The rate of photosynthesis can be followed by the oxygen a water plant gives off.

• Independent variable: light intensity, changed by moving the lamp (distance d).
• Dependent variable: oxygen made per minute: count bubbles, or collect the gas in a syringe (better: bubbles vary in size).
• Control variables: temperature (a tank of water absorbs heat from the lamp, or use an LED), carbon dioxide (add sodium hydrogencarbonate), the same piece of pondweed. Work in a darkened room and let the plant settle for a few minutes at each distance.

rate = volume of oxygen ÷ time
e.g. cm³ ÷ minutes = cm³/min

1. A syringe collects 3.6 cm³ of oxygen in 4 minutes
2. rate = 3.6 ÷ 4
3. rate = 0.9 cm³/min

Light spreads out from the lamp, so its intensity falls with the square of the distance.

light intensity ∝ 1 ÷ distance²
relative light intensity = 1 ÷ d²

1. A light meter reads 1600 lux at 10 cm
2. At 20 cm: 1600 × (10 ÷ 20)² = 1600 × 0.25
3. = 400 lux: double the distance, a quarter of the light

Do not halve the intensity when the distance doubles. Square the ratio of the distances.

Light intensity, carbon dioxide concentration and temperature all affect the rate. Whichever is in shortest supply limits the rate; the amount of chlorophyll matters too.

• On the rising part, brighter light gives a faster rate: light is limiting. Both lines are together there.
• Where a line goes flat, more light does nothing: something else is limiting. The 0.12% line is higher, so on the flat part of the 0.04% line carbon dioxide is limiting.
• Higher: growers raise these factors in greenhouses (heaters, lamps, carbon dioxide burners), but only when the extra crop is worth more than it costs.

Temperature is different: too hot and the enzymes denature, so the rate falls.