Answers to Mastering Concepts Questions



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Mastering Concepts
23.1
1. Which macro- and micronutrients do all plants require?
Macronutrients: carbon, oxygen, hydrogen, nitrogen, potassium, calcium, magnesium, phosphorus, sulfur. Micronutrients: chlorine, iron, boron, zinc, manganese, copper, molybdenum.
2. How do plants acquire C, H, O, N, and P?
C comes from CO2 absorbed from the atmosphere. H comes from water (H2O) absorbed by the roots. O comes from H2O, CO2, or O2 in the atmosphere or dissolved in water. N and P are dissolved in water absorbed by the roots.
3. How would planting nodule-producing plants be useful in crop rotations?
Nodules contain nitrogen-fixing bacteria, such as Rhizobium. These bacteria would raise the nitrogen content available for other crops and reduce the reliance on fertilizers.
23.2
1. What are the components of xylem sap?
Xylem sap contains water and dissolved minerals.
2. Trace the path of water and dissolved minerals from soil, into the root’s xylem, and up to the leaves.
Water enters the roots by osmosis; dissolved minerals enter root cells by passive or active transport. After passing through the epidermis, the water and minerals may move between or within cells of the cortex. Once the water and minerals encounter the endodermis, the Casparian strip ensures that these materials enter cells. They next enter the xylem and are pulled up the stem and into leaves. Water vapor moves out of the stomata as part of the transpiration stream.
3. How do the cuticle and stomata help plants conserve water?
The cuticle is a waxy barrier that reduces water loss from aboveground plant tissues. Guard cells close stomata when conditions are hot and dry, reducing the evaporation of water from leaf tissues.
23.3
1. What are the components of phloem sap?
Phloem sap is a solution containing organic compounds combined with water and minerals from the xylem.
2. Explain the pressure flow theory of phloem transport.
The pressure flow theory states that phloem moves under positive pressure from sources to sinks. At a source, companion cells load sugars into the phloem’s sieve tube elements. Water from adjacent xylem enters by osmosis, increasing the pressure in the phloem. This increased pressure pushes the phloem sap toward sinks, where sugar is unloaded. At a sink, water moves out of the phloem by osmosis, keeping the pressure at the sink low.
3. Distinguish between a source and a sink. How can the same plant part act as both a source and a sink?
A source is any plant part that produces or releases sugars; a sink is any plant part that does not carry out photosynthesis. The same plant part can act as both a sink and a source, because a sink may store carbohydrates that are later released to the rest of the plant.
23.4
1. Explain why the only effective treatment for mistletoe is to prune off infected branches.
Knocking off the mistletoe will only eliminate the part of the plant that emerges from the branch; the mistletoe can grow back from the parts that were not removed. Pruning away infected branches, however, eliminates all of the mistletoe.
2. Briefly describe how a parasitic plant infects a host.
After the seed of a parasitic plant germinates, the seedling secretes an adhesive material that attaches the seedling to the host. As the seedling’s roots develop, they penetrate the epidermis and vascular tissue of the host.
23.5
1. Describe the hypothesis and experimental design in Ellison and Gotelli’s study.
Ellison and Gotelli hypothesized that carnivorous plants face an evolutionary trade-off: insect traps provide nutrients but cost the plant so much leaf area that they are not adaptive in nutrient-rich environments. The experiment included nine groups of plants that received solutions with varying nutrient compositions. At the end of each year, the opening and keel size of each treated plant’s pitchers was recorded.
2. Explain the purpose of the two control solutions.
The two groups of control plants received either water or water plus nutrients. The other seven groups of plants received N and P mixed into a solution of water and micronutrients. The two control solutions eliminated the possibility that water and/or micronutrients could account for the treatment effects that Ellison and Gotelli observed.
3. Predict how the graph in figure 23.13 would look if nitrogen did not affect leaf structure.
If nitrogen content did not play a role in leaf shape, then the data points for plants receiving nitrogen would form a cluster with those for plants that did not receive nitrogen.
Write It Out
1. Explain the relationship between transpiration and how plants obtain nitrogen and phosphorus.
Nitrogen and phosphorus dissolve in water in the soil. Roots absorb these nutrients when water moves into the root hairs. This movement ultimately occurs because of transpiration occurring at the leaves. That is, because of the cohesive properties of water, transpiration at the leaves pulls water from the roots up the stem. As water moves up the plant, water and dissolved nutrients flow from the soil into the roots.
2. How might transgenic technology (see chapter 7) be used to endow plants with the ability to fix nitrogen without the aid of bacteria? In what ways would this new feature change agriculture?
In theory, genes encoding enzymes required for nitrogen fixation could be moved from nitrogen-fixing bacteria and inserted into the chromosomes of a plant. It would be potentially valuable to agriculture because plants could fix their own atmospheric nitrogen, which would reduce the need for nitrogen fertilizers.
3. Why might warm temperatures and heavy precipitation in rainforests result in topsoils with low nutrient concentrations?
Warm temperatures speed the decomposition of leaves and other organic matter, releasing nutrients into the soil. The roots of the rainforest plants quickly absorb these nutrients, leaving little in the soil. Moreover, heavy rainfall either erodes away the topsoil or leaches the remaining nutrients into soil layers below the topsoil.
4. When Chris mows the grass, she faces a choice between discarding the clippings and leaving them on the lawn. How would each choice influence the nutrient content of the soil? Explain your answer.
