Discussion
Which one of the following mature cell types is live but usually lacks nucleus?
Answer: C
The cell type is the phloem sieve element.
As a sieve element matures it loses its nucleus, vacuole membrane, ribosomes and most other organelles, leaving an open conduit for the flow of sugars through the sieve plates. It stays alive, with a functional plasma membrane, because the neighbouring companion cell, joined to it by many plasmodesmata, supplies the proteins and energy it can no longer make.
Phloem parenchyma, companion cells and phloem-pole pericycle cells are ordinary living cells that keep their nuclei; the companion cell in particular is nucleus-rich and metabolically very active, precisely because it supports the enucleate sieve element.
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