Photorespiration

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Photorespiration is the alternate pathway for production of glyceraldehyde 3-phosphate (G3P) by RuBisCO, the main enzyme of the light-independent reactions of photosynthesis (also known as the Calvin cycle or the Primary Carbon Reduction (PCR) cycle). Although RuBisCO favors carbon dioxide to oxygen,(approximately 3 carboxylations per oxygenation), oxygenation of RuBisCO occurs frequently, producing a glycolate and a glycerate. This usually occurs when oxygen levels are high; for example, when the stomata (tiny pores on the leaf) are closed to prevent water loss on dry days. It involves three cellular organelles: chloroplasts, peroxisomes, and mitochondria. Photorespiration produces no ATP.

The Oxidative photosynthetic carbon cycle reaction is catalyzed by RuBP oxygenase activity:

RuBP + O₂ → Phosphoglycolate + 3-phosphoglycerate

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 PCR cycle. One carbon dioxide molecule is produced for every 2 molecules of O₂ 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 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 dioxide 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 O₂ and carbon dioxide.

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 CO₂ in the leaves so that Rubisco is less likely to produce glycolate through reaction with O₂.

C4 plants capture carbon dioxide in cells of their mesophyll (using an enzyme called PEP carboxylase), and they release it to the bundle sheath cells (site of carbon dioxide fixation by RuBisCO) where oxygen concentration is low. The enzyme PEP carboxylase is also found in other plants such as cacti and succulents who use 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.

This ability to avoid photorespiration makes these plants more hardy than other plants in dry conditions where stomata are closed and oxygen concentrations rise. C4 plants include sugar cane, corn (maize), and sorghum.

References

• Stern, Kingsley R., Shelley Jansky, James E Bidlack. Introductory Plant Biology. Mc Graw Hill. 2003 ISBN 0-07-290941-2
• Siedow, James N., David Day. Chapter 14 "Respiration and Photorespiration". Biochemistry and Molecular Biology of Plants. American Society of Plant Physiologists. 2000.

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