If she discards the clippings, then the organic material and nutrients in the clippings are removed from the ecosystem of her yard. If she leaves the clippings, then they can be broken down by soil bacteria, and the nutrients will be released for further plant growth.
5. If left in the same pot for multiple years, a houseplant may become “root bound,” meaning that the roots grow in circles along the inner surface of the pot. Why do root bound plants eventually show signs of nutrient deprivation?
As roots grow through the soil they absorb nutrients. The densely packed roots of a root bound plant absorb most of the nutrients in the soil. If the owner does not add fertilizer or move the plant to a larger pot containing fresh soil, the plant may show signs of nutrient deprivation.
6. Explain how cohesion plays a part in xylem transport.
Cohesion is the tendency of water molecules to stick together. As water molecules evaporate from the leaf, new molecules diffuse in from the vein. Neighboring water molecules, attached by hydrogen bonds, are pulled up the stem and into the vein. In turn, water is also pulled from roots into the stem, and from the soil into the roots.
7. Suppose you use a rubber band to secure a clear plastic bag around a few leaves on a live plant. What do you think will happen?
Water from transpiration will accumulate in the bag. In addition, the composition of the gas inside the bag will change: O2 will become more abundant, and CO2 will be depleted.
8. How can phloem transport occur either with or against gravity?
Phloem sap moves under positive pressure, which can push fluids with or against gravity. As sugars enter the sieve tube at sources, water follows by osmosis, creating pressure that pushes the sap toward sinks (just as a syringe plunger pushes fluid toward the needle tip). After sugars move into the sink tissue, water diffuses out of the sieve tube by osmosis.
9. Angiosperms have both tracheids and vessel elements; conifers and ferns have only tracheids. Propose a hypothesis that explains how xylem with vessel elements might be advantageous over xylem that contains only tracheids. In what circumstances might tracheids-only xylem be adaptive?
One possible hypothesis is that in an environment with abundant water, vessels allow faster, more efficient xylem transport. Tracheids-only xylem might be adaptive in environments with frequent drought, where plants with vessels would be more subject to air bubble formation.
10. Basil is common in vegetable gardens. Many gardeners grow this plant for its leaves, which provide flavor in sauces and other dishes. Leaf production is higher when young flowers are pruned off of the plant before they have a chance to develop. Explain this observation.
Plants, like all organisms, have limited nutrients and energy. Flower production requires sugars and nutrients that could be used to produce stems and leaves. Pruning flowers therefore increases leaf production.
11. Suppose that a scientist exposes a leaf to CO2 labeled with carbon-14, and the radioactive carbon is incorporated into organic compounds in photosynthetic cells. At various times after exposure, the scientist can determine the location of the radioactive carbon in the plant. In what tissues do you expect to find the radioactive material immediately after exposure to the labeled carbon? What about during transport? When transport is complete, will the radioactive material be in plant parts above the leaf, below the leaf, or both?
The radioactive 14C enters the leaf through stomata and is used immediately in photosynthesis in a mesophyll cell. During transport, the 14C should be present in phloem sap. The phloem may transport the sugar to any sink such as a flower, fruit, or root, and therefore the 14C may end up either above or below the leaf.
12. In the early 1600s, Jean Baptista van Helmont investigated how plants acquire new mass as they grow. He weighed and planted a willow shoot into soil that he had also weighed. After 5 years of adding nothing but water to the plant, he found that the soil had lost only a little weight, while the plant had grown from 2 kilograms to about 76 kg. He therefore concluded, incorrectly, that water was the sole source of the added plant material. What other source did he fail to consider in his experiment?
He did not consider the CO2 from the atmosphere as a source of plant material.
13. Phytoremediation is the use of plants to treat environmental problems. Search the Internet for applications of phytoremediation. What are the benefits of phytoremediation? If you were trying to discover plants suitable for use in phytoremediation, what qualities would you look for? Can you foresee any problems with phytoremediation?
Some benefits could include lower cleanup costs, safety, and low levels of environmental disturbance. Desired qualities in the plants could include the ability to accumulate large amounts of the toxic material without harm to the plant, which might either store the material or convert it to a nontoxic form. Some problems could include introducing concentrated levels of the toxin into the food chain, the possibility that toxins might exist beyond the reach of the plant’s root system, and extended time requirements if the plant's growth rate is low.
14. Write nonbiological analogies for the Casparian strip and for pressure flow theory.
Many answers are possible. One possible analogy for the Casparian strip is the turnstiles that restrict the flow of people through specified openings into a football stadium. One possible analogy for the pressure flow theory would be squeezing a plastic sports bottle to force the water to squirt out.
Pull It Together
1. Add the terms soil, source, sink, pressure flow, and transpiration to the concept map.
“Roots” connects with the phrase “grow in” to “soil.” “Phloem sap” connects with the phrase “moves by” to “pressure flow,” which “pushes phloem sap from” “source” to “sink.” “Transpiration” connects with the phrase “is the loss of water through open stomata in the” to “leaves.”
2. Write a phrase connecting water to sugar.
Water flowing into sieve tubes produces the pressure that pushes sugar dissolved in phloem sap.
3. What are examples of sources and sinks in phloem transport?
Leaves are sources because they create sugars via photosynthesis; roots are sinks because they receive sugars via phloem.
4. Which nutrients do roots absorb from soil, and which do leaves absorb from the atmosphere?
Roots absorb all mineral nutrients, including nitrogen, potassium, and phosphorus, from soil. Oxygen and hydrogen also come from water absorbed from soil. Leaves absorb carbon dioxide gas from the atmosphere.
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