3. Phloem in Plants

Question 1

Structure and Function of Phloem

Answer 1

To transport solutes eg sucrose around the plant
Formed from cells arranged in tubes
Sieve tube elements are living cells that form the tube for transporting solutes. They have no nucleus and few organelles

There is a companion cell for each sieve tube element. They carry out living functions for the sieve cells eg providing energy needed for the active transport of solutes

Question 2

Translocation

Answer 2

movement of solutes e.g. amino acids and sugars like sucrose to where they are needed in a plant. Solutes are sometimes called assimilates.

Energy is required.

Translocation moves solutes from ‘sources’ to ‘sinks’

The source is where the assimilates are produced

The sink is where they are used up

Question 3

How are the solutes moved in the phloem?

At the source

Answer 3

Solutes are actively loaded from the companion cells into the sieve tubes of the phloem
This lowers the water potential inside the sieve tubes so water enters the tubes by osmosis from the xylem and companion cells.
This creates a high pressure inside the sieve tubes at the source end of the phloem

Question 4

How are the solutes moved in the phloem?
At the sink

Answer 4

Solutes are removed from the phloem to be used (diffusion or active transport depending on the sink’s concentration of solute)
This increases the water potential inside the sieve tubes so water leaves by osmosis
This lowers the pressure inside the sieve tubes at the sink end

Question 5

How are the solutes moved in the phloem?

Flow

Answer 5

There is therefore a pressure gradient from source to sink. This gradient pushes the solutes along the sieve tubes towards the sink
The higher the concentration of solutes at the source, the higher the rate of translocation

Question 6

How do xylem and phloem differ?

Answer 6