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L_0428 | protein synthesis | T_2537 | DNA and RNA are nucleic acids. DNA stores genetic information. RNA helps build proteins. Proteins, in turn, determine the structure and function of all your cells. Proteins consist of chains of amino acids. A proteins structure and function depends on the sequence of its amino acids. Instructions for this sequence are encoded in DNA. In eukaryotic cells, chromosomes are contained within the nucleus. But proteins are made in the cytoplasm at structures called ribosomes. How do the instructions in DNA reach the ribosomes in the cytoplasm? RNA is needed for this task. | text | null |
L_0428 | protein synthesis | T_2538 | RNA stands for ribonucleic acid. RNA is smaller than DNA. It can squeeze through pores in the membrane that encloses the nucleus. It copies instructions in DNA and carries them to a ribosome in the cytoplasm. Then it helps build the protein. RNA is not only smaller than DNA. It differs from DNA in other ways as well. It consists of one nucleotide chain rather than two chains as in DNA. It also contains the nitrogen base uracil (U) instead of thymine (T). In addition, it contains the sugar ribose instead of deoxyribose. You can see these differences in Figure 5.16. | text | null |
L_0428 | protein synthesis | T_2538 | RNA stands for ribonucleic acid. RNA is smaller than DNA. It can squeeze through pores in the membrane that encloses the nucleus. It copies instructions in DNA and carries them to a ribosome in the cytoplasm. Then it helps build the protein. RNA is not only smaller than DNA. It differs from DNA in other ways as well. It consists of one nucleotide chain rather than two chains as in DNA. It also contains the nitrogen base uracil (U) instead of thymine (T). In addition, it contains the sugar ribose instead of deoxyribose. You can see these differences in Figure 5.16. | text | null |
L_0428 | protein synthesis | T_2539 | There are three different types of RNA. All three types are needed to make proteins. Messenger RNA (mRNA) copies genetic instructions from DNA in the nucleus. Then it carries the instructions to a ribosome in the cytoplasm. Ribosomal RNA (rRNA) helps form a ribosome. This is where the protein is made. Transfer RNA (tRNA) brings amino acids to the ribosome. The amino acids are then joined together to make the protein. | text | null |
L_0428 | protein synthesis | T_2540 | How is the information for making proteins encoded in DNA? The answer is the genetic code. The genetic code is based on the sequence of nitrogen bases in DNA. The four bases make up the letters of the code. Groups of three bases each make up code words. These three-letter code words are called codons. Each codon stands for one amino acid or else for a start or stop signal. There are 20 amino acids that make up proteins. With three bases per codon, there are 64 possible codons. This is more than enough to code for the 20 amino acids plus start and stop signals. You can see how to translate the genetic code in Figure 5.17. Start at the center of the chart for the first base of each three-base codon. Then work your way out from the center for the second and third bases. Find the codon AUG in Figure 5.17. It codes for the amino acid methionine. It also codes for the start signal. After an AUG start codon, the next three letters are read as the second codon. The next three letters after that are read as the third codon, and so on. You can see how this works in Figure 5.18. The figure shows the bases in a molecule | text | null |
L_0428 | protein synthesis | T_2540 | How is the information for making proteins encoded in DNA? The answer is the genetic code. The genetic code is based on the sequence of nitrogen bases in DNA. The four bases make up the letters of the code. Groups of three bases each make up code words. These three-letter code words are called codons. Each codon stands for one amino acid or else for a start or stop signal. There are 20 amino acids that make up proteins. With three bases per codon, there are 64 possible codons. This is more than enough to code for the 20 amino acids plus start and stop signals. You can see how to translate the genetic code in Figure 5.17. Start at the center of the chart for the first base of each three-base codon. Then work your way out from the center for the second and third bases. Find the codon AUG in Figure 5.17. It codes for the amino acid methionine. It also codes for the start signal. After an AUG start codon, the next three letters are read as the second codon. The next three letters after that are read as the third codon, and so on. You can see how this works in Figure 5.18. The figure shows the bases in a molecule | text | null |
L_0428 | protein synthesis | T_2541 | The genetic code has three other important characteristics. The genetic code is the same in all living things. This shows that all organisms are related by descent from a common ancestor. Each codon codes for just one amino acid (or start or stop). This is necessary so the correct amino acid is always selected. Most amino acids are encoded by more than one codon. This is helpful. It reduces the risk of the wrong amino acid being selected if there is a mistake in the code. | text | null |
L_0428 | protein synthesis | T_2542 | The process in which proteins are made is called protein synthesis. It occurs in two main steps. The steps are transcription and translation. Watch this video for a good introduction to both steps of protein synthesis: http://w MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0428 | protein synthesis | T_2543 | Transcription is the first step in protein synthesis. It takes place in the nucleus. During transcription, a strand of DNA is copied to make a strand of mRNA. How does this happen? It occurs by the following steps, as shown in Figure 5.19. 1. An enzyme binds to the DNA. It signals the DNA to unwind. 2. After the DNA unwinds, the enzyme can read the bases in one of the DNA strands. 3. Using this strand of DNA as a template, nucleotides are joined together to make a complementary strand of mRNA. The mRNA contains bases that are complementary to the bases in the DNA strand. Translation is the second step in protein synthesis. It is shown in Figure 5.20. Translation takes place at a ribosome in the cytoplasm. During translation, the genetic code in mRNA is read to make a protein. Heres how it works: 1. 2. 3. 4. 5. The molecule of mRNA leaves the nucleus and moves to a ribosome. The ribosome consists of rRNA and proteins. It reads the sequence of codons in mRNA. Molecules of tRNA bring amino acids to the ribosome in the correct sequence. At the ribosome, the amino acids are joined together to form a chain of amino acids. The chain of amino acids keeps growing until a stop codon is reached. Then the chain is released from the ribosome. | text | null |
L_0428 | protein synthesis | T_2544 | Mutations have many possible causes. Some mutations occur when a mistake is made during DNA replication or transcription. Other mutations occur because of environmental factors. Anything in the environment that causes a mutation is known as a mutagen. Examples of mutagens are shown in Figure 5.21. They include ultraviolet rays in sunlight, chemicals in cigarette smoke, and certain viruses and bacteria. | text | null |
L_0428 | protein synthesis | T_2544 | Mutations have many possible causes. Some mutations occur when a mistake is made during DNA replication or transcription. Other mutations occur because of environmental factors. Anything in the environment that causes a mutation is known as a mutagen. Examples of mutagens are shown in Figure 5.21. They include ultraviolet rays in sunlight, chemicals in cigarette smoke, and certain viruses and bacteria. | text | null |
L_0428 | protein synthesis | T_2545 | Many mutations have no effect on the proteins they encode. These mutations are considered neutral. Occasionally, a mutation may make a protein even better than it was before. Or the protein might help the organism adapt to a new environment. These mutations are considered beneficial. An example is a mutation that helps bacteria resist antibiotics. Bacteria with the mutation increase in numbers, so the mutation becomes more common. Other mutations are harmful. They may even be deadly. Harmful mutations often result in a protein that no longer can do its job. Some harmful mutations cause cancer or other genetic disorders. Mutations also vary in their effects depending on whether they occur in gametes or in other cells of the body. Mutations that occur in gametes can be passed on to offspring. An offspring that inherits a mutation in a gamete will have the mutation in all of its cells. Mutations that occur in body cells cannot be passed on to offspring. They are confined to just one cell and its daughter cells. These mutations may have little effect on an organism. | text | null |
