop a climax community. | text | null |
L_0420 | biodiversity and extinction | T_2447 | Biodiversity refers to the variety of life and its processes. It includes the variation in living organisms, the genetic differences among them, and the range of communities and ecosystems in which they live. Scientists have identified about 1.9 million species alive today, but they are discovering new species all the time. How many species actually exist in the world? No one knows for sure because only a small percentage of them have already been discovered. Estimates range from 5 to 30 million total species currently in existence. Many of them live on coral reefs and in tropical rainforests (see Figure 25.14). These two ecosystems have some of the greatest biodiversity on the planet. | text | null |
L_0420 | biodiversity and extinction | T_2448 | Biodiversity is important to human beings for many reasons. For one thing, biodiversity has direct economic benefits. Here are a few of the economic benefits of biodiversity: Besides food, diverse living things provide us with many different products. Some examples include dyes, rubber, fibers, paper, adhesives, and timber. Living things are an invaluable source of medical drugs. More than half of the most important prescription drugs come from wild species. However, only a fraction of species have yet been studied for their medical potential. Certain species may warn us of toxins in the environment. Amphibians are particularly sensitive to toxins be- cause of their permeable skin. Their current high rates of extinction serve as an early warning of environmental damage and danger to us all. Wild organisms maintain a valuable pool of genetic variation. This is important because most domestic species have been bred to be genetically uniform. This puts domestic crops and animals at great risk of dying out due to disease. Some living things provide inspiration for technology. For example, water strider insects like the one in Figure water quality, among other useful purposes. | text | null |
L_0420 | biodiversity and extinction | T_2448 | Biodiversity is important to human beings for many reasons. For one thing, biodiversity has direct economic benefits. Here are a few of the economic benefits of biodiversity: Besides food, diverse living things provide us with many different products. Some examples include dyes, rubber, fibers, paper, adhesives, and timber. Living things are an invaluable source of medical drugs. More than half of the most important prescription drugs come from wild species. However, only a fraction of species have yet been studied for their medical potential. Certain species may warn us of toxins in the environment. Amphibians are particularly sensitive to toxins be- cause of their permeable skin. Their current high rates of extinction serve as an early warning of environmental damage and danger to us all. Wild organisms maintain a valuable pool of genetic variation. This is important because most domestic species have been bred to be genetically uniform. This puts domestic crops and animals at great risk of dying out due to disease. Some living things provide inspiration for technology. For example, water strider insects like the one in Figure water quality, among other useful purposes. | text | null |
L_0420 | biodiversity and extinction | T_2449 | Biodiversity is important for healthy ecosystems. It generally increases ecosystem productivity and stability. It helps ensure that at least some species will survive environmental change. Biodiversity also provides many other ecosystem services. For example: Plants and algae maintain Earths atmosphere. They add oxygen to the air and remove carbon dioxide when they undertake photosynthesis. Plants help protect the soil. Their roots grip the soil and keep it from washing or blowing away. When plants die, their organic matter improves the soil as it decomposes. Microorganisms purify water in rivers and lakes. They also decompose organic matter and return nutrients to the soil. Certain bacteria fix nitrogen and make it available to plants. Predator species such as birds and spiders control insect pests. They reduce the need for chemical pesticides, which are expensive and may be harmful to human beings and other organisms. Animals, like the bee in Figure below, pollinate flowering plants. Many crop plants depend on pollination by wild animals. | text | null |
L_0420 | biodiversity and extinction | T_2450 | Extinction is the complete dying out of a species. Once a species goes extinct, it can never return. More than 99 percent of all the species that ever lived on Earth have gone extinct. Five mass extinctions have occurred in Earths history. They were caused by major geologic and climatic events. The fifth mass extinction wiped out the dinosaurs 65 million years ago. | text | null |
L_0420 | biodiversity and extinction | T_2451 | Evidence shows that a sixth mass extinction is happening right now. Species are currently going extinct at the fastest rate since the dinosaurs died out. Dozens of species are going extinct every day. If this rate continues, as many as half of all remaining species could go extinct by 2050. Why are so many species going extinct today? Unlike previous mass extinctions, the sixth mass extinction is due mainly to human actions. | text | null |
L_0420 | biodiversity and extinction | T_2452 | The single biggest cause of the sixth mass extinction is habitat loss. A habitat is the area where a species lives and to which it has become adapted. When a habitat is disturbed or destroyed, it threatens all the species that live there with extinction. More than half of Earths land area has been disturbed or destroyed by farming, mining, forestry, or the development of cities, suburbs, and golf courses. Habitats that are rapidly being destroyed include tropical rainforests. They are being cut and burned, mainly to clear the land for farming. Half of Earths mature tropical forests have already been destroyed. At current rates of destruction, they will all be gone by 2090. In the U.S., half of the wetlands and almost all of the tall-grass prairies (see Figure 25.17) have already been destroyed for farming. | text | null |
L_0420 | biodiversity and extinction | T_2453 | There are several other causes of the sixth mass extinction. Most of them contribute to habitat destruction. The burning of fossil fuels has increased the greenhouse effect and caused global climate change. Increasing temperatures are changing basic climate factors of habitats, and rising sea levels are covering them with water. These changes threaten many species. Pollution of air, water, and soil makes habitats toxic to many organisms. A well-known example is the near extinction of the peregrine falcon in the mid-1900s due to the pesticide DDT. Humans have over-harvested trees, fish, and other wild species. This threatens not only their survival but the survival of all the other species that depend on them. Humans have introduced exotic species into new habitats. These are species that are not native to the habitat where they are introduced. They may lack predators in the new habitat so they can out-compete native species and drive them extinct. Exotic species may also carry new diseases, prey on native species, and disrupt local food webs. You can read about an example of an exotic species in Figure 25.18. | text | null |
