Investigation of limiting factors
Limiting factors
- The rate of photosynthesis depends on conditions.
- A limiting factor is the one in shortest supply that holds the rate back.
- The three main ones: light intensity, CO₂ concentration, and temperature.
Practice
A limiting factor is:
The limiting factor is whichever requirement is in shortest supply, capping the overall rate.
Reading the graph
- Raising light speeds photosynthesis — until another factor becomes limiting.
- Raising CO₂ speeds it — until another factor becomes limiting.
- Raising temperature speeds it — but only to an optimum; too hot and the enzymes denature.
- While the line rises, light is limiting. Where it levels off, something else (CO₂ or temperature) is.
Practice
On a graph of rate against light intensity, where the line levels off:
While the rate rises, light limits it; once it plateaus, light is plentiful and something else limits the rate.
Practice
Why does raising temperature only speed photosynthesis up to a point?
Higher temperature speeds the enzyme reactions until the optimum; beyond it, the enzymes denature and the rate falls.
Measuring the rate
- Use a redox indicator (DCPIP / methylene blue) with a suspension of chloroplasts: the dye loses colour as they work — test different light intensities or wavelengths.
- With a whole aquatic plant, count the oxygen bubbles given off to compare conditions.
Practice
One way to measure the rate of photosynthesis is to:
Bubble counting (aquatic plant) or DCPIP decolourising (chloroplast suspension) both track the rate.
You've got it
Key idea
- a limiting factor = the factor in shortest supply that caps the rate
- three factors: light intensity, CO₂ concentration, temperature
- on a rate-vs-light graph: rising = light limiting; plateau = another factor; too hot → enzymes denature
- measure with DCPIP/methylene blue + chloroplasts, or count O₂ bubbles from an aquatic plant