RP6: Required practical activity 6: Photosynthesis and light intensity
Not started yet — this one needs some love.
Aim: investigate the effect of light intensity on the rate of photosynthesis using an aquatic organism such as pondweed (Cabomba or Elodea).
Independent variable: distance between the lamp and the pondweed (cm), which is used as a proxy for light intensity. Light intensity is inversely proportional to distance² (HT).
Dependent variable: mean number of oxygen bubbles produced per minute (or volume of O₂ per minute if using an inverted measuring cylinder).
Control variables:
same piece of pondweed (or same mass)
CO₂ concentration (same volume of 0.2% sodium hydrogencarbonate solution)
temperature (kept constant using a heat shield or water bath between lamp and pondweed)
same time interval for counting (1 minute)
same lamp and light wavelength
Method
Fill a boiling tube with 0.2% sodium hydrogencarbonate solution (this provides CO₂ for photosynthesis).
Cut a fresh piece of pondweed approximately 10 cm long. Place it in the boiling tube with the cut end facing upwards so bubbles can escape.
Place the boiling tube in a test tube rack. Position it 10 cm from the light source. If using a tungsten lamp, place a beaker of water between the lamp and the boiling tube to act as a heat shield.
Leave the pondweed for 5 minutes to acclimatise before counting.
Count the number of bubbles produced in 1 minute. Repeat twice more at this distance. Calculate the mean.
Move the lamp (or rack) so the pondweed is 20 cm away. Wait 5 minutes, then repeat step 5.
Repeat for distances of 30 cm and 40 cm (or other planned distances).
Plot a graph of mean bubbles per minute (y-axis) against distance from lamp (x-axis). The rate decreases as distance increases because light intensity is lower.
Safety
Glassware — handle boiling tubes and glass rods carefully; report breakages immediately.
Hot lamps — tungsten lamps get hot; use a heat shield (beaker of water between lamp and pondweed) to prevent temperature changes confounding results; do not touch the lamp bulb when it is on.
Electricity near water — keep the lamp's power supply and plug away from water; do not handle plugs or switches with wet hands.
Pondweed disposal — Cabomba and Elodea are invasive non-native species; do not dispose of them in drains or natural waterways; follow teacher instructions for disposal.
Common exam mistakes
Bubble counting is only an estimate because bubbles can be different sizes; collecting the volume of oxygen per minute is better.
Sodium hydrogencarbonate provides carbon dioxide for photosynthesis; do not link it to osmosis or oxygen supply.
Use a heat shield or water bath so the lamp changes light intensity without also changing temperature.
If light is no longer limiting, the rate levels off because another factor limits photosynthesis; it does not mean photosynthesis has stopped.