L_0420 | biodiversity and extinction | T_2453 | There are several other causes of the sixth mass extinction. Most of them contribute to habitat destruction. The burning of fossil fuels has increased the greenhouse effect and caused global climate change. Increasing temperatures are changing basic climate factors of habitats, and rising sea levels are covering them with water. These changes threaten many species. Pollution of air, water, and soil makes habitats toxic to many organisms. A well-known example is the near extinction of the peregrine falcon in the mid-1900s due to the pesticide DDT. Humans have over-harvested trees, fish, and other wild species. This threatens not only their survival but the survival of all the other species that depend on them. Humans have introduced exotic species into new habitats. These are species that are not native to the habitat where they are introduced. They may lack predators in the new habitat so they can out-compete native species and drive them extinct. Exotic species may also carry new diseases, prey on native species, and disrupt local food webs. You can read about an example of an exotic species in Figure 25.18. | text | null |
L_0420 | biodiversity and extinction | T_2454 | Government policies and laws are needed to protect biodiversity. Such actions have been shown to work in the past. For example, peregrine falcons made an incredible recovery after laws were passed banning the use of DDT. Individuals can also play a role in protecting biodiversity. What can you do? Here are a few suggestions: Start a compost pile to recycle organic wastes. Use the compost to enrich yard and garden soil. It will reduce the need for chemical fertilizers and added water. Make your backyard welcoming to native wildlife. Plant native plants that will provide food and shelter for native animals such as birds and amphibians. Add a water source, such as a fountain or bird bath. Avoid the introduction of exotic species to local habitats. Avoid the use of herbicides and pesticides. In addition to killing garden weeds and pests, they may harm native organisms, such as wildflowers, honey bees, and song birds. Conserve natural resources, including energy resources. Always reduce, reuse, or recycle. Learn more about biodiversity and how to protect it. Then pass on what you learn to others. | text | null |
L_0429 | mendels discoveries | T_2547 | Mendel was an Austrian Monk who lived in the 1800s. You can see his picture in Figure 6.1. | text | null |
L_0429 | mendels discoveries | T_2548 | Mendel didnt call himself a scientist. But he had all the traits of good scientist. He was observant and curious, and he asked a lot of questions. He also tried to find answers to his questions by doing experiments. Working alone in his garden in the mid-1800s, he grew thousands of pea plants over many years. He carefully crossed plants with different traits. Then he observed what traits showed up in their offspring. He repeated each experiment many times. | text | null |
L_0429 | mendels discoveries | T_2549 | Pea plants were a good choice to study for several reasons. One reason is that they are easy to grow. They also grow quickly. In addition, peas have many traits that are easy to observe, and each trait exists in two different forms. Figure 6.2 shows the traits that Mendel studied in pea plants. For example, one trait is flower color. Flowers may be either white or violet. Another trait is stem length. Plants may be either tall or short. Pea plants reproduce sexually. The male gametes are released by tiny grains of pollen. The female gametes lie deep within the flowers. Fertilization occurs when pollen from one flower reaches the female gametes in the same or a different flower. This is called pollination. Mendel was able to control which plants pollinated each other. He transferred pollen by hand from flower to flower. | text | null |
L_0429 | mendels discoveries | T_2550 | At first, Mendel studied one trait at a time. This was his first set of experiments. These experiments led to his first law, the law of segregation. Then Mendel studied two traits at a time. This was his second set of experiments. These experiments led to his second law, the law of independent assortment. | text | null |
L_0429 | mendels discoveries | T_2551 | An example of Mendels first set of experiments is his research on flower color. He transferred pollen from a plant with white flowers to a plant with violet flowers. This is called cross-pollination. Then Mendel observed flower color in their offspring. The offspring formed the first generation (F1). You can see the outcome of this experiment in Figure 6.3. All of the F1 plants had violet flowers. Mendel wondered, "What happened to the white form of the trait?" "Did it disappear?" If so, the F1 plants should have only violet-flowered offspring. Mendel let the FI plants pollinate themselves. This is called self-pollination. Then he observed flower color in their offspring. These offspring formed the second generation (F2). Surprisingly, the trait of white flowers showed up in the F2 plants. One out of every four F2 plants had white flowers. The other three out of four had violet flowers. In other words, F2 plants with violet flowers and F2 plants with white flowers had a 3:1 ratio. Mendel repeated this experiment with each of the other traits. For each trait, he got the same results. One form of the trait seemed to disappear in the F1 plants. Then it showed up again in the F2 plants. Moreover, the two forms of the trait always showed up in the F2 plants in the same 3:1 ratio. | text | null |
L_0429 | mendels discoveries | T_2552 | Based on these results, Mendel concluded that each trait is controlled by two factors. He also concluded that one of the factors for each trait dominates the other. He described the dominating factor as dominant. He used the term recessive to describe the other factor. If both factors are present in an individual, only the dominant factor is expressed. This explains why one form of a trait always seems to disappear in the F1 plants. These plants inherit both factors for the trait, but only the dominant factor shows up. The recessive factor is hidden. When F1 plants reproduce, the two factors separate and go to different gametes. This is Mendels first law, the law of segregation. It explains why both forms of the trait show up again in the F2 plants. One out of four F2 plants inherits two of the recessive factors for the trait. In these plants, the recessive form of the trait is expressed. | text | null |
