Bioenergetics · GCSE Biology
Photosynthesis required practical
Plan the GCSE Biology pondweed practical: count oxygen bubbles or collect gas, control temperature and CO₂, vary lamp distance, and use rate as bubbles per minute as a proxy for photosynthesis.
Independent variable: light intensity (lamp distance). Dependent variable: rate of photosynthesis, measured as bubbles of oxygen per minute (or volume of gas). Controls: temperature, CO₂, time, the same sprig.
The important bits
What you need to know
- 1
Aim: investigate the effect of light intensity on the rate of photosynthesis using pondweed (often Elodea or Cabomba).
- 2
Independent variable: light intensity, changed by moving a lamp to measured distances (for example 10, 20, 30, 40, 50 cm). Higher: intensity ∝ 1/d².
- 3
Dependent variable: number of oxygen bubbles per minute, or volume of oxygen collected in a gas syringe / inverted measuring cylinder in a set time.
- 4
Control variables: temperature (water bath or a water tank as heat filter), carbon dioxide (sodium hydrogencarbonate concentration), time, pondweed mass and species, and background light (darken the room).
- 5
Oxygen is the by-product of 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, so bubble rate is a proxy for photosynthetic rate if CO₂ and temperature are constant.
- 6
Method sketch: cut the stem at an angle underwater, invert in a boiling tube of water with hydrogencarbonate, wait for a steady stream, count bubbles for one minute, repeat at each distance, calculate a mean.
- 7
Expect rate to rise as the lamp comes closer, then possibly plateau if CO₂ or temperature becomes limiting. If the lamp also heats the tube, the practical is invalid because two variables change.
- 8
Improvements: gas syringe for volume instead of counting unequal bubbles, a LED lamp to cut heat, a ruler fixed to the bench, repeats, and a heat sink of water between lamp and tube.
Quotations worth analysing
Short evidence. Real method.
“Independent variable: light intensity (distance of the lamp). Dependent variable: number of bubbles of oxygen per minute.”
State both in this language. Distance is how you change intensity; bubbles per minute is the measured rate.
“Sodium hydrogencarbonate provides carbon dioxide.”
Use it to control or to set CO₂ so light can be the only independent variable. Without it, dissolved CO₂ may run out and fake a plateau.
“Keep temperature constant so it is not a second independent variable.”
Lamps get hot. A glass tank of water or an LED reduces heat. Otherwise you cannot claim the graph shows light intensity alone.
Go deeper
How do I write a repeatable pondweed method?
Take a 5 cm sprig of pondweed, cut the stem underwater to stop air locking the xylem, and invert it in a boiling tube of dilute sodium hydrogencarbonate. Place a metre ruler on the bench. Set a lamp at 10 cm from the plant, switch off other lights, wait one minute for a steady bubble stream, then count bubbles for a further minute. Repeat the count twice more at 10 cm and mean. Move the lamp to 20, 30, 40 and 50 cm, repeating the wait-and-count each time. Record results in a table of distance, 1/d² if Higher, bubble counts, mean, and rate. Safety: water and electricity — keep sockets dry; lamps are hot. Validity: same sprig, same hydrogencarbonate concentration, same counting time, temperature monitored with a thermometer. If bubbles stop, recut the stem.
Go deeper
Why might the graph flatten even as I bring the lamp closer?
Two honest reasons: another factor is limiting (CO₂ used up, or the tube is cool), or the plant has reached its maximum enzyme rate at that temperature. A dishonest reason: you are also heating the water with the lamp, so temperature is rising at the same time as intensity — then you cannot interpret the shape. Fix CO₂ with hydrogencarbonate, fix temperature with a bath or heat filter, then a plateau means a genuine second limiter. Unequal bubble sizes also flatten or scatter the data; switching to a gas syringe measuring cm³ per minute is the upgrade. Anomalous zeros often mean the cut end is jammed against the tube. Always wait for a steady stream before timing.
Go deeper
How do Higher-tier inverse-square calculations sit on the results table?
Add a column for d in metres or centimetres (be consistent) and a column for 1/d². Plot rate against 1/d², not only against d. If light is limiting, you expect a straight-ish rise with 1/d². If the plot against 1/d² plateaus, light is not limiting at high intensity. Example: 10 cm → 1/d² = 0.010 cm⁻²; 20 cm → 0.0025 cm⁻², a quarter of the intensity. If mean bubbles fall from 40 per minute to 10, light was limiting and the data match. If they fall only to 36, something else is limiting near the lamp. State that assumption in the conclusion. Combined papers can still discuss distance and rate without the formula; Higher must use ∝ 1/d² when asked.
See the idea in action
Lamp at 10 cm: 48, 52, 50 bubbles in 1 minute, mean 50. At 20 cm: 12, 14, 13, mean 13. Distance doubled, so Higher intensity is 1/4; 50 × 1/4 = 12.5, close to 13, so light was limiting. Independent variable: light intensity (distance). Dependent: bubbles per minute. Controls: 0.2 mol/dm³ sodium hydrogencarbonate, 20 °C, same 6 cm Cabomba sprig, room blacked out. A water tank sat between lamp and tube so temperature stayed 20 °C. If mean at 10 cm had been 50 and at 20 cm still 48, light would not have been limiting.
Exam technique
Turn knowledge into marks
Name IV (light intensity via distance), DV (bubbles or volume of oxygen per unit time), and controls (temperature, CO₂, time, plant). Explain oxygen as a product of photosynthesis. Evaluate bubble size and lamp heat.
Common mistakes
Do not give these marks away
- 01
Letting the lamp heat the water so temperature and light both change.
- 02
Counting for different times at each distance, or forgetting hydrogencarbonate so CO₂ becomes an accidental limiter.
- 03
Calling the bubbles carbon dioxide, or saying a plateau means the plant is dead.
In the pondweed practical, what is the dependent variable?
ADistance of the lamp
BRate of photosynthesis measured as oxygen bubbles (or volume) per minute
CConcentration of sodium hydrogencarbonate
DThe species of pondweed
Show the answer
Rate of photosynthesis measured as oxygen bubbles (or volume) per minute. You measure oxygen production as a proxy for photosynthetic rate. Lamp distance is the independent variable. Hydrogencarbonate and species should be controlled.
Quick questions
If this is the bit you searched
What is the independent variable in the photosynthesis pondweed practical?
Light intensity, usually changed by moving a lamp to different measured distances.
Why do pondweed bubbles show the rate of photosynthesis?
The bubbles are oxygen, a product of photosynthesis. More bubbles per minute means a faster rate, if temperature and CO₂ are controlled.
Why add sodium hydrogencarbonate to the water?
It supplies carbon dioxide so CO₂ is not accidentally limiting while you investigate light intensity.
How do you keep temperature constant when using a lamp?
Use a water tank as a heat filter, an LED lamp, or a water bath, and monitor with a thermometer so heat from the bulb is not a second independent variable.