Photosynthesis

In the 1920s, the German biochemist Otto Warburg (1883-1970) discovered that plants consumed oxygen at a higher rate when they were illuminated. He also found that this increased rate of oxygen consumption inhibited photosynthesis. Stimulation of oxygen consumption by light is now referred to as photorespiration. Biochemical studies indicate that photorespiration consumes ATP and NADPH, the high-energy molecules made by the light reactions. Thus, photorespiration is a wasteful process because it prevents plants from using their ATP and NADPH to synthesize carbohydrates.

RuBISCO, the enzyme which fixes carbon dioxide during the Calvin cycle, is also responsible for oxygen fixation during photorespiration. In particular, carbon dioxide and oxygen compete for access to RuBISCO. RuBISCO's affinity for carbon dioxide is much higher than its affinity for oxygen. Thus, fixation of carbon dioxide typically exceeds fixation of oxygen, even though atmospheric carbon dioxide levels are about 0.035% whereas oxygen is about 21%.

If photorespiration is so wasteful, why does it occur at all? Many plant physiologists believe that photorespiration is an artifact of the ancient evolutionary history of photosynthesis. In particular, RuBISCO originated in bacteria several billion years ago when there was very little atmospheric oxygen present. Thus, there was little selection pressure for the ancient RuBISCO to discriminate between carbon dioxide and oxygen and RuBISCO originated with a structure that reacts with both. Even though most modern plants are under great selection pressure to reduce photorespiration, evolution cannot easily alter RuBISCO's structure so that it fixes less oxygen yet still efficiently fixes carbon dioxide.

Interestingly, photorespiration has been observed in all C-3 plants which have been examined, but is virtually nonexistent in C-4 plants. This is because C-4 plants segregate their RuBISCO enzyme in bundle sheath cells deep within the leaf and the carbon dioxide concentration in these cells is maintained at very high levels. C-4 plants generally have higher growth rates than C-3 plants simply because they do not waste their ATP and NADPH in photorespiration.