3.3 - Leaf Structure
The leaf's tissues
A leaf is a plant organ, built from several different tissues, each suited to a particular job.
Epidermis
- A thin layer of cells covering the top and bottom of the leaf.
- The upper epidermis is transparent, so light passes straight through it to the cells below.
- Apart from the guard cells around the stomata (pores), the epidermal cells contain no chloroplasts.
Palisade mesophyll
- Sits just under the upper epidermis.
- A layer of tall cells packed with chloroplasts.
- Because it's near the top of the leaf, it receives the most light - so this is where most photosynthesis happens.
Spongy mesophyll
- Sits below the palisade layer.
- A looser layer of more rounded cells with large air spaces between them.
- The air spaces let gases such as carbon dioxide and oxygen diffuse easily to and from the cells.
Vascular bundles
- Run through the leaf and make up the vein. They contain two transport tissues:
- Xylem - Delivers water and dissolved mineral ions to the leaf.
- Phloem - Carries away the sugars made in photosynthesis.
- The vascular bundles also help to support and hold the leaf together.
How the leaf is adapted for photosynthesis
The whole structure of a leaf is built around capturing light for photosynthesis, and many of its features are adaptations for exactly that.
Overall shape
- Broad - Gives a large surface area to catch as much light as possible.
- Thin - Light and gases only have to travel a short distance inside the leaf.
Palisade mesophyll and upper epidermis
- The palisade mesophyll sits in a layer near the top of the leaf and is packed with chloroplasts.
- Putting these chloroplast-rich cells near the upper surface means they receive the most light - so most photosynthesis happens there.
- Above them, the upper epidermis is transparent, so light passes straight through to the palisade layer rather than being blocked.
The vein
- The leaf contains a network of veins - the vascular bundles - made up of xylem and phloem.
- Xylem - Brings water to the leaf for photosynthesis.
- Phloem - Carries away the sugars the leaf makes, mainly as sucrose.
- Together they also support the leaf's structure.
The waxy cuticle
- A thin, waxy cuticle covers the upper and lower surfaces.
- It is transparent, so it does not block light.
- It helps reduce water loss from the leaf by evaporation.
TermDefinition
Palisade mesophyllA layer of tall, chloroplast-rich cells near the top of the leaf where most photosynthesis happens.
Spongy mesophyllA layer of rounded cells in a leaf, with air spaces between them that let gases diffuse easily.
XylemA plant transport tissue that carries water and dissolved mineral ions from the roots to the stems and leaves.
PhloemA plant transport tissue made of living cells that carries dissolved sugars and amino acids around the plant.