17.4 - Factors affecting Photosynthesis

Question 1

What is a limiting factor and give 3 examples with explanation?

Answer 1

Limiting Factor: Factor which limits the rate of a process (photosynthesis), if a factor is in short supply, it reduces photosynthetic rate

Light Intensity: Energy source, increased light intensity = increased ADP and NADPH production

CO₂ Concentration: CO₂ needed as source of carbon (in Calvin Cycle), more of it means more carbon fixation so higher rate of TP (triose phosphate) production

Temperature: Affects rate of enzyme-controlled reactions

Question 2

How does a rise in temperature affect rate of photosynthesis?

Answer 2

As temp. rises: Rate of enzyme activity rises until denaturation; Increases rate of enzyme-controlled photosynthetic reactions such as carbon fixation

Question 3

Is water a limiting factor, and why?

Answer 3

• Stomata closes to avoid water loss by transpiration during dry spells (water stress). Closure stops CO₂ diffusion affecting light independent reaction… stopping photosynthesis
• Water is required but not a limiting factor, as for the Ψ to be low enough to limit photosynthesis, the plant will already have closed the stomata and stopped photosynthesis
  
  • Plants, other than xerophytes, will die in these conditions

Question 4

What is the effect of change in light intensity on the Calvin Cycle?

Answer 4

• Low light intensity…
• Reduces rate of light dependent photosynthesis, reducing amount of ATP and NADPH produced
  
  • They are needed to convert GP > TP
  • There will be less TP to regenerate RuBP
• Therefore, RuBP and TP levels drop, and GP rises as it is not being converted to TP
• Increased light intensity has reverse effect

Question 5

What is the effect of CO₂ temperature on the Calvin Cycle?

Answer 5

• Calvin Cycle reactions are catalysed by enzymes, so temperature has direct effect (e.g. in carbon fixation).
• Lower temperatures, less kinetic energy in substrate molecules, reduced rate
  
  • Reduced about of GP, TP and RuBP
• At higher temperatures, enzymes irreversibly denature
• Carbon dioxide is an essential substrate, low conc. means reduced GP and TP. RuBP will increase as it is formed from the TP, but not being used to fix CO₂