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L_0423 | transport | T_2489 | Active transport occurs when a substance passes through the cell membrane with the help of extra energy. This happens when a substance moves from an area where it is less concentrated to an area where it is more concentrated. This is the opposite of diffusion. The substance moves up, instead of down, the concentration gradient. Like rolling a ball uphill, this requires an input of energy. The energy comes from the molecule named ATP (adenosine triphosphate). The energy allows special transport proteins called pumps to move substances to areas of higher concentration. An example is the sodium-potassium pump. | text | null |
L_0423 | transport | T_2490 | Sodium and potassium are two of the most important elements in living things. They are present mainly as positively charged ions dissolved in water. The sodium-potassium pump moves sodium ions (Na+ ) out of the cell and potassium ions (K+ ) into the cell. In both cases, the ions are moving from an area of lower to higher concentration. Energy in ATP is needed for this "uphill" process. Figure 4.4 shows how this pump works. Trace these steps from left to right in the figure: 1. Three sodium ions inside the cell bind with a carrier protein in the cell membrane. 2. The carrier protein receives a phosphate from ATP. This forms ADP (adenosine diphosphate) and releases energy. 3. The energy causes the carrier protein to change shape. As it does, it pumps the three sodium ions out of the cell. 4. Two potassium ions outside the cell next bind with the carrier protein. Then the process reverses, and the two potassium ions are pumped into the cell. | text | null |
L_0423 | transport | T_2491 | Some substances are too big to be pumped across the cell membrane. They may enter or leave the cell by vesicle transport. This takes energy, so its another form of active transport. You can see how vesicle transport occurs in Figure 4.5. Vesicle transport out of the cell is called exocytosis. A vesicle containing the substance moves through the cytoplasm to the cell membrane. Then the vesicle fuses with the cell membrane and releases the substance outside the cell. You can watch this happening in this very short animation: MEDIA Click image to the left or use the URL below. URL: Vesicle transport into the cell is called endocytosis. The cell membrane engulfs the substance. Then a vesicle pinches off from the membrane and carries the substance into the cell. | text | null |
L_0426 | cell division | T_2513 | DNA stands for deoxyribonucleic acid. It is a very large molecule. It consists of two strands of smaller molecules called nucleotides. Before learning how DNA is copied, its a good idea to review its structure. | text | null |
L_0426 | cell division | T_2514 | As you can see in Figure 5.1, each nucleotide includes a sugar, a phosphate, and a nitrogen base. The sugar in DNA is called deoxyribose. There are four different nitrogen bases in DNA: adenine (A), thymine (T), cytosine (C), and guanine (G). Chemical bonds between the bases hold the two strands of DNA together. Adenine always bonds with thymine, and cytosine always bonds with guanine. These pairs of bases are called complementary base pairs. | text | null |
L_0426 | cell division | T_2515 | As a cell prepares to divide, its DNA first forms one or more structures called chromosomes. A chromosome consists of DNA and protein molecules coiled into a definite shape. Chromosomes are circular in prokaryotes and rodlike in eukaryotes. You can see an example of a human chromosome in Figure below. The rest of the time, DNA looks like a tangled mass of strings. In this form, it would be very difficult to copy and divide. | text | null |
L_0426 | cell division | T_2516 | The process in which DNA is copied is called DNA replication. You can see how it happens in Figure 5.3. An enzyme breaks the bonds between the two DNA strands. Another enzyme pairs new, complementary nucleotides with those in the original chains. Two daughter DNA molecules form. Each contains one new chain and one original chain. | text | null |
L_0426 | cell division | T_2516 | The process in which DNA is copied is called DNA replication. You can see how it happens in Figure 5.3. An enzyme breaks the bonds between the two DNA strands. Another enzyme pairs new, complementary nucleotides with those in the original chains. Two daughter DNA molecules form. Each contains one new chain and one original chain. | text | null |
L_0426 | cell division | T_2517 | How cell division proceeds depends on whether a cell has a nucleus. Prokaryotic cells lack a nucleus. Their DNA is in the cytoplasm. It forms just one circular chromosome. Eukaryotic cells have a nucleus holding their DNA. Their DNA forms multiple rodlike chromosomes, like the one in Figure 5.2. Eukaryotic cells also have other organelles. For these reasons, cell division is more complex in eukaryotic cells. | text | null |
L_0426 | cell division | T_2518 | You can see how a prokaryotic cell divides in Figure 5.4. This type of cell division is called binary fission. The cell simply splits into two equal halves. Binary fission occurs in bacteria and other prokaryotes. It takes place in three continuous steps: 1. The cells chromosome is copied to form two identical chromosomes. This is DNA replication. 2. The copies of the chromosome separate from each other. They move to opposite poles, or ends, of the cell. This is called chromosome segregation. 3. The cell wall grows toward the center of the cell. The cytoplasm splits apart, and the cell pinches in two. This is called cytokinesis. | text | null |
L_0426 | cell division | T_2519 | Before a eukaryotic cell divides, the nucleus and other organelles must be copied. Only then will each daughter cell have all the needed structures. 1. The first step in eukaryotic cell division, as it is in prokaryotic cell division, is DNA replication. As you can see in Figure 5.5, each chromosome then consists of two identical copies. The two copies are called sister chromatids. They are attached to each other at a point called the centromere. 2. The second step in eukaryotic cell division is division of the cells nucleus. This includes division of the chromosomes. This step is called mitosis. It is a complex process that occurs in four phases. The phases of mitosis are described below. 3. The third step is the division of the rest of the cell. This is called cytokinesis, as it is in a prokaryotic cell. During this step, the cytoplasm divides, and two daughter cells form. These three steps are shown in Figure 5.6. | text | null |
L_0426 | cell division | T_2520 | Mitosis, or division of the nucleus, occurs only in eukaryotic cells. By the time mitosis occurs, the cells DNA has already replicated. Mitosis occurs in four phases, called prophase, metaphase, anaphase, and telophase. You can see what happens in each phase in Figure below. The phases are described below. You can also learn more about the phases of mitosis by watching this video: . MEDIA Click image to the left or use the URL below. URL: 1. Prophase: Chromosomes form, and the nuclear membrane breaks down. In animal cells, the centrioles near the nucleus move to opposite poles of the cell. Fibers called spindles form between the centrioles. 2. Metaphase: Spindle fibers attach to the centromeres of the sister chromatids. The sister chromatids line up at the center of the cell. 3. Anaphase: Spindle fibers shorten, pulling the sister chromatids toward the opposite poles of the cell. This gives each pole a complete set of chromosomes. 4. Telophase: The chromosomes uncoil, and the spindle fibers break down. New nuclear membranes form. | text | null |
L_0426 | cell division | T_2520 | Mitosis, or division of the nucleus, occurs only in eukaryotic cells. By the time mitosis occurs, the cells DNA has already replicated. Mitosis occurs in four phases, called prophase, metaphase, anaphase, and telophase. You can see what happens in each phase in Figure below. The phases are described below. You can also learn more about the phases of mitosis by watching this video: . MEDIA Click image to the left or use the URL below. URL: 1. Prophase: Chromosomes form, and the nuclear membrane breaks down. In animal cells, the centrioles near the nucleus move to opposite poles of the cell. Fibers called spindles form between the centrioles. 2. Metaphase: Spindle fibers attach to the centromeres of the sister chromatids. The sister chromatids line up at the center of the cell. 3. Anaphase: Spindle fibers shorten, pulling the sister chromatids toward the opposite poles of the cell. This gives each pole a complete set of chromosomes. 4. Telophase: The chromosomes uncoil, and the spindle fibers break down. New nuclear membranes form. | text | null |
L_0426 | cell division | T_2521 | Cell division is just one of the stages that a cell goes through during its lifetime. All of the stages that a cell goes through make up the cell cycle. | text | null |
L_0426 | cell division | T_2522 | The cell cycle of a prokaryotic cell is simple. The cell grows in size, its DNA replicates, and the cell divides. | text | null |
