Study Hack Lab

RP 6Biology

Photosynthesis

Write-up template

Name
Date
Course: GCSE Biology
Board and specification: AQA GCSE Biology (8461), 4.4.1.2AQA GCSE Combined Science: Trilogy (8464), 4.4.1.2Apparatus and techniques: AT 1, AT 2, AT 3, AT 4, AT 5
Class and teacher

Aim

Investigate the effect of light intensity on the rate of photosynthesis in pondweed, by changing the distance between the pondweed and a lamp.

Plan and risk assessment
Apparatus
Method
Results
Graph
Analysis
Conclusion and evaluation
Make safe and clear up

A blank write-up for use in a real lesson. The headings follow the AQA required practical; the readings are yours to take. Layout v1, practical v4. Original material; not affiliated with AQA, which has not reviewed it.

1 Plan and risk assessment

Photosynthesis uses light energy to make glucose from carbon dioxide and water, giving off oxygen. The faster the pondweed gives off oxygen, the faster it is photosynthesising. Moving the lamp away spreads its light over a bigger area, so less light reaches the pondweed: light intensity is proportional to 1 ÷ distance² (Higher). When light is the limiting factor, less light means a slower rate.

carbon dioxide + water → glucose + oxygen (light energy needed); light intensity ∝ 1 ÷ distance²

Variables

Independent (you change)
Dependent (you measure)
Control (you keep the same)

Prediction

As you move the lamp further from the pondweed (10, 20, 30 then 40 cm), what will happen to the rate of photosynthesis?

The lamp moves from 10 cm to 20 cm. What happens to the light intensity at the pondweed? (Higher)

What gas is in the bubbles that come out of the pondweed?

Risk assessment

Hazard	Harm it could cause	Control

2 Apparatus

Pondweed (Cabomba or Elodea), about 10 cm sprigs The plant that photosynthesises.
Scalpel and white tile To cut a fresh end on each stem.
Boiling tube in a clamp, or a 400 cm³ beaker with a glass funnel Holds the pondweed in the solution.
10 cm³ measuring cylinder (resolution 0.2 cm³) Upturned over the funnel: collects the oxygen.
Glass rod Pushes the pondweed down into the solution.
0.2 % sodium hydrogencarbonate solution Supplies the carbon dioxide the pondweed needs.
Lamp (LED, or filament) The light source.
Glass tank of water A heat shield between a filament lamp and the pondweed.
Metre rule (resolution 1 mm) Sets the distance from the pondweed to the lamp.
Thermometer (resolution 0.5 °C) Checks the temperature of the solution stays the same.
Stopclock (resolution 1 s) Times each count or collection.
Hand tally counter Counts the bubbles without losing count.

3 Method

Write numbered steps someone else could follow. Say what you measure, with which instrument, and how you keep it a fair test.

4 Results

The table is drawn before you measure. Record each reading to the resolution of the instrument, then work out the mean of your repeats, leaving out any anomaly.

Oxygen collected at each distance · At each distance: wait 5 minutes, read the thermometer, then collect the oxygen for 5 minutes, three times (refill the cylinder each time). Light intensity is 10 000 ÷ distance²: you work out the 30 cm value yourself in Analyse; the others fill in as each mean is done.

Distance from the lamp in cm	Temperature of the solution in °C	Volume 1 in cm³	Volume 2 in cm³	Volume 3 in cm³	Mean volume of oxygen in 5 minutes in cm³	Light intensity in a.u.
10
20
30
40

Anomalies

Ring any anomalous result. Say what probably went wrong and whether you repeated it.

5 Graph

Rate against distance · Choose your axes: the independent variable goes on the x-axis. Label each axis with the quantity and unit.

Rate against light intensity (Higher) · Choose your axes: the independent variable goes on the x-axis. Label each axis with the quantity and unit.

6 Analysis

Reading your curve
Go up from 25 cm to your curve, then across to the y-axis.
Answer
Relative light intensity (Higher)
light intensity ∝ 1 ÷ distance², so relative light intensity = 10 000 ÷ distance² (100 at 10 cm)
Answer
Four times the light, four times the rate?
rate at 10 cm ÷ rate at 20 cm
Answer

7 Conclusion and evaluation

Conclusion

Conclusion: what do your results show? Use your own numbers.

Sources of error

For each, say whether it is systematic or random, and how it affected your results.

Counting bubbles that are different sizes

Heat from a filament lamp

Light from the room

Not waiting after moving the lamp

Measuring the distance to the lamp’s stand

Not enough carbon dioxide (tap water)

A squashed cut end

Only four distances, 10 cm apart

Improvement

How could you improve the method? Say why it would help.

8 Make safe and clear up

Switch the lamp off at the wall and unplug it. Let a filament lamp cool before you move it
Put the pondweed back in the class tank. Never put it down the sink or into a pond or stream
Pour the solution down the sink and rinse the glassware
Put the scalpel and tile away, blade pointing down. Wipe up any spilt water, then switch the room lights back on