Phloem: Sieve Tubes, Companion Cells and Parenchyma
Biology · Anatomy of Flowering Plants · NEET
Phloem is a complex tissue that carries food (sugar) made in leaves to the rest of the plant. It has four parts: sieve tube elements, companion cells, phloem parenchyma, and phloem fibres. Memory hook: think "SCPF" — Sieve tube, Companion, Parenchyma, Fibre. Key NEET fact: a mature sieve tube element has NO nucleus, so the living companion cell next to it controls its work.
Phloem has four parts. Sieve tube elements (linked by sieve plates) carry food but have no nucleus; the companion cell beside them has a nucleus and controls them. Phloem parenchyma is living and stores food, while phloem fibres are dead with thick walls.
Your doubts, answered
Does the sieve tube element have a nucleus?
No. A mature sieve tube element loses its nucleus and most organelles at maturity. It is still a LIVING cell, but it cannot control itself. The nearby companion cell (which has a nucleus) controls it. NEET loves this: the sieve tube is 'enucleate' but 'living'. Do not confuse enucleate with dead.
Why is phloem called living tissue if sieve tubes have no nucleus?
Because most phloem cells are living. Sieve tube elements, companion cells and phloem parenchyma are all living. Only phloem fibres are dead (they have thick lignified walls). So phloem is mostly living tissue. Compare this with xylem, which is mostly dead.
What is the difference between a sieve tube and a companion cell?
A sieve tube element is a long tube-like cell that carries food and has NO nucleus at maturity. A companion cell is a small, thin cell with a dense cytoplasm and a nucleus. It sits beside the sieve tube and controls it. They are connected by pit-fields (plasmodesmata). One companion cell keeps the enucleate sieve tube alive and working.
What do sieve plates do?
Sieve tube elements are joined end to end. The end walls have many pores and are called sieve plates. Food (sugar solution) moves from one cell to the next through these pores. Think of sieve plates as strainers that let sap flow up and down the plant.
Is phloem parenchyma present in monocots?
No, phloem parenchyma is absent in most monocots (like the monocot stem). This is a common NEET point. Phloem parenchyma is made of living cells that store food and other substances like resin and mucilage, and it is found in dicots but absent in monocots.
⚠️ The NEET trap ✗ Mature sieve tube elements have a clear nucleus and normal cytoplasmic organelles. ✓ Mature sieve tube elements have NO nucleus and lose most organelles; the companion cell controls them. 🧠 Sieve tube = living but headless. It works, but the companion cell is its 'brain'. NEET 2019/2022 asked exactly this as the WRONG statement.
Real NEET questions
NEET 2019 / 2022
Which of the following is an incorrect statement?
A · The perinuclear space forms a barrier between the materials present inside the nucleus and that of the cytoplasm
B · Nuclear pores act as passages for proteins and RNA molecules in both directions between nucleus and cytoplasm
C · Mature sieve tube elements possess a conspicuous nucleus and usual cytoplasmic organelles ✓
D · Microbodies are present both in plant and animal cells
Solution: A mature sieve tube element is a living but enucleate cell — it loses its nucleus and most organelles at maturity, and the adjacent companion cell controls its functions. So statement (C) is incorrect. The other three statements are correct.
Solved Anatomy of Flowering Plants NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Only phloem fibres are dead. The sieve tube elements, companion cells and phloem parenchyma are all living.
What is the function of phloem?
Phloem carries food (mainly sucrose) made in the leaves to storage organs and growing parts. This movement of food is called translocation, and it can go both up and down the plant.
Why does the sieve tube depend on the companion cell?
Because the sieve tube has no nucleus at maturity, it cannot make its own proteins or control its activity. The companion cell, which has a nucleus, does this job for it. They are linked by plasmodesmata.
Is phloem parenchyma present in monocots?
No. Phloem parenchyma is usually absent in monocots and present in dicots. This is a frequently tested NEET point.
