S-Cool Revision Summary

S-Cool Revision Summary

Structure of Leaves

The leaves are the part of a plant where most photosynthesis takes place.

Structure of leaves


  1. Waxy cuticle, this gives the leaf a waterproof layer, which lets in light.
  2. Upper epidermis - provides an upper surface.
  3. Palisade cells, which are packed full of chloroplasts.
  4. Spongy mesophyll. Collection of damp, loosely packed cells.
  5. Lower epidermis is the layer of cells on the lower surface.
  6. Air space inside the leaf, allows contact between air and moist cell surfaces.
  7. Stoma, a hole in the leaf which gases diffuse through.
  8. Guard cells, which change shape to close the stoma.

One amazing feature of leaves is that they have tiny holes in them to let the gases carbon dioxide and oxygen enter and exit. One of the holes is called a 'stoma', although just to be confusing, when there are more than one they are called 'stomata'.

A stoma is just a hole. It is controlled by two guard cells which change shape to either open or close the hole. Something makes water enter the cells by osmosis and so they swell up and change shape, but no-one is quite sure of the trigger.


Photosynthesis is the way that plants make their food using energy from sunlight.

What is the word equation for photosynthesis?

Carbon Dioxide + Water >>sunlight>> Sugar + Oxygen

Chlorophyll: green dye (or pigment) plants use to pick up the energy from the sunlight.

Plants make sugar and use some of it for energy to keep them alive (respiration) but they also use some for growth and repair by making fats and proteins.

Plants can use starch or glucose. Starch is insoluble (it does not dissolve in water) while glucose is soluble. This means that if starch is used, less water is required to keep its food stored.

Influencing Factors

Things which might effect how well plants can carry out photosynthesis. The amounts of water, carbon dioxide, sunlight and temperature.

Amount of water: effected by how much is taken up through the roots and how much is lost from the leaves. If less water is available in the leaf then photosynthesis will occur more slowly.

Carbon Dioxide: if there is less carbon dioxide around then photosynthesis will occur more slowly. There wont be enough of the fuel (substrate) to get the reaction to work

Sun: If there is less sun, which usually means it is cooler too, then there is less energy for photosynthesis and it occurs more slowly. So photosynthesis works best when it is warm and sunny - don't we all!

Photosynthesis and Transpiration

When plants have their stoma open during the day, they can let carbon dioxide and oxygen diffuse through them. However, water can also diffuse out from the moist inside of the leaf to the drier air outside. This movement of water is called 'transpiration'.

How do plants stop themselves drying out?

They close their stomata. This happens because the guard cells lose water too and go floppy. A very neat trick!