L_0429 | mendels discoveries | T_2553 | Mendel wondered whether different traits are inherited together. For example, are seed form and seed color passed together from parents to offspring? Or do the two traits split up in the offspring? To answer these questions, Mendel studied two traits at a time. For example, he crossed plants that had round, yellow seeds with plants that had wrinkled, green seeds. Then he observed how the two traits showed up in their offspring (F1). You can see the results of this cross in Figure 6.4. All of the F1 plants had round, yellow seeds. The wrinkled and green forms of the traits seemed to disappear in the F1 plants. Then Mendel let the F1 plants self-pollinate. Their offspring, the F2 plants, had all possible combinations of the two traits. You can see this in Figure 6.5. For example there were plants that had round, green seeds, as well as plants that had wrinkled, yellow seeds. In this case the ratios were 9:3:3:1. The ratios are shown across the bottom of Figure 6.5. Mendel repeated this experiment with other combinations of two traits. In each case, he got the same results. One form of each trait seemed to disappear in the F1 plants. Then these forms reappeared in the F2 plants in all possible combinations. Moreover, the different combinations of traits always occurred in the same 9:3:3:1 ratio. | text | null |
L_0429 | mendels discoveries | T_2553 | Mendel wondered whether different traits are inherited together. For example, are seed form and seed color passed together from parents to offspring? Or do the two traits split up in the offspring? To answer these questions, Mendel studied two traits at a time. For example, he crossed plants that had round, yellow seeds with plants that had wrinkled, green seeds. Then he observed how the two traits showed up in their offspring (F1). You can see the results of this cross in Figure 6.4. All of the F1 plants had round, yellow seeds. The wrinkled and green forms of the traits seemed to disappear in the F1 plants. Then Mendel let the F1 plants self-pollinate. Their offspring, the F2 plants, had all possible combinations of the two traits. You can see this in Figure 6.5. For example there were plants that had round, green seeds, as well as plants that had wrinkled, yellow seeds. In this case the ratios were 9:3:3:1. The ratios are shown across the bottom of Figure 6.5. Mendel repeated this experiment with other combinations of two traits. In each case, he got the same results. One form of each trait seemed to disappear in the F1 plants. Then these forms reappeared in the F2 plants in all possible combinations. Moreover, the different combinations of traits always occurred in the same 9:3:3:1 ratio. | text | null |
L_0429 | mendels discoveries | T_2554 | The results of Mendels two-trait experiments led to the law of independent assortment. This law states that factors controlling different traits go to gametes independently of each other. This explains why F2 plants have all possible combinations of the two traits. | text | null |
L_0429 | mendels discoveries | T_2555 | You might think that Mendels discoveries would have made him an instant science rock star. Hed found the answers to age-old questions about heredity. In fact, Mendels work was largely ignored until 1900. Thats when three other scientists independently arrived at Mendels laws. Only then did people appreciate what a great contribution to science Mendel had made. Mendels discoveries form the basis of the modern science of genetics. Genetics is the science of heredity. For his discoveries, Mendel is now called the "father of genetics." Watch this entertaining, upbeat video for an excellent review of Mendels life and work. Its also a good introduction to the next lesson, "Introduction to Genetics." MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0430 | introduction to genetics | T_2556 | Today we know that the traits of organisms are controlled by genes on chromosomes. A gene can be defined as a section of a chromosome that contains the genetic code for a particular protein. The position of a gene on a chromosome is called its locus. Each gene may have different versions. The different versions are called alleles. Figure 6.6 shows an example in pea plants. It shows the gene for flower color. The gene has two alleles. There is a purple-flower allele and a white-flower allele. Different alleles account for most of the variation in the traits of organisms within a species. In sexually reproducing species, chromosomes are present in cells in pairs. Chromosomes in the same pair are called homologous chromosomes. They have the same genes at the same loci. These may be the same or different alleles. During meiosis, when gametes are produced, homologous chromosomes separate. They go to different gametes. Thus, the alleles for each gene also go to different gametes. | text | null |
L_0430 | introduction to genetics | T_2557 | When gametes unite during fertilization, the resulting zygote inherits two alleles for each gene. One allele comes from each parent. | text | null |
L_0430 | introduction to genetics | T_2558 | The two alleles that an individual inherits make up the individuals genotype. The two alleles may be the same or different. Look at Table 6.1. It shows alleles for the flower-color gene in peas. The alleles are represented by the letters B (purple flowers) and b (white flowers). A plant with two alleles of the same type (BB or bb) is called a homozygote. A plant with two different alleles (Bb) is called a heterozygote. Genotypes BB (homozygote) Bb (heterozygote) bb (homozygote) Phenotypes purple flowers purple flowers white flowers | text | null |
L_0430 | introduction to genetics | T_2559 | The expression of an organisms genotype is called its phenotype. The phenotype refers to the organisms traits, such as purple or white flowers. Different genotypes may produce the same phenotype. This will be the case if one allele is dominant to the other. Both BB and Bb genotypes in Table 6.1 have purple flowers. Thats because the B allele is dominant to the b allele, which is recessive. The terms dominant and recessive are the terms Mendel used to describe his "factors." Today we use them to describe alleles. In a Bb heterozygote, only the dominant B allele is expressed. The recessive b allele is expressed only in the bb genotype. | text | null |
