Answers to Review Questions

1. Heterotrophs are organisms that cannot make their own food. These organisms obtain their food from the environment by ingesting it. Chemoautotrophs are organisms that extract energy and build organic compounds from inorganic acids that were plentiful in the primordial seas; they can make their own food. Photoautotrophs make their own food as well but they tap into the energy of sunlight to acquire the energy needed. The heterotrophs probably evolved first. They were able to feed on the ready-made organic materials that had built up in the seas abiotically before the first living heterotrophs evolved. Once enough of the heterotrophs were present in the seas and enough time had passed, the supplies of these organic materials began to dwindle. This set up competition and selective pressure that favored the evolution of organisms that could synthesize their own food, the autotrophs. Probably the chemoautotrophs evolved first after the heterotrophs and the photoautotrophs evolved after the chemoautotrophs.

3. There are a number of pieces of evidence that support the correctness of the endosymbiotic theory. First of all, the DNA of prokaryotic cells is more like that of mitochondria and chloroplasts than the DNA found in the nucleus of the cell of which these organelles are a part. The DNA in mitochondria and chloroplasts is a single loop of DNA lacking histones like that in prokaryotic cells and unlike the DNA in the eukaryotic cell's nucleus. The nuclear DNA of a eukaryotic cell is found in the linear chromosomes of these cells, associated with many histones. Mitochondria and chloroplasts are self-replicating organelles; they divide within the cytoplasm of a cell by a process resembling binary fission that prokaryotic cells use to divide. They do not exhibit mitosis as do the nuclear chromosomes of the eukaryote. Mitochondria and chloroplasts have their own protein-synthesizing machinery (tRNAs, enzymes, ribosomes); it is more similar to that of prokaryotes than that of cells in which they reside. For example, mitochodrial and chloroplast ribosomes are smaller than the ribosomes in the cytoplasm of the cell that surrounds them; these ribosomes are very similar in size to the ribosomes of prokaryotes. There is evidence of living prokaryotes that share many features with mitochondria and chloroplasts of eukaryotes e.g. cyanobacteria photosynthesis is similar to that of chloroplast. Also, the metabolic processes of aerobic bacteria are similar to those of mitochondria.

5. Human beings do exhibit an alternation of generations, but the haploid phase is extremely deemphasized while the diploid phase is overwhelmingly dominant. The haploid phase in humans that results from meiosis is restricted to only one cell, either an egg in the female or a sperm in the male. There is never a multicellular haploid structure as there usually are in plants. If such a multicellular haploid structure is required for a true alternation of generations, then humans do not exhibit alternation of generations. The human organism grows by mitosis (the diploid phase) only after fertilization. A plant sporophyte is diploid and it produces spores by meiosis. Humans, like plant sporophytes, are diploid and make haploid gametes by meiosis.

7. A flower is the structure that produces seeds. A flower structurally is a stem that bears leaves. The leaves are often highly colored and modified into petals, sepals, and reproductive structures called stamens and carpels. Stamens and carpels are the places where angiosperm gametophytes are enclosed and where the gametes are made. They are analogous to the gonads of animals. Animals have nothing, however, that is analogous to the plant gametophyte. The vast array of flower varieties and types reflects the many strategies that plants have evolved to assure the delivery of sperm from one plant to another. The flowers are, therefore, the sex organs of the angiosperms.

9. Photosynthesizing cells need a constant supply of CO₂ that they must obtain from the atmosphere. Leaves exhibit adaptations that prevent desiccation like the waxy cuticle and the epidermis. While preventing water loss, these same adaptations act as barriers to gas diffusion across the leaf surface as well. Pores called stomata, which are located on the underside of the leaf, solve this problem. These pores allow the passage of CO₂ and O₂ into and out of the leaves, respectively. However, when the pores are open to allow CO₂ into the leaves and O₂ out, water will also leave the leaf causing it to dry out if the stomata are open too long. The stomata open when a plant's greatest need is the acquisition of CO₂. They close when the greatest need is saving water. Conflict arises in hotter, drier weather. Stomata stay closed more often in this type of environment to prevent water loss. This means that CO₂ is used up as the plant photosynthesizes. Normally, stomata would open as CO₂ levels dropped too low, but in hot, dry weather the stomata stay closed to prevent water loss. If the stomata open to allow CO₂ into the leaf, water loss will increase as will the risk of desiccation. Consequently, many plants do not do well in hot' dry weather. Plants have evolved two strategies to get around this problem. The first is C4 photosynthesis. CO₂ is fixed from the atmosphere in these plants and attached to a 3-carbon compound thus producing a 4-carbon molecules, hence the name C4 photosynthesis. This reaction can occur when stomata are closed and CO₂ levels are low. Thus, carbon fixation can occur with the stomata closed and reduce water loss at the same time. Under identical circumstances, plants that cannot perform C4 photosynthesis will have to open their stomata in order to continue photosynthesizing; they will, therefore, be more susceptible to desiccation. Desert plants resolve the conflict in a different way. They keep their stomata closed all day to prevent water loss. They open their stomata only at night when the risk of desiccation is much reduced. CO₂ enters the plant during the night and is stored as part of a 4-carbon (as in C4 plants). During daylight, the stored CO₂ is released behind closed stomata, thus reducing the risk of desiccation. They are called CAM (Crassulacean Acid Metabolism) plants.

