Phloem Structure
Biology: IB Diploma Higher Level, topic 7.2.8
Phloem 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.
Test yourself on Phloem Structure
Which of the following statements about phloem structure are true?
What is the role of sieve plates?