How do desert plants prepare their own food?
Answers
In their chloroplasts they use photosystem II and photosystem I to make ATP and NADPH2 which are then used to make glucose from CO2 and water using the Calvin cycle. The connection between the Light reactions and the carbon fixation reactions can be direct or indirect.
(a) Plants with a direct system are called C3 plants. They have problems in desert climates to balance their need for water and access to CO2 in the atmosphere. This is because stomates in the leaves have to be open for the plant to get access to atmospheric CO2 but that leads to water loss. Clover, peas, wheat and rice are examples.
(b) C4 plants have a CO2 supercharging mechanism to essentially concentrate CO2 from the atmosphere and then convert it into glucose using the Calvin cycle. The biochemistry occurs in different cells in the leaves and so is a separation in space. This allows them to get by on less water. Corn and many desert grasses are examples. C4 has evolved independently several times from ordinary C3 plants.
(c) There is another form of C4 photosynthesis that uses the same biochemistry but separates absorption of CO2 from the atmosphere in time rather than space. Plants like cacti open their stomates at night and store CO2 as a 4 carbon organic acid in their vacuoles. During the day they close their stomates and so lose almost no water during the day. They break down the 4-carbon organic acids to release CO2 that is sealed inside the leaf because the stomates are closed and use the Calvin cycle to make glucose. This form of C4 photosynthesis is called CAM (Crassulacean Acid Metabolism). Cactuses and Pineapples are examples. CAM has evolved independently several times from ordinary C3 plants.