Many plant species that evolved in hot, dry climates fix carbon through alternative pathways. Under hot and dry conditions, plants can rapidly lose water to the air through small pores called stomata. Stomata (singular, stoma), are usually located on the undersurface of the leaves. Plants can reduce water loss by partially closing their stomata when the air is hot and dry. Stomata are the major passageways through which CO2 enters and O2 leaves a plant. When a plant’s stomata are partly closed, the level of CO2 in the plant falls as CO2 is consumed in the Calvin cycle. At the same time, the level of O2 in the plant rises as the light reactions generate O2. Both a low CO2 level and a high O2 level inhibit carbon fixation by the Calvin cycle. Alternative pathways for carbon fixation help plants deal with this problem.
The C4 Pathway
One alternative pathway enables certain plants to fix CO2 into four-carbon compounds. This pathway is thus called the C4 pathway, and plants that use it are known as C4 plants. During the hottest part of the day, C4 plants have their stomata partially closed. However, certain cells in C4 plants have an enzyme that can fix CO2 into four-carbon compounds even when the CO2 level is low and the O2 level is high. These compounds are then transported to other cells, where CO2 is released and enters the Calvin cycle. C4 plants include corn, sugar cane, and crab grass. Such plants lose only about half as much water as C3 plants when producing the same amount of carbohydrates. Many plants that use the C4 pathway evolved in tropical climates.
The CAM Pathway
Cactuses, pineapples, and certain other plants have a different adaptation to hot, dry climates. Such plants fix carbon through a pathway called the CAM pathway. CAM is an abbreviation for crassulacean acid metabolism, because this water-conserving pathway was first discovered in plants of the family Crassulaceae, such as the jade plant. Plants that use the CAM pathway open their stomata at night and close them during the day—just the opposite of what other plants do. At night, CAM plants take in CO2 and fix it into a variety of organic compounds. During the day, CO2 is released from these compounds and enters the Calvin cycle. Because CAM plants have their stomata open at night, when the temperature is lower, they grow fairly slowly. However, they lose less water than either C or C plants.