L_0426 | cell division | T_2523 | In eukaryotes, the cell cycle is more complicated. The diagram in Figure 5.7 shows the stages that a eukaryotic cell goes through in its lifetime. There are two main stages: interphase and mitotic phase. They are described below. You can watch a eukaryotic cell going through the phases of the cell cycle at this link: Interphase is longer than mitotic phase. Interphase, in turn, is divided into three phases: Mitotic phase is when the cell divides. It includes mitosis (M) and cytokinesis (C). | text | null |
L_0427 | reproduction | T_2524 | Asexual reproduction is simpler than sexual reproduction. It involves just one parent. The offspring are genetically identical to each other and to the parent. All prokaryotes and some eukaryotes reproduce this way. There are several different methods of asexual reproduction. They include binary fission, fragmentation, and budding. | text | null |
L_0427 | reproduction | T_2525 | Binary fission occurs when a parent cell simply splits into two daughter cells. This method is described in detail in the lesson "Cell Division." Bacteria reproduce this way. You can see a bacterial cell reproducing by binary fission in Figure 5.9. | text | null |
L_0427 | reproduction | T_2526 | Fragmentation occurs when a piece breaks off from a parent organism. Then the piece develops into a new organism. Sea stars, like the one in Figure 5.10, can reproduce this way. In fact, a new sea star can form from a single arm. | text | null |
L_0427 | reproduction | T_2527 | Budding occurs when a parent cell forms a bubble-like bud. The bud stays attached to the parent while it grows and develops. It breaks away from the parent only after it is fully formed. Yeasts can reproduce this way. You can see two yeast cells budding in Figure 5.11. | text | null |
L_0427 | reproduction | T_2528 | Sexual reproduction is more complicated. It involves two parents. Special cells called gametes are produced by the parents. A gamete produced by a female parent is generally called an egg. A gamete produced by a male parent is usually called a sperm. An offspring forms when two gametes unite. The union of the two gametes is called fertilization. You can see a human sperm and egg uniting in Figure 5.12. The initial cell that forms when two gametes unite is called a zygote. | text | null |
L_0427 | reproduction | T_2529 | In species with sexual reproduction, each cell of the body has two copies of each chromosome. For example, human beings have 23 different chromosomes. Each body cell contains two of each chromosome, for a total of 46 chromosomes. You can see the 23 pairs of human chromosomes in Figure 5.13. The number of different types of chromosomes is called the haploid number. In humans, the haploid number is 23. The number of chromosomes in normal body cells is called the diploid number. The diploid number is twice the haploid number. In humans, the diploid number is two times 23, or 46. | text | null |
L_0427 | reproduction | T_2530 | The two members of a given pair of chromosomes are called homologous chromosomes. We get one of each homologous pair, or 23 chromosomes, from our father. We get the other one of each pair, or 23 chromosomes, from our mother. A gamete must have the haploid number of chromosomes. That way, when two gametes unite, the zygote will have the diploid number. How are haploid cells produced? The answer is meiosis. | text | null |
L_0427 | reproduction | T_2531 | Meiosis is a special type of cell division. It produces haploid daughter cells. It occurs when an organism makes gametes. Meiosis is basically mitosis times two. The original diploid cell divides twice. The first time is called meiosis I. The second time is called meiosis II. However, the DNA replicates only once. It replicates before meiosis I but not before meiosis II. This results in four haploid daughter cells. Meiosis I and meiosis II occurs in the same four phases as mitosis. The phases are prophase, metaphase, anaphase, and telophase. However, meiosis I has an important difference. In meiosis I, homologous chromosomes pair up and then separate. As a result, each daughter cell has only one chromosome from each homologous pair. Figure 5.14 is a simple model of meiosis. It shows both meiosis I and II. You can read more about the stages below. You can also learn more about them by watching this video: . MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0427 | reproduction | T_2532 | After DNA replicates during interphase, the nucleus of the cell undergoes the four phases of meiosis I: 1. Prophase I: Chromosomes form, and the nuclear membrane breaks down. Centrioles move to opposite poles of the cell. Spindle fibers form between the centrioles. Heres whats special about meiosis: Homologous chromosomes pair up! You can see this in Figure below. 2. Metaphase I: Spindle fibers attach to the centromeres of the paired homologous chromosomes. The paired chromosomes line up at the center of the cell. 3. Anaphase I: Spindle fibers shorten, pulling apart the chromosome pairs. The chromosomes are pulled toward opposite poles of the cell. One of each pair goes to one pole. The other of each pair goes to the opposite pole. 4. Telophase I: The chromosomes uncoil, and the spindle fibers break down. New nuclear membranes form. Phases of meiosis I Meiosis I is followed by cytokinesis. Thats when the cytoplasm of the cell divides. Two haploid daughter cells result. Both of these cells go on to meiosis II. | text | null |
L_0427 | reproduction | T_2533 | Meiosis II is just like mitosis. 1. Prophase II: Chromosomes form. The nuclear membrane breaks down. Centrioles move to opposite poles. Spindle fibers form. 2. Metaphase II: Spindle fibers attach to the centromeres of sister chromatids. Sister chromatids line up at the center of the cell. 3. Anaphase II: Spindle fibers shorten. They pull the sister chromatids to opposite poles. 4. Telophase II: The chromosomes uncoil. The spindle fibers break down. New nuclear membranes form. Meiosis II is also followed by cytokinesis. This time, four haploid daughter cells result. Thats because both daughter cells from meiosis I have gone through meiosis II. The four daughter cells must continue to develop before they become gametes. For example, in males, the cells must develop tails, among other changes, in order to become sperm. | text | null |
L_0427 | reproduction | T_2534 | Both types of reproduction have certain advantages. | text | null |
L_0427 | reproduction | T_2535 | Asexual reproduction can happen very quickly. It doesnt require two parents to meet and mate. Under ideal conditions, 100 bacteria can divide to produce millions of bacteria in just a few hours! Most bacteria dont live under ideal conditions. Even so, rapid reproduction may allow asexual organisms to be very successful. They may crowd out other species that reproduce more slowly. | text | null |
L_0427 | reproduction | T_2536 | Sexual reproduction is typically slower. However, it also has an advantage. Sexual reproduction results in offspring that are all genetically different. This can be a big plus for a species. The variation may help it adapt to changes in the environment. How does genetic variation arise during sexual reproduction? It happens in three ways: crossing over, independent assortment, and the random union of gametes. Crossing over occurs during meiosis I. It happens when homologous chromosomes pair up during prophase I. The paired chromosomes exchange bits of DNA. This recombines their genetic material. You can see where crossing over has occurred in Figures 5.15 and 5.16. Independent assortment occurs when chromosomes go to opposite poles of the cell in anaphase I. Which chromosomes end up together at each pole is a matter of chance. You can see this in Figures 5.15 and 5.16 as well. In sexual reproduction, two gametes unite to produce an offspring. Which two gametes is a matter of chance. The union of gametes occurs randomly. Due to these sources of variation, each human couple has the potential to produce more than 64 trillion unique offspring. No wonder we are all different! | text | null |
L_0436 | introduction to prokaryotes | T_2634 | Prokaryotes are currently placed in two domains. A domain is the highest taxon in the classification of living things. Its even higher than the kingdom. | text | null |
L_0436 | introduction to prokaryotes | T_2635 | The prokaryote domains are the Bacteria Domain and Archaea Domain, shown in Figure 8.2. All other living things are eukaryotes and placed in the domain Eukarya. (Unlike prokaryotes, eukaryotes have a nucleus in their cells.) | text | null |
L_0436 | introduction to prokaryotes | T_2635 | The prokaryote domains are the Bacteria Domain and Archaea Domain, shown in Figure 8.2. All other living things are eukaryotes and placed in the domain Eukarya. (Unlike prokaryotes, eukaryotes have a nucleus in their cells.) | text | null |