L_0430 | introduction to genetics | T_2560 | Each trait Mendel studied was controlled by one gene with two alleles. In each case, one of the alleles was dominant to the other. This resulted in just two possible phenotypes for each trait. Each trait Mendel studied was also controlled by a gene on a different chromosome. As a result, each trait was inherited independently of the others. With traits like these, its easy to predict which forms of a trait will show up in the offspring of a given set of parents. | text | null |
L_0430 | introduction to genetics | T_2561 | Consider a purple-flowered pea plant with the genotype Bb. Half the gametes produced by this parent will have a B allele. The other half will have a b allele. You can see this in Figure 6.7. This is similar to tossing a coin. There is a 50 percent chance of a head and a 50 percent chance of a tail. Like a head or tail, there is a 50 percent chance that any gamete from this parent will have the B allele. There is also a 50 percent chance that any gamete will have the b allele. | text | null |
L_0430 | introduction to genetics | T_2562 | Now lets see what happens if two parent pea plants have the Bb genotype. What genotypes are possible for their offspring? And what ratio of genotypes would you expect? The easiest way to find the answer to these questions is with a Punnett square. A Punnett square is a chart that makes it easy to find the possible genotypes in offspring of two parents. Figure of the chart. The gametes produced by the female parent are along the left side of the chart. The different possible combinations of alleles in their offspring can be found by filling in the cells of the chart. If the parents had four offspring, their most likely genotypes would be one BB, two Bb, and one bb. But the genotype ratios of their actual offspring may differ. Thats because which gametes happen to unite is a matter of chance, like a coin toss. The Punnett square just shows the possible genotypes and their most likely ratios. | text | null |
L_0430 | introduction to genetics | T_2563 | You know that the B allele is dominant to the b allele. Therefore, you can also use the Punnett square in Figure 6.8 to predict the most likely offspring phonotypes. If the parents had four offspring, their most likely phenotypes would be three with purple flowers (1 BB + 2 Bb) and one with white flowers (1 bb). | text | null |
L_0430 | introduction to genetics | T_2564 | Inheritance is often more complex than it is for traits like those Mendel studied. Several factors can complicate it. | text | null |
L_0430 | introduction to genetics | T_2565 | If a gene has two alleles, one may not be dominant to the other. There are other possibilities. One possibility is called codominance. Another is called incomplete dominance. With codominance, both alleles are expressed equally in heterozygotes. The red and white flower in Figure With incomplete dominance, a dominant allele is not completely dominant. Instead, it is influenced by the recessive allele in heterozygotes. The pink flower in Figure 6.9 is an example. It has an incompletely dominant allele for red petals. It also has a recessive allele for white petals. This results in a flower with pink petals. | text | null |
L_0430 | introduction to genetics | T_2566 | Many genes have more than two alleles. An example is ABO blood type in people. There are three common alleles for the gene that controls this trait. The allele for type A is codominant with the allele for type B. Both of these alleles are dominant to the allele for type O. The possible genotypes and phenotypes for this trait are shown in Table below Genotype AA AO BB BO AB OO Phenotype Type A Type A Type B Type B Type AB Type O | text | null |
L_0430 | introduction to genetics | T_2567 | Some traits are controlled by more than one gene. They are called polygenic traits. Each gene for a polygenic trait may have two or more alleles. The genes may be on the same or different chromosomes. Polygenic traits may have many possible phenotypes. Skin color and adult height are examples of polygenic traits in humans. Think about all the variation in the heights of adults you know. Normal adults may range from less than 5 feet tall to more than 7 feet tall. There are people at every gradation of height in between these extremes. | text | null |
L_0430 | introduction to genetics | T_2568 | Genes play an important role in determining an organisms traits. However, for many traits, phenotype is influenced by the environment as well. For example, skin color is controlled by genes but also influenced by exposure to sunlight. You can see the effect of sunlight on skin in Figure 6.10. | text | null |
L_0430 | introduction to genetics | T_2569 | Animals and most plants have two special chromosomes. They are called sex chromosomes. These are chromo- somes that determine the sex of the organism. All of the other chromosomes are called autosomes. Genes on sex chromosomes may be inherited differently than genes on autosomes. | text | null |
L_0430 | introduction to genetics | T_2570 | In people, the sex chromosomes are called X and Y chromosomes. Individuals with two X chromosomes are normally females. Individuals with one X and one Y chromosome are normally males. As you can see in Figure sons. | text | null |
L_0430 | introduction to genetics | T_2571 | Traits controlled by genes on the sex chromosomes are called sex-linked traits. One gene on the Y chromosome determines male sex. There are very few other genes on the Y chromosome, which is the smallest human chromo- some. There are hundreds of genes on the much larger X chromosome. None is related to sex. Traits controlled by genes on the X chromosome are called X-linked traits. X-linked traits have a different pattern of inheritance than traits controlled by genes on autosomes. With just one X chromosome, males have only one allele for any X-linked trait. Therefore, a recessive X-linked allele is always expressed in males. With two X chromosomes, females have two alleles for any X-linked trait, just as they do for autosomal traits. Therefore, a recessive X-linked allele is expressed in females only when they inherit two copies of it. This explains why X-linked recessive traits show up less often in females than males. | text | null |
L_0430 | introduction to genetics | T_2572 | An example of a recessive X-linked trait is red-green color blindness. People with this trait cant see red or green colors. This trait is fairly common in males but rare in females. Figure 6.12 is a pedigree for this trait. A pedigree is a chart that shows how a trait is inherited in a family. The mother has one allele for color blindness. She doesnt have color blindness because she also has a dominant normal allele for the gene. Instead, she is called a carrier for the trait. She passes the allele to half of her children. One daughter is a carrier, and one son has the color blindness trait. No matter how many children this couple has, none of the daughters will have color blindness, but half of the sons, on average, will have the trait. Can you explain why? | text | null |
