Question 1

How does photorespiration begin?

Accepted Answer

Photorespiration begins when RuBisCO acts as an oxygenase in the chloroplast stroma. One oxygenated RuBP molecule forms one 3-phosphoglycerate molecule and one 2-phosphoglycolate molecule. In the lab, two oxygenation events are tracked together so the later 2-glycine mitochondrial step is represented consistently.

Question 2

Which organelles participate in photorespiratory recovery?

Accepted Answer

Recovery involves the chloroplast, peroxisome and mitochondrion. The chloroplast converts 2-phosphoglycolate to glycolate, the peroxisome converts glycolate through glyoxylate to glycine, and the mitochondrion processes two glycine molecules.

Question 3

Why is photorespiration called wasteful?

Accepted Answer

Recovery reactions consume ATP and reducing power without producing ATP or NADPH for the plant. The pathway also releases previously fixed carbon, so it lowers the net carbon gain from photosynthesis.

Question 4

How much carbon is lost during mitochondrial glycine decarboxylation?

Accepted Answer

Two glycine molecules bring four carbon atoms into this step. One carbon atom is released as CO2 while the remaining three form serine, so the carbon loss at this step is 25%.

Question 5

How does the C4 pathway reduce photorespiration?

Accepted Answer

The C4 pathway concentrates CO2 around RuBisCO in bundle-sheath cells. This higher local CO2 concentration favours carboxylation over the oxygenase reaction and suppresses photorespiration.

Photorespiration 3D — RuBisCO Oxygenase Across Three Organelles

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