L_0436 | introduction to prokaryotes | T_2636 | Prokaryotes were the first living things to evolve on Earth, probably around 3.8 billion years ago. They were the only living things until the first eukaryotic cells evolved about 2 billion years ago. Prokaryotes are still the most numerous organisms on Earth. Its not certain how the three domains of life are related. Archaea were once thought to be offshoots of Bacteria that were adapted to extreme environments. For their part, Bacteria were considered to be ancestors of Eukarya. Scientists now know that Archaea share several traits with Eukarya that Bacteria do not share. How can this be explained? One hypothesis is that the first Eukarya formed when an archaean cell fused with a bacterial cell. By fusing, the two prokaryotic cells became the nucleus and cytoplasm of a new eukaryotic cell. If this hypothesis is correct, both prokaryotic domains are ancestors of Eukarya. | text | null |
L_0436 | introduction to prokaryotes | T_2637 | All prokaryotes consist of just one cell. They share a number of other traits as well. Watch this entertaining video from the Amoeba Sisters to see how prokaryotes differ in structure from eukaryotes: MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0436 | introduction to prokaryotes | T_2638 | Most prokaryotic cells are much smaller than eukaryotic cells. Prokaryotic cells are typically only 0.2-2.0 microm- eter in diameter. Eukaryotic cells are about 50 times as big. Prokaryotic cells have a variety of different cell shapes. Figure 8.3 shows three of the most common shapes: spirals (helices), spheres, and rods. Bacteria may be classified by their shape. | text | null |
L_0436 | introduction to prokaryotes | T_2639 | Most prokaryotes have one or more long, thin "whips" called flagella (flagellum, plural). You can see flagella in Figure 8.4. Flagella help prokaryotes move toward food or away from toxins. Each flagellum spins around a fixed base. This causes the cell to roll and tumble. | text | null |
L_0436 | introduction to prokaryotes | T_2640 | The cells of prokaryotes have two or three outer layers. Like all other living cells, prokaryotes have a cell membrane. It controls what enters and leaves the cell. Its also the site of many metabolic reactions. For example, cellular respiration takes place in the cell membrane. Most prokaryotes also have a cell wall. It lies just outside the cell membrane. It makes the cell stronger and more rigid. Many prokaryotes have another layer, called a capsule, outside the cell wall. The capsule protects the cell from chemicals and drying out. It also allows the cell to stick to surfaces and to other cells. You can see a model of a prokaryotic cell in Figure 8.5. Find the cell membrane, cell wall, and capsule in the figure. | text | null |
L_0436 | introduction to prokaryotes | T_2641 | Several other prokaryotic cell structures are also shown in Figure 8.5. They include: cytoplasm. Like all other cells, prokaryotic cells are filled with cytoplasm. It includes watery cytosol and other structures. ribosomes. This is the site where proteins are made. cytoskeleton. This is a network of fibers and tubules that crisscrosses the cytoplasm. The cytoskeleton helps the cell keep its shape. pili. These are hair-like projections from the surface of the cell. They help the cell hold on to surfaces or do other jobs for the cell. | text | null |
L_0436 | introduction to prokaryotes | T_2642 | All prokaryotic cells contain DNA, as you can see in Figure 8.6. Most of the DNA is in the form of a single large loop. This DNA coils up in the cytoplasm to form a structure called a nucleoid. There is no membrane surrounding it. Most prokaryotes also have one or more small loops of DNA. They are called plasmids. | text | null |
L_0436 | introduction to prokaryotes | T_2642 | All prokaryotic cells contain DNA, as you can see in Figure 8.6. Most of the DNA is in the form of a single large loop. This DNA coils up in the cytoplasm to form a structure called a nucleoid. There is no membrane surrounding it. Most prokaryotes also have one or more small loops of DNA. They are called plasmids. | text | null |
L_0436 | introduction to prokaryotes | T_2643 | Some prokaryotes form structures consisting of many individual cells, like the cells in Figure 8.7. This is called a biofilm. A biofilm is a colony of prokaryotes that is stuck to a surface. The surface might be a rock or a hosts tissues. The sticky plaque that collects on your teeth between brushings is a biofilm. It consists of millions of prokaryotic cells. | text | null |
L_0436 | introduction to prokaryotes | T_2644 | Like all living things, prokaryotes need energy and carbon. They meet these needs in a variety of ways and in a range of habitats. | text | null |
L_0436 | introduction to prokaryotes | T_2645 | Prokaryotes may have just about any type of metabolism. They may get energy from light or from chemical compounds. They may get carbon from carbon dioxide or from other living things. Most prokaryotes get both energy and carbon from other living things. Many of them are decomposers. They break down wastes and remains of dead organisms. In this way, they help to recycle carbon and nitrogen through ecosystems. Some prokaryotes use energy in sunlight to make food from carbon dioxide. They do this by the process of photosynthesis. They are important producers in aquatic ecosystems. Look at the green streaks on the lake in Figure 8.8. They consist of billions of photosynthetic bacteria called cyanobacteria. | text | null |
L_0436 | introduction to prokaryotes | T_2646 | Prokaryotes live in a wide range of habitats. For example, they may live in habitats with or without oxygen. Prokaryotes that need oxygen are described as aerobic. They use oxygen for cellular respiration. Examples include the prokaryotes that live on your skin. Prokaryotes that dont need oxygen or are poisoned by it are described as anaerobic. They use fermentation or other anaerobic processes rather than cellular respiration. Examples include many of the prokaryotes that live inside your body. Like most other living things, prokaryotes have a temperature range that they "like" best. Thermophiles are prokaryotes that prefer a temperature above 45 C (113 F). They might be found in a compost pile. Mesophiles are prokaryotes that prefer a temperature of about 37 C (98 C). They might be found inside the body of an animal such as you. Psychrophiles are prokaryotes that prefer a temperature below 20 C (68 F). They might be found deep in the ocean. | text | null |
L_0436 | introduction to prokaryotes | T_2647 | Prokaryotes reproduce asexually. This can happen by binary fission or budding. In binary fission, a cell splits in two. First, the large circular chromosome is copied. Then the cell divides to form two new daughter cells. Each has a copy of the parent cells chromosome. In budding, a new cell grows from a bud on the parent cell. It only breaks off to form a new cell when it is fully formed. | text | null |
L_0436 | introduction to prokaryotes | T_2648 | For natural selection to take place, organisms must vary in their traits. Asexual reproduction results in offspring that are all the same. They are also identical to the parent cell. So how can prokaryotes increase genetic variation? They can exchange plasmids. This is called genetic transfer. It may happen by direct contact between cells. Or a "bridge" may form between cells. Genetic transfer mixes the genes of different cells. It creates new combinations of alleles. | text | null |
L_0439 | protists | T_2666 | Protists are placed in the Protist Kingdom. This kingdom is one of four kingdoms in the Eukarya domain. The other three Eukarya kingdoms are the Fungi, Plant, and Animal Kingdoms. | text | null |
L_0439 | protists | T_2667 | The Protist Kingdom is hard to define. It includes many different types of organisms. You can see some examples of protists in Figure 9.1. The Protist Kingdom includes all eukaryotes that dont fit into one of the other three eukaryote kingdoms. For that reason, its sometimes called the trash can kingdom. The number of species in the Protist Kingdom is unknown. It could range from as few as 60,000 to as many as 200,000 species. For a beautiful introduction to the amazing world of protists, watch this video: MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0439 | protists | T_2668 | Scientists think that protists are the oldest eukaryotes. If so, they must have evolved from prokaryotes. How did this happen? How did cells without organelles acquire them? What was the origin of mitochondria, chloroplasts, and other organelles? The most likely way organelles evolved is shown in Figure 9.2. First, smaller prokaryotic cells invaded, or were engulfed by, larger prokaryotic cells. The smaller cells benefited by getting nutrients and a safe place to live. The larger cells benefited by getting some of the organic molecules or energy released by the smaller cells. Eventually, the smaller cells evolved into organelles in the larger cells. After that, neither could live without the other. | text | null |
