Part 1 of 5What Is a Limiting Factor?
Definition
- A limiting factor is an environmental factor that limits the rate of a reaction when it is in short supply.
Three factors
- The rate of photosynthesis can be limited by carbon dioxide concentration, light intensity and temperature.
Effect on growth
- Because photosynthesis makes the sugar that fuels growth, limiting factors also limit how fast a plant grows.
Plateau
- A rate stops rising when a different factor becomes the one in shortest supply.
Part 2 of 5How Each Factor Acts
How Each Factor Acts
Carbon dioxide concentration
- Carbon dioxide is needed for carbon fixation, so more of it raises the rate until another factor limits.
Light intensity
- Brighter light lets chlorophyll absorb more energy to split water and make ATP in the light reactions.
Temperature rising
- Warmer conditions give more successful collisions between enzymes and substrates in carbon fixation, so the rate increases.
Temperature too high
- Above the optimum, enzymes denature because the shape of the active site is lost, so the rate drops steeply.
Part 3 of 5Reading a Limiting Factor Graph
1
Find the factor on the x axis and the rate of photosynthesis on the y axis.
2
Where the line slopes upwards, the factor on the x axis is the limiting factor.
3
Where the line becomes flat, the factor on the x axis is no longer limiting.
4
Name a different factor, such as temperature or light intensity, that must now be limiting.
5
If a second curve levels off higher, a warmer temperature may have given enzymes more successful collisions with substrate.
Part 4 of 5Exam Language
Be specific
- Write carbon dioxide concentration and light intensity rather than just carbon dioxide or light.
Explain, not describe
- An explain question needs a reason, such as more successful enzyme and substrate collisions at higher temperature.
Name the other factors
- When a graph levels off, say temperature or light intensity is now limiting, not that the reaction has stopped.
Denaturing
- Link a steep fall at high temperature to enzymes denaturing and the loss of the complementary active site.
Part 5 of 5Exam Checklist & Tips
3.3.4 – Key Points for the Exam
- Limiting factor A factor in short supply that limits the rate of a reaction.
- Rate of photosynthesis How quickly a plant produces sugar and oxygen.
- Optimum temperature The temperature at which enzymes work fastest.
- Denature To lose the shape of an enzyme active site, so it stops working.
- Plateau The flat part of a graph where the rate no longer increases.
Common Mistakes
- Writing that light or carbon dioxide alone is the limiting factor, without saying intensity or concentration.
- Saying the reaction has stopped at a plateau, when really another factor has become limiting.
- Claiming enzymes are killed at high temperature; they are denatured because the active site changes shape.
- Leaving out the reason in an explain question, such as the extra successful collisions at a higher temperature.