Investigation of limiting factors
Introduced| English | Meaning |
|---|---|
| limiting factor/ˈlɪmɪtɪŋ ˈfæktə/ | A factor whose availability restricts the rate or extent of a process. |
| light intensity/laɪt ɪnˈtensɪti/ | The amount of light energy incident per unit area per unit time, expressed using an appropriate physical measure. |
| denature/ˈdenətʃə/ | To alter a biological molecule's structure enough to disrupt its normal function, without necessarily breaking its primary covalent structure. |
| greenhouse/ˈɡriːnhaʊs/ | A structure with transparent walls or roofing that provides a controlled environment for growing plants. |
What holds photosynthesis back?
- The rate of photosynthesis depends on conditions.
- A limiting factor 限制因素 is the one in shortest supply that holds the rate back.
- The three main ones: light intensity 光照强度, CO₂ concentration, and temperature.
A limiting factor is:
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The limiting factor is whichever requirement is in shortest supply, capping the overall rate.
Adding carbon dioxide can raise the rate of photosynthesis when CO₂ is the limiting factor.
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Only the factor in shortest supply limits the rate, so increasing it speeds things up.
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 变性.
- While the line rises, light is limiting. Where it levels off, something else (CO₂ or temperature) is.
Limiting factors
rate rises, then plateaus
Drag the sliders. More light speeds photosynthesis — until CO₂ or temperature caps it (the plateau).
On a graph of rate against light intensity, where the line levels off:
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While the rate rises, light limits it; once it plateaus, light is plentiful and something else limits the rate.
Why does raising temperature only speed photosynthesis up to a point?
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Higher temperature speeds the enzyme reactions until the optimum; beyond it, the enzymes denature and the rate falls.
Match each factor to its role in photosynthesis.
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Light powers the light reactions, CO₂ feeds the Calvin cycle, temperature controls the enzymes — the scarcest one caps the rate.
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.
One simple way to measure the rate of photosynthesis is to count the ______ bubbles given off by an aquatic plant.
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Oxygen is a product of photosynthesis; counting (or collecting) the bubbles tracks the rate.
Using limiting factors commercially
- Growers raise yields by removing the limiting factor in greenhouses 温室: adding light, warmth and extra CO₂.
Worked example. On a bright warm day, CO₂ is often limiting. A grower pipes extra CO₂ into the greenhouse, and the rate of photosynthesis — and the crop yield — rises.
You've got it
- 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