Question

Explain Photorespiration In Briefly.

Class XI .

Answer 1

Definition

(1) The process by which in the presence of light plant consumes oxygen and releases carbon dioxide (in stead of fixing carbon dioxide) during photosynthesis, resulting in a decrease in photosynthetic output since no ATP is produced and carbon (as well as nitrogen in the form of ammonia) is lost inevitably.

(2) An alternative counter-productive metabolic pathway wherein oxygen molecule is fixed rather than carbon dioxide to produce glyceraldehyde 3-phosphate (G3P).

Supplement

Plants, especially C3 plants, face the problem of photorespiration. In hot dry days, these plants tend to close their stomata to prevent excessive loss of water (from transpiration). Inevitably, the carbon dioxide cannot enter the leaves (via the stomata) resulting in the levels of carbon dioxide within the leaves to become low. Since there are few carbon dioxide molecules to fix, the oxygen molecules are used as a substitute to produce G3P. Because of photorespiration, in stead of about 2 molecules of G3P, only one G3P is produced and a toxic phosphoglycolate (which the plant must get rid of) is also formed. Some plants such as CAM plants and C4 plants have evolved mechanisms to avoid photorespiration.

Answer 2

ANSWER.....

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☆☆ PHOTORESPIRATION ☆☆

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▪ In Calvin Pathway, RuBP combines with CO2 .

RuBP + CO2 --RuBISCO-------> 2 × 3-PGA

•• RuBISCO = Ribulase Biphospate Carboxylase Oxygenase Enzyme.

•• PGA = Phosphogylceric Acid.

▪ RuBISCO is the abundant enzyme in the world. Its Active site can bind to both CO2 & O2 -- Hence the name.

▪ RuBISCO has a much greater affinity for CO2 than for O2. This binding is competitive. Relative Concentration of O2 and CO2 determines which one will bind to the Enzyme.

▪ In C3 plants some O2 bind to RuBISCO. Hence CO2 fixation is decreased. Here RuBP binds with O2 to form one molecule of phosphoglycerate and phosphoglycolate. Hence this Pathway is known as PHOTORESPIRATION.

▪ In this pathway, there is no synthesis of sugers, ATP and NADPH. Hence, Photorespiration is a wasteful process. Rather it causes the realeses of CO2 by using ATP.

▪ In C4 plants photorespiration does not occur because they can increase CO2 concentration at the enzyme site. This takes place when C4 Acid From the mesophyll is broken down in the bundle cells to release CO2. This minimises the oxygenase activity of RuBISCO.

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