Photorespiration Is a Wasteful Process

Assignment 03

Photorespiration is a wasteful process.

Plant Biochemistry
Department of Life sciences
Name- Mahnaz Reza
ID- 072 398 047
Date- the 16th of March 2011, Wednesday

Photorespiration, as the name suggests is the process by which the most abaundant protein on earth, RubisCO, uses up oxygen or adds oxygen to the 5 carbon sugar rubisco bisphosphate, instead of carbon dioxide as in the process of photosynthesis. It is the alternate pathway used by the enzyme. This enzyme being both a carboxylase and an oxygenase does not have the capability to differentiate between the carbon dioxide molecule or the oxygen molecule as a result of which it can assist in both the reaction of photorespiration and photosynthesis, even though they are the opposite pathways. In general RubisCO favours carbon dioxide to oxygen, and approximately 3 carboxylations occur per oxygenation. It is interesting to note that photorespiration only occurs in C3 plants where the oxygenase plays its role. It is important because it 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 Photorespiration occurs when carbon dioxide levels within the leaf tissue dwindle to about 50 parts per million, usually on a hot, dry day where a plant has closed its stomates to prevent water loss. At this concentration of CO2, oxygen reacts with ribulose bisphosphate (RuBP) in the presence of RubisCO, and creates 2 molecules of 3-phosphoglycerate and 1 molecule of 2-phosphoglycolate. Under normal oxygen and carbon dioxide concentrations, the reaction is between CO2, RuBP, and RubisCO to produce 12 molecules of 3-phosphoglycerate. 2-phosphoglycolate is toxic, so the plant has to convert it into a non-detrimental compound through a number of steps, each of which costs energy and CO2. This processes occur in 3 organelles. Namely the chloroplast, peroxisome and mitochondria. RuBP + O2 → Phosphoglycolate + 3-phosphoglycerate
The reactions that take place are. Chloroplast 1. Rubisco + ribulose-1,5-bisphosphate (RuBP) + oxygen→ Phosphoglycolate + 3- phosphoglycerate 2. 2-phosphoglycolate → glycolate (by phosphoglycolate phosphatase) and phosphate Peroxisome 1. Glycolate(oxygenated by oxygen gas)→ glyoxylate + H2O2 (by glycolate oxydase) 2. Glyoxylate→ glycine (amino acid). The amino group is transferred to the glyoxylate from glutamate. 3. glutamate →α-ketoglutarate
Mitochondria
1. Glycine + glycine decarboxylase→ removes carbon dioxide, and amino group. 2. As a result, the methylene group that is left, is placed on a folate molecule in the mitochondrion. 3. second glycine + methylene-folate → serine (by serine hydroxymethyltransferase)
Peroxisome
1. The serine loses its amino group to α-ketoglutarate → regenerate glutamate ( by serine aminotransferase) 2. Serine→ hydroxypyruvate (reduced by hydroxypyruvate reductase)→ glycerate
Chloroplast
1. Glycerate (by glycerate kinase) → 3-phosphoglycerate

Photorespiration is wasteful, in that it does not produce any 3-phosphoglyceraldehyde (PGAL), which is used by the plant for a large number of function. Photorespiration produces no ATP. And leads to a net loss of carbon and nitrogen, as ammonia, which are the requirements for plant growth, as a result of which the plants growth is hampered. Moreover photorespiration lowers the efficiency of photosynthesis by removing carbon molecules from the Calvin Cycle. The reduction process requires NADH and it utilizes ATP as well, which are mainly produced from the light reaction, causing it to be wasteful for the plant. The carbon dioxide which would other wise be used for the generation of sugar for the plant is siphoned off from the calvin cycle and since it takes two glycines in photorespiration to complete the pathway, two glycolates must be taken from the Calvin cycle. Of these four carbons taken, one is lost as carbon dioxide and three are returned to the Calvin cycle thus the 25% loss of carbon. Protorespiration as a whole reduces the net productivity of the plant.

Reference
Class lecture http://plantphys.info/plant_physiology/photoresp.shtml http://www.answerbag.com/ http://en.wikipedia.org/wiki/Photorespiration http://www2.mcdaniel.edu