Q.48 Which one of the following mature cell types is live but usually lacks nucleus?
(A) Phloem parenchyma
(B) Phloem companion
(C) Phloem sieve element
(D) Phloem–pole pericycle

The correct answer is (C) Phloem sieve element.​

Mature phloem sieve elements remain alive to conduct sugars but lose their nucleus during development, relying on adjacent companion cells for metabolic support. This enucleation maximizes flow efficiency in phloem transport.​

Option Analysis

Phloem Parenchyma (A): These storage cells retain a prominent nucleus, dense cytoplasm, and protoplast at maturity for food storage and lateral transport.​

Phloem Companion (B): Companion cells maintain a nucleus, dense cytoplasm, ribosomes, and mitochondria to support sieve elements metabolically via plasmodesmata.​

Phloem Sieve Element (C): These conducting cells are live with peripheral cytoplasm and sieve plates but lack a nucleus, ribosomes, and tonoplast for unobstructed sap flow.​

Phloem-Pole Pericycle (D): Pericycle cells adjacent to phloem poles have nuclei and show cell cycle activity, often initiating lateral roots.​

Phloem sieve elements represent the key mature cell type that is live but lacks nucleus, essential for sugar transport in plants and a frequent CSIR NET Life Sciences topic. These cells undergo programmed enucleation to form efficient conduits.​

Phloem Cell Functions

Phloem conducts photoassimilates via sieve tubes, formed by stacked sieve elements connected by sieve plates. Parenchyma stores reserves, companion cells provide ATP, and pericycle supports root development.​

Why Sieve Elements Lack Nucleus

During maturation, sieve elements degrade nucleus, ribosomes, and vacuole to minimize flow resistance while retaining cytoplasm for P-proteins that seal wounds. Companion cells supply mRNA and proteins through plasmodesmata.​

• Live status confirmed by active transport and responsiveness.​

• Gymnosperms use sieve cells similarly, lacking tubes.​