Required practical 5 · Biology · Biology Paper 1, both tiers
Photosynthesis and light intensity
Investigate the effect of light intensity on the rate of photosynthesis using an aquatic organism such as pondweed.
The exam trap. Light intensity is proportional to 1/d², not 1/d. Bubbles are not all the same size.
Variables
- Independent (you change)
- Light intensity, changed by the distance of the lamp from the pondweed
- Dependent (you measure)
- Rate of photosynthesis (bubbles or volume of oxygen per minute)
- Control (you keep the same)
- Temperature (LED lamp or heat shield)
- Carbon dioxide concentration (same sodium hydrogencarbonate solution)
- The same piece of pondweed
- Time allowed to adjust at each distance
Apparatus
- Pondweed and a boiling tube of dilute sodium hydrogencarbonate solution
- Lamp (LED if possible)
- Metre rule
- Stopwatch
- Gas syringe or inverted measuring cylinder (optional)
- Beaker of water as a heat shield
Method, and why each step matters
Put a piece of pondweed of a set length in the boiling tube of sodium hydrogencarbonate solution.
Why The hydrogencarbonate supplies carbon dioxide, so it is not limiting.
Place the lamp 10 cm away, measuring from the pondweed with a metre rule.
Why Distance sets the light intensity.
Leave the pondweed for 5 minutes before counting.
Why The rate needs time to settle at the new light intensity.
Count the bubbles (or measure the gas volume) for one minute. Repeat three times and take a mean.
Why Repeats reduce the effect of random error.
Move the lamp to 20, 30, 40 and 50 cm and repeat.
Why A range of distances shows the pattern.
Keep the temperature constant with an LED lamp or a beaker of water between lamp and tube.
Why A hot lamp would change the temperature as well as the light.
Risks
| Hazard | How to reduce the risk |
|---|---|
| Water near an electrical lamp. | Keep the lamp dry and away from the edge; use a low-voltage LED. |
| A hot lamp can burn. | Do not touch the bulb; let it cool. |
A worked set of results
| Distance in cm | Bubbles per minute (mean) | Relative light intensity (10 000 ÷ d²) |
|---|---|---|
| 10 | 60 | 100 |
| 20 | 32 | 25 |
| 30 | 15 | 11 |
| 40 | 8 | 6.3 |
| 50 | 5 | 4 |
- Rate falls as distance increases: the fall is not proportional to distance.
- Higher: light intensity ∝ 1/d², so doubling the distance gives a quarter of the intensity.
- At high light intensity another factor, such as carbon dioxide or temperature, becomes limiting.
Mistakes that cost marks
Using 1/d instead of 1/d² for light intensity.
Instead Square the distance: intensity ∝ 1/d².
Reading the water displaced rather than the gas collected in the measuring cylinder.
Instead Read the volume of gas at the top of the inverted cylinder.
Assuming every bubble is the same size.
Instead Collect and measure the gas volume for a more accurate rate.
Practice questions on this practical
- A student put some pondweed in a beaker of water under a lamp.In Photosynthesis and its rate
- A student investigated how the distance of a lamp from some pondweed affects the rate of photosynthesis.In Photosynthesis and its rate
- Pondweed releases bubbles of oxygen when it photosynthesises.In Photosynthesis and its rate