L_0431 | advances in genetics | T_2573 | A species genome consists of all of its genetic information. The human genome consists of the complete set of genes in the human organism. Its all the DNA of a human being. | text | null |
L_0431 | advances in genetics | T_2574 | The Human Genome Project was launched in 1990. It was an international effort to sequence all 3 billion bases in human DNA. Another aim of the project was to identify the more than 20,000 human genes and map their locations on chromosomes. The logo of the Human Genome Project in Figure 6.13 shows that the project brought together experts in many fields. The Human Genome Project was completed in 2003. It was one of the greatest feats of modern science. It provides a complete blueprint for a human being. Its like having a very detailed manual for making a human organism. | text | null |
L_0431 | advances in genetics | T_2575 | Knowing the sequence of the human genome is very useful. For example, it helps us understand how humans evolved. Another use is in medicine. It is helping researchers identify and understand genetic disorders. You can learn more about the Human Genome Project and its applications by watching this funny, fast-paced video: http://w MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0431 | advances in genetics | T_2576 | Sequencing the human genome has increased our knowledge of genetic disorders. Genetic disorders are diseases caused by mutations. Many genetic disorders are caused by mutations in a single gene. Others are caused by abnormal numbers of chromosomes. | text | null |
L_0431 | advances in genetics | T_2577 | Table 6.3 lists some genetic disorders caused by mutations in just one gene. It include autosomal and X-linked disorders. It also includes dominant and recessive disorders. Genetic Disorder Marfan syndrome Cystic fibrosis Sickle Cell Anemia Hemophilia A Effect of Mutation Defective protein in tis- sues such as cartilage and bone Defective protein needed to make mucus Defective hemoglobin protein that is needed to transport oxygen in red blood cells Reduced activity of a pro- tein needed for blood to clot Signs of the Disorder Heart and bone defects; unusually long limbs Type of Trait Autosomal dominant Unusually thick mucus that clogs airways in lungs and ducts in other organs Sickle-shaped red blood cells that block blood ves- sels and interrupt blood flow Excessive bleeding that is difficult to control Autosomal recessive Autosomal recessive X-linked recessive Relatively few genetic disorders are caused by dominant alleles. A dominant allele is expressed in everybody who inherits even one copy of it. If it causes a serious disorder, affected people may die young and fail to reproduce. They wont pass the allele to the next generation. As a result, the allele may die out of the population. One of the exceptions is Marfan syndrome. It is thought to have affected Abraham Lincoln. Hes pictured in Figure 6.14. His very long limbs are one reason for the suspicion of Marfan syndrome in this former U.S. president. Recessive disorders are more common than dominant ones. Why? A recessive allele is not expressed in heterozy- gotes. These people are called carriers. They dont have the genetic disorder but they carry the recessive allele. They can also pass this allele to their offspring. A recessive allele is more likely than a dominant allele to pass to the next generation rather than die out. | text | null |
L_0431 | advances in genetics | T_2578 | In the process of meiosis, paired chromosomes normally separate from each other. They end up in different gametes. Sometimes, however, errors occur. The paired chromosomes fail to separate. When this happens, some gametes get an extra copy of a chromosome. Other gametes are missing a chromosome. If one of these gametes is fertilized and survives, a chromosomal disorder results. You can see examples of such disorders in Table 6.4 Genetic Disorder Down syndrome Genotype Extra copy (complete or partial) of chromosome 21 Turners syndrome One X chromosome and no other sex chromosome (XO) One Y chromosome and two or more X chromosomes (XXY, XXXY) Klinefelters syndrome Phenotypic Effects Developmental delays, distinctive facial appearance, and other abnor- malities Female with short height and inabil- ity to reproduce Male with abnormal sexual devel- opment and reduced level of male sex hormone Most chromosomal disorders involve the sex chromosomes. Can you guess why? The X and Y chromosomes are very different in size. The X is much larger than the Y. This difference in size creates problems. It increases the chances that the two chromosomes will fail to separate properly during meiosis. | text | null |
L_0431 | advances in genetics | T_2579 | Treating genetic disorders is one use of biotechnology. Biotechnology is the use of technology to change the genetic makeup of living things for human purposes. Its also called genetic engineering. Besides treating genetic disorders, biotechnology is used to change organisms so they are more useful to people. | text | null |
L_0431 | advances in genetics | T_2580 | Biotechnology uses a variety of methods, but some are commonly used in many applications. A common method is the polymerase chain reaction. Another common method is gene cloning. The polymerase chain reaction is a way of making copies of a gene. It uses high temperatures and an enzyme to make new DNA molecules. The process keeps cycling to make many copies of a gene. Gene cloning is another way of making copies of a gene. A gene is inserted into the DNA of a bacterial cell. Figure 6.15 shows how this is done. Bacteria multiply very rapidly by binary fission. Each time a bacterial cell divides, the inserted gene is copied. | text | null |
L_0431 | advances in genetics | T_2581 | Biotechnology has many uses. It is especially useful in medicine and agriculture. Biotechnology is used to treat genetic disorders. For example, copies of a normal gene might be inserted into a patient with a defective gene. This is called gene therapy. Ideally, it can cure a genetic disorder. create genetically modified organisms (GMOs). Many GMOs are food crops such as corn. Genes are inserted into plants to give them desirable traits. This might be the ability to get by with little water. Or it might be the ability to resist insect pests. The modified plants are likely to be healthier and produce more food. They may also need less pesticide. produce human proteins. Insulin is one example. This protein is needed to treat diabetes. The human insulin gene is inserted into bacteria. The bacteria reproduce rapidly. They can produce large quantities of the human protein. You can see another example in Figure 6.16. | text | null |
