Question

The process that demarcates C3 and C4 plants is:

• A. Calvin Cycle
• B. Kerb`s Cycle
• C. Photorespiration
• D. Glycolysis

Answer 1

Answer:

C. Photorespiration

• Plants which use only the calvin cycle for fixing the carbon dioxide from the air are known as C3 plants.

• C3 plants have the disadvantage that in hot dry conditions their photosynthetic efficiency suffers because of a process called photorespiration.

• When the CO2 concentration in the chloroplast drops below about 50 ppm, the catalyst rubisco that helps to fix carbon begins to fix oxygen instead.

• This is highly wasteful of the energy that has been collected from the light, and causes the rubisco to operate at perhaps a quarter of its maximal rate.

• The problem of photorespiration is overcome in C4 plants by a two-stage strategy that keeps CO2 high and oxygen low in the chloroplast where the Calvin cycle operates.

• The class of plants called C3-C4 intermediates and the CAM plants also have better strategies than C3 plants for the avoidance of photorespiration.