fotorespiratio

Photorespiration is a metabolic pathway that occurs in plants and algae. It consumes oxygen and releases carbon dioxide. This process is often seen as a wasteful byproduct of photosynthesis, particularly under conditions of high light intensity and high temperatures.

The primary enzyme involved in photorespiration is RuBisCO, the same enzyme that catalyzes the initial fixation of carbon dioxide during the Calvin cycle. However, RuBisCO has a dual activity: it can bind to either carbon dioxide or oxygen. When RuBisCO binds to oxygen instead of carbon dioxide, it initiates the photorespiratory pathway.

This oxygenation reaction leads to the formation of a toxic compound called phosphoglycolate. Photorespiration is essentially a salvage pathway that recovers some of the carbon from phosphoglycolate, converting it back into a molecule that can re-enter the Calvin cycle. However, this recovery is incomplete, and a net loss of fixed carbon and energy in the form of ATP and NADPH occurs.

Several factors influence the rate of photorespiration. High temperatures increase the oxygenase activity of RuBisCO relative to its carboxylase activity, thus promoting photorespiration. Similarly, low carbon dioxide concentrations and high oxygen concentrations favor photorespiration. Plants have evolved different strategies to minimize photorespiration, such as the C4 and CAM photosynthetic pathways, which concentrate carbon dioxide around RuBisCO.