A Limiting Factor Sets the Rate
Definition
Limiting factor
A factor in shortest supply that restricts the rate of a process.
- The rate of photosynthesis is the amount of photosynthetic product formed, or reactant used, per unit time.
- A limiting factor is the factor in shortest supply that restricts the rate of a process.
- Increasing the limiting factor increases the rate until a different factor becomes limiting.
- The three factors assessed here are light intensity, carbon dioxide concentration and temperature.
Key Idea
A factor is limiting only when changing it changes the rate.
Light and Carbon Dioxide Reach Plateaus
- At low light intensity, light limits the rate because little energy is absorbed by chlorophyll.
- Increasing light intensity supplies more energy, so the rate rises.
- The curve eventually reaches a plateau, where light is no longer limiting and another factor restricts the rate.
- At low carbon dioxide concentration, carbon dioxide limits the rate because it is a reactant.
- Increasing carbon dioxide concentration increases the frequency with which the photosynthetic reactions can use this reactant, so the rate rises.
- The carbon dioxide curve also plateaus when light intensity or temperature becomes limiting.
Example
- A plant produces 12 cm312\,\mathrm{cm^3}12cm3 of oxygen in 4 min4\,\mathrm{min}4min.
- Its mean rate is 12 cm34 min=3 cm3 min−1\dfrac{12\,\mathrm{cm^3}}{4\,\mathrm{min}} = 3\,\mathrm{cm^3\,min^{-1}}4min12cm3=3cm3min−1.
Temperature Has an Optimum
Definition
Denaturation
A permanent change in the shape of an enzyme's active site, caused by high temperature or extreme pH, so that the substrate no longer fits.
- At low temperature, molecules have less kinetic energy, so enzyme-controlled reactions proceed slowly.
- As temperature rises, particles move faster and successful collisions involving enzymes occur more often, so the rate increases.
- The rate reaches an optimum, which is the temperature at which it is highest.
- Above the optimum, bonds maintaining enzyme shape break and the active site changes shape.
- The substrate no longer fits the active site, so fewer enzyme-substrate complexes form and the rate falls rapidly.
Common Mistake
High temperature denatures enzymes; it does not kill them.
Graphs Show What Is Limiting
- A rising line shows that the factor on the horizontal axis is limiting because increasing it increases the rate.
- A plateau shows that the factor on the horizontal axis is no longer limiting.
- A temperature graph rises to an optimum and then falls rather than forming a simple plateau.
- When comparing points, use values from the graph and describe both the change in the factor and the change in rate.
Exam technique
To explain a plateau, state that the named factor is no longer limiting and identify a plausible factor that now limits the rate.
Rate Calculations
- Rate is calculated using ** rate=quantity changedtime\text{rate} = \dfrac{\text{quantity changed}}{\text{time}}rate=timequantity changed **.
- Suitable measures include oxygen volume per minute, carbon dioxide uptake per minute or bubble count per minute.
- Oxygen volume is more reliable than bubble count because bubbles can have different volumes.
Example
- A plant releases 18 cm318\,\mathrm{cm^3}18cm3 of oxygen in 6 min6\,\mathrm{min}6min.
- rate=186=3 cm3 min−1\text{rate} = \dfrac{18}{6} = 3\,\mathrm{cm^3\,min^{-1}}rate=618=3cm3min−1.
Recall the Relationships
- Light intensity and carbon dioxide concentration give rising curves that level off when another factor becomes limiting.
- Temperature gives a curve with an optimum because high temperatures denature enzymes.
Self review
- What is a limiting factor?
- Why does a light-intensity graph reach a plateau?
- Why does photosynthesis slow above the optimum temperature?
- How is a rate calculated?
