Under conditions of high oxygen concentration and low carbon dioxide concentration, RuBisCO acts as an oxygenase, initiating photorespiration. What are the immediate products of this oxygenation reaction?

Answer and explanation

Correct answer: D. One molecule of 3-phosphoglyceric acid (3C) and one molecule of 2-phosphoglycolate (2C)

  1. Step 1: When binds competitively to the active site of RuBisCO, it oxygenates ribulose 1,5-bisphosphate (5C).
  2. Step 2: The oxygenase cleavage splits RuBP (5C) into one 3-carbon molecule of 3-phosphoglyceric acid (3-PGA) and one 2-carbon molecule of 2-phosphoglycolate.
  3. Step 3: 3-PGA enters the Calvin cycle, while 2-phosphoglycolate initiates the photorespiratory C2 cycle across chloroplast, peroxisome, and mitochondrion.
Answer
One molecule of 3-phosphoglyceric acid (3C) and one molecule of 2-phosphoglycolate (2C)
Why not the others: 'Two molecules of 3-phosphoglyceric acid (3C)' is the product of carboxylation (), not oxygenation; 'Two molecules of 2-phosphoglycolate (2C)' accounts for only 4 carbons, violating carbon balance since RuBP has 5 carbons; 'One molecule of oxaloacetate...' confuses the C4 carboxylation reaction with RuBisCO oxygenation.

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