photorespiration

Photorespiration (or "photo-respiration") is the process by which RuBP, (a sugar) has oxygen added to it by the main enzyme involved in photosynthesis, rubisco, instead of carbon dioxide as happens during photosynthesis. Rubisco favours carbon dioxide to oxygen^[1]; photorespiration tends to occur when there is a high concentration of oxygen relative to carbon dioxide. The first reaction produces glyceraldehyde 3-phosphate (G3P) and phosphoglycolate (PPG), G3P 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. Photorespiration can occur when carbon dioxide levels are low; for example, when the stomata (tiny pores on the leaf) are closed to prevent water loss during drought. In most plants it occurs more as temperatures increase as the ratio of oxygenation to carboxylation reactions increases. Photorespiration produces no ATP (energy for cells) and leads to a net loss of carbon and nitrogen (as ammonia) which slows the growth of plants.

Biochemistry of photorespiration

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.

Photorespiration

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 O₂ 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. Crucially, the photorespiratory pathway is a major source of H₂O₂ in photosynthetic cells. Through H₂O₂ production and pyridine nucleotide interactions, photorespiration makes a key contribution to cellular redox homeostasis. In so doing, it influences multiple signaling pathways, particularly those that govern plant hormonal responses controlling growth, environmental and defense responses, and programmed cell death.^[2]

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)

Maize uses the C4 pathway, minimizing photorespiration.

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 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. The Calvin cycle then proceeds as normal.

The enzyme PEP carboxylase (which catalyzes the combination of carbon dioxide with a compound called Phosphoenolpyruvate or PEP) 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 and hot environments where stomata are closed and internal carbon dioxide levels are low. C4 plants include sugar cane, corn (maize), and sorghum.

References

• Stern, Kingsley (2003). Introductory Plant Biology. New York: McGraw-Hill. ISBN 0072909412. 
• 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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