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3.8.1 State that photosynthesis involves the conversion of light energy into chemical energy.

Photosynthesis involves the conversion of light energy into chemical energy.

3.8.2 State that light from the Sun is composed of a range of wavelengths (colours).

The light from the sun is composed of a range of wavelengths (colours).

3.8.3 State that chlorophyll is the main photosynthetic pigment.

Chlorophyll is the main photosynthetic pigment.

3.8.4 Outline the differences in absorption of red, blue and green light by chlorophyll.

Chlorophyll can absorb red and blue light more than green. Chlorophyll cannot absorb green light and so instead reflects it making leaves look green.

3.8.5 State that light energy is used to produce ATP, and to split water molecules (photolysis) to form oxygen and hydrogen.

Light energy is used to produced ATP and to split water molecules (photolysis) to form oxygen and hydrogen.

3.8.6 State that ATP and hydrogen (derived from the photolysis of water) are used to fix carbon dioxide to make organic molecules.

ATP and hydrogen derived from photolysis of water are used to fix carbon dioxide to make organic molecules.

3.8.7 Explain that the rate of photosynthesis can be measured directly by the production of oxygen or the uptake of carbon dioxide, or indirectly by an increase in biomass.

Photosynthesis can be measured in many ways as it involves the production of oxygen, the uptake of carbon dioxide and an increase in biomass. For example, aquatic plants release oxygen bubbles during photosynthesis and so these can be collected and measured. The uptake of carbon dioxide is more difficult to measure so it is usually done indirectly. When carbon dioxide is absorbed from water the pH of the water rises and so this can be measured with pH indicators or pH meters. Finally, photosynthesis can be measured through an increase in biomass. If batches of plants are harvested at a series of times and the biomass of these batches is calculated, the rate increase in biomass gives an indirect measure of the rate of photosynthesis in the plants.

3.8.8 Outline the effects of temperature, light intensity and carbon dioxide concentration on the rate of photosynthesis.

As temperature increases, the rate of photosynthesis increases more and more steeply until the optimum temperature is reached. If temperature keeps increasing above the optimum temperature then photosynthesis starts to decrease very rapidly.

As light intensity increases so does photosynthesis until a certain point. At a high light intensities photosynthesis reaches a plateau and so does not increase any more. At low and medium light intensity the rate of photosynthesis is directly proportional to the light intensity.

As the carbon dioxide concentration increases so does the rate of photosynthesis. There is no photosynthesis at very low levels of carbon dioxide and at high levels the rate reaches a plateau.

8.2.2 State that photosynthesis consists of light-dependent and light- independent reactions.

Photosynthesis consists of light-dependent and light-independent reactions.

8.2.3 Explain the light-dependent reactions.

Photosynthesis occurs inside chloroplasts. Chloroplasts contain chlorophyll, a green pigment found…


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