6.3 - 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:
  1. Xylem - Delivers water and dissolved mineral ions to the leaf.
  2. 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.

The leaf adapted for gas exchange

As well as capturing light, a leaf must let gases move in and out efficiently. Several features adapt it for gas exchange.

Spongy mesophyll and air spaces

• The spongy mesophyll contains large air spaces between its cells.
• These air spaces give a large internal surface area and let gases like carbon dioxide and oxygen move freely between the cells.
• This speeds up the rate of diffusion to and from the photosynthesising cells.

Stomata in the lower epidermis

• The lower epidermis is dotted with tiny pores called stomata (a single one is a stoma).
• They let carbon dioxide diffuse into the leaf for photosynthesis, and oxygen (a product of photosynthesis) diffuse out.
• Most stomata are on the lower surface, which is shaded and cooler, so less water is lost through them than would be lost from the upper surface.

Broad and thin shape

The large surface area and the short distance gases have to travel both make gas exchange faster.

Guard cells and the stomata

• Each stoma (pore in the leaf) is bordered by a pair of guard cells.
• The guard cells open and close the pore by changing shape.

Why the stomata open and close

• Open - Lets carbon dioxide and oxygen be exchanged for photosynthesis.
• Closed - Reduces the amount of water vapour escaping from the leaf.

When the stomata open and close

• During the day - Guard cells tend to open the stomata, when light is available for photosynthesis.
• At night, or when short of water - Guard cells close the stomata when the plant is losing water faster than the roots can replace it.
• This is why a plant that is short of water closes its stomata rather than wilt and dry out.