Photorespiration

Photorespiration, or "'photo-respiration'", is a process in plant metabolism by which RuBP (a sugar) has oxygen added to it by the enzyme (rubisco), instead of carbon dioxide during normal photosynthesis. This process reduces efficiency of photosynthesis in C3 plants.

Simplified biochemistry
Rubisco favors carbon dioxide as a ligand to oxygen, however, photorespiration occurs when there is a high concentration of oxygen relative to carbon dioxide. The first reaction produces phosphoglycerate (PGA) and phosphoglycolate; PGA re-enters the Calvin cycle and is simply converted back to RuBP.

PPG, however, is more difficult to recycle and has to move from the chloroplast to the peroxisomes, and then to the mitochondria, undergoing many reactions on the way, before the atoms can return into the Calvin cycle.

Conditions under which photorespiration occurs
Photorespiration can occur when carbon dioxide levels are low, for example, when the stomata are closed to prevent water loss during drought. In most plants, photorespiration increases as temperature increases. Photorespiration produces no ATP and leads to a net loss of carbon and nitrogen (as ammonia), slowing plant growth.

Potential photosynthetic output may be reduced by photorespiration by up to 25% in C3 plants.

More or less efficient?
However, reduction in photorespiration may not result in increasing growth rates for plants. Some research has suggested, for example, that photorespiration may be necessary for the assimilation of nitrate from soil. Thus, a reduction in photorespiration by genetic engineering or because of increasing atmospheric carbon dioxide due to fossil fuel burning may not benefit plants as has been proposed.

Biochemistry of photorespiration
The oxidative photosynthetic carbon cycle reaction is catalyzed by RuBP oxygenase activity:


 * RuBP + &rarr; Phosphoglycolate + 3-phosphoglycerate + 2

The phosphoglycolate is salvaged by a series of reactions in the peroxisome, mitochondria, and again in the peroxisome where it is converted into serine and later glycerate. Glycerate reenters the chloroplast and, subsequently, the Calvin cycle by the same transporter that exports glycolate. A cost of 1 ATP is associated with conversion to 3-phosphoglycerate (PGA) (Phosphorylation), within the chloroplast, which is then free to reenter the Calvin cycle. One carbon dioxide molecule is produced for every 2 molecules of that are  taken up by RuBisCO.

Photorespiration is a wasteful process because G3P is created at a reduced rate and higher metabolic cost (2ATP and one NAD(P)H) compared with RuBP carboxylase activity. G3P produced in the chloroplast is used to create "nearly all" of the food and structures in the plant. While photorespiratory carbon cycling results in the formation of G3P eventually, it also produces waste ammonia that must be detoxified at a substantial cost to the cell in ATP and reducing equivalents.

Role of photorespiration
Photorespiration is said to be an evolutionary relic. Photorespiration lowers the efficiency of photosynthesis by removing carbon molecules from the Calvin Cycle. The early atmosphere in which primitive plants originated contained very little oxygen, so it is hypothesized that the early evolution of RuBisCO was not influenced by its lack of discrimination between and carbon dioxide.

Although the functions of photorespiration remain controversial, it is widely accepted that this pathway influences a wide range of processes from bioenergetics, photosystem II function, and carbon metabolism to nitrogen assimilation and respiration. The photorespiratory pathway is a major source of in photosynthetic cells. Through production and pyridine nucleotide interactions, photorespiration makes a key contribution to cellular redox homeostasis. In so doing, it influences multiple signaling pathways, in particular, those that govern plant hormonal responses controlling growth, environmental and defense responses, and programmed cell death.

Another theory postulates that it may function as a "safety valve", preventing excess NADPH and ATP from reacting with oxygen and producing free radicals, as these can damage the metabolic functions of the cell by subsequent reactions with lipids or metabolites of alternate pathways.

Minimization of photorespiration (C4 and CAM plants)


Since photorespiration requires additional energy from the light reactions of photosynthesis, some plants have mechanisms to reduce uptake of molecular oxygen by RuBisCO. They increase the concentration of in the leaves so that Rubisco is less likely to produce glycolate through reaction with.

C4 plants capture carbon dioxide in cells of their mesophyll (using an enzyme called PEP carboxylase), and oxaloacetate is formed. This oxaloacetate is then converted to malate and is released into the bundle sheath cells (site of carbon dioxide fixation by RuBisCO) where oxygen concentration is low to avoid photorespiration. Here, carbon dioxide is removed from the malate and combined with RuBP in the usual way, and the Calvin cycle proceeds as normal.

This ability to avoid photorespiration makes these plants more hardy than other plants in dry and hot environments, wherein stomata are closed and internal carbon dioxide levels are low. C4 plants include sugar cane, corn (maize), and sorghum.

CAM plants, such as cacti and succulent plants, use the enzyme PEP carboxylase (which catalyzes the combination of carbon dioxide with a compound called Phosphoenolpyruvate or PEP) in a mechanism called Crassulacean acid metabolism, or CAM, in which PEP carboxylase sequesters carbon at night and releases it to the photosynthesizing cells during the day. This provides a mechanism for reducing high rates of water loss (transpiration) by stomata during the day.