Phloem
Biology: Cambridge International A Level, topic 7.1.5
Structure
The phloem are plant vessels that are responsible for transporting sugars. The structure of the phloem is specialised for this role.
Sieve tube elements
- Sieve tube elements are the cells that make up the phloem vessel.
- The vessel consists of many sieve tube elements that run up the stem on top of one another.
- Sieve tube elements are living cells that contain a cytoplasm but no nucleus.
- The walls of sieve tube elements are made of cellulose.
Companion cells
- Companion cells are connected to sieve tube elements via plasmodesmata.
- Plasmodesmata allows the cytoplasm to be shared between companion cells and sieve tube elements.
- Companion cells have a nucleus.
Sieve plates
- At either end of the sieve tube elements are sieve plates.
- Sieve plates have large pores that allow sap to move through the sieve tube elements.
- Sieve plates allow sugars to be transported through the phloem.
Translocation
Translocation is the process where sugars produced in photosynthesis are transported from the leaves to other parts of a plant. The steps involved are:
1) Active transport of sucrose
- Sucrose is produced during photosynthesis in the leaves.
- The leaves are called the source.
- Sucrose is actively transported into the companion cells in the phloem.
- Energy is provided for active transport from mitochondria in the companion cells.
2) Diffusion of sucrose
- The loading of sucrose into the phloem causes the concentration of sucrose to increase.
- Sucrose diffuses from the companion cells into the sieve tube elements.
3) Osmosis
- The increase in the concentration of sucrose in the phloem causes water potential in the phloem to decrease.
- There is a water potential gradient between the outside of the phloem and the inside.
- Water diffuses into the phloem by osmosis.
4) Unloading of sucrose
- At the bottom of the phloem, sucrose concentration is low because it is being used up in the cells.
- The cells at the bottom of the phloem are called the sink.
- Sucrose diffuses out of the phloem and into the sink cells.
- This lowers the water potential of the sink cells.
5) Osmosis
- Water diffuses down its water potential gradient out of the phloem by osmosis.
- The diffusion of water into the phloem at the source and out of the phloem at the sink creates a hydrostatic pressure gradient.
- The hydrostatic pressure gradient allows mass transport of sucrose from source to sink.
- This is called mass flow.
Test yourself on Phloem
Which of the following statements about phloem structure are true?
What is the role of sieve plates?
Which of the following statements about translocation are true?
During translocation, what type of cell is sucrose actively transported into?
Steps Involved in Translocation