L_0431 | advances in genetics | T_2582 | Biotechnology has many benefits. Its pros are obvious. It helps solve human problems. However, biotechnology also raises many concerns. For example, some people worry about eating foods that contain GMOs. They wonder if GMOs might cause health problems. The person in Figure 6.17 favors the labeling of foods that contain GMOs. That way, consumers can know which foods contain them and decide for themselves whether to eat them. Another concern about biotechnology is how it may affect the environment. Negative effects on the environment have already occurred because of some GMOs. For example, corn has been created that has a gene for a pesticide. The corn plants have accidentally cross-pollinated nearby milkweeds. Monarch butterfly larvae depend on milkweeds for food. When they eat milkweeds with the pesticide gene, they are poisoned. This may threaten the survival of the monarch species as well as other species that eat monarchs. Do the benefits of the genetically modified corn outweigh the risks? What do you think? | text | null |
L_0438 | archaea | T_2657 | Archaeans are prokaryotes in the Archaea Domain. They were first discovered in extreme environments such as hot springs. For a long time, they were classified as bacteria. As more was learned about them, they were found to be quite different from bacteria. They were finally placed in their own domain in the late 1970s. You can see the incredible story of their discovery in this brief video: . MEDIA Click image to the left or use the URL below. URL: The study of archaeans is in its infancy. Scientists still know relatively little about them. New species of archaeans are being discovered all the time. | text | null |
L_0438 | archaea | T_2658 | Many archaeans are extremophiles. Extremophiles are organisms that live in extreme conditions. For example, some archaeans live around hydrothermal vents. A hydrothermal vent is a crack on the ocean floor. You can see one in Figure 8.16. Boiling hot, highly acidic water pours out of the vent. These extreme conditions dont deter archaeans. They have evolved adaptations for coping with them. These conditions are like those on ancient Earth. This suggests that archaeans may have evolved very early in Earths history. There are four types of archaean extremophiles. Each type is described below. Extreme conditions pose many challenges to living cells. Archaeans have evolved adaptations that allow them to deal with the challenges. | text | null |
L_0438 | archaea | T_2659 | Halophiles are organisms that "love" salt. They can survive in very salty water. For example, they have been found in the Great Salt Lake in Utah and the Dead Sea between Israel and Jordan. Both of these bodies of water are much saltier than the ocean. | text | null |
L_0438 | archaea | T_2660 | Hyperthermophiles are organisms that "love" heat. Some archaeans can survive at very high temperatures. For example, they can grow in hot springs and geysers. One archaean species can even reproduce at 122 C (252 F). This is higher than the boiling point of water. It is the highest recorded temperature for any organism. | text | null |
L_0438 | archaea | T_2661 | Acidophiles are organisms that "love" acids. They live in very acidic environments, such as acid mine drainage. They are also found near vents of volcanoes. The most acidophilic archaeans can thrive at negative pH values. No other organisms can survive in such acidic conditions. | text | null |
L_0438 | archaea | T_2662 | Alkaliphiles are organisms that "love" bases. Bases are like the opposite of acids. Basic environments where archaeans are found include Mono Lake in California, pictured in Figure 8.17. Mono Lake is the oldest lake in North America. The water is not only unusually basic. Its also saltier than the ocean. So archaeans that live in the water of Mono Lake must have adaptations to both salty and basic conditions. They are haloalkaliphiles. | text | null |
L_0438 | archaea | T_2663 | Not all archaeans live in extreme conditions. In fact, archaeans are now known to live just about everywhere on Earth. They make up as much as 20 percent of Earths total mass of living things. | text | null |
L_0438 | archaea | T_2664 | Archaeans have been found in a broad range of habitats. For example, they live in soils, bodies of water, and marshlands. They even live in the human belly button! Archaeans are very common in the ocean. Archaeans in plankton may be some of the most abundant organisms on Earth. | text | null |
L_0438 | archaea | T_2665 | Like bacteria, archaeans are important decomposers. For example, archaeans help break down sewage in waste treatment plants. As decomposers, they help recycle carbon and nitrogen. Many archaeans live in close relationships with other organisms. For example, large numbers live inside animals, including humans. Unlike many bacteria, archaeans dont harm their hosts. None of them is known to cause human disease. Archaeans are more likely to help their hosts. For example, archaeans called methanogens live inside the gut of cows (see Figure 8.18). They help cows digest tough plant fibers made of cellulose. They produce methane gas as a waste product. | text | null |
L_0443 | alligators and crocodiles | T_2694 | Crocodilia, containing both alligators and crocodiles, is an order of large reptiles. Reptiles belonging to Crocodilia are the closest living relatives of birds. Reptiles and birds are the only known living descendants of the dinosaurs. Some would day that alligators and crocodiles actually look like small dinosaurs. Dinosaurs that evolved wings are the ancestors of birds. The basic crocodilian body plan ( Figure 1.1) is a very successful one and has changed little over time. Modern species actually look very similar to their Cretaceous ancestors of 84 million years ago. All species of crocodilians have similar body structures, including an elongated snout, powerful jaws, muscular tail, large protective scales, streamlined body, and eyes and nostrils that are positioned on top of the head. | text | null |
