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Compared with a C3 plant, how many additional ATP molecules are needed by a C4 plant for the net production of one hexose molecule?
Your answer:
Correct answer: (c)
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Explanation
Why (c) is correct
A C3 plant uses 18 ATP per hexose. A C4 plant also spends 2 ATP per CO2 to regenerate PEP, that is 12 more ATP for six CO2, so it needs 30 ATP in all: 12 extra.
Why the other options are wrong
- (a) · your answer Two extra ATP are needed per CO2, not per hexose; for six CO2 it is 12.
- (b) · your answer Six would be one ATP per CO2; regenerating PEP costs two.
- (d) · your answer C4 plants do need extra ATP to regenerate PEP.
NCERT point: The C4 pathway costs more ATP, but it avoids the larger losses of photorespiration.
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