L_0428 | protein synthesis | T_2546 | The effect of a mutation is likely to depend as well on the type of mutation that occurs. A mutation that changes all or a large part of a chromosome is called a chromosomal mutation. This type of mutation tends to be very serious. Sometimes chromosomes are missing or extra copies are present. An example is the mutation that causes Down syndrome. In this case, there is an extra copy of one of the chromosomes. Deleting or inserting a nitrogen base causes a frameshift mutation. All of the codons following the mutation are misread. This may be disastrous. To see why, consider this English-language analogy. Take the sentence The big dog ate the red cat. If the second letter of big is deleted, then the sentence becomes: The bgd oga tet her edc at. Deleting a single letter makes the rest of the sentence impossible to read. Some mutations change just one or a few bases in DNA. A change in just one base is called a point mutation. Table 5.1 compares different types of point mutations and their effects. Type Silent Missense Nonsense Description mutated codon codes for the same amino acid mutated codon codes for a different amino acid mutated codon is a prema- ture stop codon Example CAA (glutamine) ! CAG (glutamine) CAA (glutamine) ! CCA (proline) CAA (glutamine) ! UAA (stop) Effect none variable serious | text | null |
L_0432 | darwins theory of evolution | T_2583 | Darwins theory of evolution by natural selection contains two major ideas: One idea is that evolution happens. Evolution is a change in the inherited traits of organisms over time. Living things have changed as descendants diverged from common ancestors in the past. The other idea is that evolution occurs by natural selection. Natural selection is the process in which living things with beneficial traits produce more offspring. As a result, their traits increase in the population over time. | text | null |
L_0432 | darwins theory of evolution | T_2584 | How did Darwin come up with the theory of evolution by natural selection? A major influence was an amazing scientific expedition he took on a ship called the Beagle. Darwin was only 22 years old when the ship set sail. The trip lasted for almost five years and circled the globe. Figure 7.2 shows the route the ship took. It set off from Plymouth, England in 1831. It wouldnt return to Plymouth until 1836. Imagine setting out for such an incredible adventure at age 22, and youll understand why the trip had such a big influence on Darwin. Darwins job on the voyage was to observe and collect specimens whenever the ship went ashore. This included plants, animals, rocks, and fossils. Darwin loved nature, so the job was ideal for him. During the long voyage, he made many observations that helped him form his theory of evolution. Some of his most important observations were made on the Galpagos Islands. The 16 Galpagos Islands lie 966 kilometers (about 600 miles) off the west coast of South America. (You can see their location on the map in Figure 7.2.) Some of the animals Darwin observed on the islands were giant tortoises and birds called finches. Watch this video for an excellent introduction to Darwin, his voyage, and the Galpagos: | text | null |
L_0432 | darwins theory of evolution | T_2585 | The Galpagos Islands are still famous for their giant tortoises. These gentle giants are found almost nowhere else in the world. Darwin was amazed by their huge size. He was also struck by the variety of shapes of their shells. You can see two examples in Figure 7.3. Each island had tortoises with a different shell shape. The local people even could tell which island a tortoise came from based on the shape of its shell. Darwin wondered how each island came to have its own type of tortoise. He found out that tortoises with dome- shaped shells lived on islands where the plants they ate were abundant and easy to reach. Tortoises with saddle- shaped shells, in contrast, lived on islands that were drier. On those islands, food was often scarce. The saddle shape of their shells allowed tortoises on those islands to reach up and graze on vegetation high above them. This made sense, but how had it happened? | text | null |
L_0432 | darwins theory of evolution | T_2585 | The Galpagos Islands are still famous for their giant tortoises. These gentle giants are found almost nowhere else in the world. Darwin was amazed by their huge size. He was also struck by the variety of shapes of their shells. You can see two examples in Figure 7.3. Each island had tortoises with a different shell shape. The local people even could tell which island a tortoise came from based on the shape of its shell. Darwin wondered how each island came to have its own type of tortoise. He found out that tortoises with dome- shaped shells lived on islands where the plants they ate were abundant and easy to reach. Tortoises with saddle- shaped shells, in contrast, lived on islands that were drier. On those islands, food was often scarce. The saddle shape of their shells allowed tortoises on those islands to reach up and graze on vegetation high above them. This made sense, but how had it happened? | text | null |
L_0432 | darwins theory of evolution | T_2586 | Darwin also observed that each of the Galpagos Islands had its own species of finches. The finches on different islands had beaks that differed in size and shape. You can see four examples in Figure 7.4. Darwin investigated further. He found that the different beaks seemed to suit the birds for the food available on their island. For example, finch number 1 in Figure 7.4 used its large, strong beak to crack open and eat big, tough seeds. Finch number 4 had a long, pointed beak that was ideal for eating insects. This seemed reasonable, but how had it come about? | text | null |
L_0432 | darwins theory of evolution | T_2587 | Besides his observations on the Beagle, other influences helped Darwin develop his theory of evolution by natural selection. These included his knowledge of plant and animal breeding and the ideas of other scientists. | text | null |
L_0432 | darwins theory of evolution | T_2588 | Darwin knew that people could breed plants and animals to have useful traits. By selecting which individuals were allowed to reproduce, they could change an organisms traits over several generations. Darwin called this type of change in organisms artificial selection. You can see an example in Figure 7.5. Keeping and breeding pigeons was a popular hobby in Darwins day. Both types of pigeons in the bottom row were bred from the common rock pigeon at the top of the figure. | text | null |
L_0432 | darwins theory of evolution | T_2589 | There were three other scientists in particular that influenced Darwin. Their names are Lamarck, Lyell, and Malthus. All three were somewhat older than Darwin, and he was familiar with their writings. Jean Baptiste Lamarck was a French naturalist. He was one of the first scientists to propose that species change over time. In other words, he proposed that evolution occurs. Lamarck also tried to explain how it happens, but he got that part wrong. Lamarck thought that the traits an organism developed during its life time could be passed on to its offspring. He called this the inheritance of acquired characteristics. Charles Lyell was an English geologist. He wrote a famous book called Principles of Geology. Darwin took the book with him on the Beagle. Lyell argued that geological processes such as erosion change Earths surface very gradually. To account for all the changes that had occurred on the planet, Earth must be a lot older than most people believed. Thomas Malthus was an English economist. He wrote a popular essay called On Population. He argued that human populations have the potential to grow faster than the resources they need. When populations get too big, disease and famine occur. These calamities control population size by killing off the weakest people. | text | null |
L_0432 | darwins theory of evolution | T_2590 | Darwin spent many years thinking about his own observations and the writings of Lamarck, Lyell, and Malthus. What did it all mean? How did it all fit together? The answer, of course, is the theory of evolution by natural selection. | text | null |
L_0432 | darwins theory of evolution | T_2591 | Heres how Darwin thought through his theory: Like Lamarck, Darwin assumed that species evolve, or change their traits over time. Fossils Darwin found on his voyage helped convince him that evolution occurs. From Lyell, Darwin realized that Earth is very old. This meant that living things had a long time in which to evolve. There was enough time to produce the great diversity of living things that Darwin had observed. From Malthus, Darwin saw that populations could grow faster than their resources. This overproduction of offspring led to a struggle for existence, in Darwins words. In this struggle, only the fittest survive. From Darwins knowledge of artificial selection, he knew how traits can change over time. Breeders artificially select the traits that they find beneficial. These traits become more common over many generations. In nature, Darwin reasoned, individuals with certain traits might be more likely to survive the struggle for existence and have offspring. Their traits would become more common over time. In this case, nature selects the traits that are beneficial. Thats why Darwin called this process natural selection. Darwin used the word fitness to refer to the ability to reproduce and pass traits to the next generation | text | null |