L_0439 | protists | T_2669 | Despite the diversity of protists, they do share some traits. The cells of all protists have a nucleus. They also have other membrane-bound organelles. For example, all of them have mitochondria, and some of them have chloroplasts. Most protists consist of a single cell. Some are multicellular but they lack specialized cells. Most protists live in wet places. They are found in oceans, lakes, swamps, or damp soils. Many protists can move. Most protists also have a complex life cycle. The life cycle of an organism is the cycle of phases it goes through until it returns to the starting phase. The protist life cycle includes both sexual and asexual reproduction. Why reproduce both ways? Each way has benefits. Asexual reproduction is fast. It allows rapid population growth when conditions are stable. Sexual reproduction increases genetic variation. This helps ensure that some organisms will survive if conditions change. | text | null |
L_0439 | protists | T_2670 | Protists are classified based on traits they share with other eukaryotes. There are animal-like, plant-like, and fungus- like protists. The three groups differ mainly in how they get carbon and energy. | text | null |
L_0439 | protists | T_2671 | Animal-like protists are called protozoa (protozoan, singular). Most protozoa consist of a single cell. Protozoa are probably ancestors of animals. Protozoa are like animals in two ways: 1. Protozoa are heterotrophs. Heterotrophs get food by eating other organisms. Some protozoa prey on bacteria. Some are parasites of animals. Others graze on algae. Still others are decomposers that break down dead organic matter. 2. Almost all protozoa can move. They have special appendages for this purpose. You can see different types in Figure 9.3. Cilia (cilium, singular) are short, hair-like projections. Pseudopods are temporary extensions of the cytoplasm. Flagella are long, whip-like structures. Flagella are also found in most prokaryotes. | text | null |
L_0439 | protists | T_2672 | Plant-like protists are commonly called algae (alga, singular). Some algae consist of single cells. They are called diatoms. Other algae are multicellular. An example is seaweed. Seaweed called kelp can grow as large as trees. You can see both a diatom and kelp in Figure 9.4. Algae are probably ancestors of plants. Algae are like plants mainly because they contain chloroplasts. This allows them to make food by photosynthesis. Algae are important producers in water-based ecosystems such as the ocean. On the other hand, algae lack other plant structures. For example, they dont have roots, stems, or leaves. Also unlike plants, some algae can move. They may move with pseudopods or flagella. | text | null |
L_0439 | protists | T_2673 | Fungus-like protists include slime molds and water molds, both shown in Figure 9.5. They exist as individual cells or as many cells that form a blob-like colony. They are probably ancestors of fungi. Like fungi, many fungus-like protists are decomposers. They absorb nutrients from dead logs, compost, and other organic remains Slime molds are commonly found on rotting organic matter such as compost. Swarms of cells move very slowly over the surface. They digest and absorb nutrients as they go. Water molds are commonly found in moist soil and surface water. Many water molds are plant pathogens or fish parasites. | text | null |
L_0439 | protists | T_2674 | Many human diseases are caused by protists. Most of them are caused by protozoa. They are parasites that invade and live in the human body. The parasites get a place to live and nutrients from the human host. In return, they make the host sick. Examples of human diseases caused by protozoa include giardiasis and malaria. Protozoa that cause giardiasis are spread by contaminated food or water. They live inside the intestine. They may cause abdominal pain, fever, and diarrhea. Protozoa that cause malaria are spread by a vector. They enter the blood through the bite of an infected mosquito. They live inside red blood cells. They cause overall body pain, fever, and fatigue. Malaria kills several million people each year. Most of the deaths occur in children. | text | null |
L_0440 | fungi | T_2675 | Fungi (fungus, singular) are relatively simple eukaryotic organisms. They are placed in their own kingdom, the Fungus Kingdom. Most fungi are multicellular organisms. These fungi are called molds. However, some fungi exist as single cells. These fungi are called yeasts. You can see examples of different types of fungi in Figure 9.7. For a funny, fast-paced overview of fungi, watch this video: . MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0440 | fungi | T_2676 | For a long time, scientists classified fungi as members of the Plant Kingdom. Fungi share several obvious traits with plants. For example, both fungi and plants lack the ability to move. Both grow in soil, and both have cell walls. Some fungi even look like plants. | text | null |
L_0440 | fungi | T_2677 | Today, fungi are no longer classified as plants. We now know that they have important traits that set them apart from plants. Thats why they are placed in their own kingdom. How do fungi differ from plants? The cell walls of fungi are made of chitin. Chitin is a tough carbohydrate that also makes up the outer skeleton of insects. The cell walls of plants are made of cellulose. Fungi are heterotrophs that absorb food from other organisms. Plants are autotrophs that make their own food. The Fungus Kingdom is large and diverse. It may contain more than a million species. However, fewer than 100,000 species of fungi have been identified. | text | null |
L_0440 | fungi | T_2678 | The earliest fungi evolved about 600 million years ago. They lived in the water. Fungi colonized the land around the same time as plants. That was probably between 400 and 500 million years ago. After that, fungi became very abundant on land. By 250 million years ago, they may have been the dominant life forms on land. | text | null |
L_0440 | fungi | T_2679 | Yeasts grow as single cells. Other fungi grow into multicellular, thread-like structures. These structures are called hyphae (hypha, singular). You can see a photo of hyphae in Figure 9.8. They resemble plant roots. Each hypha consists of a group of cells surrounded by a tubular cell wall. A mass of hyphae make up the body of a fungus. The body is called the mycelium (mycelia, plural). A mycelium may range in size from microscopic to very large. In fact, the largest living thing on Earth is the mycelium of a single fungus. Nicknamed the humongous fungus, it grows in a forest in Oregon. A small part of the fungus is pictured in Figure 9.9. The giant fungus covers an area of 2384 acres. Thats about the size of 1,665 football fields! The fungus is estimated to be at least 2400 years old, but it could be much older. | text | null |
L_0440 | fungi | T_2680 | Most fungi reproduce both asexually and sexually. In both types of reproduction, they produce spores. A spore is a special reproductive cell. When fungi reproduce asexually, they can spread quickly. This is good when conditions are stable. They can increase their genetic variation by sexual reproduction. This is beneficial when conditions are changing. Variation helps ensure that at least some organisms survive the changing conditions. Figure 9.10 shows how asexual and sexual reproduction occur in fungi. Refer to the figure as you read about each of them below. | text | null |
L_0440 | fungi | T_2680 | Most fungi reproduce both asexually and sexually. In both types of reproduction, they produce spores. A spore is a special reproductive cell. When fungi reproduce asexually, they can spread quickly. This is good when conditions are stable. They can increase their genetic variation by sexual reproduction. This is beneficial when conditions are changing. Variation helps ensure that at least some organisms survive the changing conditions. Figure 9.10 shows how asexual and sexual reproduction occur in fungi. Refer to the figure as you read about each of them below. | text | null |
L_0440 | fungi | T_2681 | During asexual reproduction, fungi produce haploid spores by mitosis of a haploid parent cell. A haploid cell has just one of each pair of chromosomes. The haploid spores are genetically identical to the parent cell. Spores may be spread by moving water, wind, or other organisms. Wherever the spores land, they will develop into new hyphae only when conditions are suitable for growth. Yeasts are an exception. They reproduce asexually by budding instead of by producing spores. An offspring cell forms on a parent cell. After it grows and develops, it buds off to form a new cell. The offspring cell is genetically identical to the parent cell. You can see yeast cells budding in Figure 9.11. | text | null |
L_0440 | fungi | T_2681 | During asexual reproduction, fungi produce haploid spores by mitosis of a haploid parent cell. A haploid cell has just one of each pair of chromosomes. The haploid spores are genetically identical to the parent cell. Spores may be spread by moving water, wind, or other organisms. Wherever the spores land, they will develop into new hyphae only when conditions are suitable for growth. Yeasts are an exception. They reproduce asexually by budding instead of by producing spores. An offspring cell forms on a parent cell. After it grows and develops, it buds off to form a new cell. The offspring cell is genetically identical to the parent cell. You can see yeast cells budding in Figure 9.11. | text | null |
