factors affecting photosynthesis

• Limiting factors

  these operate simultaneously

  • light intensity
  • carbon dioxide
  • water
• Light intensity

  provides energy to power the first stage of photosynthesis, producing ATP + reduced NADP needed for the next stage

  • causes stoma to open so that gaseous echange can occur

  →when stomata are open transpiration also occurs, leading to uptake of water + its delivery to leaves
• Light intensity 2

  • at a constant favourable temperature + constant suitable carbon dioxide concentration, light intensity is the limiting factor

  → when light intensity is low, rate of photosynthesis is low vice versa

  • at a certain point, even when light intensity increases, the rate of photosynthesis does not increase but a factor other than light intensity is the limiting factor
• The effect of changing light intensity on the Calvin Cycle, when there is little or no light:

  1. GP cannot be reduced to TP
  2. TP levels fall + GP accumulates
  3. If TP levels fall, RuBP cannot be regenerated
• Carbon dioxide concentration

  levels of carbon dioxide in atmosphere + aquatic habitats are high enough that carbon dioxide is not usually a limiting factor

  The effect of changing the carbon dioxide concentration on the Calvin Cycle if concentration falls below 0.01% :

  1. RuBP cannot accept it, + accumulates
  2. GP cannot be made
  3. Therefore TP cannot be made
• Temperature

  Calvin Cycle involves many enzyme-catalysed reactions + therefore is sensitive to temperature

  effects are :

  • from low temperatures to temperatures of 25-30 degrees, if plants have enough water + carbon dioxide + a sufficient light intensity, rate of photosynthesis increases a s temperature increases
• Temperature 2.

  • at temperatures above 30 degrees, for most plants, growth rates may reduce to photorespiration: oxygen competes with carbon dioxide for the enzyme RuBisCO’s active site, reducing the amount of carbon dioxide being accepted by RuBP + subsequently reduces the quantity of GP + therefore of TP being produced whilst initially causing an accumulation of RuBP

  →however due to lack of TP, RuBP cannot be regenerated.
• Temperature 3

  • at temperature above 45 degrees, enzymes involved in photosynthesis may be denatured

  →this would reduce the concentrations of GP + TP, and eventually of RuBP as it could not be regenerated due to lack of TP
• Water Stress

  if a plant has access to sufficient water in the soil, the the transpiration stream has a cooling effect on the plant

  • the water passing up xylem to leave also keeps plant cells turgid so they can function
  • turgid guard cells keep the stomata open for gaseous exchange
• Water Stress 2

  if not enough water is available to the plant (water stress):

  1. roots are unable to take up enough water to replace that lost via transpiration
  2. cells lose water + become plasmolysed
  3. plant roots produce abscisic acid that, when translocated to leaves, cause stomata to close, reducing gaseous exchange
  4. tissues become flaccid + leaves wilt
  5. rate of photosynthesis greatly reduces