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Photosynthesis and the Global Greenhouse

With the beginning of the Industrial Revolution around 1850, the atmospheric concentration of CO2 started to increase. This increase has resulted largely from the burning of fossil fuels, which releases CO2 as a byproduct. You might expect plants to benefit from the buildup of CO2 in the atmosphere. In fact, the rise in CO2 levels may harm photosynthetic organisms more than it helps them. CO2 and other gases in the atmosphere retain some of the Earth's heat, causing the Earth to become warmer. This warming could reduce the amount of worldwide precipitation, creating deserts that would be inhospitable to most plants.

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 C3 or C4 plants.

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