Given below are two statements: one is labelled as Assertion (A) and the other as Reason (R):
Assertion (A): Sieve tube elements in phloem are living cells but lack nuclei at maturity.
Reason (R): Companion cells control the metabolic activities of sieve tube elements through plasmodesmata, ensuring the continuous pressure gradient necessary for phloem loading and unloading of sugars.
In the light of the above statements, choose the most appropriate answer from the options given below:
- ABoth (A) and (R) are correct and (R) is the correct explanation of (A)
- B(A) is correct but (R) is not correct
- CBoth (A) and (R) are not correct
- DNone of the above
Show answer and explanation
Correct answer: (a) Both (A) and (R) are correct and (R) is the correct explanation of (A)
Explanation
Both statements are true, and (R) explains (A).
- (A): sieve tube elements are living cells, but as they mature they lose their nucleus, ribosomes and vacuole. This leaves an open tube through which sugars can flow easily.
- (R): without a nucleus, a sieve tube element cannot keep itself alive. Its neighbouring companion cell, joined to it by many plasmodesmata, has a nucleus and many mitochondria. It carries out the metabolism of the sieve tube, including the active loading and unloading of sugars that keeps the pressure flow going.
So the sieve tube can lose its nucleus only because the companion cell supports it.