The $C_{4}$ plants are photosynthetically more efficient than $C_{3}$ plants because:

The

C O_{2}

compensation point is low.

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C O_{2}

generated during photorespiration is trapped and recycled through PEP carboxylase.

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The

C O_{2}

efflux is not prevented.

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They have more chloroplasts.

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Solution

The correct option is A The $C O_{2}$ compensation point is low.
In C $_{3}$ plants, three molecules of ribulose monophosphate (RuMP), 5C compound, are phosphorylated by ATP to form three molecules of ribulose bisphosphate (RuBP). RuBP is the first CO $_{2}$ acceptor. It absorbs CO $_{2}$ in presence of enzyme RuBP-carboxylase to form transitory or additive 6C compound, which is unstable. Three molecules of additive 6C compound immediately split up by hydrolysis into six molecules of 3C compound called 3-phosphoglyceric acid (3-PGA). C $_{3}$ plants can do photosynthesis only at high concentrations of CO $_{2}$ . In C $_{4}$ plants, the CO $_{2}$ is not directly absorbed by RuBP in presence of enzyme RuBP carboxylase as this enzyme helps to absorb CO $_{2}$ having concentration of 50 ppm or more. However, C $_{4}$ plants, due to high temperature and high light intensity, keep their stomata partially closed due to which the CO $_{2}$ concentration goes down to 2 ppm. Hence the C $_{4}$ plants can conduct photosynthesis at low concentrations of CO $_{2}$ .
So the correct option is 'the CO $_{2}$ compensation point is low'.

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