7. Transport in Plants - Phloem

What does phloem do?

transports organic solutes (sucrose).

Draw and label a phloem diagram

Describe the structure of it

• Made up of sieve tube elements which are living cells that form the tube for transporting solutes. They have no nucleus and a few organelles.

• There’s a companion cell for each sieve tube element. They carry out living functions for sieve cells, e.g. providing energy needed for the active transport of solutes.

Sieve plates

inside the sieve tube

Translocation

The movement of solutes to where they’re needed in a plant.

Solutes are sometimes called assimilates.

translocations moves… to …

sources to sinks

sink

where the assimilates are used up and therefore is a lower concentration of them.

Describe the mass flow hypothesis: 3

1. Source

   • Solutes are actively transported from the companion cells to the sieve tube elements. This lowers the water potential of the sieve tube elements, causing water to move into the tube from the xylem by osmosis.

2. Sink

   • The solutes are removed from the phloem and used up in the sink, increasing the water potential of the sieve tubes, so water leaves the tubes by osmosis, lowering the pressure inside the tubes.

3. Flow

   • The result is a pressure gradient from the source end to the sink end. This gradient pushes solutes along the sieve tubes towards the sink.

Evidence supporting mass flow

• If a ring of bark is removed from the woody stem, the top bit starts to bulge showing that there is a higher concentration at the top of the tree than the bottom, showing evidence of downward movement of sugars.

• You can cut phloem closer to the leaves and at the bottom and sap flows faster at the top than at the bottom.

• radioactive tracer can be used to track the movement of organic substances in a plant, such as radioactive carbon.

• If a metabolic inhibitor is used to stop ATP production, translocation stops, showing evidence for active transport

Evidence against mass flow hypothesis

• sugar travels to many different sinks, not just not the one with the highest water potential as the model shows.

• The sieve plates would create a barrier to mass flow. A lot of pressure would be needed for the solutes to get through at a reasonable rate.