Limiting factor = A factor which, if in short supply limits or reduces the rate of photosynthesis 

Factors affecting the rate of photosynthesis:

  • Light intensity - This is the distance between the plant and the light source. As it increases, so does the rate of photosynthesis as more energy is available for the chemical reaction. The rate becomes constant when another factor is limiting.
  • Carbon dioxide concentration - CO2 is one of the reactants of photosynthesis, the rate becomes constant when another factor is limiting.
  • Temperature - Phtotsynthesis is an enzyme controlled reaction. If the temperature increases too much, the enzymes become denatured and the reaction rate will decrease and stop. 
  • Amount of chlorophyll - This absorbs the light energy to make photosynthesis happen, so leaves with more chlorophyll can absorb more light energy. The rate becomes constant when another factor is limiting.


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  • Trap the suns heat / have shades & ventilation for when it's too hot
  • Artificial light for night time (used by mainly commercial farmers)
  • Paraffin heater - burns and produces carbon dioxide
  • Enclosed, keeping pests and diseases away
  • Cost-effective for farmers
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Glucose + Oxygen -> Carbon dioxide + Water + Energy

Respiration is exothermic - it transfers energy to the environment.

Why do organisms need energy:

  • to drive the chemical reactions needed to keep them alive
  • movement
  • cell division
  • to maintain constant conditions in cells and the body - homeostasis
  • active transport
  • transmition of nerve impulses

Metabolism = all the chemical reactions in the cells of an organism.

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Aerobic: glucose + oxygen -> carbon dioxide + water

  • respiration using oxygen
  • most efficient way to transfer energy from glucose
  • goes on all the time in plants and animals
  • most reactions happen inside mitochondria

Anaerobic: glucose -> lactic acid

  • respiration without oxygen
  • it's the incomplete breakdown of glucose 
  • plant and yeast cells can do it too: glucose -.> ethanol + carbon dioxide
  • in yeast cells = fermentation, can be used to make bread and wine
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Muscles need energy to contract. While exercising the muscles need additional energy as:

  • breathing rate/volume of breaths increase to bring more oxygen in to the body and remove the carbon dioxide produced.
  • heart rate increases to supply muscles with extra oxygen and remove carbon dioxide produced 

During long periods of vigorous activity:

  • lactic acid levels build up
  • gycogen reserves in the muscles become low as more glucose is used for respiration, and additional glucose is transported from the liver

The build up of lactic acid produces oxygen debt.

As the body stores of glycogen become low, the person suffers from muscle fatigue.

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