L_0432 | darwins theory of evolution | T_2592 | Darwin finally published his theory of evolution by natural selection in 1859. He presented it in his book On the Origin of Species. The book is very detailed and includes a lot of evidence for the theory. Darwins book changed science forever. The theory of evolution by natural selection became the unifying theory of all life science. | text | null |
L_0433 | evidence for evolution | T_2593 | Fossils are the preserved remains or traces of organisms that lived during earlier ages. Remains that become fossils are generally the hard parts of organismsmainly bones, teeth, or shells. Traces include any evidence of life, such as footprints like the dinosaur footprint in Figure 7.7. Fossils are like a window into the past. They provide direct evidence of what life was like long ago. A scientist who studies fossils to learn about the evolution of living things is called a paleontologist. | text | null |
L_0433 | evidence for evolution | T_2594 | The soft parts of organisms almost always decompose quickly after death. Thats why most fossils consist of hard parts such as bones. Its rare even for hard parts to remain intact long enough to become fossils. Fossils form when water seeps through the remains and deposits minerals in them. The remains literally turn to stone. Remains are more likely to form fossils if they are covered quickly by sediments. Once in a while, remains are preserved almost unchanged. For example, they may be frozen in glaciers. Or they may be trapped in tree resin that hardens to form amber. Thats what happened to the wasp in Figure 7.8. The wasp lived about 20 million years ago, but even its fragile wings have been preserved by the amber. | text | null |
L_0433 | evidence for evolution | T_2595 | Fossils are useful for reconstructing the past only if they can be dated. Scientists need to determine when the organisms lived who left behind the fossils. Fossils can be dated in two different ways: absolute dating and relative dating. Absolute dating determines about how long ago a fossil organism lived. This gives the fossil an approximate age in years. Absolute dating is often based on the amount of carbon-14 or other radioactive element that remains in a fossil. You can learn how carbon-14 dating works by watching this short video: Relative dating determines which of two fossils is older or younger than the other but not their age in years. Relative dating is based on the positions of fossils in rock layers. Lower rock layers were laid down earlier, so they are assumed to contain older fossils. This is illustrated in Figure 7.9. | text | null |
L_0433 | evidence for evolution | T_2595 | Fossils are useful for reconstructing the past only if they can be dated. Scientists need to determine when the organisms lived who left behind the fossils. Fossils can be dated in two different ways: absolute dating and relative dating. Absolute dating determines about how long ago a fossil organism lived. This gives the fossil an approximate age in years. Absolute dating is often based on the amount of carbon-14 or other radioactive element that remains in a fossil. You can learn how carbon-14 dating works by watching this short video: Relative dating determines which of two fossils is older or younger than the other but not their age in years. Relative dating is based on the positions of fossils in rock layers. Lower rock layers were laid down earlier, so they are assumed to contain older fossils. This is illustrated in Figure 7.9. | text | null |
L_0433 | evidence for evolution | T_2596 | The evolution of whales is a good example of how fossils can help us understand evolution. Scientists have long known that mammals first evolved on land about 200 million years ago. Its been a mystery, however, how whales evolved. Whales are mammals that live completely in the water. Did they evolve from earlier land mammals? Or did they evolve from animals that already lived in the water? Starting in the late 1970s, a growing number of fossils have allowed scientists to piece together the story of whale evolution. The fossils represent ancient, whale-like animals. They show that an ancient land mammal made its way back to the sea more than 50 million years ago. It became the ancestor of modern whales. In doing so, it lost its legs and became adapted to life in the water. In Figure 7.10 you can see an artists rendition of such a whale ancestor. It had legs and could walk on land, but it was also a good swimmer. Watch this short video to learn more about the amazing story of whale evolution based on the fossils: | text | null |
L_0433 | evidence for evolution | T_2596 | The evolution of whales is a good example of how fossils can help us understand evolution. Scientists have long known that mammals first evolved on land about 200 million years ago. Its been a mystery, however, how whales evolved. Whales are mammals that live completely in the water. Did they evolve from earlier land mammals? Or did they evolve from animals that already lived in the water? Starting in the late 1970s, a growing number of fossils have allowed scientists to piece together the story of whale evolution. The fossils represent ancient, whale-like animals. They show that an ancient land mammal made its way back to the sea more than 50 million years ago. It became the ancestor of modern whales. In doing so, it lost its legs and became adapted to life in the water. In Figure 7.10 you can see an artists rendition of such a whale ancestor. It had legs and could walk on land, but it was also a good swimmer. Watch this short video to learn more about the amazing story of whale evolution based on the fossils: | text | null |
L_0433 | evidence for evolution | T_2597 | Scientists have learned a lot about evolution by comparing living organisms. They have compared body parts, embryos, and molecules such as DNA and proteins. | text | null |
L_0433 | evidence for evolution | T_2598 | Comparing body parts of different species may reveal evidence for evolution. For example, all mammals have front limbs that look quite different and are used for different purposes. Bats use their front limbs to fly, whales use them to swim, and cats use them to run and climb. However, the front limbs of all three animalsas well as humanshave the same basic underlying bone structure. You can see this in Figure 7.11. The similar bones provide evidence that all four animals evolved from a common ancestor. | text | null |
L_0433 | evidence for evolution | T_2599 | Some of the most interesting evidence for evolution comes from vestigial structures. These are body parts that are no longer used but are still present in modern organisms. Examples in humans include tail bones and the appendix. Human beings obviously dont have tails, but our ancestors did. We still have bones at the base of our spine that form a tail in other, related animals, such as monkeys. The appendix is a tiny remnant of a once-larger organ. In a distant ancestor, it was needed to digest food. If your appendix becomes infected, a surgeon can remove it. You wont miss it because it no longer has any purpose in the human body. | text | null |
L_0433 | evidence for evolution | T_2600 | An embryo is an organism in the earliest stages of development. Embryos of different species may look quite similar, even when the adult forms look very different. Look at the drawings of embryos in Figure 7.12. They represent very early life stages of a chicken, turtle, pig, and human being. The embryos look so similar that its hard to tell them apart. Such similarities provide evidence that all four types of animals are related. They help document that evolution has occurred. | text | null |
L_0433 | evidence for evolution | T_2601 | Scientists can compare the DNA or proteins of different species. If the molecules are similar, this shows that the species are related. The more similar the molecules are, the closer the relationship is likely to be. When molecules are used in this way, they are called molecular clocks. This method assumes that random mutations occur at a constant rate for a given protein or segment of DNA. Over time, the mutations add up. The longer the amount of time since species diverged, the more differences there will be in their DNA or proteins. Table 7.1 compares the DNA of four different organisms with modern human DNA. The DNA of chimpanzees is almost 99 percent the same as the DNA of modern humans. This shows that chimpanzees are very closely related to us. We are less closely related to the other organisms in the table. Its no surprise that grapes, which are plants, are less like us than the animals in the table. Organism Chimpanzee Cow Chicken Honeybee Grape Similarity with Human DNA (percent the same) 98.8 85 65 44 24 | text | null |
L_0433 | evidence for evolution | T_2602 | The best evidence for evolution comes from actually observing changes in organisms through time. In the 1970s, biologists Peter and Rosemary Grant went to the Galpagos Islands to do fieldwork. They wanted to re-study Darwins finches. They spent the next 40 years on the project. Their hard work paid off. They were able to document evolution by natural selection taking place in the finches. A period of very low rainfall occurred while the Grants were on the islands. The drought resulted in fewer seeds for the finches to eat. Birds with smaller beaks could eat only the smaller seeds. Birds with bigger beaks were better off. They could eat seeds of all sizes. Therefore, there was more food available to them. Many of the small-beaked birds died in the drought. More of the big-beaked birds survived and reproduced. Within just a couple of years, the average beak size in the finches increased. This was clearly evolution by natural selection. | text | null |
