Chapter 29 Summary

Core Concepts Summary

29.1 The evolution of land plants from aquatic ancestors introduced a major challenge for photosynthesis: acquiring carbon dioxide without drying out.

Early-branching groups of land plants, commonly grouped as bryophytes, balance CO2 gain and water loss passively, photosynthesizing in wet conditions and tolerating desiccation in dry ones. page 600

Vascular plants maintain the hydration of their photo-synthetic cells with water pulled from the soil. page 600

29.2 Leaves have a waxy cuticle and stomata, both of which are important in regulating carbon dioxide gain and water loss.

The low concentration of CO2 in the atmosphere forces plants to expose their photosynthetic cells directly to the air. The outward diffusion of water vapor leads to a massive loss of water. page 601

The waxy cuticle on the outside of the epidermis slows rates of water loss from leaves but also slows the diffusion of CO2 into leaves. page 602

Stomata are pores in the epidermis that open and close, allowing CO2 to enter into the leaf and also allowing water vapor to diffuse out of the leaf. page 602

CAM plants capture CO2 at night when evaporative rates are low. During the day, they close their stomata and use this stored CO2 to supply the Calvin cycle, resulting in increases in the exchange ratio of CO2 and H2O. page 604

C4 plants suppress photorespiration by concentrating CO2 in bundle-sheath cells. The buildup of CO2 in bundle-sheath cells results from the rapid production of C4 compounds in the mesophyll cells, which then diffuse into the bundle-sheath cells and release CO2 that can be used in the Calvin cycle. page 605

29.3 Xylem allows vascular plants to replace water evaporated from leaves with water pulled from the soil.

Water flows through xylem conduits from the soil to the leaves. page 607

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Xylem is formed from cells that lose all their cell contents as they mature. page 607

Xylem conduits have thick, lignified cell walls. Water flows into xylem conduits across small thin-walled regions called pits. page 607

Xylem conduits are of two types: tracheids and vessels. Tracheids are unicellular xylem conduits; vessels are formed from many cells. page 607

The evaporation of water from leaves generates the driving force for water movement in xylem. Because of the strong hydrogen bonds between water molecules, water can be pulled from the soil and transported through xylem. page 609

Pulling water through xylem creates the risk of mechanical failure, either by the collapse inward of conduit walls or by cavitation in which a gas bubble expands to fill the entire conduit. page 610

29.4 Phloem transports carbohydrates to the non-photosynthetic portions of the plant for use in growth and respiration.

Phloem transport occurs through living cells, which lack a nucleus or vacuole at maturity. In angiosperms, these cells are connected end to end by sieve plates, forming multicellular sieve tubes. Sieve plates contain large pores that allow phloem sap to flow from one sieve tube cell to another. page 611

The active loading of solutes brings water into sieve tubes by osmosis, increasing the turgor pressure. At sites of use, the removal of solutes leads to an outflow of water and a drop in turgor pressure. page 612

The difference in turgor pressure drives the movement of phloem sap from source to sink. page 612

Carbohydrates exuded from roots provide carbon and energy sources for the rhizosphere microbial community, stimulating decomposition of organic matter and increasing nutrient availability. page 612

29.5 Roots expend energy to obtain nutrients from the soil.

The endodermis allows roots to control which materials enter the xylem. The uptake of solutes by roots is therefore highly selective. page 614

Plants exchange carbohydrates with symbiotic fungi in return for assistance obtaining nutrients from the soil. page 615

Plants supply carbohydrates to symbiotic nitrogen-fixing bacteria in exchange for available forms of nitrogen. page 616

Agricultural productivity is closely linked to nitrogen supply. page 616

Self-Assessment

  1. Explain why vascular plants are better able to sustain photosynthesis than are bryophytes.

    Self-Assessment 1 Answer

    Vascular plants are better able to sustain photosynthesis than bryophytes because they are able to take up water from the ground through their root system instead of being dependent on the level of water on the surface. Bryophytes dry out when not enough water is available at the surface, and, as a result, they lose the ability to photosynthesize.

  2. Explain why transpiration is best explained as a consequence of acquiring CO2 from the atmosphere, as opposed to temperature regulation, nutrient transport, or the use of water as a substrate in photosynthesis.

    Self-Assessment 2 Answer

    Transpiration is the loss of water vapor from the leaves through evaporation. It is a consequence of acquiring CO2 from the atmosphere. As CO2 diffuses into the leaf through stomata, water vapor diffuses out at a faster rate. Thus, several hundred molecules of water are lost for every molecule of CO2 acquired for photosynthesis.

  3. Draw and label the features of a leaf that are necessary to maintain a well-hydrated interior while still allowing CO2 uptake from the atmosphere.

    Self-Assessment 3 Answer

  4. Diagram how the movement of solutes results in the opening and closing of stomata.

    Self-Assessment 4 Answer

  5. Diagram how the enzyme PEP carboxylase allows CAM plants to reduce transpiration rates and C4 plants to suppress photorespiration.

    Self-Assessment 5 Answer

  6. Contrast how transport is generated in xylem and in phloem.

    Self-Assessment 6 Answer

    Water transport in xylem is driven by an evaporative pump whose driving force is generated in the leaves. Thus, water moves from the roots to the leaves in xylem. In contrast, the transport of sap in the phloem can go in either direction, from tubers to young leaves or from leaves to roots, for example.

    Phloem transport is driven by differences in turgor pressure. Water is drawn into the phloem by osmosis, in response to the active transport of sugar molecules into the phloem. The influx of water increases turgor pressure at the source. Water leaves the phloem by osmosis when sugar molecules are transported out into the other plant tissues. The outflow of water reduces turgor pressure at the sink. Water flows from the region of high turgor pressure to the region of low turgor pressure.

  7. Describe the structure of xylem conduits and sieve tubes in relation to their function.

    Self-Assessment 7 Answer

    Xylem conduits are basically hollow tubes made up of thick cell walls. There are no cytosol or internal organelles. Water is able to flow easily through the hollow tubes.

    Phloem consists of sieve tubes that transport carbohydrates. They are also elongated, but they contain cytoplasm and a few organelles. The cytoplasm provides a medium in which to transport the carbohydrates, called phloem sap.

  8. Explain why phloem is necessary to produce roots.

    Self-Assessment 8 Answer

    Phloem transports sugars and other nutrients to non-photosynthetic organs, like the roots.

  9. List four adaptations of roots that enhance the uptake of nutrients from the soil.

    Self-Assessment 9 Answer

    Four adaptations of roots that enhance the uptake of nutrients from the soil are (1) root hairs of the epidermis; (2) the ability of endodermal cells to control which nutrients enter the vascular system; (3) the beneficial association of mycorrhizal fungi; and (4) the beneficial association of bacteria in the root nodules of some plants like legumes.