Leaf structure

Last time

Before we start: last time, in Investigating photosynthesis, limiting factors and minerals, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

A food factory

A leaf is thin, flat and green, and every part of it is there for a reason. It is a factory for photosynthesis: it must catch light, take in carbon dioxide, and get water in and sugar out. Today: each layer of the leaf and what it is adapted for.

The outside of a leaf

A leaf is broad, giving a large surface area to absorb as much light as possible. A leaf is thin, so carbon dioxide only has a short distance to diffuse to the cells. Veins carry water in and sugars out, and also support the leaf.

The cross-section

Here is a cross-section of a leaf. From the top: the waxy cuticle, the upper epidermis, the palisade mesophyll, the spongy mesophyll with air spaces, the lower epidermis with stomata and guard cells. A vascular bundle runs through the middle.

The top layers

The waxy cuticle is waterproof, so it reduces water loss by evaporation. The upper epidermis is one layer of transparent cells with no chloroplasts, so light passes straight through.

Palisade mesophyll

The palisade mesophyll is just under the upper epidermis, where the light is brightest. Its cells are long, closely packed and full of chloroplasts. So this is where most photosynthesis takes place.

Spongy mesophyll

The spongy mesophyll has round, loosely packed cells with large air spaces between them. The air spaces let carbon dioxide diffuse quickly to the cells, and oxygen out. The cells also have a large surface area, wet with water, for gas exchange.

Stomata and guard cells

Stomata are tiny pores, mostly in the lower epidermis. Carbon dioxide diffuses in through them, and oxygen and water vapour diffuse out. Each stoma is surrounded by two guard cells, which open it in light and close it in the dark.

The vascular bundle

The vein is a vascular bundle, containing xylem and phloem. Xylem brings water and mineral ions to the leaf. It is usually on the upper side of the bundle. Phloem carries sucrose and amino acids away to the rest of the plant. It is on the lower side.

Structure and function

Here is the table to learn. Each structure, and how it helps photosynthesis. The examiner always wants the feature AND its function.