GCSE Biology Photosynthesis revision

Photosynthesis transfers light energy into chemical energy stored in glucose. The word equation: carbon dioxide + water → glucose + oxygen. The carbon dioxide diffuses in through the stomata, water arrives from the roots through the xylem, and the reaction happens in the chloroplasts, where the green pigment chlorophyll absorbs the light.

Photosynthesis is endothermic — it takes in energy from the environment (the light) rather than releasing it. That word earns marks: it explains why light intensity and temperature both matter, since the reaction needs a continuous energy supply.

Three factors limit the rate: light intensity, carbon dioxide concentration and temperature. The limiting factor is the one in shortest supply — raise it and the rate rises; raise the others and nothing happens until the limiting one changes.

The plateau is where marks live: on a flat section of a light graph, adding more light changes nothing, so the limiter must be CO₂ or temperature. State the factor and the evidence (the flat line).

Light spreads out as it travels, so light intensity is inversely proportional to the square of the distance from the source: intensity ∝ 1/d². Move the lamp from 10 cm to 20 cm and the intensity falls to a quarter — 1/10² to 1/20², from 1/100 to 1/400.

GCSE questions usually give you intensity in relative units. If the intensity is 100 units at 10 cm, then at 20 cm it is 100 ÷ 2² = 25 units, and at 40 cm it is 100 ÷ 4² = 6.25 units. Every doubling of the distance divides the intensity by four.

The practical measures how light intensity affects the photosynthesis rate of an aquatic plant such as pondweed (*Cabomba* or *Elodea*). The rate is tracked through its oxygen output — count bubbles per minute from the cut stem, or collect the gas and measure its volume.

1. 1Set up

   Place a fresh piece of pondweed in a beaker or boiling tube of water, cut stem upwards, under a lamp at a chosen distance.

2. 2Equilibrate

   Leave the shoot several minutes at each distance so it adjusts before counting — early readings are unreliable.

3. 3Count

   Count bubbles for one minute; repeat three times at each distance and use the mean.

4. 4Change distance

   Move the lamp to the next distance, re-equilibrate, and repeat the counts.

5. 5Control

   Keep temperature constant (water bath or LED lamp that stays cool), keep the CO₂ level constant (same water, added sodium hydrogencarbonate), and use the same piece of pondweed throughout.

Results rise steeply as the lamp closes in, then plateau once light stops limiting. Sources of error include heat from the lamp changing the temperature, bubbles of different sizes, and gas dissolving in the water; using a gas syringe and a heat shield improves the measurement.

The glucose made in photosynthesis has five documented destinations, and examiners routinely ask for any three:

• Respiration — glucose is broken down to transfer energy for every cell process.
• Storage as starch — glucose is converted into insoluble starch, stored in leaves, roots and tubers; being insoluble, it does not dissolve away or disturb cell chemistry.
• Storage as fat or oil — seeds and some storage organs pack energy into lipids.
• Cellulose for cell walls — glucose units are chained into cellulose, the strong structural fibre of plant walls.
• Amino acids and proteins — glucose is combined with nitrate ions absorbed from the soil to build amino acids, which are assembled into proteins for growth.

Note the soil link in the last item: a plant short of nitrates grows poorly because it cannot make enough protein — photosynthesis supplies the carbon skeleton, the soil supplies the nitrogen.

Commercial growers manage all three limiting factors. Light: glass or polythene transmits light, and artificial lighting extends the day. Carbon dioxide: burners or paraffin heaters enrich the air with CO₂ (paraffin burning releases it directly). Temperature: heaters keep the crop near the photosynthesis optimum, especially overnight.

Each input costs money — fuel, electricity, CO₂ systems — and the grower’s decision is an economic one: the extra yield must be worth more than the extra cost. The optimum is not "as much as possible" but the point where the next pound of input returns more than a pound of crop. A balanced evaluation names a boost, its effect on the rate, its cost, and the judgement about whether it pays.