37. An organized and differentiated cell having cytoplasm but no nucleus is found in
(A) Companion cell
(B) Xylem parenchyma
(C) Sieve tube element
(D) Phloem parenchyma

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In plant biology, understanding specialized cells in vascular tissues is crucial for topics like transport mechanisms and phloem structure. A common exam question asks: “An organized and differentiated cell having cytoplasm but no nucleus is found in” with options (A) Companion cell, (B) Xylem parenchyma, (C) Sieve tube element, (D) Phloem parenchyma.

The correct answer is (C) Sieve tube element. These cells are mature, functional units in the phloem that lose their nucleus during differentiation but retain cytoplasm for sucrose transport. Let’s break down all options with clear explanations, diagrams in mind, and why they’re right or wrong.

Why Sieve Tube Elements Fit Perfectly

Sieve tube elements are elongated, living cells in the phloem, stacked end-to-end to form sieve tubes. Key features include:

• No nucleus: During maturation, the nucleus degenerates, leaving the cell organized and differentiated.

• Cytoplasm present: A thin layer of cytoplasm (including plasmodesmata at sieve plates) enables mass flow of photosynthates.

• Function: Pressure-flow hypothesis relies on these nucleate-free cells for efficient sap transport without nuclear interference.

This makes sieve tube elements the classic example of a cell having cytoplasm but no nucleus. Without a companion cell’s nucleus, they couldn’t survive—highlighting their interdependence.

(Imagine a labeled diagram showing sieve plate, cytoplasm, and absent nucleus)

Analyzing All Options: Right and Wrong Choices

(A) Companion Cell – Incorrect

Companion cells are nucleated parenchyma-like cells adjacent to sieve tube elements. They have:

• Dense cytoplasm and a prominent nucleus.

• Role: Provide metabolic support (e.g., ATP, proteins) to the enucleate sieve tube.

These retain both nucleus and cytoplasm, so they don’t match a cell having cytoplasm but no nucleus.

(B) Xylem Parenchyma – Incorrect

Xylem parenchyma cells store food and aid radial transport in wood tissue. Characteristics:

• Fully nucleated with abundant cytoplasm.

• Living cells, unlike dead tracheids/vessels.

They possess a nucleus, disqualifying them as an organized cell without one.

(C) Sieve Tube Element – Correct (As Explained Above)

The defining choice: differentiated, cytoplasm-filled, but nucleus-free for optimal phloem conduction.

(D) Phloem Parenchyma – Incorrect

These are storage and support cells in phloem bundles, similar to ground tissue. They feature:

• Intact nucleus and cytoplasm.

• Functions like starch storage and lateral transport.

Nucleus presence rules them out.

Exam Relevance and Tips for Plant Biology Learners

This question tests phloem ultrastructure, often appearing in NEET, CSIR-NET, or ICAR exams. Remember: Sieve tubes = sieve tube elements + companion cells (the “dynamic duo”). Protoplasm persists post-nuclear degeneration via sieve pores.

For deeper study:

• Read Esau’s Plant Anatomy for micrographs.

• Visualize: Nucleus breakdown allows unobstructed flow, like a highway without toll booths.

Mastering cells having cytoplasm but no nucleus strengthens your grasp of secondary growth and translocation.