Photosynthesis (B1.4)

Photosynthesis is one of the most heavily examined topics on Gateway A — appearing in both Paper 1 and Paper 3 as a 6-marker on limiting factors and again as a graph-interpretation task.

The equation

Word equation:

carbon dioxide + water → glucose + oxygen

Symbol equation:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Conditions: light energy is required, and chlorophyll (in chloroplasts) absorbs that energy.

Photosynthesis is endothermic — it transfers energy from the surroundings (light) into the chemical bonds of glucose.

Where it happens

Photosynthesis takes place in chloroplasts, found mainly in palisade mesophyll cells in the leaf. Spongy mesophyll cells with their air spaces allow gas exchange. Stomata on the underside of the leaf let CO₂ in and O₂ out.

Uses of glucose by plants

Plants don't waste glucose. They use it for:

• Respiration — releasing energy.
• Storage as starch (insoluble; can be tested with iodine — turns blue-black if starch present).
• Storage as oils/fats — especially in seeds.
• Building cellulose for cell walls.
• Making amino acids by combining glucose with nitrate ions absorbed by roots → proteins.

Limiting factors — the key concept

A limiting factor is whatever is in shortest supply at a given moment. As you increase that factor, rate of photosynthesis rises until something else becomes limiting and the rate plateaus.

The three main limiting factors are:

1. Light intensity
2. Carbon dioxide concentration
3. Temperature

A fourth factor — chlorophyll concentration — only becomes limiting if leaves are damaged (e.g. by disease, mineral deficiency).

Reading rate-of-photosynthesis graphs

Typical Gateway graph: rate vs light intensity at three temperatures.

• Low light → rate is light-limited: graph rises linearly.
• High light → rate plateaus → CO₂ or temperature is now limiting.
• A higher-temperature curve plateaus at a higher rate (until ~40°C, where enzymes denature).

⚠ Always justify the limiting factor by saying what is happening on the graph (e.g. "the curve has flattened, so light intensity is no longer limiting; the rate is now limited by CO₂ or temperature").

Temperature

Photosynthesis is enzyme-controlled. Below ~10°C the enzymes work slowly; the optimum is typically around 25–30°C; above ~40°C the enzymes start to denature and rate drops sharply. (Be careful: the rate doesn't slowly fall — it crashes once enzymes change shape.)

Inverse-square law for light intensity

If you double the distance between a lamp and a plant, the light intensity falls to a quarter (not half). This is the inverse-square law:

light intensity ∝ 1 / d²

Where d is distance. So at 2d, intensity = 1/4 × original; at 3d, intensity = 1/9.

Required practical: investigating rate

Standard set-up: a piece of pondweed (Cabomba or Elodea) in sodium hydrogen carbonate solution (provides CO₂), with a lamp at varying distances. Count bubbles of oxygen per minute, OR measure displacement of water in an inverted measuring cylinder.

Key control variables: temperature (water bath), CO₂ concentration (constant NaHCO₃), same length of pondweed.

Common Gateway-paper mistakes

1. Forgetting that photosynthesis is endothermic and requires light.
2. Saying "more light = more photosynthesis" without mentioning the plateau.
3. Confusing limiting factor justification — describe the graph!
4. Forgetting that at >40°C the enzymes denature (some students think rate just slows).
5. Forgetting that doubling distance quarters intensity (inverse-square).