L_0434 | the scale of evolution | T_2603 | We now know how variation in traits is inherited. Variation in traits is controlled by different alleles for genes. Alleles, in turn, are passed to gametes and then to offspring. Evolution occurs because of changes in alleles over time. How long a time? That depends on the time scale of evolution you consider. Evolution that occurs over a short period of time is known as microevolution. It might take place in just a couple of generations. This scale of evolution occurs at the level of the population. The Grants observed evolution at this scale in populations of Darwins finches. Beak size in finch populations changed in just two years because of a serious drought. Evolution that occurs over a long period of time is called macroevolution. It might take place over millions of years. This scale of evolution occurs above the level of the species. Fossils provide evidence for evolution at this scale. The evolution of the horse family, shown in Figure 7.13, is an example of macroevolution. | text | null |
L_0434 | the scale of evolution | T_2604 | Individuals dont evolve. Their alleles dont change over time. The unit of microevolution is the population. | text | null |
L_0434 | the scale of evolution | T_2605 | A population is a group of organisms of the same species that live in the same area. All the genes in all the members of a population make up the populations gene pool. For each gene, the gene pool includes all the different alleles in the population. The gene pool can be described by its allele frequencies for specific genes. The frequency of an allele is the number of copies of that allele divided by the total number of alleles for the gene in the gene pool. A simple example will help you understand these concepts. The data in Table 7.2 represent a population of 100 individuals. For each gene, the gene pool has a total of 200 alleles (2 per individual x 100 individuals). The gene in question exists as two different alleles, A and a. The number of A alleles in the gene pool is 140. Of these, 100 are in the 50 AA homozygotes. Another 40 are in the 40 Aa heterozygotes. The number of a alleles in the gene pool is 60. Of these, 40 are in the 40 Aa heterozygotes. Another 20 are in the 10 aa homozygotes. The frequency of the A allele is 140/200 = 0.7. The frequency of the a allele is 60/200 = 0.3. Genotype AA Aa aa Totals Number of Individuals 50 40 10 100 Number of A Alleles 100 (50 x 2) 40 (40 x 1) 0 (10 x 0) 140 Number of a Alleles 0 (50 x 0) 40 (40 x 1) 20 (10 x 2) 60 Evolution occurs in a population when its allele frequencies change over time. For example, the frequency of the A allele might change from 0.7 to 0.8. If that happens, evolution has occurred. What causes allele frequencies to change? The answer is forces of evolution. | text | null |
L_0434 | the scale of evolution | T_2606 | There are four major forces of evolution that cause allele frequencies to change. They are mutation, gene flow, genetic drift, and natural selection. Mutation creates new genetic variation in a gene pool This is how all new alleles first arise. Its the ultimate source of new genetic variation, so it is essential for evolution. However, for any given gene, the chance of a mutation occurring is very small. Therefore, mutation alone does not have much effect on allele frequencies. Gene flow is the movement of genes into or out of a gene pool It occurs when individuals migrate into or out of the population. How much gene flow changes allele frequencies depends on how many migrants there are and their genotypes. Genetic drift is a random change in allele frequencies. It occurs in small populations. Allele frequencies in the offspring may differ by chance from those in the parents. This is like tossing a coin just a few times. You may, by chance, get more or less than the expected 50 percent heads or tails. In the same way, you may get more or less than the expected allele frequencies in the small number of individuals in the next generation. The smaller the population is, the more allele frequencies may drift. Natural selection is a change in allele frequencies that occurs because some genotypes are more fit than others. Genotypes with greater fitness produce more offspring and pass more copies of their alleles to the next generation. This is the force of evolution that Darwin identified. Figure 23.12 shows how Darwin thought natural selection led to variation in finches on the Galpagos Islands. | text | null |
L_0434 | the scale of evolution | T_2607 | What happens when forces of evolution work over a long period of time? The answer is macroevolution. An example is the evolution of a new species. | text | null |
L_0434 | the scale of evolution | T_2608 | The evolution of a new species is called speciation. A species is a group of organisms that can mate and produce fertile offspring together but not with members of other such groups. What must happen for a new species to arise? Some members of an existing species must change so they can no produce fertile offspring with the rest of the species. Speciation often occurs when some members of a species break off from the rest. The splinter group evolves in isolation from the original species. The original species also continues to evolve. Sooner or later, the splinter group becomes too different to breed with members of the original species. At that point, a new species has formed. A good example of speciation involves anole lizards, like the one pictured in Figure 7.15. There are about 150 different species of anole lizards in the Caribbean Islands. Scientists think that a single species of lizard first colonized one of the islands about 50 million years ago. A few lizards from this original species eventually reached each of the other islands, where they evolved in isolation. Anoles in different habitats evolved traits that affected mating. For example, they evolved skin flaps of different colors. Females didnt respond to male anoles with the wrong color skin flap. This prevented them from mating. Eventually, all the different species of anoles known today evolved. Watch this interesting video to learn more about anole speciation in the Caribbean: | text | null |
L_0434 | the scale of evolution | T_2609 | Sometimes two species evolve the same traits. It happens because they live in similar habitats. This is called convergent evolution. Caribbean Anoles demonstrate this as well. On each Caribbean island, anoles in similar habitats evolved the same traits. For example, anoles that lived on the forest floor evolved long legs for leaping and running on the ground. Anoles that lived on tree branches evolved short legs that helped them cling to small branches and twigs. Anoles that lived at the tops of trees evolved large toe pads that allowed them to walk on leaves without falling. On each of the islands, there were anole species that evolved in each of these same ways. | text | null |
L_0434 | the scale of evolution | T_2610 | Two species may often interact with each other and have a close relationship. Examples include flowers and the animals that pollinate them. When one of the two species evolves new traits, the other species may evolve matching traits. This is called coevolution. You can see an example of this in Figure 7.16. The very long beak of this hummingbird co-evolved with the tubular flowers it pollinates. Only this species of hummingbird can reach nectar deep in the flowers. | text | null |
L_0434 | the scale of evolution | T_2611 | Darwin thought that evolution occurs very slowly. This is likely if conditions are stable. But what if conditions are changing rapidly? Evolution is likely to occur more rapidly as well. For example, the Grants showed that evolution occurred in just a couple of years in Darwins finches. This happened when a severe drought killed off a lot of the plants that the birds needed for food. Millions of fossils have been found since Darwins time. They show that evolution may occur in fits and starts. Long period of little or gradual change may be interrupted by bursts of rapid change. The rate of evolution is influenced by how the environment is changing. Today, Earths climate is changing rapidly. How do you think this might affect the rate of evolution? | text | null |
L_0435 | history of life on earth | T_2612 | Its hard to grasp the vast amounts of time since Earth formed and life first appeared. It may help to think of Earths history as a 24-hour day. | text | null |
L_0435 | history of life on earth | T_2613 | Figure 7.17 shows the history of Earth in a day. In this model, the planet forms at midnight. The first prokaryotes evolve around 3:00 am. Eukaryotes evolve at about 1:00 pm. Animals dont evolve until almost 8:00 pm. Humans appear only in the last minute of the day. Relating these major events in Earths history to a 24-hour day helps to put them in perspective. | text | null |
