Limiting factors on the Calvin Cycle

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Light intensity

Increase in light intensity:

Alters the rate of the light-dependent reaction. 

  • The electrons take part in photophosphorylation, so increasing light intensity means more ATP and reduced NADP will be produced.
  • ATP and reduced NADP are both used in the light-independent stage to reduce glycerate-3-phosphate to triose phosphate. 
  • ATP is also used to help 5 out of every 6 TP regenerate RuBP. 

Decrease in light intensity:

The light-dependent stage will stop.

  • This will also stop the light-independent as this stage uses products from the light-dependent stage. 
  • GP cannot be converted to TP as there wil be no ATP or reduced NADP availlable.
  • This will reduce the amount of RuBP, whih reduces the fixation of carbon dioxide and the formation of more GP.
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Carbon dioxide concentration

Increase in carbon dioxide concentration:

Leads to an increase in carbon dioxide fixation in the Calvin Cycle. 

  • More carbon fixation leads to more molecules of GP and so more being converted into TP. Also more regeneration of RuBP.
  • However, the number of stomata open to allow gaseous exchange leads to more transpiration and wilting. This will lead to a stress response and the stomata will close. 
  • This will reduce the uptake of carbon dioxide and reduce the rate of photosynthesis. 

Decrease in carbon dioxide concentration:

  • If carbon dioxide concentration is reduced below 0.01% then RuBP, the carbon dioxide acceptor will accumulate. 
  • As a result, levels of GP and therefore TP will fall. 
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Temperature

Increasing the temperature:

Have little effect on the light-dependent reaction, because it is not dependent on enzymes.

  • Will affect the rate of the light-independent stage as this is a series of biochemical steps catalysed by a specific enzyme.
  • Increasing temperature will at first increase the rate of photosynthesis.

Temperature above 25 degrees:

  • Photorespiration exceeds photosynthesis. 
  • As a result, ATP and reduced NADP from the light-dependent reaction are wasted.
  • Very high temperatures also damage proteins involved in photosynthesis. 
  • Increased temperatures cause an increase in water loss from leaves by transpiration.
  • This may lead to closure of stomata and a reduction in the rate of photosynthesis. 
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