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If ¹⁴CO₂ is supplied to a C₃ plant for a few seconds, the radioactivity will first appear in

(a) 3-PGA Correct
(b) malic acid Your answer
(c) glucose Your answer
(d) RuBP Your answer

Your answer: Correct answer: (a)

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Explanation

Why (a) is correct

In a C₃ plant, RuBisCO fixes CO₂ into 3-PGA first, so the label appears in 3-PGA within seconds. This is how Calvin identified the first product.

Why the other options are wrong

(b) · your answer Malic acid is labelled first in C₄ and CAM plants, not C₃ plants.
(c) · your answer Glucose is labelled only later.
(d) · your answer RuBP is the acceptor; it is labelled only after the cycle turns.

NCERT point: Short exposure to ¹⁴CO₂ labels the earliest products.

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