L_0435 | history of life on earth | T_2614 | Another tool for understanding the history of Earth and its life is the geologic time scale. You can see this time scale in Figure 7.18. It divides Earths history into eons, eras, and periods. These divisions are based on major changes in geology, climate, and the evolution of life. The geologic time scale organizes Earths history on the basis of important events instead of time alone. It also puts more focus on recent events, about which we know the most. | text | null |
L_0435 | history of life on earth | T_2615 | The Precambrian Supereon is the first major division of Earths history (see Figure 7.18). It covers the time from Earths formation 4.6 billion years ago to 544 million years ago. To see how life evolved during the Precambrian and beyond, watch this wonderful video. Its a good introduction to the rest of the lesson. MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0435 | history of life on earth | T_2616 | When Earth first formed, it was a fiery hot, barren ball. It had no oceans or atmosphere. Rivers of melted rock flowed over its surface. Gradually, the planet cooled and formed a solid crust. Gases from volcanoes formed an atmosphere, although it contained only a trace of oxygen. As the planet continued to cool, clouds formed and rain fell. Rainwater helped form oceans. The ancient atmosphere and oceans would be toxic to modern life, but they set the stage for life to begin. | text | null |
L_0435 | history of life on earth | T_2617 | All living things consist of organic molecules. Many scientists think that organic molecules evolved before cells, perhaps as early as 4 billion years ago. Its possible that lightning sparked chemical reactions in Earths early atmosphere. This could have created a soup of organic molecules from inorganic chemicals. Some scientists think that RNA was the first organic molecule to evolve. RNA can not only encode genetic instructions. Some RNA molecules can carry out chemical reactions. All living things are made of one or more cells. How the first cells evolved is not known for certain. Scientists speculate that lipid membranes grew around RNA molecules. The earliest cells may have consisted of little more than RNA inside a lipid membrane. You can see a model of such a cell in Figure 7.19. The first cells probably evolved between 3.8 and 4 billion years ago. Scientists think that one cell, called the Last Universal Common Ancestor (LUCA), gave rise to all of the following life on Earth. LUCA may have existed around 3.5 billion years ago. | text | null |
L_0435 | history of life on earth | T_2618 | The earliest cells were heterotrophs. They were unable to make food. Instead, they got energy by "eating" organic molecules in the soup around them. The earliest cells were also prokaryotes. They lacked a nucleus and other organelles. Gradually, these and other traits evolved. Photosynthesis evolved about 3 billion years ago. After that, certain cells could use sunlight to make food. These were the first autotrophs. They made food for themselves and other cells. They also added oxygen to the atmosphere. The oxygen was a waste product of photosynthesis. Oxygen was toxic to many cells. They had evolved in its absence. Many of them died out. The few that survived evolved a new way to use oxygen. They used it to get energy from food. This is the process of cellular respiration. The first eukaryotic cells probably evolved about 2 billion years ago. Thats when cells evolved organelles and a nucleus. Figure 7.20 shows one theory about the origin of organelles. According to this theory, a large cell engulfed small cells. The small cells took on special roles that helped the large cell function. In return, the small cells got nutrients from the large cell. Eventually, the large and small cells could no longer live apart. With their specialized organelles, eukaryotic cells were powerful and efficient. Eukaryotes would go on to evolve sexual reproduction. They would also evolve into multicellular organisms. The first multicellular organisms evolved about 1 billion years ago. | text | null |
L_0435 | history of life on earth | T_2619 | At the end of the Precambrian, a mass extinction occurred. In a mass extinction, the majority of species die out. The Precambrian mass extinction was the first of six mass extinctions that occurred on Earth. Its not certain what caused this first mass extinction. Changes in Earths geology and climate were no doubt involved. | text | null |
L_0435 | history of life on earth | T_2620 | The Paleozoic Era lasted from 544 to 245 million years ago. It is divided into six periods. | text | null |
L_0435 | history of life on earth | T_2621 | The Precambrian mass extinction opened up many niches for new organisms to fill. As a result, the Cambrian Period began with an explosion of new kinds of living things. For example, many types of simple animals called sponges evolved. Trilobites were also very common. Sponges and trilobites were small ocean invertebrates. These are animals without a backbone. You can see examples of them in Figure 7.21. | text | null |
L_0435 | history of life on earth | T_2622 | During the Ordovician Period, the oceans became filled with many kinds of invertebrates. The first fish also evolved. Plants colonized the land for the first time. However, animals remained in the water. | text | null |
L_0435 | history of life on earth | T_2623 | Corals appeared in the oceans during the Silurian period. Fish continued to evolve. On land, vascular plants appeared. These are plants that have special tissues to circulate water and other substances. This allowed plants to become larger and colonize drier habitats. | text | null |
L_0435 | history of life on earth | T_2624 | During the Devonian Period, the first seed plants evolved. Seeds have a protective coat and contain stored food. This was a big advantage over other types of plant reproduction. Seed plants eventually became the most common type of plants on land. In the oceans, fish with lobe fins evolved. These fish could breathe air when they raised their head above water. This was a step in the evolution of animals that could live on land. | text | null |
L_0435 | history of life on earth | T_2625 | In the Carboniferous Period, forests of huge ferns and trees were widespread. You can see how these first forests might have looked in Figure 7.22. After the ferns and trees died, their remains eventually turned to coal. The first amphibians also evolved during this period. They could live on land but had to return to the water to lay their eggs. After amphibians, the earliest reptiles appeared. They were the first animals that could reproduce on land and move away from the water. | text | null |
L_0435 | history of life on earth | T_2626 | During the Permian Period, all the major landmasses moved together to form one supercontinent. The supercontinent has been named Pangaea. You can see how it looked in Figure 7.23. At this time, temperatures were extreme and the climate became very dry. As a result, plants and animals evolved ways to cope with dryness. For example, reptiles evolved leathery skin. This helped prevent water loss. Plants evolved waxy leaves for the same purpose. The Permian Period ended with Earths second mass extinction. During this event, most of Earths species went extinct. It was the most massive extinction ever recorded. Its not clear why it happened. One possible reason is that a very large meteorite struck Earth. Another possibility is the eruption of enormous volcanoes. Either event could create a huge amount of dust. The dust might block out sunlight for months. This would cool the planet and prevent photosynthesis. | text | null |
L_0435 | history of life on earth | T_2627 | The Permian mass extinction paved the way for another burst of new life at the start of the Mesozoic Era. This era is known as the age of dinosaurs. It is divided into three periods. | text | null |
L_0435 | history of life on earth | T_2628 | During the Triassic Period, the first dinosaurs evolved from reptile ancestors. They eventually colonized the air and water in addition to the land. There were also forests of huge seed ferns and cone-bearing conifer trees in the Triassic Period. Modern corals, fish, and insects all evolved in this period as well. The supercontinent of Pangea started to break up. The Triassic Period ended in a mass extinction. The majority of species died out, but dinosaurs were spared. | text | null |
L_0435 | history of life on earth | T_2629 | The Triassic mass extinction gave dinosaurs the opportunity to really flourish during the Jurassic Period. Thats why this period is called the golden age of dinosaurs. The earliest birds also evolved during the Jurassic from dinosaur ancestors. In addition, all the major groups of mammals appeared. Flowering plants also appeared for the first time. New insects evolved to pollinate them. The continents continued to move apart. | text | null |
