Xylem and phloem

Overview

What xylem and phloem transport and in which direction, the structure of a xylem vessel, lignin and why dead empty xylem cells help, sieve plates, living sieve tube cells and their companion cells, why transport in phloem needs energy, and how movement in phloem differs from movement in xylem.

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

   What does xylem transport, and in which direction?

   Model answer

   Water and dissolved mineral ions, upwards from the roots to the leaves.

2. 2

   What does phloem transport, and where does it come from?

   Model answer

   Dissolved sugars, from the leaves where they are made to the rest of the plant.

3. 3

   Describe the structure of a xylem vessel.

   Model answer

   It is a column of dead cells stacked end to end with their end walls broken down, forming a continuous hollow tube.

4. 4

   What is lignin and why is it useful in xylem?

   Model answer

   It is a tough substance in the vessel walls that strengthens and waterproofs them, holding the tube open and supporting the plant.

5. 5

   Why is it an advantage that xylem cells are dead and empty?

   Model answer

   There is nothing inside to obstruct the water, so it can flow freely up the tube.

6. 6

   What are sieve plates?

   Model answer

   They are the perforated end walls between phloem cells, which let the sugar solution pass from one cell into the next.

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