Transport in Plants - Phloem

Question 1

What are sieve tube elements and companion cells?

Answer 1

Sieve tube elements are living cells that form the tube for transporting solutes. There’s a companion cell for each sieve tube element that carry out the living functions for the sieve cells

Question 2

What is translocation?

Answer 2

Translocation is the movement of solutes to where they’re needed in a plant. Solutes are sometimes called assimilates

Question 3

Where does translocation move solutes?

Answer 3

From ‘sources’ to ‘sinks’. Sources are where solutes are made, sinks are where they’re used up

Question 4

What maintains a conc gradient from source to sink?

Answer 4

Enzymes - they change the solutes at the sink, ensuring there’s always a lower conc at the sink than the source

Question 5

What theory explains translocation?

Answer 5

Mass flow hypothesis

Question 6

Explain the mass flow hypothesis

Answer 6

1. Active transport loads the solutes from companion cells into sieve tubes at the source of the phloem
2. This lowers water potential inside the sieve tubes, so water enters tube via osmosis from xylem and companion cells
3. This creates high pressure inside sieve tube at source end of phloem
4. At the sink end solutes are removed from phloem and used
5. This increases water potential inside the sieve gives, so water leaves the tubes by osmosis
6. This lowers pressure inside the sieve tubes
7. This creates a pressure gradient from source to sink which pushes solutes along the sieve tubes from source to sink
8. When they reach the sink solutes will be used up