L_0440 | fungi | T_2682 | Sexual reproduction also occurs in most fungi. It happens when two haploid hyphae mate. During mating, two haploid parent cells fuse. The single fused cell that results is a diploid spore. It is genetically different from both parents. The spore undergoes meiosis to form haploid daughter cells. These haploid cells develop into new hyphae. | text | null |
L_0440 | fungi | T_2683 | Most fungi grow on moist soil or rotting vegetation such as dead logs. Some fungi live in water. Others live in or on other organisms. Fungi get their nutrition by absorbing organic compounds from other organisms. The other organisms may be dead or alive, depending on the fungus. | text | null |
L_0440 | fungi | T_2684 | Most fungi get organic compounds from dead organisms. Fungi use their hyphae to penetrate deep into decaying organic matter. They produce enzymes at the tips of their hyphae. The enzymes digest the organic matter so the fungal cells can absorb it. Fungi are the main decomposers in forests. They are the only decomposers that can break down cellulose and wood. They have special enzymes for this purpose. | text | null |
L_0440 | fungi | T_2685 | Many fungi get organic compounds from living organisms. They have close relationships with other species. A close relationship between two species is called a symbiotic relationship. Two symbiotic relationships in fungi are mycorrhiza and lichen. These relationships are beneficial for both species. Mycorrhiza is a relationship between a fungus and a plant. The fungus grows in or on the plants roots. The fungus benefits from easy access to food made by the plant. The plant benefits because the fungal hyphae absorb water and nutrients from the soil that the plant needs. Lichen is a relationship between a fungus and cyanobacteria or green algae. The fungus grows around the bacterial or algal cells. The fungus benefits by getting some of the food made by the photosynthetic cells. The bacteria or algae benefit by getting some of the water and nutrients absorbed by the fungus. You can see a picture of lichen in Figure 9.12. Some fungi have a different kind of relationship with plants. They are plant parasites. They get food from the plants and cause harm to the plants in return. Fungi are the major causes of disease in agricultural crops. They may eventually kill their plant hosts. Some fungi are animal parasites. The wasp in Figure 9.13 is infected with a fungus. The fungus is the white fuzzy matter on the dark brown moth. | text | null |
L_0440 | fungi | T_2685 | Many fungi get organic compounds from living organisms. They have close relationships with other species. A close relationship between two species is called a symbiotic relationship. Two symbiotic relationships in fungi are mycorrhiza and lichen. These relationships are beneficial for both species. Mycorrhiza is a relationship between a fungus and a plant. The fungus grows in or on the plants roots. The fungus benefits from easy access to food made by the plant. The plant benefits because the fungal hyphae absorb water and nutrients from the soil that the plant needs. Lichen is a relationship between a fungus and cyanobacteria or green algae. The fungus grows around the bacterial or algal cells. The fungus benefits by getting some of the food made by the photosynthetic cells. The bacteria or algae benefit by getting some of the water and nutrients absorbed by the fungus. You can see a picture of lichen in Figure 9.12. Some fungi have a different kind of relationship with plants. They are plant parasites. They get food from the plants and cause harm to the plants in return. Fungi are the major causes of disease in agricultural crops. They may eventually kill their plant hosts. Some fungi are animal parasites. The wasp in Figure 9.13 is infected with a fungus. The fungus is the white fuzzy matter on the dark brown moth. | text | null |
L_0440 | fungi | T_2686 | Fungi may cause disease in people as well as other organisms. On the other hand, people have been using fungi for thousands of years. | text | null |
L_0440 | fungi | T_2687 | One way we use fungi is by eating them. Many species of mushrooms are edible. Yeasts are used for break making. Other fungi are used to ferment foods, such as soy sauce and cheeses. You can see the fungus growing through the blue cheese in Figure 9.14. The fungus gives the cheese its distinctive appearance and taste. People also use fungi: to produce antibiotics. to produce human hormones such as insulin. as natural pesticides. as model research organisms. | text | null |
L_0440 | fungi | T_2688 | Several common human diseases are caused by fungi. They include ringworm and athletes foot, both shown in Figure 9.15. Ringworm isnt caused by a worm. Its a skin infection by a fungus that leads to a ring-shaped rash. The rash may occur on the head, neck, trunk, arms, or legs. Athletes foot is caused by the same fungus as ringworm. But in athletes foot, the fungus infects the skin between the toes. Athletes foot is the second most common skin disease in the U.S. | text | null |
L_0441 | active transport | T_2689 | During active transport, molecules move from an area of low concentration to an area of high concentration. This is the opposite of diffusion, and these molecules are said to flow against their concentration gradient. Active transport is called "active" because this type of transport requires energy to move molecules. ATP is the most common source of energy for active transport. As molecules are moving against their concentration gradients, active transport cannot occur without assistance. A carrier protein is always required in this process. Like facilitated diffusion, a protein in the membrane carries the molecules across the membrane, except this protein moves the molecules from a low concentration to a high concentration. These proteins are often called "pumps" because they use energy to pump the molecules across the membrane. There are many cells in your body that use pumps to move molecules. For example, your nerve cells (neurons) would not send messages to your brain unless you had protein pumps moving molecules by active transport. The sodium-potassium pump ( Figure 1.1) is an example of an active transport pump. The sodium-potassium pump uses ATP to move three sodium (Na+ ) ions and two potassium (K+ ) ions to where they are already highly concentrated. Sodium ions move out of the cell, and potassium ions move into the cell. How do these ions then return to their original positions? As the ions now can flow down their concentration gradients, facilitated diffusion returns the ions to their original positions either inside or outside the cell. | text | null |
L_0451 | archaea | T_2724 | For many years, archaea were classified as bacteria. Like the bacteria, archaea lacked a nucleus and membrane- bound organelles and, therefore, were prokaryotic cells. However, when scientists compared the DNA of the two prokaryotes, they found that there were distinct differences. They concluded that there must be two distinct types of prokaryotes, which they named archaea and bacteria. Even though the two groups might seem similar, archaea have many features that distinguish them from bacteria: 1. The cell walls of archaea are distinct from those of bacteria. While bacteria have cell walls made up of the polymer peptidoglycan, most archaea do not have peptidoglycan in their cell walls. 2. The plasma membranes of the archaea are also made up of lipids that are distinct from those in bacteria. 3. The ribosomal proteins of the archaea are similar to those in eukaryotic cells, not those in bacteria. Although archaea and bacteria share some fundamental differences, they are still similar in many ways: 1. They both are single-celled, microscopic organisms that can come in a variety of shapes ( Figure 1.1). 2. Both archaea and bacteria have a single circular chromosome of DNA and lack membrane-bound organelles. 3. Like bacteria, archaea can have flagella to assist with movement. Archaea shapes can vary widely, but some are bacilli (rod-shaped). | text | null |
L_0451 | archaea | T_2725 | Most archaea are chemotrophs and derive their energy and nutrients from breaking down molecules in their envi- ronment. A few species of archaea are photosynthetic and capture the energy of sunlight. Unlike bacteria, which can be parasites and are known to cause a variety of diseases, there are no known archaea that act as parasites. Some archaea do live within other organisms. But these archea form mutualistic relationships with their host, where both the archaea and the host benefit. In other words, they assist the host in some way, for example by helping to digest food. | text | null |
