The photosynthesis rate required practical uses pondweed (often Elodea or Cabomba) submerged in water: as the plant photosynthesises, it releases oxygen bubbles that you count or collect to measure the rate. By changing one variable at a time — light intensity, CO₂ concentration or temperature — you test which factors limit the rate.
What is the aim of this required practical?
The aim is to investigate the effect of a named factor on the rate of photosynthesis, using pondweed as a model organism. The overall equation for photosynthesis is:
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (using light energy and chlorophyll)
The rate of oxygen production is used as a proxy for the rate of photosynthesis — more oxygen bubbles per minute = faster photosynthesis.
Three common factor investigations at GCSE:
- Light intensity (most commonly examined)
- CO₂ concentration
- Temperature
What equipment is needed?
| Equipment | Purpose |
|---|---|
| Pondweed (Elodea or Cabomba) | The photosynthesising organism |
| Large beaker/container of water | Aquatic environment for the plant |
| Lamp (adjustable or moveable) | Controllable light source |
| Ruler | Measuring distance from lamp to plant (proxy for light intensity) |
| Sodium hydrogencarbonate (NaHCO₃) | Added to water to provide a constant CO₂ supply (as a control when investigating light) |
| Thermometer | Monitoring water temperature |
| Stopwatch | Timing bubble counts |
| Syringe/gas syringe (optional) | Collecting oxygen gas for a more accurate volume measurement |
How do you carry out the light intensity investigation?
- Cut a fresh 5 cm sprig of pondweed, cut the stem at an angle, and place it upright in a beaker of water containing dissolved sodium hydrogencarbonate.
- Position a lamp at a measured distance (e.g. 10 cm) from the pondweed. Allow 2 minutes for the plant to equilibrate at this light level.
- Count the number of oxygen bubbles released in 1 minute. Repeat three times and calculate the mean.
- Move the lamp to 20 cm, wait 2 minutes, count bubbles again.
- Repeat at 30 cm, 40 cm, 50 cm.
- Record results and plot a graph of bubble count per minute against distance (or against 1/d², which is proportional to light intensity).
Independent variable: distance from lamp (or light intensity). Dependent variable: number of oxygen bubbles per minute (rate of photosynthesis). Control variables: CO₂ concentration (sodium hydrogencarbonate), temperature (keep beaker in ice-water jacket if warm lamp is used), same pondweed sprig.
How does light intensity affect photosynthesis rate?
Light intensity and distance follow an inverse square law: if you double the distance from the lamp, the light intensity falls to one quarter:
$$I \propto \frac{1}{d^2}$$
where I = light intensity and d = distance from lamp to plant.
Expected results:
| Distance (cm) | Relative light intensity (1/d²) | Mean bubbles/min |
|---|---|---|
| 10 | 1/100 = 0.010 | 32 |
| 20 | 1/400 = 0.0025 | 18 |
| 30 | 1/900 = 0.0011 | 10 |
| 40 | 1/1600 = 0.0006 | 6 |
| 50 | 1/2500 = 0.0004 | 3 |
Interpretation: bubble rate increases as the lamp is moved closer (light intensity increases), because light is a limiting factor. At very high light intensities, the rate plateaus — another factor (CO₂ or temperature) becomes limiting.
What are the sources of error and how can you reduce them?
| Error | How to reduce it |
|---|---|
| Lamp heats water (temperature changes) | Use an LED lamp (less heat); place a flat-sided glass water container between lamp and beaker (heat filter) |
| CO₂ concentration falls as plant photosynthesises | Add sodium hydrogencarbonate to maintain CO₂ |
| Bubbles are difficult to count accurately | Collect gas in inverted syringe and measure volume; use a data logger with oxygen sensor |
| Pondweed output changes over time (fatigue) | Use the same plant throughout; repeat each distance three times and take the mean |
| Background light | Conduct in a darkened room; use a lightproof box if available |
What do the results tell you about limiting factors?
When you plot rate of photosynthesis against light intensity, you get a curve that:
- Rises steeply at low light intensities (light is limiting — more light means more photosynthesis).
- Levels off at high light intensities (another factor becomes limiting — usually CO₂ concentration or temperature).
If you then increase CO₂ concentration and repeat, the plateau rises higher — confirming CO₂ was the next limiting factor. If you increase temperature (up to the enzyme optimum, ~25–30 °C), the plateau may rise further — confirming temperature was limiting.
Frequently asked questions
Why is sodium hydrogencarbonate added to the water?
Sodium hydrogencarbonate (NaHCO₃) dissolves in water and releases CO₂, maintaining a constant dissolved CO₂ concentration for the pondweed to use in photosynthesis. Without it, the pondweed would use up the dissolved CO₂ in the water as it photosynthesises, and CO₂ would become a confounding variable — falling as the experiment proceeds. Adding NaHCO₃ keeps CO₂ controlled so that any change in bubble rate is due only to the factor you are varying (e.g. light intensity).
Why is Cabomba sometimes preferred over Elodea?
Both are aquatic pondweeds that produce visible oxygen bubbles when photosynthesising. Cabomba (a feathery aquatic plant) often produces more vigorous, more consistently sized bubbles, making counting easier and more reliable. Elodea is more commonly available but can produce irregular bubble sizes. Neither is significantly better scientifically — the choice is largely practical. What matters more is using a freshly cut stem, cutting it under water to avoid introducing air, and maintaining consistent conditions throughout the experiment.
How would increasing CO₂ concentration change the results?
Increasing CO₂ concentration (for example by adding more sodium hydrogencarbonate) at a constant light intensity shifts the limiting factor. At low CO₂, CO₂ is limiting — more CO₂ means faster photosynthesis because more substrate is available for the Calvin cycle. Once light intensity becomes the limiting factor, increasing CO₂ has no further effect. On a graph of rate vs light intensity, adding more CO₂ raises the plateau level and extends the rising portion to higher light intensities — because CO₂ no longer limits the reaction until a higher rate is reached.
Why does the rate of photosynthesis not keep increasing indefinitely as light intensity rises?
At some point, a factor other than light becomes the bottleneck. Most commonly, this is CO₂ concentration (the substrate for the Calvin cycle) or temperature (which affects enzyme activity). Even with unlimited light, if there is insufficient CO₂ to react or if RuBisCO and other enzymes are too slow (limited by temperature), the photosynthesis rate cannot increase further. This is the concept of a limiting factor: the variable in shortest supply relative to the current rate. Removing one limiting factor raises the rate only until the next one is encountered.
For Socratic GCSE biology with Professor Darwin — investigating photosynthesis rate through experimental design and interpreting limiting factor graphs — visit aitutors.me.