L_0443 | alligators and crocodiles | T_2695 | Crocodilians have a flexible, semi-erect posture. They can walk either in a low, sprawled belly walk, or hold their legs more directly underneath them to perform the high walk. Most other reptiles can only walk in a sprawled position. All crocodilians have, like humans, teeth set in bony sockets. But unlike mammals, they replace their teeth through- out life. Crocodiles and gharials (large crocodilians with longer jaws) have salivary glands on their tongue, which are used to remove salt from their bodies. This helps with life in a saltwater environment. Crocodilians are often seen lying with their mouths open, a behavior called gaping. One of its functions is probably to cool them down. The crocodilian digestive system is highly adapted to their lifestyle. Crocodilians are known to swallow stones, known as gastroliths, which help digest their prey. The crocodilian stomach is divided into two chambers. The first is powerful and muscular. The other stomach is the most acidic digestive system of any animal. It can digest mostly everything from their prey, including bones, feathers, and horns! All crocodilians are carnivores. They feed on live animals such as birds, small mammals and fish. Crocodilians use several methods of attack when pursuing live prey. One approach is that of the ambush. The crocodilian lies motionless beneath the waters surface with only their nostrils above the water line. This keeps them concealed while they watch for prey that approaches the waters edge. The crocodilian then lunges out of the water, taking their prey by surprise and dragging it from the shoreline into deep water where the prey is killed. The sex of developing crocodilians is determined by the temperature of the eggs during incubation, when eggs are kept warm before they hatch. This means that the sex of crocodilians is not determined genetically. If the eggs are kept at a cold or a hot temperature, then their offspring may be all male or all female. To get both male and female offspring, the temperature must be kept within a narrow range. Female crocodilians care for the young after they hatch, providing them with protection until they grow large enough to defend themselves. In many species of crocodilians, the female carries her tiny offspring in her mouth. | text | null |
L_0443 | alligators and crocodiles | T_2696 | Like all reptiles, crocodilians have a relatively small brain, but the crocodilian brain is more advanced than those of other reptiles. Because of their aquatic habitat, the eyes, ears, and nostrils are all located on the same "face" in a line one after the other. The crocodiles have advanced sensory organs. They see well during the day and may even have color vision, and they also have excellent night vision. A third transparent eyelid, the nictitating membrane, protects their eyes underwater. The eardrums are located behind the eyes and are covered by a movable flap of skin. This flap closes, along with the nostrils and eyes, when they dive. This prevents water from entering their external head openings. Their jaws are covered with sensory pits, which hold bundles of nerve fibers that respond to the slightest disturbance in surface water. Crocodiles can detect vibrations and small pressure changes in water. This makes it possible for them to sense prey and danger even in total darkness, and becomes very useful when the animal is submerged in the water. Like mammals and birds, and unlike other reptiles, crocodiles have a four-chambered heart. But, unlike mammals, blood with and without oxygen can be mixed. See Supersize Crocs at | text | null |
L_0444 | amphibians | T_2697 | What group of animals begins its life in the water, but then spends most of its life on land? Amphibians! Amphibians are a group of vertebrates that has adapted to live in both water and on land. Amphibian larvae are born and live in water, and they breathe using gills. The adults live on land for part of the time and breathe both through their skin and with their lungs as their lungs are not sufficient to provide the necessary amount of oxygen. There are approximately 6,000 species of amphibians. They have many different body types, physiologies, and habitats, ranging from tropical to subarctic regions. Frogs, toads, salamanders ( Figure 1.1), newts, and caecilians are all types of amphibians. | text | null |
L_0444 | amphibians | T_2698 | Transition to life on land meant significant changes to both external and internal features. In order to live on land, amphibians replaced gills with another respiratory organ, the lungs. Other adaptations include: One of the many species of amphibian is this dusky salamander. Skin that prevents loss of water. Eyelids that allow them to adapt to vision outside of the water. An eardrum developed to separate the external ear from the middle ear. A tail that disappears in adulthood (in frogs and toads). | text | null |
L_0444 | amphibians | T_2699 | Like fish, amphibians are ectothermic vertebrates. They belong to the class Amphibia. There are three orders: 1. Urodela, containing salamanders and newts. 2. Anura, containing frogs and toads. 3. Apoda, containing caecilians. | text | null |
L_0444 | amphibians | T_2700 | Most amphibians live in fresh water, not salt water. Their habitats can include areas close to springs, streams, rivers, lakes, swamps and ponds. They can be found in moist areas in forests, meadows and marshes. Amphibians can be found almost anywhere there is a source of fresh water. Although there are no true saltwater amphibians, a few can live in brackish (slightly salty) water. Some species do not need any water at all, and several species have also adapted to live in drier environments. Most amphibians still need water to lay their eggs. | text | null |
L_0444 | amphibians | T_2701 | Amphibians reproduce sexually. The life cycle of amphibians happens in the following stages: 1. Egg Stage: Amphibian eggs are fertilized in a number of ways. External fertilization, employed by most frogs and toads, involves a male gripping a female across her back, almost as if he is squeezing the eggs out of her. The male releases sperm over the females eggs as they are laid. Another method is used by salamanders, whereby the male deposits a packet of sperm onto the ground. The female then pulls it into her cloaca, a single opening for her internal organ systems. Therefore, fertilization occurs internally. By contrast, caecilians and tailed frogs use internal fertilization, just like reptiles, birds, and mammals. The male deposits sperm directly into the females cloaca. 2. Larval stage: When the egg hatches, the organism is legless, lives in water, and breathes with gills, resembling their evolutionary ancestors (fish). 3. During the larval stage, the amphibian slowly transforms into an adult by losing its gills and growing four legs. Once development is complete, it can live on land. | text | null |
L_0445 | angiosperms | T_2702 | Angiosperms, in the phylum Anthophyta, are the most successful phylum of plants. This category also contains the largest number of individual plants ( Figure 1.1). Angiosperms evolved the structure of the flower, so they are also called the flowering plants. Angiosperms live in a variety of different environments. A water lily, an oak tree, and a barrel cactus, although different, are all angiosperms. | text | null |
