investigation of limiting factors

How can the rate of photosynthesis be measured (4)

• measure the volume of oxygen released using a gas syringe

• Measure increase in pH

• Measure decrease in carbon dioxide

• Indirectly - measure increase in biomass

Define a limiting factor

The factor that will determine the rate of photosynthesis when at sub-optimal levels

Which limiting factors affect the rate of photosynthesis

• Temperature

• Light intensity

• Carbon dioxide concentration

How does light intensity affect the rate of photosynthesis

• Increase in light increases rate

• Plateau is reached at higher light intensity when another factor is limiting

• Light needed for light dependant reactions

• Low intensities mean insufficient ATP and NADPH is produced so prevents Calvin cycle from occurring at sufficient rate

How does temperature affect the rate of photosynthesis

• increase in temperature increases rate

• Above optimum temperature, rate drops as enzymes denature

• Temperature affects rate of rubisco activity in Calvin cycle

How does CO2 concentration affect rate of photosynthesis

• increases CO2 increases rate until plateau where another factor is limiting

• CO2 needed for carbon fixation/carboxylation for light independent reactions (calvin cycle)

Explain why rates of photosynthesis are low in the early morning

• low light intensity

• Rate of photolysis low and excitement of electrons is limited

• Light needed to produce NADPH and ATP

Explain which factor is most likely to be limiting in the middle of the day

• carbon dioxide concentration

• Enzymes (RUBISCO) cannot fix carbon dioxide effectively at low concentrations

• ATP and NADPH not used as quickly as they are made

Explain why decreasing decreasing CO2 concentration leads to an increase in RuBP

• co2 required to generate GP and TP in Calvin cycle

• Low co2 conc means less GP and TP synthesised

• RuBP still generated from existing TP

• RuBP no longer being used to fix CO2 so RuBP levels increase

Explain why reducing light intensity leads to an increase in GP

• Light required to produce ATP and NADPH in light dependent reactions Low

• ATP and NADPH required to convert GP into TP

• Low light intensity means less GP converted into TP

• GP levels build up

• TP and RuBP decrease

What other feature of light influences the rate of oxygen production in photosynthesis

Wavelength, not all wavelengths are absorbed as some reflected

What is the role of dehydrogenase enzymes in light-dependant stage

• NADP+ acts as electron acceptor and is reduced

• Reaction catalysed by dehydrogenase enzyme

How can the activity of dehydrogenase enzyme be measured

Using DCPIP

What is DCPIP

An electron acceptor that acts as an indicator of dehydrogenase enzyme

How does DCPIP work

• changes from blue to colourless when reduced

• Occurs when light is shone on chloroplasts

Describe how to investigate the effect of temperature on the rate of photosynthesis

• independent variable: temperature (eg 0˚C,10˚C, 20˚C, 30˚C, 40˚C)

• Dependant variable: volume of oxygen produced in 1 minute

• Control variables: CO2 conc, light intensity, species of pond week (Elodea), length of pond weed