Photosynthesis Under Extreme Conditions In order to conserve water, most plants under bright, hot conditions (of the sorts often found in the tropics) close the small openings in their leaves that normally admit carbon dioxide. While this keeps the plants from drying out, it causes carbon dioxide within the leaves to fall to very low levels. When this happens to most plants, photosynthesis slows down or even stops. However, some plants have adapted to extremely bright, hot conditions. There are two major groups of these specialized plants: C4 plants and CAM plants. C4 and CAM plants have biochemical adaptations that minimize water loss while still allowing photosynthesis to take place in intense sunlight.

▸ C4 Photosynthesis C4 plants have a specialized chemical pathway that allows them to capture even very low levels of carbon dioxide and pass it to the Calvin cycle. The name “C4 plant” comes from the fact that the first compound formed in this pathway contains 4 carbon atoms. The C4 pathway enables photosynthesis to keep working under intense light and high temperatures, but it requires extra energy in the form of ATP to function. C4 organisms include important crop plants like corn, sugar cane, and sorghum.

▸ CAM Plants Other plants adapted to dry climates use a different strategy to obtain carbon dioxide while minimizing water loss. These include members of the family Crassulaceae. Because carbon dioxide becomes incorporated into organic acids during photosynthesis, the process is called Crassulacean Acid Metabolism (CAM). CAM plants admit air into their leaves only at night. In the cool darkness, carbon dioxide is combined with existing molecules to produce organic acids, “trapping” the carbon within the leaves. During the daytime, when leaves are tightly sealed to prevent the loss of water, these compounds release carbon dioxide, enabling carbohydrate production. CAM plants include pineapple trees, many desert cacti, and also the fleshy “ice plants” shown in Figure 8–12, which are frequently planted near freeways along the west coast to retard brush fires and prevent erosion.

FIGURE 8–12 CAM Plants Plants like this ice plant can survive in dry conditions due to their modified light-independent reactions. Air is allowed into the leaves only at night, minimizing water loss.

8.3 Assessment

Review Key Concepts

1. 1. Review Summarize what happens during the light-dependent reactions of photosynthesis.

   2. Sequence Put the events of the light-dependent reactions in the order in which they occur and describe how each step is dependent on the step that comes before it.

2. 1. Review What is the Calvin cycle?

   2. Compare and Contrast List at least three differences between the light-dependent and light-independent reactions of photosynthesis.

3. 1. Review What are the three primary factors that affect the rate of photosynthesis?

   2. Interpret Graphs Look at the graph on page 240. What are the independent and dependent variables being tested?

BUILD VOCABULARY

4. The word carbohydrate comes from the prefix carbo-, meaning “carbon,” and the word hydrate. Based on the reactants of the photosynthesis equation, what does hydrate mean?

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