L_0435 | history of life on earth | T_2630 | During the Cretaceous Period, the dinosaurs reached their maximum size and distribution. For example, the well- known Tyrannosaurus rex weighed at least 7 tons! You can get an idea of how big it was from the T. rex skeleton in Figure 7.24. (Notice how small the person looks in the bottom left of the photo.) By the end of the Cretaceous, the continents were close to their present locations. The period ended with another mass extinction. This time, the dinosaurs went extinct. What happened to the dinosaurs? Some scientists think that a comet or asteroid may have crashed into Earth. This could darken the sky, shut down photosynthesis, and cause climate change. Other factors probably contributed to the mass extinction as well. | text | null |
L_0435 | history of life on earth | T_2631 | The extinction of the dinosaurs at the end of the Mesozoic Era paved the way for mammals to take over. Thats why the Cenozoic Era is called the age of mammals. They soon became the dominant land animals on Earth. The Cenozoic is divided into two periods. | text | null |
L_0435 | history of life on earth | T_2632 | During the Tertiary Period, many new kinds of mammals evolved. For example, primates and human ancestors first appeared during this period. Many mammals also increased in size. Modern rain forests and grasslands appeared. Flowering plants and insects increased in numbers. | text | null |
L_0435 | history of life on earth | T_2633 | During the Quaternary Period, the climate cooled. This caused a series of ice ages. Glaciers advanced southward from the North Pole. They reached as far south as Chicago and New York City. Sea levels fell because so much water was frozen in glaciers. This exposed land bridges between continents. The land bridges allowed land animals to move to new areas. Some mammals adapted to the cold by evolving very large size and thick fur. An example is the woolly mammoth, shown in Figure 7.25. Other mammals moved closer to the equator. Those that couldnt adapt or move went extinct, along with many plants. The last ice age ended about 12,000 years ago. By then, our own species, Homo sapiens, had evolved. After that, we were eyewitnesses to the story of life. As a result, the recent past is less of a mystery than the billions of years before it. | text | null |
L_0437 | bacteria | T_2649 | Bacteria are the most abundant living things on Earth. They live in almost all environments. They are found in the air, ocean, soil, and intestines of animals. They are even found in rocks deep below Earths surface. Any surface that has not been sterilized is likely to be covered with bacteria. The total number of bacteria in the world is amazing. Its estimated to be about 5 million trillion trillion. If you write that number in digits, it has 30 zeroes! | text | null |
L_0437 | bacteria | T_2650 | Bacteria are the most diverse organisms on Earth. Thousands of species of bacteria have been discovered. Many more are thought to exist. The known species are classified on the basis of various traits. For example, they may be classified by the shape of their cells. They may also be classified by how they react to a dye called Gram stain. | text | null |
L_0437 | bacteria | T_2651 | Bacteria come in several different shapes. The different shapes can be seen by examining bacteria under a light microscope. Therefore, its relatively easy to classify them by shape. There are three types of bacteria based on shape: bacilli (bacillus, singular), or rod shaped. cocci (coccus, singular), or sphere shaped. spirilli (spirillus, singular), or spiral shaped. You can see a common example of each type of bacteria in Figure 8.10. | text | null |
L_0437 | bacteria | T_2652 | Different types of bacteria stain a different color when Gram stain is applied to them. This makes them easy to identify. Some stain purple and some stain red, as you can see in Figure 8.11. The two types differ in their outer layers. This explains why they stain differently. Bacteria that stain purple are called gram-positive bacteria. They have a thick cell wall without an outer membrane. Bacteria that stain red are called gram-negative bacteria. They have a thin cell wall with an outer membrane. | text | null |
L_0437 | bacteria | T_2652 | Different types of bacteria stain a different color when Gram stain is applied to them. This makes them easy to identify. Some stain purple and some stain red, as you can see in Figure 8.11. The two types differ in their outer layers. This explains why they stain differently. Bacteria that stain purple are called gram-positive bacteria. They have a thick cell wall without an outer membrane. Bacteria that stain red are called gram-negative bacteria. They have a thin cell wall with an outer membrane. | text | null |
L_0437 | bacteria | T_2653 | Bacteria and people have many important relationships. Bacteria make our lives easier in a variety of ways. In fact, we could not survive without them. On the other hand, many bacteria can make us sick. Some of them are even deadly. For a dramatic overview of the many roles of bacteria, watch this stunning video: MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0437 | bacteria | T_2654 | Bacteria help usand all other living thingsby decomposing wastes. In this way, they recycle carbon and nitrogen in ecosystems. In addition, photosynthetic cyanobacteria are important producers. On ancient Earth, they added oxygen to the atmosphere and changed the course of evolution forever. There are billions of bacteria inside the human digestive tract. They help us digest food. They also make vitamins and play other important roles. We use bacteria in many other ways as well. For example, we use them to: create medical products such as vaccines. transfer genes in gene therapy. make fuels such as ethanol. clean up oil spills. kill plant pests. ferment foods. Do you eat any of the fermented foods pictured in Figure 8.12? If so, you are eating bacteria and their wastes. Yum! | text | null |
L_0437 | bacteria | T_2655 | You have ten times as many bacterial cells as human cells in your body. Luckily for you, most of these bacteria are harmless. However, some of them can cause disease. Any organism that causes disease is called a pathogen. Diseases caused by bacterial pathogens include food poisoning, strep throat, and Lyme disease. Bacteria that cause disease may spread directly from person to person. For example, they may spread when people shake hands with, or sneeze on, other people. Bacteria may also spread through food, water, or objects that have become contaminated with them. Some bacteria are spread by vectors. A vector is an organism that spreads bacteria or other pathogens. Most vectors are animals, commonly insects. For example, deer ticks like the one in Figure 8.13 spread Lyme disease. Ticks carry Lyme disease bacteria from deer to people when they bite them. | text | null |
L_0437 | bacteria | T_2656 | Bacteria in food or water usually can be killed by heating it to a high temperature. Generally, this temperature is at least 71 C (160 F). Bacteria on surfaces such as countertops and floors can be killed with disinfectants, such as chlorine bleach. Bacterial infections in people can be treated with antibiotic drugs. These drugs kill bacteria and may quickly cure the disease. If youve ever had strep throat, you were probably prescribed an antibiotic to treat it. Some bacteria have developed antibiotic resistance. They have evolved traits that make them resistant to one or more antibiotic drugs. You can see how this happens in Figure 8.14. Its an example of natural selection. Some bacteria are now resistant to most common antibiotic drugs. These infections are very hard to treat. | text | null |
L_0442 | adulthood and aging | T_2690 | When is a person considered an adult? That depends. Most teens become physically mature by the age of 16 or so. But they are not adults in a legal sense until they are older. For example, in the U.S., you must be 18 to vote. Once adulthood begins, it can be divided into three stages: (1) early, (2) middle, and (3) late adulthood. | text | null |
L_0442 | adulthood and aging | T_2691 | Early adulthood starts at age 18 or 21. It continues until the mid-30s. During early adulthood, people are at their physical peak. They are also usually in good health. The ability to have children is greatest during early adulthood, as well. This is the stage of life when most people complete their education. They are likely to begin a career or take a full-time job. Many people also marry and start a family during early adulthood. | text | null |
L_0442 | adulthood and aging | T_2692 | Middle adulthood begins in the mid-30s. It continues until the mid-60s. During middle adulthood, people start to show signs of aging. Their hair slowly turns gray. Their skin develops wrinkles. The risk of health problems also increases during middle adulthood. For example, heart disease, cancer, and diabetes become more common during this time. This is the stage of life when people are most likely to achieve career goals. Their children also grow up and may leave home during this stage. | text | null |
