Photosynthesis

Green plants make their own food in a chemical process called photosynthesis.

Light energy from the Sun is transferred into chemical energy in glucose sugar.

Light energy is absorbed by a green substance called chlorophyll.  This is found inside some plant cells in structures called chloroplasts.

Plant cells that carry out a lot of photosynthesis, like the palisade cells in a leaf, contain many chloroplasts to absorb lots of light.

This is a diagram of a palisade cell.

The energy is used to convert carbon dioxide and water into glucose and oxygen.

Carbon dioxide is obtained from the air by the leaves.

Water is obtained from the soil through the roots.

The reaction can be summarised in the word equation:

The products of photosynthesis are important for all other organisms.

Oxygen is used by plants and animals in respiration.

The glucose is a source of energy for animals.

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Uses of Glucose

Some of the glucose produced is used by the plant for respiration.

The energy released is used to build up smaller molecules into larger ones:

• Excess glucose may be converted into insoluble starch for storage
• Sugars are converted into cellulose for cell walls
• Sugars, nitrates and other nutrients are converted into amino acids to make proteins
• Sugars are converted into lipids (fats and oils) for storage in seeds
  

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Mineral Requirements

Mineral salts are needed for healthy growth.

These are absorbed by the plant in the form of mineral ions from the soil.

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Photosynthesis Experiments

Experiments can be carried out to show that light, carbon dioxide and chlorophyll are all needed for photosynthesis.

If any one of these is removed, a plant cannot photosynthesise.

To see if a plant has photosynthesised, a leaf can be removed and tested for the presence of starch. 

• The leaf is killed by dipping it into boiling water. This stops all chemical reactions inside the leaf
• The green chlorophyll is removed by boiling the leaf in ethanol for a few minutes
• The leaf is softened and the ethanol is washed off in warm water
• Iodine solution is then put onto the leaf to test for the presence of starch

This is the result for a variegated leaf.

Only the central part of the leaf contained chlorophyll. After testing, only part of the leaf was stained black with iodine solution. This shows that chlorophyll is needed for photosynthesis.

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Rate of Photosynthesis

Several factors affect the rate of photosynthesis:

• Light intensity – the more light, the faster the rate
• Carbon dioxide concentration – the more carbon dioxide, the faster the rate
• Temperature – higher temperatures increase the rate of photosynthesis because it is controlled by enzymes. However, too high a temperature can denature enzymes, thus slowing the rate down

Whichever of these factors is in short supply will limit the rate of photosynthesis.

Line A on the graph shows that as light intensity is increased, the rate of photosynthesis increases.

Eventually, the line levels off and increasing the light intensity has no further affect on the rate.

However, line B shows that if the temperature is increased, the rate can be increased further. Therefore the temperature must have been the limiting factor.

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