Photosynthesis

Photosynthesis is the chemical change which happens in the leaves of green plants. It is the first step towards making food, not just for plants but ultimately every animal on the planet as well. During this reaction, carbon dioxide and water are converted into glucose and oxygen. The reaction requires light energy, which is absorbed by a green substance called chlorophyll.

Photosynthesis takes place in leaf cells. These contain chloroplasts - tiny objects that contain chlorophyll.

Here are the equations for photosynthesis:

Higher tier only for the following equation

Products of photosynthesis

Glucose is soluble. It is transported in the plant as soluble sugars but stored as starch - which is insoluble, so it cannot escape from the cells. The stored starch can be turned back into glucose later and used to release energy by respiration. Starch and glucose can also be used by the plant to make:

• cellulose for their cell walls
• proteins for growth and repair
• fats and oils for storage by the plant

Factors affecting photosynthesis

Three factors can limit the speed of photosynthesis: light intensity, carbon dioxide concentration and temperature.

Light intensity

Without enough light, a plant cannot photosynthesise very quickly, even if there is plenty of water and carbon dioxide. Increasing the light intensity will boost the speed of photosynthesis.

Carbon dioxide concentration

Sometimes photosynthesis is limited by the concentration of carbon dioxide in the air. Even if there is plenty of light, a plant cannot photosynthesise if there is insufficient carbon dioxide.

Temperature

If it gets too cold, the rate of photosynthesis will decrease. Plants cannot photosynthesise if it gets too hot.

If you plot the rate of photosynthesis against the levels of these three limiting factors, you get graphs like the ones above.

In practice, any one of these factors could limit the rate of photosynthesis.

Maximising growth

Farmers can use their knowledge of these limiting factors to increase crop growth in greenhouses. They may use artificial light so that photosynthesis can continue beyond daylight hours, or in a higher-than-normal light intensity. The use of paraffin lamps inside a greenhouse increases the rate of photosynthesis because the burning paraffin produces carbon dioxide, and heat too.

Respiration

To unlock the energy in the carbohydrate [carbohydrate: Food belonging to the food group consisting of sugars, starch and cellulose. It is vital for energy in humans, and is stored as fats if eaten in excess. In plants, carbohydrates are important for photosynthesis. ] produced in photosynthesis, green plants need to respire [respire: to engage in respiration - the energy-producing process inside living cells ], just as animals do. Respiration takes place in the plant's cells, using oxygen to produce energy and giving off carbon dioxide as a waste product. So in terms of the gas taken in and the gas given out, respiration is the opposite of photosynthesis.

The result is that during the day when the plant is both respiring and photosynthesising there is a two-way traffic of oxygen and carbon dioxide both into and out of the plant. During the night when the plant is respiring but not photosynthesising, oxygen is being taken in but not given out - and carbon dioxide is being given out but not taken in.

Luckily, plants use up more carbon dioxide in photosynthesis than they produce in respiration, and produce more oxygen while photosynthesising than they use up while respiring - otherwise there would not be enough oxygen in the atmosphere for us animals to breathe!

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