L_0451 | archaea | T_2726 | Like bacteria, reproduction in archaea is asexual. Archaea can reproduce through binary fission, where a parent cell divides into two genetically identical daughter cells. Archaea can also reproduce asexually through budding and fragmentation, where pieces of the cell break off and form a new cell, also producing genetically identical organisms. | text | null |
L_0453 | asexual vs. sexual reproduction | T_2730 | Animals and other organisms cannot live forever. They must reproduce if their species is to survive. But what does it mean to reproduce? Reproduction is the ability to make the next generation, and it is one of the basic characteristics of life. Two methods of reproduction are: 1. Asexual reproduction, the process of forming a new individual from a single parent. 2. Sexual reproduction, the process of forming a new individual from two parents. There are advantages and disadvantages to each method, but the result is always the same: a new life begins. | text | null |
L_0453 | asexual vs. sexual reproduction | T_2731 | When humans reproduce, there are two parents involved. DNA must be passed from both the mother and father to the child. Humans cannot reproduce with just one parent; humans can only reproduce sexually. But having just one parent is possible in other eukaryotic organisms, including some insects, fish, and reptiles. These organisms can reproduce asexually, meaning the offspring ("children") have a single parent and share the exact same genetic material as the parent. This is very different from reproduction in humans. Bacteria, being a prokaryotic, single- celled organism, must reproduce asexually. The advantage of asexual reproduction is that it can be very quick and does not require the meeting of a male and female organism. The disadvantage of asexual reproduction is that organisms do not receive a mix of traits from both parents. An organism that is born through asexual reproduction only has the DNA from the one parent. In fact, the offspring is genetically an exact copy of the parent. This can cause problems for the individual. For example, if the parent has a gene that causes a particular disease, the offspring will also have the gene that causes that disease. Organisms produced sexually may or may not inherit the disease gene because they receive a mix of their parents genes. Types of organisms that reproduce asexually include: 1. Prokaryotic organisms, like bacteria. Bacteria reproduce through binary fission, where they grow and divide in half ( Figure 1.1). First, their chromosome replicates and the cell enlarges. The cell then divides into two cells as new membranes form to separate the two cells. After cell division, the two new cells each have one identical chromosome. This simple process allows bacteria to reproduce very rapidly. 2. Flatworms, an invertebrate animal species. Flatworms divide in two, then each half regenerates into a new flatworm identical to the original, a process called fragmentation. 3. Different types of insects, fish, and lizards. These organisms can reproduce asexually through a process called parthenogenesis. Parthenogenesis happens when an unfertilized egg cell grows into a new organism. The resulting organism has half the amount of genetic material of the parent. Parthenogenesis is common in honeybees. In a hive, the sexually produced eggs become workers, while the asexually produced eggs become drones. | text | null |
L_0453 | asexual vs. sexual reproduction | T_2732 | During sexual reproduction, two parents are involved. Most animals are dioecious, meaning there is a separate male and female sex, with the male producing sperm and the female producing eggs. When a sperm and egg meet during fertilization, a zygote, the first cell of a new organism, is formed ( Figure 1.2). This process combines the genetic material from both parents. The resulting organism will be genetically unique. The zygote will divide by mitosis and grow into the embryo. Lets explore how animals, plants, and fungi reproduce sexually: Animals often have gonads, organs that produce eggs or sperm. The male gonads are the testes, and the female gonads are the ovaries. Testes produce sperm; ovaries produce eggs. Sperm and egg, the two sex cells, are known as gametes, and can combine two different ways, both of which combine the genetic material from the two parents. Gametes have half the amount of the genetic material of a regular body cell; they are haploid cells. In humans, gametes have one set of 23 chromosomes. Gametes are produced through a special type of cell division known as meiosis. Normal human cells have 46 chromosomes. They are diploid cells, with two sets of 23 chromosomes (23 pairs). Bacteria reproduce by binary fission. Shown is one bacterium reproducing and becoming two bacteria. During sexual reproduction, a sperm fer- tilizes an egg. Fish and other aquatic animals release their gametes in the water, which is called external fertilization ( Figure by internal fertilization. Typically males have a penis that deposits sperm into the vagina of the female. Birds do not have penises, but they do have a chamber called the cloaca that they place close to another birds cloaca to deposit sperm. Amphibians must live close to water as they must lay their eggs in a moist or wet environment prior to external fertilization. This fish guards her eggs, which will be fertilized externally. Plants can also reproduce sexually, but their reproductive organs are different from animals gonads. Plants that have flowers have their reproductive parts in the flower. The sperm is contained in the pollen, while the egg is contained in the ovary, deep within the flower. The sperm can reach the egg two different ways: 1. In self-pollination, the egg is fertilized by the pollen of the same flower. 2. In cross-pollination, sperm from the pollen of one flower fertilizes the egg of another flower. Like other types of sexual reproduction, cross-pollination allows new combinations of traits. Cross-pollination occurs when pollen is carried by the wind to another flower. It can also occur when animal pollinators, like honeybees or butterflies ( Figure 1.4), carry the pollen from flower to flower. Butterflies receive nectar when they de- posit pollen into flowers, resulting in cross-pollination. | text | null |
L_0453 | asexual vs. sexual reproduction | T_2732 | During sexual reproduction, two parents are involved. Most animals are dioecious, meaning there is a separate male and female sex, with the male producing sperm and the female producing eggs. When a sperm and egg meet during fertilization, a zygote, the first cell of a new organism, is formed ( Figure 1.2). This process combines the genetic material from both parents. The resulting organism will be genetically unique. The zygote will divide by mitosis and grow into the embryo. Lets explore how animals, plants, and fungi reproduce sexually: Animals often have gonads, organs that produce eggs or sperm. The male gonads are the testes, and the female gonads are the ovaries. Testes produce sperm; ovaries produce eggs. Sperm and egg, the two sex cells, are known as gametes, and can combine two different ways, both of which combine the genetic material from the two parents. Gametes have half the amount of the genetic material of a regular body cell; they are haploid cells. In humans, gametes have one set of 23 chromosomes. Gametes are produced through a special type of cell division known as meiosis. Normal human cells have 46 chromosomes. They are diploid cells, with two sets of 23 chromosomes (23 pairs). Bacteria reproduce by binary fission. Shown is one bacterium reproducing and becoming two bacteria. During sexual reproduction, a sperm fer- tilizes an egg. Fish and other aquatic animals release their gametes in the water, which is called external fertilization ( Figure by internal fertilization. Typically males have a penis that deposits sperm into the vagina of the female. Birds do not have penises, but they do have a chamber called the cloaca that they place close to another birds cloaca to deposit sperm. Amphibians must live close to water as they must lay their eggs in a moist or wet environment prior to external fertilization. This fish guards her eggs, which will be fertilized externally. Plants can also reproduce sexually, but their reproductive organs are different from animals gonads. Plants that have flowers have their reproductive parts in the flower. The sperm is contained in the pollen, while the egg is contained in the ovary, deep within the flower. The sperm can reach the egg two different ways: 1. In self-pollination, the egg is fertilized by the pollen of the same flower. 2. In cross-pollination, sperm from the pollen of one flower fertilizes the egg of another flower. Like other types of sexual reproduction, cross-pollination allows new combinations of traits. Cross-pollination occurs when pollen is carried by the wind to another flower. It can also occur when animal pollinators, like honeybees or butterflies ( Figure 1.4), carry the pollen from flower to flower. Butterflies receive nectar when they de- posit pollen into flowers, resulting in cross-pollination. | text | null |
