Photorespiration
ஒளிசுவாசம்
RuBisCO is the most abundant protein on Earth, yet it is fundamentally flawed. It cannot distinguish between CO2 and O2. This evolutionary blunder is the reason C3 plants lose 25% of their fixed carbon.
The idea
In C3 plants, RuBisCO acts as an oxygenase when O2 concentration is high. It reacts with RuBP to form one molecule of phosphoglycolate and one molecule of 3-PGA. This pathway involves the chloroplast, peroxisome, and mitochondria. It consumes ATP and releases CO2 without synthesizing sugars or ATP.
Why it matters
It is an evolutionary byproduct of the dual nature of the RuBisCO enzyme, which can bind to both CO2 and O2.
NEET lens
NEET asks about the cell organelles involved in this pathway: chloroplast, peroxisome, and mitochondria. Remember, no ATP is produced here; it is a wasteful process. Do not confuse this with C4 photosynthesis, which evolved specifically to bypass this limitation.
Clinical significance
Photorespiration is a metabolic sink that limits crop yield, directly impacting global food security. In human physiology, the inability of enzymes to discriminate between substrates is analogous to metabolic errors like Phenylketonuria, where a single enzyme failure disrupts the entire pathway.
Introduction
Photorespiration is a metabolic process in C3 plants where RuBisCO reacts with O₂ instead of CO₂ under high O₂ / low CO₂ conditions, leading to loss of fixed carbon as CO₂ without producing ATP or NADPH.
Core concept
C3 plants: RuBisCO (ribulose bisphosphate carboxylase/oxygenase) can react with both CO₂ (carboxylation, useful) and O₂ (oxygenation, wasteful). Oxygenation pathway: RuBP + O₂ → phosphoglycolate + PGA. Phosphoglycolate is processed in peroxisomes and mitochondria → CO₂ released + ATP consumed with NO sugar gain. Occurs in chloroplast + peroxisome + mitochondria (three organelles). Temperature-dependent: increases at high temperature.
Go deeper
C4 plants eliminate photorespiration: Mesophyll cells fix CO₂ into C4 acids (OAA, malate) via PEP carboxylase (high CO₂ affinity, doesn't react with O₂) → C4 acids transported to bundle sheath cells → CO₂ released → enters Calvin cycle with CO₂ concentrated around RuBisCO. CO₂ concentration mechanism prevents photorespiration.
From Class 10
Photosynthesis fixes CO₂. But sometimes the enzyme RuBisCO reacts with O₂ instead of CO₂, wasting energy. This wasteful process is called photorespiration.