L_0442 | adulthood and aging | T_2693 | Late adulthood begins in the mid-60s. It continues until death. This is the stage of life when most people retire from work. They are also likely to reflect on their life. They may focus on their grandchildren. During late adulthood, people are not as physically able. For example, they usually have less muscle and slower reflexes. Their immune system also doesnt work as well as it used to. As a result, they have a harder time fighting diseases like the flu. The risk of developing diseases such as heart disease and cancer continues to rise. Arthritis is also common. In arthritis, joints wear out and become stiff and painful. As many as one in four late adults may develop Alzheimers disease. In this disease, brain changes cause mental abilities to decrease. This family picture shows females in each of the three stages of life. Which stage does each represent? Despite problems such as these, many people remain healthy and active into their 80s or even 90s. Do you want to be one of them? Then adopt a healthy lifestyle now and follow it for life. Doing so will increase your chances of staying healthy and active to an old age. Exercising the body and brain help prevent the physical and mental effects of aging. | text | null |
L_0449 | aquatic biomes | T_2716 | Recall that terrestrial biomes are defined by their climate. Thats because plants and animals are adapted for certain amounts of temperature and moisture. However, would aquatic biomes be classified in the same way? No, that wouldnt make much senseall parts of an aquatic environment have plenty of water. Aquatic biomes can be generally classified based on the amount of salt in the water. Freshwater biomes have less than 1% salt and are typical of ponds and lakes, streams and rivers, and wetlands. Marine biomes have more salt and are characteristic of the oceans, coral reefs, and estuaries. Most aquatic organisms do not have to deal with extremes of temperature or moisture. Instead, their main limiting factors are the availability of sunlight and the concentration of dissolved oxygen and nutrients in the water. | text | null |
L_0449 | aquatic biomes | T_2717 | Aquatic biomes in the ocean are called marine biomes. Organisms that live in marine biomes must be adapted to the salt in the water. For example, many have organs for excreting excess salt. Marine biomes include the oceans, coral reefs, and estuaries ( Figure 1.1). The oceans are the largest of all the ecosystems. They can be divided into four separate zones based on the amount of sunlight. Ocean zones are also divided based on their depth and their distance from land. Each zone has a great diversity of species. Within a coral reef, the dominant organisms are corals. Corals consist partially of algae, which provide nutrients via photosynthesis. Corals also extend tentacles to obtain plankton from the water. Coral reefs include several species of microorganisms, invertebrates, fishes, sea urchins, octopuses, and sea stars. Estuaries are areas where freshwater streams or rivers merge with the ocean. An example of a marine biome, a kelp for- est, from Anacapa Island in the Channel Islands National Marine Sanctuary. | text | null |
L_0449 | aquatic biomes | T_2718 | Freshwater biomes are defined by their low salt concentration, usually less than 1%. Plants and animals in freshwater regions are adjusted to the low salt content and would not be able to survive in areas of high salt concentration, such as the ocean. There are different types of freshwater biomes: ponds and lakes ( Figure 1.2), streams and rivers, and wetlands. Ponds and lakes range in size from just a few square meters to thousands of square kilometers. Streams and rivers are bodies of flowing water moving in one direction. They can be found everywhere. They get their starts at headwaters, which may be springs, melting snow, or even lakes, and then travel all the way to their mouths, emptying into another water channel or the ocean. Wetlands are areas of standing water that support aquatic plants. Wetlands include marshes, swamps, and bogs. Lake Tahoe in Northern California is a freshwater biome. | text | null |
L_0449 | aquatic biomes | T_2719 | In large bodies of water, such as the ocean and lakes, the water can be divided into zones based on the amount of sunlight it receives: 1. The photic zone extends to a maximum depth of 200 meters (656 feet) below the surface of the water. This is where enough sunlight penetrates for photosynthesis to occur. Algae and other photosynthetic organisms can make food and support food webs. 2. The aphotic zone is water deeper than 200 meters. This is where too little sunlight penetrates for photosyn- thesis to occur. As a result, producers must make "food" by chemosynthesis, or the food must drift down from the water above. | text | null |
L_0449 | aquatic biomes | T_2720 | Water in lakes and the ocean also varies in the amount of dissolved oxygen and nutrients it contains: 1. Water near the surface of lakes and the ocean usually has more dissolved oxygen than does deeper water. This is because surface water absorbs oxygen from the air above it. 2. Water near shore generally has more dissolved nutrients than water farther from shore. This is because most nutrients enter the water from land. They are carried by runoff, streams, and rivers that empty into a body of water. 3. Water near the bottom of lakes and the ocean may contain more nutrients than water closer to the surface. When aquatic organisms die, they sink to the bottom. Decomposers near the bottom of the water break down the dead organisms and release their nutrients back into the water. | text | null |
L_0454 | autoimmune diseases | T_2733 | The immune system usually protects you from pathogens and other causes of disease. When the immune system is working properly, it keeps you from getting sick. But the immune system is like any other system of the body. It can break down or develop diseases. AIDS is an infectious disease of the immune system caused by a virus. Some diseases of the immune system are noninfectious. They include autoimmune diseases and allergies. | text | null |
L_0454 | autoimmune diseases | T_2734 | Does it make sense for an immune system to attack the cells it is meant to protect? No, but an immune system that does not function properly will attack its own cells. An autoimmune disease is a disease in which the immune system attacks the bodys own cells. One example is type 1 diabetes. In this disease, the immune system attacks cells of the pancreas. Other examples are multiple sclerosis and rheumatoid arthritis. In multiple sclerosis, the immune system attacks nerve cells. This causes weakness and pain. In rheumatoid arthritis, the immune system attacks the cells of joints. This causes joint damage and pain. Autoimmune diseases cannot be cured. But they can be helped with medicines that weaken the immune systems attack on normal cells. Other autoimmune diseases include celiac disease (damages to the small intestine), inflam- matory bowel disease (damage to the digestive tract), psoriasis (damage to the skin), and lupus (damage to the joints, skin, kidneys, heart, and lungs). | text | null |
L_0454 | autoimmune diseases | T_2735 | An allergy occurs when the immune system attacks a harmless substance that enters the body from the outside. A substance that causes an allergy is called an allergen. It is the immune system, not the allergen, that causes the symptoms of an allergy. Did you ever hear of hay fever? Its not really a fever at all. Its an allergy to plant pollens. People with this type of allergy have symptoms such as watery eyes, sneezing, and a runny nose. A common cause of hay fever is the pollen of ragweed. Many people are also allergic to poison ivy ( Figure 1.2). Skin contact with poison ivy leads to an itchy rash in people who are allergic to the plant. Ragweed is a common roadside weed found throughout the United States. Many people are allergic to its pollen. Some people are allergic to certain foods. Nuts and shellfish are common causes of food allergies. Other common causes of allergies include: Drugs, such as penicillin. Mold. Dust. The dead skin cells of dogs and cats, called dander. Stings of wasps and bees. Most allergies can be treated with medicines. Medicines used to treat allergies include antihistamines and corticos- teroids. These medicines help control the immune system when it attacks an allergen. Sometimes, allergies cause severe symptoms, a condition known as anaphylaxis. For example, they may cause the throat to swell so it is hard to breathe. Severe allergies may be life threatening. They require emergency medical care. | text | null |
