Biology 6.1 - 6.4 - Photosynthesis and limiting factors
Plants and algae use light energy to make the organic food that supports most food chains. Learn how light, carbon dioxide and temperature limit that reaction, then explain how the limiting factor can change.
Photosynthesis, food and biomass
A producer makes organic food from substances in its surroundings instead of obtaining all of its food by eating other organisms. Photosynthetic organisms, especially plants and algae, are the main producers in most food chains.
Their starting substances, carbon dioxide and water, contain matter but are not food. During photosynthesis, light energy is transferred into the chemical reaction that forms glucose. Glucose contains carbon and can be used to build more living material. This gives the causal chain:
photosynthesis → glucose and other organic material → producer biomass → food for consumers
Here, biomass means the mass of living material. Producers do not create energy; they transfer light energy into a chemical store while making the organic material that becomes biomass.
Photosynthesis word equation
carbon dioxide + water → glucose + oxygen
Light energy is transferred into the reaction.
Photosynthesis is therefore endothermic: overall, energy is taken in from the surroundings. Light is the energy input, not an extra chemical reactant in the word equation. Oxygen is a product, so oxygen production can later be used as evidence that photosynthesis is occurring.
How one factor limits the rate
The rate of photosynthesis is how much photosynthesis occurs per unit time. It can be estimated by measuring a product, such as the number of oxygen bubbles per minute or the volume of oxygen produced per minute.
A limiting factor is the factor in shortest effective supply: changing it changes the rate, while increasing a different non-limiting factor has little or no effect. The limiting factor can change as conditions change.
| Factor | When the factor is low | As the factor increases | Why the pattern eventually changes |
|---|---|---|---|
| Light intensity | Little light energy is available, so the rate is low. | More light energy is available, so the rate rises. | The rate reaches a plateau when temperature or carbon dioxide concentration becomes limiting. |
| Carbon dioxide concentration | There is little carbon dioxide reactant, so the rate is low. | More reactant is available, so the rate rises. | The rate reaches a plateau when light intensity or temperature becomes limiting. |
| Temperature | Enzyme-controlled reactions are slow because particles have less kinetic energy and there are fewer successful collisions. | The rate rises as temperature approaches the enzymes' optimum. | Above the optimum, enzymes denature and their active sites change shape, so the rate falls. |
The graph shapes are not identical. Increasing light intensity or carbon dioxide concentration usually gives a rising curve followed by a plateau. Increasing temperature gives a rise to an optimum and then a fall. A plateau means the tested factor is no longer limiting; it does not mean photosynthesis has stopped.
| Light intensity (arbitrary units) | 100 | 200 | 300 |
|---|---|---|---|
| Rate of photosynthesis (arbitrary units) | 5 | 10 | 10 |
When limiting factors interact
Higher tier only
Light intensity, carbon dioxide concentration and temperature do not act independently. The factor that limits the rate depends on the values of the other two, and it can change during an investigation.
| Conditions | Current limiting factor | Effect of changing another factor |
|---|---|---|
| Low light, enough carbon dioxide, suitable temperature | Light intensity | Adding more carbon dioxide has little effect because too little light energy is available. |
| High light, low carbon dioxide, suitable temperature | Carbon dioxide concentration | Adding carbon dioxide increases the rate until a different factor limits it. |
| High light, enough carbon dioxide, low temperature | Temperature | Warming towards the optimum increases the rate of the enzyme-controlled reactions. |
| High light, enough carbon dioxide, temperature above the optimum | Temperature is too high | Cooling towards the optimum can raise the rate; further heating increases denaturation and lowers it. |
This interaction changes graph curves. At low light intensity, plants at low and high carbon dioxide concentration may photosynthesise at similar low rates because light is limiting both. At greater light intensity, the high-carbon-dioxide plant can reach a higher rate, while the low-carbon-dioxide plant reaches a lower plateau because carbon dioxide has become limiting.
The key reasoning pattern is condition → identify the factor in shortest effective supply → predict whether changing a named factor can alter the rate. Increasing every factor does not guarantee a faster rate, especially when temperature is already above its optimum.