Factors affecting rate of photosynthesis

What factors affect the rate of photosynthesis?

1. Light Intensity

2. CO2 concentration

3. Temperature

How does light intensity affect the rate of photosynthesis

Light needed as an energy source. As light intensity increases, ATP & red.NADP produced at higher rate (faster LDS occurs)

• More ATP & red.NADP produced so Calvin Cycle occurs faster

How does CO2 concentration affect the rate of photosynthesis

Increasing CO2 conc. increases rate of carbon fixation in Calvin Cycle, and rate of TP production

How does temperature affect the rate of photosynthesis

As temp increases, rate of enzyme-controlled reactions in photosynthesis increases, until protein denatures, then decreases

• E.g. carbon fixation (RuBisCo)

NOTE: Photorespiration increases above 25 degrees - higher photosynthetic rates may not be seen at higher temps even if enzymes still functioning

Implications of water stress on rate of photosynthesis

Stomata close to avoid water loss by transpiration during water stress e.g. during drought, when too hot

• Stops CO2 diffusing in, reduces rate of LIR, eventually stops photosynthesis

• LDS relies on H+ gradient forming across thylakoid membrane

  • Membrane permeability can change w. temperature

  • May lead to dissipation of H+ gradient, photosynthesis slows

Why is water not considered a limiting factor?

If water potential low enough to limit photosynthesis rate, plant will have closed stomata & stopped photosynthesis

Effect of reduced light intensity on Calvin Cycle

Reduces rate of light-dependent stage

• Reduced ATP & red.NADP quantity

  • Needed to convert GP to TP

• Conc. of GP increases (slight), conc. of TP decrease

• Less TP to regenerate RuBP = conc. of RuBP decreased

• Over time, CO2 fix. stops, conc. of GP plateaus

Effect of CO2 conc. on Calvin Cycle

Low conc. of CO2 = reduced GP conc. (less C to fix), & less TP

• RuBP conc. increase as is still formed from TP but isn’t being used to fix CO2

  • RuBP unfixed & builds up

Effect of Temperature on Calvin Cycle

At low temps or too high temps, rate is reduced, as light independent reactions are enzyme-controlled (RuBisCo)

• Enzyme & substrates have less KE

• Fewer successful collisions, reduced rate of reaction

• Lower conc. of GP, TP, & RuBP

NOTE: As long there’s enough light to produce ATP + red.NADP, increasing temp to optimum, increases rate of LIR & so photosynthesis rate