Photorespiration

Photosynthetic organisms must cope with a competing reaction that inhibits photosynthesis known as photorespiration. Unlike photosynthesis, this process involves the uptake of oxygen and the release of carbon dioxide.

Recall that mitochondrial respiration involves the uptake of O₂ and the evolution of CO₂ and is associated with the burning of cellular fuel to obtain energy in the form of ATP. In contrast, photorespiration starts in the chloroplast and wastes energy.

Photorespiration can be defined as the light-dependent uptake of O₂ in the chloroplast. It is caused by a fundamental “inefficiency” of RuBisCO.

During photosynthesis RuBisCO catalyzes the carboxylation of RuBP to give two molecules of PGA. However, it can also catalyze the oxygenation of RuBP to give one molecule of PGA and one molecule of a 2-carbon compound called phosphoglycolate. This reaction occurs because O₂ can compete with CO₂ at the active site of RuBisCO. As oxygenation of RuBP competes with carboxylation, it lowers the efficiency of photosynthesis. A significant portion (25%) of the carbon in phosphoglycolate is lost as CO₂. Algae must use energy to “recover” the remaining 75% of this carbon, which further limits the efficiency of photosynthesis.

If photorespiration lowers the yield of photosynthesis, why has such a process been maintained throughout the course of evolution? The answer to this intriguing question has to do with the origin of RuBisCO and the CBB cycle. RuBisCO is an ancient enzyme, having evolved over 2.5 billion years ago in cyanobacteria. During this period in Earth’s history, the atmosphere contained high levels of CO₂ and very little oxygen.

Thus, photorespiration did not present a problem for early photosynthetic organisms. By the time oxygen levels accumulated to significant levels in the atmosphere (ironically, by the process of photosynthesis!), the catalytic mechanism of RuBisCO was apparently “fixed.” In other words, because both O₂ and CO₂ compete for the same active site of the enzyme, algae could not decrease the efficiency of oxygenation without also decreasing the efficiency of carboxylation. To compensate, algae evolved an elaborate pathway, known as the photorespiratory pathway, to recover at least some of the carbon that would otherwise be lost. This pathway involved biochemical reactions in the chloroplast, mitochondria, and peroxisome. The importance of photorespiration is easily demonstrated by the fact that nearly all plants grow better under high CO₂ versus low CO₂. Conditions that favor carboxylation (photosynthesis) over oxygenation (photorespiration) include high CO₂, moderate light intensities, and moderate temperatures. Conditions that favor oxygenation over carboxylation include low CO₂ levels, high temperatures, and high light intensities.