L_0453 | asexual vs. sexual reproduction | T_2732 | During sexual reproduction, two parents are involved. Most animals are dioecious, meaning there is a separate male and female sex, with the male producing sperm and the female producing eggs. When a sperm and egg meet during fertilization, a zygote, the first cell of a new organism, is formed ( Figure 1.2). This process combines the genetic material from both parents. The resulting organism will be genetically unique. The zygote will divide by mitosis and grow into the embryo. Lets explore how animals, plants, and fungi reproduce sexually: Animals often have gonads, organs that produce eggs or sperm. The male gonads are the testes, and the female gonads are the ovaries. Testes produce sperm; ovaries produce eggs. Sperm and egg, the two sex cells, are known as gametes, and can combine two different ways, both of which combine the genetic material from the two parents. Gametes have half the amount of the genetic material of a regular body cell; they are haploid cells. In humans, gametes have one set of 23 chromosomes. Gametes are produced through a special type of cell division known as meiosis. Normal human cells have 46 chromosomes. They are diploid cells, with two sets of 23 chromosomes (23 pairs). Bacteria reproduce by binary fission. Shown is one bacterium reproducing and becoming two bacteria. During sexual reproduction, a sperm fer- tilizes an egg. Fish and other aquatic animals release their gametes in the water, which is called external fertilization ( Figure by internal fertilization. Typically males have a penis that deposits sperm into the vagina of the female. Birds do not have penises, but they do have a chamber called the cloaca that they place close to another birds cloaca to deposit sperm. Amphibians must live close to water as they must lay their eggs in a moist or wet environment prior to external fertilization. This fish guards her eggs, which will be fertilized externally. Plants can also reproduce sexually, but their reproductive organs are different from animals gonads. Plants that have flowers have their reproductive parts in the flower. The sperm is contained in the pollen, while the egg is contained in the ovary, deep within the flower. The sperm can reach the egg two different ways: 1. In self-pollination, the egg is fertilized by the pollen of the same flower. 2. In cross-pollination, sperm from the pollen of one flower fertilizes the egg of another flower. Like other types of sexual reproduction, cross-pollination allows new combinations of traits. Cross-pollination occurs when pollen is carried by the wind to another flower. It can also occur when animal pollinators, like honeybees or butterflies ( Figure 1.4), carry the pollen from flower to flower. Butterflies receive nectar when they de- posit pollen into flowers, resulting in cross-pollination. | text | null |
L_0453 | asexual vs. sexual reproduction | T_2732 | During sexual reproduction, two parents are involved. Most animals are dioecious, meaning there is a separate male and female sex, with the male producing sperm and the female producing eggs. When a sperm and egg meet during fertilization, a zygote, the first cell of a new organism, is formed ( Figure 1.2). This process combines the genetic material from both parents. The resulting organism will be genetically unique. The zygote will divide by mitosis and grow into the embryo. Lets explore how animals, plants, and fungi reproduce sexually: Animals often have gonads, organs that produce eggs or sperm. The male gonads are the testes, and the female gonads are the ovaries. Testes produce sperm; ovaries produce eggs. Sperm and egg, the two sex cells, are known as gametes, and can combine two different ways, both of which combine the genetic material from the two parents. Gametes have half the amount of the genetic material of a regular body cell; they are haploid cells. In humans, gametes have one set of 23 chromosomes. Gametes are produced through a special type of cell division known as meiosis. Normal human cells have 46 chromosomes. They are diploid cells, with two sets of 23 chromosomes (23 pairs). Bacteria reproduce by binary fission. Shown is one bacterium reproducing and becoming two bacteria. During sexual reproduction, a sperm fer- tilizes an egg. Fish and other aquatic animals release their gametes in the water, which is called external fertilization ( Figure by internal fertilization. Typically males have a penis that deposits sperm into the vagina of the female. Birds do not have penises, but they do have a chamber called the cloaca that they place close to another birds cloaca to deposit sperm. Amphibians must live close to water as they must lay their eggs in a moist or wet environment prior to external fertilization. This fish guards her eggs, which will be fertilized externally. Plants can also reproduce sexually, but their reproductive organs are different from animals gonads. Plants that have flowers have their reproductive parts in the flower. The sperm is contained in the pollen, while the egg is contained in the ovary, deep within the flower. The sperm can reach the egg two different ways: 1. In self-pollination, the egg is fertilized by the pollen of the same flower. 2. In cross-pollination, sperm from the pollen of one flower fertilizes the egg of another flower. Like other types of sexual reproduction, cross-pollination allows new combinations of traits. Cross-pollination occurs when pollen is carried by the wind to another flower. It can also occur when animal pollinators, like honeybees or butterflies ( Figure 1.4), carry the pollen from flower to flower. Butterflies receive nectar when they de- posit pollen into flowers, resulting in cross-pollination. | text | null |
L_0455 | b and t cell response | T_2736 | Some defenses, like your skin and mucous membranes, are not designed to ward off a specific pathogen. They are just general defenders against disease. Your body also has defenses that are more specialized. Through the help of your immune system, your body can generate an army of cells to kill that one specific pathogen. There are two different types of specific immune responses. One type involves B cells. The other type involves T cells. Recall that B cells and T cells are types of white blood cells that are key in the immune response. Whereas the immune systems first and second line of defense are more generalized or non-specific, the immune response is specific. It can be described as a specific response to a specific pathogen, meaning it uses methods to target just one pathogen at a time. These methods involve B and T cells. | text | null |
L_0455 | b and t cell response | T_2737 | B cells respond to pathogens and other cells from outside the body in the blood and lymph. Most B cells fight infections by making antibodies. An antibody is a large, Y-shaped protein that binds to an antigen, a protein that is recognized as foreign. Antigens are found on the outside of bacteria, viruses and other foreign microorganisms. Each antibody can bind with just one specific type of antigen ( Figure 1.1). They fit together like a lock and key. Once an antigen and antibody bind together, they signal for a phagocyte to destroy them. Phagocytes are white blood cells that engulf targeted antigens by phagocytosis. As the antigen is on the outside of a pathogen, the pathogen is destroyed by this process. At any one time the average human body contains antibodies that can react with about 100,000,000 different antigens. This means that there can be 100,000,000 different antibody proteins in the body. | text | null |
L_0455 | b and t cell response | T_2738 | There are different types of T cells, including killer T cells and helper T cells. Killer T cells destroy infected, damaged, or cancerous body cells ( Figure 1.2). When the killer T cell comes into contact with the infected cell, it releases poisons. The poisons make tiny holes in the cell membrane of the infected cell. This causes the cell to burst open. Both the infected cell and the pathogens inside it are destroyed. Helper T cells do not destroy infected or damaged body cells. But they are still necessary for an immune response. They help by releasing chemicals that control other lymphocytes. The chemicals released by helper T cells switch on both B cells and killer T cells so they can recognize and fight specific pathogens. | text | null |
L_0456 | bacteria characteristics | T_2739 | Bacteria are the most successful organisms on the planet. They lived on this planet for two billion years before the first eukaryotes and, during that time, evolved into millions of different species. | text | null |
L_0456 | bacteria characteristics | T_2740 | Bacteria are so small that they can only be seen with a microscope. When viewed under the microscope, they have three distinct shapes ( Figure 1.1). Bacteria can be identified and classified by their shape: 1. Bacilli are rod-shaped. 2. Cocci are sphere-shaped. 3. Spirilli are spiral-shaped. Bacteria come in many different shapes. Some of the most common shapes are bacilli (rods), cocci (spheres), and spirilli (spirals). Bacteria can be identified and classified by their shape. | text | null |