11. Nitrogen fixation is the process in which nitrogen gas is used by some microbes to make organic compounds. Nitrogen-fixing microbes include free-living species and species that form close symbiotic relationships with other organisms. The free-living forms are seen in soils and sediments, on leaf and bark surfaces, and in animal intestinal tracts. The symbiotic species have a special, intimate relationship with their hosts in which the host provides chemical food energy to the bacterium in return for nitrogen in a usable form. Such partnerships are seen in legumes like alfalfa, clover, peas, beans, and peanuts. As legume seedlings develop, their roots secrete substances into the soil that attract bacteria called rhizobia. Rhizobia are a group including three genera of bacteria: Rhizobium, Bradyrhizobium, and Azorhizobium; these are further divided into several species. Animals and plants are unable to use nitrogen gas in the air. The nitrogen gas must be made part of a solid chemical compound before it can be used by animals and plants, a process called nitrogen fixation. In order to fix nitrogen gas from the atmosphere, the two nitrogen atoms of nitrogen gas must be broken apart. The two nitrogen atoms of nitrogen gas are attached by three bonds (a triple bond). Such a bond is very strong and neither plants nor animals can break it. Thus, they cannot use or fix nitrogen gas.

13. The forces responsible for moving water upward through the body of a vascular plant come from negative pressure created in the leaves when they lose water by transpiration. It is similar to what happens when a child drinks through a straw. By sucking on the straw, a child creates negative pressure. Water and minerals from the roots move up into the xylem to replace water that was lost to the atmosphere during transpiration. The polarity of water molecules gives water a great deal of cohesion or tensile strength. As water moves up through the xylem, it pulls other water up behind it in a thin, unbroken column. The force that moves sap through the plant body is called pressure-flow movement. It works by loading carbohydrates into one region of plant (near leaves) called the source. The carbohydrates in that region attract water into that region. This, in turn, raises pressure in the area above that of other areas surrounding the source. The water and carbohydrates move elsewhere toward areas of the plant where pressure is lower. In certain areas of the plant, the sinks, the sugars are removed and stored. Water leaves following the carbohydrates out of the phloem and lowering the pressure in the area of the sink. This maintains the pressure differential between the sources and sinks and keeps the sap moving around the plant.

15. A non-woody plant is unlikely to grow very tall. Non-woody plants must rely on turgor pressure to make cells rigid and hold the plants erect. Plants relying on such hydrostatic support cannot grow too large or they will begin to bend under their own weight. Thus, soft-stemmed plants do not grow to great heights. Furthermore, if such a plant could grow taller, it would need to grow in a very moist area in order to assure itself enough water to maintain the turgor pressure that serves as their mechanical and structural support.

17. A meristem tissue is a cluster of cells within a plant that retains the ability to divide (undergo mitosis), thus creating new cells. Meristem tissues are found at root tips and shoot tips; these are called apical meristems. There is also a meristem tissue called an axillary bud meristem; this tissue is found at the base of leaves in the angle formed by the leaf and the stem. The vascular cambium is also a meristem tissue. It is a ring of meristematic tissue found between the primary xylem and primary phloem. There is another meristem tissue called the cork cambium that is produced from cells of ground tissues. It produces a new layer of cells called cork that is involved in the formation of bark.

19. Auxins have a number of effects on plant growth. Generally, auxins have a positive influence on growth causing it to occur more rapidly in tissues exposed to the hormone. One effect of auxins is to stimulate cell elongation and it plays a role in phototropism. When light shines on one side of a plant, auxins diffuse down the stem to the shaded side. Cells on the shaded side elongate faster than those on the lighted side of the stem. Consequently, the stem bends toward the light. Auxins also play a role in causing the growing root of a plant to bend downward in aresponse called gravitropism. Cells facing upward, away from the direction of gravitational pull, elongate faster than those situated downward. Thus, the root bends into the Earth. Terminal bud auxins inhibit axillary bud growth on side branches. They promote apical dominance and cause the lead shoot to grow faster than side shoots. This results in the pyramid shape that is most familiar in evergreens. For shrubs in which a bushier shape is preferred, horticulturists remove the terminal buds. This removes growth-inhibiting auxins from the terminal bud and hastens the growth of the axillary buds. This causes the plant to grow outward instead of upward and the plant grows in a more pleasing, bushier form. Another effect is an involvement in fruit development. It prevents fruits from dropping prematurely. Abscisic acid (ABA), on the other hand, has as its general effect the slowing or inhibition of growth. More specifically, as its name suggests, it is involved in a process called abscission, in which leaves or fruit are separated from the rest of the plant at the end of the growing season. Its exact role in this process is poorly defined. This process is directly opposite to one effect of auxins, their ability to prevent the dropping of fruits. When water is scarce, ABA levels rise inhibiting growth; the same happens when temperatures are very high or very low, conditions that are not as conducive to growth as more moderate temperatures. High ABA levels have also been correlated with seed dormancy.

21. The male gametophyte of flowering plants is found inside the pollen grain. The female gametophyte of flowering plants is found in the ovule. The female gametophyte is housed in a layer of diploid protective cells from the parent cell.

23. Double fertilization refers to the fact that in flowering plants there are two fertilizations for every for each embryo created. This occurs only in plants and mainly in angiosperms. One of the fertilizations -that of the egg nucleus by one of the sperm from the pollen grain- results in the formation of a zygote. The zygote will become the plant embryo and ultimately the new plant or sporophyte of the next generation. The second fertilization involves the second sperm cell from the pollen grain contributing its nucleus to two nuclei in the female gametophyte (the polar nuclei). This creates an unusual triploid nucleus and the endosperm develops as a result of this fertilization. The endosperm is a nutritive tissue that feeds the developing embryo within the seed.