Xylem & Phloem
A xylem vessel
Xylem cells lose their top and bottom walls to form a continuous tube through which water moves through from the roots to the leaves
- Function: transport tissue for water and dissolved mineral ions
- Adaptations:
- No top and bottom walls between cells to form continuous hollow tubes through which water is drawn upwards towards the leaves by transpiration
- Cells are essentially dead, without organelles or cytoplasm, to allow free passage of water
- Outer walls are thickened with a substance called lignin, strengthening the tubes, which helps support the plant
Phloem cells
Phloem cells form tubes similar to xylem vessels, except the cells still retain some subcellular structures and are therefore living
- Function: transport of dissolved sugars (e.g. sucrose) and amino acids
- Adaptations:
- Made of living cells (as opposed to xylem vessels, which are made of dead cells) that are supported by companion cells
- Cells are joined end-to-end and contain holes in the end cell walls (sieve plates) forming tubes which allow sugars and amino acids to flow easily through (by translocation)
- Cells also have very few subcellular structures to aid the flow of materials
Exam Tip
You may be given some information (including an image) about an unfamiliar cell in an exam, and asked to describe how it’s able to carry out its function. This shouldn’t faze you – just look at the shape of the cell and its subcellular structures.
Does the cell have a shape which increases its surface area? Are there lots of ribosomes to make proteins (such as enzymes or hormones), or lots of mitochondria (to transfer lots of energy via respiration)?