L_0457 | bacteria in the digestive system | T_2745 | Your large intestine is not just made up of cells. It is also an ecosystem, home to trillions of bacteria known as the "gut flora" ( Figure 1.1). But dont worry, most of these bacteria are helpful. Friendly bacteria live mostly in the large intestine and part of the small intestine. The acidic environment of the stomach does not allow bacterial growth. Gut bacteria have several roles in the body. For example, intestinal bacteria: Produce vitamin B12 and vitamin K. Control the growth of harmful bacteria. Break down poisons in the large intestine. Break down some substances in food that cannot be digested, such as fiber and some starches and sugars. Bacteria produce enzymes that digest carbohydrates in plant cell walls. Most of the nutritional value of plant material would be wasted without these bacteria. These help us digest plant foods like spinach. Your intestines are home to trillions of bacteria. A wide range of friendly bacteria live in the gut. Bacteria begin to populate the human digestive system right after birth. Gut bacteria include Lactobacillus, the bacteria commonly used in probiotic foods such as yogurt, and E. coli bacteria. About a third of all bacteria in the gut are members of the Bacteroides species. Bacteroides are key in helping us digest plant food. It is estimated that 100 trillion bacteria live in the gut. This is more than the human cells that make up you. It has also been estimated that there are more bacteria in your mouth than people on the planet. There are over 7 billion people on the planet. The bacteria in your digestive system are from anywhere between 300 and 1000 species. As these bacteria are helpful, your body does not attack them. They actually appear to the bodys immune system as cells of the digestive system, not foreign invaders. The bacteria actually cover themselves with sugar molecules removed from the actual cells of the digestive system. This disguises the bacteria and protects them from the immune system. As the bacteria that live in the human gut are beneficial to us, and as the bacteria enjoy a safe environment to live, the relationship that we have with these tiny organisms is described as mutualism, a type of symbiotic relationship. Lastly, keep in mind the small size of bacteria. Together, all the bacteria in your gut may weight just about 2 pounds. | text | null |
L_0458 | bacteria nutrition | T_2746 | Like all organisms, bacteria need energy, and they can acquire this energy through a number of different ways. | text | null |
L_0458 | bacteria nutrition | T_2747 | Photosynthetic bacteria use the energy of the sun to make their own food. In the presence of sunlight, carbon dioxide and water are turned into glucose and oxygen. The glucose is then turned into usable energy. Glucose is like the "food" for the bacteria. An example of photosynthetic bacteria is cyanobacteria, as seen in the opening image. These bacteria are sometimes called blue-green algae, although they are not algae, due to their numerous chlorophyll molecules. | text | null |
L_0458 | bacteria nutrition | T_2748 | Bacteria known as decomposers break down wastes and dead organisms into smaller molecules. These bacteria use the organic substrates they break down to get their energy, carbon, and nutrients they need for survival. | text | null |
L_0458 | bacteria nutrition | T_2749 | Bacteria can also be chemotrophs. Chemosynthetic bacteria, or chemotrophs, obtain energy by breaking down chemical compounds in their environment. An example of one of these chemicals broken down by bacteria is nitrogen-containing ammonia. These bacteria are important because they help cycle nitrogen through the environ- ment for other living things to use. Nitrogen cannot be made by living organisms, so it must be continually recycled. Organisms need nitrogen to make organic compounds, such as DNA. | text | null |
L_0458 | bacteria nutrition | T_2750 | Some bacteria depend on other organisms for survival. For example, some bacteria live in the roots of legumes, such as pea plants ( Figure 1.1). The bacteria turn nitrogen-containing molecules into nitrogen that the plant can use. Meanwhile, the root provides nutrients to the bacteria. In this relationship, both the bacteria and the plant benefit, so it is known as a mutualism. Other mutualistic bacteria include gut microbes. These are bacteria that live in the intestines of animals. They are usually beneficial bacteria, needed by the host organism. These microbes obviously dont kill their host, as that would kill the bacteria as well. | text | null |
L_0458 | bacteria nutrition | T_2751 | Other bacteria are parasitic and can cause illness. In parasitism, the bacteria benefit, and the other organism is harmed. Harmful bacteria will be discussed in another concept. | text | null |
L_0460 | barriers to pathogens | T_2755 | It is the immune systems job to protect the body. Your body has many ways to protect you from pathogens. Your bodys defenses are like a castle. The outside of a castle was protected by a moat and high walls. Inside the castle, soldiers were ready to fight off any enemies that made it across the moat and over the walls. Like a castle, your body has a series of defenses. Only pathogens that get through all the defenses can harm you. The first line of defence includes both physical and chemical barriers that are always ready and prepared to defend the body from infection. Pathogens must make it past this first line of defense to cause harm. If this defense is broken, the second line of defense within your body is activated. Your bodys first line of defense is like a castles moat and walls. It keeps most pathogens out of your body. This is a non-specific type of defense, in that it tries to keep all pathogens out. The first line of defense includes different types of barriers. Being the "first line", it starts with the skin. The first line also includes tears, mucus, cilia, stomach acid, urine flow, and friendly bacteria. | text | null |
L_0460 | barriers to pathogens | T_2756 | The skin is a very important barrier to pathogens. The skin is the bodys largest organ. In adults, it covers an area of about 16 to 22 square feet! The skin is also the bodys most important defense against disease. It forms a physical barrier between the body and the outside world. The skin has several layers that stack on top of each other ( Figure The mouth and nose are not lined with skin. Instead, they are lined with mucous membranes. Other organs that are exposed to the outside world, including the lungs and stomach, are also lined with mucous membranes. Mucous membranes are not tough like skin, but they have other defenses. One defense of mucous membranes is the mucus they release. Mucus is a sticky, moist substance that covers mucous membranes. Most pathogens get stuck in the mucus before they can do harm to the body. Many mucous membranes also have cilia. Cilia in the lungs are pictured below ( Figure 1.2). Cilia are tiny finger-like projections. They move in waves and sweep mucus and trapped pathogens toward body openings. When you clear your throat or blow your nose, you remove mucus and pathogens from your body. | text | null |
L_0460 | barriers to pathogens | T_2757 | Most body fluids that you release from your body contain chemicals that kill pathogens. For example, mucus, sweat, tears, and saliva contain enzymes called lysozymes that kill pathogens. These enzymes can break down the cell walls of bacteria to kill them. The stomach also releases a very strong acid, called hydrochloric acid. This acid kills most pathogens that enter the stomach in food or water. Urine is also acidic, so few pathogens can grow in it. This is what the cilia lining the lungs look like when they are magnified. Their movements constantly sweep mucus and pathogens out of the lungs. Do they remind you of brushes? | text | null |
L_0460 | barriers to pathogens | T_2758 | You are not aware of them, but your skin is covered by millions (or more!) of bacteria. Millions more live inside your body. Most of these bacteria help defend your body from pathogens. How do they do it? They compete with harmful bacteria for food and space. This prevents the harmful bacteria from multiplying and making you sick. | text | null |
L_0467 | blood types | T_2774 | Do you know what your blood type is? Maybe you have heard people say that they have type A or type O blood. Blood type is a way to describe the type of antigens, or proteins, on the surface of red blood cells (RBCs). There are four blood types; A, B, AB, and O. 1. Type A blood has type A antigens on the RBCs in the blood. 2. Type AB blood has A and B antigens on the RBCs. 3. Type B has B antigens on the RBCs. 4. Type O does not have either A or B antigens. The ABO blood group system is important if a person needs a blood transfusion. A blood transfusion is the process of putting blood or blood products from one person into the circulatory system of another person. The blood type of the recipient needs to be carefully matched to the blood type of the donor. Thats because different blood types have different types of antibodies, or proteins, released by the blood cells. Antibodies attack strange substances in the body. This is a normal part of your immune response, which is your defense against disease. For example, imagine a person with type O blood was given type A blood. First, what type of antibodies do people with type O blood produce? They produce anti-A and anti-B antibodies. This means, if a person with type O blood received type A blood, the anti-A antibodies in the persons blood would attack the A antigens on the RBCs in the donor blood ( Figure 1.1). The antibodies would cause the RBCs to clump together, and the clumps could block a blood vessel. This clumping of blood cells could cause death. A person with type O blood has A and B antibodies in his/her plasma; if the person was to get type A blood instead of type O, his/her A antibodies would attach to the antigens on the RBCs and cause them to clump together. People with type A blood produce anti-B antibodies, and people with type B blood produce anti-A antibodies. People with type AB blood do not produce either antibody. | text | null |
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