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S E C T I O N 8.5 Gas Exchange in Plants E X P E C TAT I O N S Describe the major processes and mechanisms for gas exchange in plants. Describe how leaves are modified to reduce water loss while enabling gas exchange. Compare gas exchange mechanisms in plants and animals. Figure 8.28 Plants are complex organisms — but they have no lungs. How can such organisms survive without a respiratory system? At the beginning of this chapter, you saw that single-celled organisms such as protists do not require any specialized respiratory structures. The small size and moist habitat of these organisms mean that a direct exchange of gases across the cell membrane is sufficient to meet the organisms’ metabolic needs. You also saw that as animal organisms became larger and more complex, different kinds of respiratory systems evolved to increase the respiratory surface area and facilitate the exchange of gases. Like animals, plants must respire to supply their cells with oxygen and remove waste carbon dioxide. Many terrestrial plants are large and complex organisms. Although plants have specialized structures for many functions, including nutritive and reproductive functions, they have neither a distinct respiratory surface area nor a circulatory system that carries gases from one part of a plant to the other. This means that each plant organ must rely on a direct exchange of gases with its environment. Like terrestrial animals, terrestrial plants also face the challenge of maintaining a moist respiratory surface for gas exchange while preventing excessive water loss. Gas Exchange in Roots Like the soil-borne organisms you saw in section 8.1, the roots and rhizoids of plants depend on gas exchange with the air in the soil. The surface of the root is covered with many outgrowths called root hairs (Figure 8.29). These outgrowths add considerable surface area to the root, and provide a moist surface for gas exchange. As long as the soil is sufficiently aerated and contains water, oxygen will diffuse from the air into the air spaces of the soil and then into the moisture film surrounding the soil particles and root hairs. The dissolved oxygen then enters the root hair cell and by diffusion passes to the other cells of the root. At the same time, carbon dioxide diffuses from the cells of the root through the root hairs and out into the soil. The rate of respiration in roots is relatively low, so this mechanism is sufficient to meet the roots’ gas exchange needs. FAST FORWARD To review plant anatomy turn to Chapter 15, Section 15.2. Figure 8.29 Root hairs on the surface of the root of a germinating radish. 274 MHR • Internal Systems and Regulation Root cells can also obtain oxygen from the intercellular spaces often found in plant tissues. Some plants, including certain plants found in swampy areas, have large air canals linking the root and stem. These canals allow oxygen to diffuse internally to the roots directly from other regions of the plant. Gas Exchange in Leaves In most plants, leaves are the primary organs responsible for photosynthesis, the process that produces plant nutrients (in a few species, such as cacti, the stem is the main site of photosynthesis). During photosynthesis, plants absorb carbon dioxide and give off oxygen. But plants also respire constantly, absorbing oxygen and giving off carbon dioxide. During the day, both photosynthesis and respiration occur simultaneously in the leaf. Both photosynthesis and respiration require the leaf to exchange gases with its environment. At the same time, the leaf must guard against losing too much water. Most of the surface of the leaf is covered with a waterproof coating, or cuticle. In order to allow for the gases to pass into and out of the leaf, the cuticle is perforated with small structures called stomata (singular stoma). Each stoma consists of a pore bordered by a pair of guard cells. The guard cells allow the pore to be opened to permit the exchange of gases, or closed to prevent the loss of water. You will learn more about how the stomata function in Unit 5. As shown in Figure 8.30, the stomata open into intracellular air spaces within the leaf. The arrangement of different tissues within the leaf ensures that all cells are close to, or directly in contact with, these air spaces. The surfaces of the leaf cells are moist to permit the gaseous exchange to occur by diffusion across the cell membranes. As the plant photosynthesizes, oxygen is released into the intracellular air space and can be reused by the cells for respiration. Similarly, carbon dioxide released by the respiring leaf cells enters the intracellular air space and can be taken up by cells for photosynthesis. Additional oxygen reaches the cells from the external environment through the stomata, while excess carbon dioxide leaves the plant through the same openings. Gas Exchange in Stems The stem of a plant — whether it is the flexible stem of a tulip or grass, or the thick woody bark of a giant sequoia — contains small pores. The flexible green stems of grasses and other nonwoody plants perform photosynthesis and, like leaves, contain stomata through which gases can enter the leaf. On woody stems you can often see the small white markings of lenticels. These are arrangements of cells that provide openings from the environment into the tissue of the stem. leaf hair cuticle air space leaf vein O2 enters leaf through stomata cuticle guard cell CO2 exits leaf through stomata Figure 8.30 A cross section of a leaf. When the stomata Figure 8.31 The bark of the giant sequoia tree can protect are open, gases can enter and leave the air spaces within the leaf. the plant from forest fires. Even this thick bark has pores through which the stem exchanges gases with the environment. The Breath of Life • MHR 275 p ant transport vesse s stoma CO2 out Figure 8.32 A lenticel. On the woody stems of tree trunks, lenticels usually appear as small white spots. Lenticels often occur at the same place where a stoma was once situated when the stem was younger and capable of carrying on photosynthesis. Oxygen diffuses through the stomata or lenticels into the intercellular air space of the plant, and from these air spaces can reach every cell in the stem. The arrangement of cells in the stem is shown in Figure 8.33. Inside the plant, oxygen dissolves in the water of the moist cell membrane and then diffuses across the cell membrane into the cell. Carbon dioxide follows the opposite path, diffusing across the cell membrane into the intracellular air space and then out through the pores. Looking at the stem of a large tree, it may be hard to imagine that diffusion alone can allow gas exchange to take place at a sufficient rate to meet the needs of all the cells of the stem. In woody stems, however, only a relatively thin layer at the outer surface of the stem is made up of living cells. The central portion of the stem is composed of dead cells that do not require oxygen. SECTION 276 100 µm O2 in air spaces Figure 8.33 Portion of a cross section of the stem of a young corn plant. Gases are exchanged through the stomata. The air channels in the stem help respiratory gases reach all the cells of the stem. In this chapter you learned how different organisms obtain oxygen from their environments. As you saw, most animals need a mechanism to deliver that oxygen to all the cells of the organism. In most organisms, substances such as nutrients, waste materials, and hormones must also be moved from one tissue or organ to another. Both plants and animals have intricate transport networks that help maintain their vital processes. In the next chapter, you will explore how some of these transport systems operate. REVIEW 1. K/U How are plants adapted to reduce water loss while still allowing for gas exchange? 2. K/U What are some of the features of roots that assist plants with the exchange of gases? 3. With respect to water loss and gas exchange, what are some of the features you would expect to find in plants that live on the forest floor? 4. MC Would you expect a shade-tolerant plant to have more or fewer stomata per leaf than a plant that grows best in full sunlight? Why? MC MHR • Internal Systems and Regulation 5. MC You are planning a vegetable garden. What are some of the things you should do, both as you prepare the garden bed and as you care for the plants, to make sure the plants can respire properly? 6. MC Beaver dams often flood large areas of wooded land. Knowing what you do about respiration in plant stems and roots, what effect would you expect this flooding to have on trees? Explain.