Measuring the rate of photosynthesis

1. The rate of photosynthesis is the amount of product a plant makes in a given time, and it is what changes when the conditions change.
2. The most convenient thing to measure is the oxygen given off, because it is a gas and comes off a water plant as visible bubbles.
3. The rate can be worked out as $rate=\frac{volume\; of\; oxygen}{time}$, giving units such as ${cm}^3{min}^{-1}$.
4. Counting bubbles per minute is quicker but less precise, because bubbles vary in size and a large one carries far more gas than a small one.
5. Two other measures are possible: the volume of carbon dioxide used up, and the increase in dry mass of the plant over a longer period.
6. Whichever measure is used, the plant is also respiring, so what you actually record is the net effect of photosynthesis minus respiration.
7. In bright light photosynthesis is so much faster than respiration that this correction hardly matters, but in dim light it does.
8. Pondweed such as Elodea or Cabomba is used because it lives underwater, so the oxygen it releases collects as bubbles instead of escaping into the air.

The effect of light intensity

1. Light supplies the energy for the first stage of photosynthesis, in which water is split to release the hydrogen the second stage needs.
2. Raising the light intensity means more energy is absorbed by the chlorophyll each second, so water is split faster and glucose is built faster.
3. In the dark the rate is zero, because no water is being split, and the only gas exchange you can measure is the plant respiring.
4. At low light intensities the rate is roughly proportional to the light intensity, so doubling the intensity roughly doubles the rate.
5. As the intensity keeps rising the line begins to level off, because light is no longer the factor holding the rate back.
6. Light intensity is hard to measure directly in a school laboratory, so the distance from a lamp is used instead.
7. Light spreads out as it travels, so the inverse square law applies: $I\propto \frac{1}{d^2}$, where $d$ is the distance from the lamp.
8. Because the distance is squared, doubling the distance cuts the intensity to a quarter, not to a half.
9. Plotting the rate against $\frac{1}{d^2}$ rather than against $d$ is therefore what reveals the real relationship.

The effect of carbon dioxide concentration

1. Carbon dioxide is a raw material, and it is used in the second stage of photosynthesis where it is joined with hydrogen to build glucose.
2. Raising the carbon dioxide concentration means more molecules reach the chloroplasts each second, so more glucose can be assembled.
3. The air contains only about $0.04\%$ carbon dioxide, which is a very small proportion, so carbon dioxide is often the factor holding the rate back on a warm bright day.
4. The rate rises steeply as the concentration is increased from a low value, because every extra molecule that arrives can be used.
5. The line then levels off, because the chloroplasts cannot process carbon dioxide any faster once light or temperature has become the limit.
6. Carbon dioxide has to reach the mesophyll cells by diffusion through the stomata, so anything that closes the stomata also slows photosynthesis.
7. A plant short of water closes its stomata to save it, which cuts the carbon dioxide supply and is one reason drought reduces growth.

The effect of temperature

1. Every step of photosynthesis is controlled by an enzyme, so temperature affects the rate in the same way it affects any enzyme-controlled reaction.
2. Raising the temperature gives the enzymes and their substrates more kinetic energy, so they move faster and collide more often.
3. More frequent collisions with enough energy to react means more successful reactions per second, so the rate rises.
4. The rate keeps rising until the optimum temperature is reached, which is the temperature at which the process runs fastest.
5. Above the optimum the rate falls sharply, because the enzymes begin to denature as the bonds holding their folded shape are broken.
6. A denatured enzyme has an active site that no longer fits its substrate, so it stops catalysing its step and the whole pathway slows.
7. For most plants the optimum is somewhere around $25{}^{\circ }C$ to $30{}^{\circ }C$, and denaturation becomes serious above roughly $40{}^{\circ }C$ to $45{}^{\circ }C$.
8. At low temperatures nothing is denatured, so the rate is slow but recovers as soon as the plant is warmed again.
9. That difference is what makes the temperature graph asymmetrical: it climbs to a peak and then drops away steeply, rather than levelling off.

The shape of a single-factor graph

1. A graph of rate against light intensity or against carbon dioxide concentration has the same two-part shape.
2. The first part is a steep straight rise, and along that stretch the factor on the horizontal axis is the limiting factor.
3. The gradient of that straight stretch tells you how much extra rate each extra unit of that factor buys.
4. The second part is a plateau, where the line runs flat and adding more of that factor changes nothing at all.
5. A plateau means the factor on the axis has stopped being the limiting one, and something else is now holding the rate back.
6. The point where the line stops rising and starts to flatten is therefore the point at which the limiting factor changes over.
7. A graph of rate against temperature looks different, rising to a peak at the optimum and then falling, because the enzymes are being denatured rather than simply working at full stretch.
8. Reading the rate from any of these graphs means finding the value on the vertical axis, and finding a rate of change means taking a gradient.