Biology-AQA-Additional Science- How plants produce food

Photosynthesis is the chemical change which happens in the leaves of green plants. 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.

carbon dioxide + water (+light energy) --------> glucose +oxygen

Plants absorb water through their roots and carbon dioxide through their leaves. Some glucose is used for respiration while some is converted into insoluble starch for storage.

The structure of a leaf

There are four main parts to a leaf which is the upper epidermis, palisade layer, spongy layer and the lower epidermis. In the upper epidermis there are waxy cuticles which is a waterproof layer which stops water loss. In the palisade layer there are palisade cells at the top of the leaf which is closer to the light and is tightly packed together and is full of chloroplasts. Between the palisade layer and spongy layer there are air spaces and in the spongy layer the cells are not tightly packed and have a large surface area available for gas exchange and some chloroplasts. In the lower epidermis the guard cells open and close the stomata to control water loss.

Light intensity, carbon dioxide concentration and temperature are three factors which can limit the speed of photosynthesis.

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.

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.

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

The product of photosynthesis is glucose. Glucose is used for respiration.

Glucose is also combined with other nutrients (mineral ions) by the plant to produce new materials.

Glucose is stored, by some plants, as insoluble starch. It is stored as an insoluble substance so that it has no effect on osmosis.

Key points

Plants use glcose for energy(respiration)

Most plants can store glucose as starch

Plants need a number of elements to stay alive. The most important are hydrogen, oxygen and carbon. Plants get hydrogen and oxygen from the water in the soil and carbon and oxygen from carbon dioxide. and oxygen in the atmosphere. Water and carbon dioxide are used to synthesise food during photosynthesis. Oxygen is used to release energy from food during respiration.

Plants need minerals for healthy growth which are absorbed through the roots as mineral ions dissolved in the soil water. The important minerals needed are nitrate and magnesium. Nitrates are taken from the soil for producing amino acids. These are used to make proteins for growth. A plant that does not take up enough nitrate (is nitrate deficient) will have stunted growth. Magnesium ions are essential to make chlorophyll. If the plant is deficient in chlorophyll it will have yellow leaves.

Key points - Plants produce sugars through photosynthesis. However, just like animals, they need other substances to grow properly.