L_0456 | bacteria characteristics | T_2741 | Like eukaryotic cells, bacterial cells have: 1. 2. 3. 4. Cytoplasm, the fluid inside the cell. A plasma or cell membrane, which acts as a barrier around the cell. Ribosomes, in which proteins are put together. DNA. By contrast though, bacterial DNA is contained in a large, circular strand. This single chromosome is located in a region of the cell called the nucleoid. The nucleoid is not an organelle, but a region within the cytoplasm. Many bacteria also have additional small rings of DNA known as plasmids. See bacterial cell pictured below ( Figure 1.2). The structure of a bacterial cell is dis- tinctive from a eukaryotic cell because of features such as an outer cell wall, the circular DNA of the nucleoid, and the lack of membrane-bound organelles. | text | null |
L_0456 | bacteria characteristics | T_2742 | Bacteria lack many of the structures that eukaryotic cells contain. For example, they dont have a nucleus. They also lack membrane-bound organelles, such as mitochondria or chloroplasts. The DNA of a bacterial cell is also different from a eukaryotic cell. Bacterial DNA is contained in one circular chromosome, located in the cytoplasm. Eukaryotes have several linear chromosomes. Bacteria also have two additional unique features: a cell wall and flagella. Some bacteria also have a capsule outside the cell wall. | text | null |
L_0456 | bacteria characteristics | T_2743 | Bacteria are surrounded by a cell wall consisting of peptidoglycan. This complex molecule consists of sugars and amino acids. The cell wall is important for protecting bacteria. The cell wall is so important that some antibiotics, such as penicillin, kill bacteria by preventing the cell wall from forming. Some bacteria depend on a host organism for energy and nutrients. These bacteria are known as parasites. If the host starts attacking the parasitic bacteria, the bacteria release a layer of slime that surrounds the cell wall. This slime offers an extra layer of protection. | text | null |
L_0456 | bacteria characteristics | T_2744 | Some bacteria also have tail-like structures called flagella ( Figure 1.3). Flagella help bacteria move. As the flagella rotate, they spin the bacteria and propel them forward. It is often said the flagella looks like a tiny whip, propelling the bacteria forward. Though some eukaryotic cells do have a flagella, a flagella in eukaryotes is rare. The flagella facilitate movement in bacte- ria. Bacteria may have one, two, or many flagellaor none at all. | text | null |
L_0459 | bacteria reproduction | T_2752 | Bacteria, being single-celled prokaryotic organisms, do not have a male or female version. Bacteria reproduce asexually. In asexual reproduction, the "parent" produces a genetically identical copy of itself. | text | null |
L_0459 | bacteria reproduction | T_2753 | Bacteria reproduce through a process called binary fission. During binary fission, the chromosome copies itself, forming two genetically identical copies. Then, the cell enlarges and divides into two new daughter cells. The two daughter cells are identical to the parent cell. Binary fission can happen very rapidly. Some species of bacteria can double their population in less than ten minutes! This process makes it possible for a tremendous bacterial colony to start from a single cell. | text | null |
L_0459 | bacteria reproduction | T_2754 | Are there male and female bacteria? Of course the answer is no. So, sexual reproduction does not occur in bacteria. But not all new bacteria are clones. This is because bacteria can acquire new DNA. This process occurs in three different ways: 1. Conjugation: In conjugation, DNA passes through an extension on the surface of one bacterium and travels to another bacterium ( Figure 1.1). Bacteria essential exchange DNA via conjugation. 2. Transformation: In transformation, bacteria pick up pieces of DNA from their environment. 3. Transduction: In transduction, viruses that infect bacteria carry DNA from one bacterium to another. | text | null |
L_0465 | blood diseases | T_2767 | Problems can occur with red blood cells, white blood cells, platelets, and other parts of the blood. Many blood disorders are genetic, meaning they are inherited from a parent. Some blood diseases are caused by not getting enough of a certain nutrient, while others are cancers of the blood. | text | null |
L_0465 | blood diseases | T_2768 | Anemia is a disease that occurs when there is not enough hemoglobin in the blood to carry oxygen to body cells. Hemoglobin is the blood protein that normally carries oxygen from the lungs to the tissues. Anemia leads to a lack of oxygen in organs. Anemia is usually caused by one of the following: A loss of blood from a bleeding wound or a slow leak of blood. The destruction of red blood cells. A lack of red blood cell production. Anemia may not have any symptoms. Some people with anemia feel weak or tired in general or during exercise. They also may have poor concentration. People with more severe anemia often get short of breath during times of activity. Iron-deficiency anemia is the most common type of anemia. It occurs when the body does not receive enough iron. Since there is not enough iron, hemoglobin, which needs iron to bind oxygen, cannot function properly. In the United States, 20% of all women of childbearing age have iron-deficiency anemia, compared with only 2% of adult men. The most common cause of iron-deficiency anemia in young women is blood lost during menstruation. Iron deficiency anemia can be avoided by getting the recommended amount of iron in ones diet. Anemia is often treated or prevented by taking iron supplements. Boys and girls between the ages of 9 and 13 should get 9 mg of iron every day. Girls between the ages of 14 and 18 should get 15 mg of iron every day. Boys between the ages of 14 and 18 should get 11 mg of iron every day. Pregnant women need the most iron27 mg daily. Good sources of iron include shellfish, such as clams and oysters. Red meats, such as beef, are also a good source of iron. Non-animal sources of iron include seeds, nuts, and legumes. Breakfast cereals often have iron added to them in a process called fortification. Some good sources of iron are listed below ( Table 1.1). Eating vitamin C along with iron-containing food increases the amount of iron that the body can absorb. Food Canned clams, drained, 3 oz. Fortified dry cereals, about 1 oz. Roasted pumpkin and squash seeds, 1 oz. Cooked lentils, 12 cup Cooked fresh spinach, 21 cup Cooked ground beef, 3 oz. Cooked sirloin beef, 3 oz. Milligrams (mg) of Iron 23.8 1.8 to 21.1 4.2 3.3 3.2 2.2 2.0 | text | null |
L_0465 | blood diseases | T_2769 | Sickle-cell anemia is a blood disease that is caused by an abnormally shaped hemoglobin protein in red blood cells. Many of the red blood cells of a person with sickle-cell anemia are long and curved (sickle-shaped) ( Figure 1.1). The long, sickle shape of the cells can cause them to get stuck in narrow blood vessels. This clotting means that oxygen cannot reach the cells. People with sickle-cell anemia are most often well but can occasionally have painful attacks. The disease is not curable, but it can be treated with medicines. The red blood cells of a person with sickle-cell anemia (left) are long and pointed, rather than straight, like normal cells (right). The abnormal cells cannot carry oxygen properly and can get stuck in capillaries. | text | null |
L_0465 | blood diseases | T_2770 | Blood cancers affect the production and function of your blood cells. Most of these cancers start in your bone marrow where blood is produced. In most blood cancers, the normal production of blood cells is replaced by uncontrolled growth of an abnormal type of blood cell. These abnormal blood cells are cancerous cells, and prevent your blood from performing many of its functions, like fighting off infections or preventing serious bleeding. Leukemia is a cancer of the blood or bone marrow. It is characterized by an abnormal production of blood cells, usually white blood cells. Lymphoma is a cancer of a type of white blood cell called lymphocytes. There are many types of lymphoma. | text | null |
L_0465 | blood diseases | T_2771 | Hemophilia is the name of a group of hereditary diseases that affect the bodys ability to control blood clotting. Hemophilia is caused by a lack of clotting factors in the blood. Clotting factors are normally released by platelets. Since people with hemophilia cannot produce clots, any cut can put a person at risk of bleeding to death. The risk of internal bleeding is also increased in hemophilia, especially into muscles and joints. This disease affected the royal families of Europe. | text | null |
L_0475 | cell biology | T_2801 | A cell is the smallest structural and functional unit of an organism. Some organisms, like bacteria, consist of only one cell. Big organisms, like humans, consist of trillions of cells. Compare a human to a banana. On the outside, they look very different, but if you look close enough youll see that their cells are actually very similar. | text | null |
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