Photosynthesis

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

If you cut a leaf in half and look at the cut end you will see something like this:

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Key:

  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.

See the section on for more details on plant structure.

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'.

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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.

Why do the swollen cells bend?

One side is thicker than the other, a bit like having a piece of tape on the side of long balloon and then blowing it up. Try this at home!

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The stomata (air holes) on plants are normally open during the day and closed at night. Photosynthesis happens during the day.

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

What is the word equation for photosynthesis?

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Plants use the green dye (or pigment) called chlorophyll 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.

However, it is not always sunny so plants need to be able to store some ofthe sugar they make, so they convert it to a storage carbohydrate.

Why bother storing it as starch?

Plants could 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.

What things might effect how well plants can carry out photosynthesis?

The amounts of water, carbon dioxide, sunlight and temperature.

The amount of water is 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.

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Similarly, 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.

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!

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How could you measure how quickly photosynthesis is occurring?

You could wait ages and then see how much glucose is in the plant. Boring and slow!

Alternatively, one favourite way is to measure how much oxygen gas is given off. This can be done easiest by counting the bubbles of gas as they come off a water plant like the pondweed, Elodea.

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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'?.

Transpiration is a bit like the sweat on our skin and may also help to cool the leaves. Large trees may lose nearly 200 litres of water a day by transpiration.

On a hot sunny day plants could become dehydrated as they lose water even though photosynthesis works best in these conditions. They cannot draw up water from their roots quickly enough so photosynthesis starts to slow down. A big problem!

So 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!

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