Phloem: sieve-tube elements and companion cells

Overview

Function of phloem tissue, how a sieve tube is formed and sieve plate.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   State the function of phloem tissue.

   Model answer

   Phloem transports dissolved organic solutes such as sucrose and amino acids through the plant.

2. 2

   Describe how a sieve tube is formed.

   Model answer

   It is a column of elongated living cells called sieve-tube elements, joined end to end through perforated sieve plates.

3. 3

   What is a sieve plate?

   Model answer

   A sieve plate is the perforated end wall between two sieve-tube elements, through which the contents flow from one to the next.

4. 4

   Which organelle is absent from a mature sieve-tube element?

   Model answer

   A mature sieve-tube element has no nucleus.

5. 5

   Describe the cytoplasm of a mature sieve-tube element.

   Model answer

   It is reduced to a thin layer pressed against the cell wall, with few organelles, leaving the centre of the cell largely open.

6. 6

   Why is a sieve-tube element described as living even though it lacks a nucleus?

   Model answer

   It retains its cell surface membrane and a layer of cytoplasm, and it depends on active processes to keep working.

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