These questions ask you to interpret changes in the rate of photosynthesis and decide which requirement is holding the process back. Focus on what changes, what stays controlled, and whether the rate rises, levels off, or falls.
Reason one factor at a time
The three main rate factors are light intensity, carbon dioxide concentration, and temperature. Light supplies energy, carbon dioxide is a reactant, and temperature affects the enzymes controlling the reaction. In complete darkness the rate of photosynthesis is zero, although respiration continues.
A limiting factor is the requirement in shortest supply relative to the plant's needs. Imagine a process that needs several inputs: adding more of an already plentiful input cannot help while another is scarce. When the scarce input is increased, the rate rises until that input is no longer limiting. A different requirement then sets the ceiling.
| Pattern | Reasoning |
|---|---|
| More light and a rising rate | Light was limiting, so extra energy allows faster photosynthesis |
| A graph becomes horizontal as light rises | Light is no longer limiting; another requirement now caps the rate |
| Temperature rises towards an optimum | The enzyme-controlled reaction becomes faster |
| Temperature rises beyond the optimum | Enzymes denature and the rate falls |
| Extra carbon dioxide raises the rate | Carbon dioxide was limiting until another requirement became limiting |
Apply the same logic to practical work. Growers can raise light, temperature, and carbon dioxide together, while checking that each change is useful. Pondweed releases oxygen during photosynthesis, so counting or collecting its bubbles over a fixed time gives an estimate of rate. A shortage of water can close stomata, preventing carbon dioxide from entering. The amount of chlorophyll can also cap how much light a leaf absorbs.
Common reasoning traps
| Tempting route | How to reject it |
|---|---|
| Extend a rising light graph upwards forever | Every increase has a ceiling because another requirement eventually becomes limiting. |
| Assume hotter always means faster or that chlorophyll melts | The rate rises only to an optimum; above it, enzymes denature. |
| Define limiting as the most abundant requirement, a dispensable one, or one already at its maximum | Limiting means the currently scarce requirement that sets the rate. |
| Treat a plateau as the plant choosing to stop | The reaction continues at a capped rate because some other requirement is scarce. |
| Use fertiliser as a direct speed control or blame dry soil only on consumed water | Mineral ions support growth, while water shortage mainly slows photosynthesis by closing stomata and restricting carbon dioxide. |
| Treat chlorophyll as a reactant that is used up | Chlorophyll absorbs light without being consumed; having less of it reduces light capture. |
Try it
Which set names the three main factors that affect the rate of photosynthesis?
- Light intensity, carbon dioxide concentration and temperature
- Amount of soil, amount of fertiliser and soil depth
- Oxygen concentration, wind speed and air pressure
- Light intensity, oxygen concentration and amount of fertiliser
A potted plant is shut in a completely dark cupboard for a night and a day. What is happening in its leaves?
- Photosynthesis and respiration both stop until light returns
- Photosynthesis slows down but continues at a low rate
- Photosynthesis stops altogether, while respiration carries on
- Photosynthesis continues, using stored light from the day before
A class measures the rate of photosynthesis of pondweed by watching bubbles rise from the cut stem. What are those bubbles, and what do they show?
- Bubbles of carbon dioxide, showing how much gas the plant takes in
- Bubbles of water vapour, showing how fast water is being used
- Bubbles of oxygen, showing how fast the plant is photosynthesising
- Bubbles of air pushed out of the stem as the plant grows