Photosynthesis
Senior 2 · Biology
Photosynthesis
Lesson 2 of 4
20 min
Let's begin
Farmers who grow tomatoes and sweet peppers in greenhouses want every plant to photosynthesise as fast as possible, because faster photosynthesis means faster growth and bigger harvests. But adding more of one thing, for example more light, does not always help. Knowing which factor is holding the plant back saves money and raises yields.
A water plant such as Elodea (pondweed) gives off bubbles of oxygen from its cut stem. Counting bubbles per minute gives a simple measure of the rate of photosynthesis (how fast it happens).
Worked example
A student moved a lamp away from a pondweed and counted bubbles for one minute at each distance. A beaker of water between the lamp and the plant kept the temperature steady.
| Distance of lamp (cm) | 10 | 20 | 30 | 40 | 50 |
|---|---|---|---|---|---|
| Bubbles per minute | 48 | 30 | 16 | 9 | 6 |
Reasoning: as the lamp moves away, the light reaching the plant becomes weaker, and the rate falls. Temperature and carbon dioxide were kept the same, so light intensity must be the factor controlling the rate here.
Early in the 20th century Frederick Blackman and Gabrielle Matthaei showed that the rate of photosynthesis is controlled by whichever condition is in shortest supply. This is the idea of a limiting factor: the one factor that, at a given moment, holds the rate back. Increasing the limiting factor increases the rate; increasing any other factor does not.
As light intensity rises, the rate rises, then levels off (a plateau). On the rising part of the graph, light is limiting. On the flat part, something else, usually carbon dioxide or temperature, has become limiting.
Air contains only about 0.04% carbon dioxide, so on a bright, warm day carbon dioxide is often the limiting factor. Raising it raises the rate until another factor limits. Greenhouse growers sometimes burn paraffin, which both releases carbon dioxide and warms the air.
Worked example
Two graphs of rate against light intensity are drawn, one at 0.04% and one at 0.1% carbon dioxide. At low light the two lines are the same. At high light the 0.1% line levels off much higher. Conclusion: at low light, light was limiting (extra carbon dioxide made no difference); at high light, carbon dioxide was limiting the 0.04% plant.
The light-independent stage is run by enzymes. Warming increases the rate up to an optimum temperature (around 25–35 °C for many crop plants). Above it the rate falls quickly because the enzymes are denatured (their shape is damaged permanently). Blackman also found that when light is weak, warming the plant makes little difference, because light, not temperature, is then limiting.
Tip
Reading a rate graph: where the line is rising, the factor on the x-axis is limiting. Where the line is flat, something else is limiting, so name it from the conditions given in the question (for example "carbon dioxide concentration" or "temperature").
Always name the factor precisely: "light intensity", not just "light"; "carbon dioxide concentration", not just "gas".
A pondweed gives off 36 bubbles in 3 minutes. What is the rate in bubbles per minute?
Solution: 36 ÷ 3 = 12 bubbles per minute.
At 6 a.m. in Kigali the air is cool and the sky is only just light. Which factors are most likely to limit photosynthesis in a maize field?
Solution: Light intensity is the main limit because the light is dim. Low temperature may also slow the enzymes. Carbon dioxide is unlikely to be limiting at such a low rate.
On a hot, dry afternoon a bean plant wilts. Explain why its rate of photosynthesis falls even though the light is bright.
Solution: Short of water, the plant closes its stomata to reduce water loss. Carbon dioxide can no longer diffuse into the leaf, so carbon dioxide becomes the limiting factor.
In the pondweed experiment, why is a beaker of water placed between the lamp and the plant?
Solution: Lamps give off heat. The water absorbs heat so the temperature stays constant, and only light intensity changes. This makes it a fair test.
Remember
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
