Many plants, called C-4 plants, in warm areas use a different strategy to fix carbon dioxide. This strategy involves first fixing the carbon dioxide in specially arranged mesophyll cells using a seperate cycle and then transferring the carbon dioxide to cells where the normal Calvin cycle happens. The function of this arrangement is to isolate the cells where the Calvin cycle takes place from the high concentration of oxygen that interfere with the enzyme involved in carbon fixation.
Comparison of C-3 and
C-4 plant structures.
The diagram shows on the left a typical C-3 plant with a Kranz or C-4 plant on the right. Kranz is German for wreath and refers to the wreath like arrangement of the mesophyll cells that specialize in the light reaction around the vascular bundle. In C-3 plants this mesophyll is arranged in a columnar layer that is called palisade mesphyll. In C-4 plants carbon dioxide is actively pumped into the mesophyll cells and fixed to a three carbon compound. The now four carbon compound is sent to the bundle sheath cells where the C-4 compound gives up the carbon atom to the normal Calvin-Benson cycle.
Advantages C-3 vs C-4. In warm climates C-4 plants have an advantage because during the day they can keep their stoma closed thus conserve water. The C-4 adaptation allows oxygen to build up to much higher levels without seriously affecting the rate of carbon fixation. In cooler climates, C-3 plants have the edge because it takes less energy for them to fix carbon dioxide.
pgd revised 2/17/01