Factors affecting photosynthesis

• Light intensity

• Light wavelength

• Light duration

• Water availability

  • Humidity (indirect)

• Temperature

• Carbon dioxide concentration

• Amount of chlorophyll

• Wind (indirect)

Indirect factors - stress response of plans in these conditions = stomatal closure which affects carbon dioxide concentration

• Respiration reduces the overall rate of photosynthesis

  Gross rate of photosynthesis - rate of respiration = net rate of photosynthesis

• Usually the rate of respiration in plants is quite low and on a sunny day the rate of photosynthesis is much higher than the rate of respiration

Limiting factor: The factor that is present at the lowest or least favourable value (and so is therefore limiting the rate)

• The law of limiting factors states that the rate of a metabolic process is limited (prevents the reaction from being faster) by the factor that is at its least favourable value

Light intensity

Rate of photosynthesis in mm³CO₂cm^-2h^-1

1. Describe the effect of increasing light intensity on the rate of photosynthesis

   • Firstly, the rate increases in a linear fashion - light intensity limiting factor

   • Then in the non-linear zone, the rate of photosynthesis is increasing at a slower rate - light intensity still limiting factor, but other factors are also starting to be limiting

   • Finally, the rate plateaus, or becomes constant, while light intensity still continues to increase - light intensity is no longer the limiting factor

2. Explain

   • More light is absorbed by photosystems 1 and 2

   • As light intensity increases, more electrons can be excited out of chlorophyll for non-cyclic and cyclic phosphorylation

   • More electrons passed down electron transport chain, so more ATP and NADH

   • More protons being pumped from stroma into thylakoid space

   • More photolysis of water

   • More/faster rate of GP → TP → RuBP

   • Greater rate of fixation of CO2 by ribisco combining CO₂ and RuBp

3. Why does it level off?

   • Light no longer limiting factor

   • Most likely carbon dioxide

   • Rate of fixation of carbon dioxide by rubisco is limited because the collision rate and formation of ESCs with RuBp and carbon dioxide is liited by carbon dioxide conc

   • Rate of production of GP is limiting

   • ATP and reduced NADP from LDR is not limiting

How light affects concentrations of GP, RuBP and TP

• No light independent stage so no ATP or reduced NADP

• RUBp reacts with carbon dioxide, producing GP

• GP cannot be converted to TP also cannot regenerate RuBP because ATP and reduced NADP are required from LDR

• So GO builds up and plateaus as no more RuBP is made whilst the RuBP falls and then plateaus as it is used up

• Low carbon dioxide concentration = low rate/no reaction of RuBP and carbon dioxide so little/no GP made

• However ATP and reduced NADP from LDR are available

• So GP is used up being converted to TP and then back to RUBp

• So GP levels fall and RuBp rise then plateau, once all the GP is used up