• Created by: holly6901
  • Created on: 25-03-19 19:24


Photosynthesis takes place inside chloroplasts found in plants and algae. The reaction is endothermic, meaning that it requires energy. The source of this energy is sunlight, which is trapped by a chemical called chlorophyll inside chloroplasts in plant cells.

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Equation for photosynthesis


During photosynthesis, carbon dioxide reacts with water to produce glucose and oxygen.

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Factors that affect the rate of photosynthesis 1

Carbon dioxide

  • Increasing carbon dioxide concentration increases the rate of photosynthesis.
  • However, above a certain threshold, further increases in the carbon dioxide concentration do not increase the rate of photosynthesis because another factor (such as light intensity) is limiting the rate of reaction.


  • Increasing the temperature increases the rate of photosynthesis because more energy is provided.
  • However, if the temperature is increased to above about 45°C, the enzymes that catalyse (speed-up) the reaction begin to denature (not work anymore).
  • This causes the rate of the reaction to drop sharply until it stops altogether.
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Factors that affect the rate of photosynthesis 2

Chlorophyll concentration

  • High chlorophyll concentration gives a high rate of photosynthesis

Light intensity

  • Increasing light intensity increases the rate of photosynthesis because more energy is provided.
  • However, if the light intensity is increased above a certain threshold, the rate of photosynthesis will not increase because another factor (such as temperature) is limiting the rate of the reaction.
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Testing the Rate of Photosynthesis

  • You can easily investigate the effect of light intensity on the rate of photosynthesis by using an aquatic (lives in water) plant like pondweed.
  • To do this, change the distance between the lamp and the pondweed and count the number of bubbles produced.
  • In this experiment, the light intensity is the independent variable and the number of bubbles is the dependent variable.
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Uses of Glucose


  • Starch is insoluble (cannot be dissolved) and is stored in stems, leaves or roots.


  • Cellulose is used to strengthen cell walls. It is particularly important for fast-growing plants.


  • Proteins are needed for cell growth and repair.
  • A source of nitrogen is also needed for glucose to be converted into proteins.
  • Plants get this nitrogen by absorbing nitrate ions (minerals) from the soil.

Fats and oils

Fats and oils are stored in structures such as seeds.

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Why organisms need energy


To make larger molecules from smaller molecules. For example:

  • Glucose molecules can be joined together to produce starch (in plants).
  • Proteins can be made from amino acids in plants and animals.


  • To keep the body temperature of warm-blooded animals (mammals and birds) relatively constant.


  • To contract the muscles of animals, allowing them to move.
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Anaerobic Respiration

Anaerobic respiration in plants

  • In plant and yeast cells, glucose is converted into ethanol and carbon dioxide.In yeast cells, this is called fermentation. This is an important step in the manufacture (making) of both bread and alcohol.
    • Glucose → ethanol + carbon dioxide

Anaerobic respiration in animals

  • Glucose is not broken down completely, making it a less efficient way of transferring energy than aerobic respiration.
  • In animals, glucose is converted to lactic acid:Anaerobic respiration happens when insufficient (not enough) oxygen reaches the muscles during periods of intense activity.
    • Glucose → lactic acid
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Aerobic respiration

Aerobic respiration

In aerobic respiration, glucose reacts with oxygen in the mitochondria of cells to give carbon dioxide, water and energy.

  • Glucose + oxygen → carbon dioxide + water (+ energy)
  • C6H12O6 + 6O2 → 6CO2 + 6H20 (+ energy)
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Metabolism is the sum of all the chemical reactions that happen in an organism. Many of these chemical reactions create new molecules with the help of enzymes. Metabolic reactions include:

  • Breaking down excess proteins to form urea
  • Converting glucose into starch, glycogen or cellulose
  • Combining glucose and nitrate ions to make amino acids
  • Combining glycerol and fatty acids to make lipids
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Respiration during exercise

Exercise increases the body's energy demands. The rate of respiration must increase in order to meet these new demands. The main responses to increase the rate of respiration are:

  • An increase in breath volume
  • An increase in breathing rate
  • The blood vessels dilate
  • An increased heart rate
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Anaerobic Respiration

If muscles are not supplied with enough oxygen during long periods of activity (like exercise), then muscle cells may start to respire anaerobically. This can lead to oxygen debt:

Lactic acid

  • Lactic acid is a by-product of anaerobic respiration.
  • Lactic acid is toxic and can build up in muscles, leading to muscular pain and fatigue.
  • Lactic acid also stops muscles from contracting efficiently.

Oxygen debt

  • Oxygen debt refers to the extra oxygen required after exercise to oxidise (combine with oxygen) lactic acid and remove it from cells.
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