Additional Science-Biology Chapter 2

GCSE AQA Additional Science-Biology Chapter 2

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  • Created by: Mel
  • Created on: 24-05-12 10:51


The equation for photosynthesis is:


The carbon dioxide is taken in by the leaves and the water by the roots.

During photosynthesis, light energy is absorbed by the chlorophyll in the chloroplasts in the leaves. The energy is then used to convert carbon dioxide from the air plus water from soil into glucose. In photosynthesis the sugar glucose (carbohydrate) is made. Oxygen is given off as a waste gas.

Iodine solution is a yellowy-brown liquid which turns dark blue when it reacts with starch.

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Plant Leaf Adaptations

  • Most leaves are broad, they have a big surface area for light to fall on
  • They contain chlorophyll in the chloroplasts to absorb the light energy
  • They have air spaces which allow carbon dioxide to get to the cells and oxygen to leave them
  • They have veins, which bring water to the cells of the leaves
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Plant Leaf Structure


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Factors limiting photosynthesis

A lack of light would slow down the rate of photosynthesis as light provides the energy for the process.

rate of photosynthesis plotted against light intensity. the rate begins to slow as the light intensity continues to increase (

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Factors limiting photosynthesis

If there is too little carbon dioxide, then the rate will slow down. Carbon dioxide may be limited in an enclosed space, e.g. in a greenhouse on a sunny day or in a rapidly photosynthesising rain forest. The atmosphere contains about 0.04% CO2. The CO2 levels around a plant tend to rise in the night as it respires but doesn't photosynthesise.

rate of photosynthesis plotted against carbon dioxide concentration. the rate begins to slow as the carbon dioxide concentration continues to increase (

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Factors limiting photosynthesis

Temperature affects all chemical reactions, including photosynthesis. Photosynthesis is controlled by enzymes because it takes place in living organisms. Enzymes are destroyed once the temperature rises to around 40°C to 50°C.

rate of photosynthesis plotted against temperature. the rate begins to slow as the temperature continues to increase (

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How Plants Use Glucose

Glucose is the product of photosynthesis used for respiration and to provide energy for cell functions, growth and reproduction. Glucose is combined with other nutrients by the plant to produce new materials.

Glucose is stored by some plants as insoluble starch so that it has no effect on osmosis. Plants also build up sugars into more complex carbohydrates like cellulose which is used to make new plant cell walls.

Starch is also stored in special storage areas of a plant. Many plants produce tubers and bulbs which are full of starch to help them survive through the winter.

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Transport and Storage in Plants

There are two separate transport systems in plants. The phloem is made up of living tissue. It transports sugars made by photosynthesis from the leaves to the rest of the plant which are carried to all areas of the plant.

Xylem is the other, it carries water and mineral ions from the soil around the plant.


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Why Do Plants Need Minerals?

Plant roots take up mineral salts for healthy growth. Nitrates are taken from the soil for producing amino acids which are used to make proteins for growth.

A plant that does not take up enough nitrate (nitrate deficient) will have stunted growth.

Plants also take up magnesium that is essential to produce chlorophyll. If the plant is deficient in chlorophyll it will have yellow and pale leaves.

Glucose and starch are made of carbon, hydrogen and oxygen. Proteins are made of amino acids containing carbon, hydrogen, oxygen and nitrogen.

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