L_0445 | angiosperms | T_2703 | Even though flowers may look very different from each other, they do have some structures in common. The structures are explained in the picture below ( Figure 1.2). The green outside of a flower that often looks like a leaf is called the sepal ( Figure 1.3). All of the sepals together are called the calyx, which is usually green and protects the flower before it opens. All of the petals ( Figure 1.3) together are called the corolla. They are bright and colorful to attract a particular pollinator, an animal that carries pollen from one flower to another. Examples of pollinators include birds and insects. Angiosperms are the flowering plants. A complete flower has sepals, petals, sta- mens, and one or more carpels. The next structure is the stamen, consisting of the stalk-like filament that holds up the anther, or pollen sac. The pollen, which is found at the top of the stamen, is the male gametophyte. At the very center is the carpel, which is divided into three different parts: (1) the sticky stigma, where the pollen lands, (2) the tube of the style, and (3) the large, bottom part, known as the ovary. The ovary holds the ovules, the female gametophytes. When the ovules are fertilized, the ovule becomes the seed and the ovary becomes the fruit. The following table summarizes the parts of the flower ( Table 1.1). Part sepals calyx corolla stamens filament anther carpel Definition The green outside of the flower. All of the sepals together, or the outside of the flower. The petals of a flower collectively. The part of the flower that produces pollen. Stalk that holds up the anther. The structure that contains pollen in a flower. Female part of the flower; includes the stigma, style, and ovary. style ovary ovules This image shows the difference between a petal and a sepal. | text | null |
L_0445 | angiosperms | T_2703 | Even though flowers may look very different from each other, they do have some structures in common. The structures are explained in the picture below ( Figure 1.2). The green outside of a flower that often looks like a leaf is called the sepal ( Figure 1.3). All of the sepals together are called the calyx, which is usually green and protects the flower before it opens. All of the petals ( Figure 1.3) together are called the corolla. They are bright and colorful to attract a particular pollinator, an animal that carries pollen from one flower to another. Examples of pollinators include birds and insects. Angiosperms are the flowering plants. A complete flower has sepals, petals, sta- mens, and one or more carpels. The next structure is the stamen, consisting of the stalk-like filament that holds up the anther, or pollen sac. The pollen, which is found at the top of the stamen, is the male gametophyte. At the very center is the carpel, which is divided into three different parts: (1) the sticky stigma, where the pollen lands, (2) the tube of the style, and (3) the large, bottom part, known as the ovary. The ovary holds the ovules, the female gametophytes. When the ovules are fertilized, the ovule becomes the seed and the ovary becomes the fruit. The following table summarizes the parts of the flower ( Table 1.1). Part sepals calyx corolla stamens filament anther carpel Definition The green outside of the flower. All of the sepals together, or the outside of the flower. The petals of a flower collectively. The part of the flower that produces pollen. Stalk that holds up the anther. The structure that contains pollen in a flower. Female part of the flower; includes the stigma, style, and ovary. style ovary ovules This image shows the difference between a petal and a sepal. | text | null |
L_0445 | angiosperms | T_2703 | Even though flowers may look very different from each other, they do have some structures in common. The structures are explained in the picture below ( Figure 1.2). The green outside of a flower that often looks like a leaf is called the sepal ( Figure 1.3). All of the sepals together are called the calyx, which is usually green and protects the flower before it opens. All of the petals ( Figure 1.3) together are called the corolla. They are bright and colorful to attract a particular pollinator, an animal that carries pollen from one flower to another. Examples of pollinators include birds and insects. Angiosperms are the flowering plants. A complete flower has sepals, petals, sta- mens, and one or more carpels. The next structure is the stamen, consisting of the stalk-like filament that holds up the anther, or pollen sac. The pollen, which is found at the top of the stamen, is the male gametophyte. At the very center is the carpel, which is divided into three different parts: (1) the sticky stigma, where the pollen lands, (2) the tube of the style, and (3) the large, bottom part, known as the ovary. The ovary holds the ovules, the female gametophytes. When the ovules are fertilized, the ovule becomes the seed and the ovary becomes the fruit. The following table summarizes the parts of the flower ( Table 1.1). Part sepals calyx corolla stamens filament anther carpel Definition The green outside of the flower. All of the sepals together, or the outside of the flower. The petals of a flower collectively. The part of the flower that produces pollen. Stalk that holds up the anther. The structure that contains pollen in a flower. Female part of the flower; includes the stigma, style, and ovary. style ovary ovules This image shows the difference between a petal and a sepal. | text | null |
L_0445 | angiosperms | T_2704 | Flowering plants can reproduce two different ways: 1. Self-pollination: Pollen falls on the stigma of the same flower. This way, a seed will be produced that can turn into a genetically identical plant. 2. Cross-fertilization: Pollen from one flower travels to a stigma of a flower on another plant. Pollen travels from flower to flower by wind or by animals. Flowers that are pollinated by animals such as birds, butterflies, or bees are often colorful and provide nectar, a sugary reward, for their animal pollinators. | text | null |
L_0445 | angiosperms | T_2705 | Angiosperms are important to humans in many ways, but the most significant role of angiosperms is as food. Wheat, rye, corn, and other grains are all harvested from flowering plants. Starchy foods, such as potatoes, and legumes, such as beans, are also angiosperms. And, as mentioned previously, fruits are a product of angiosperms that increase seed dispersal and are nutritious. There are also many non-food uses of angiosperms that are important to society. For example, cotton and other plants are used to make cloth, and hardwood trees are used for lumber. | text | null |
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