Factors Affecting the Rate of Photosynthesis

Graphs with Multiple Lines (Interacting Factors) Exams often test your understanding of interacting limiting factors by using graphs with multiple lines.

• For example, a graph may show light intensity on the x-axis with two lines representing different conditions (such as 15°C and 25°C).
• If the line for 25°C reaches a higher plateau than the line for 15°C, it proves that temperature was the limiting factor at the lower level.
• By comparing the different plateaus, you can identify which additional factors are interacting to limit the overall rate of photosynthesis.

Temperature and Enzyme Action

Photosynthesis is an endothermic reaction controlled by photosynthetic enzymes. Because it relies on enzymes, temperature heavily influences the rate of reaction based on collision theory.

• Increasing temperature: As temperature rises, the reacting molecules ($CO_2$ and $H_2O$) and the enzymes gain kinetic energy.
• They move faster, leading to a higher frequency of collisions between the substrate and the enzyme's active site.
• A higher proportion of these collisions will have enough energy to react (meeting or exceeding the activation energy), so the rate of photosynthesis increases.
• Beyond the optimum: If the temperature gets too high (typically above 45°C), the thermal energy breaks the bonds holding the enzyme together.
• This causes denaturation. The active site permanently changes shape and is no longer complementary to the substrate, causing photosynthesis to stop entirely.

Carbon Dioxide ($CO_2$) Concentration

Carbon dioxide is a reactant in the photosynthesis equation:

• The normal atmospheric concentration of $CO_2$ is very low (around 0.04%).
• Increasing the concentration of $CO_2$ increases the number of reactant particles in a given volume.
• This increases the frequency of successful collisions with enzyme active sites, increasing the overall rate of reaction until another factor becomes limiting.

Light Intensity and the Inverse Square Law

Photosynthesis requires an input of light energy from the surroundings. This energy is transferred to chloroplasts and absorbed by the pigment chlorophyll.

• Higher light intensity means more photons hit the chlorophyll per second, trapping energy faster.
• The Plateau Mechanism: As light intensity continues to rise, the graph eventually levels off (plateaus). At this saturation point, light is no longer the limiting factor. Even though more photons are absorbed, the enzymes cannot process the reactants any faster. The rate is now restricted by another factor, such as a lack of $CO_2$ limiting substrate concentration, or low temperature restricting the kinetic energy and frequency of collisions between the substrate and the enzymes' active site.
• However, light intensity is inversely proportional to the square of the distance from the light source. This is called the inverse square law.
• The Double/Quarter Rule: If you double the distance from a light source, the light intensity falls by a factor of four ($2^2 = 4$).

Calculating Light Intensity Light intensity is often measured in arbitrary units (a.u.) using this formula:

Worked Example:

Compare the light intensity when a lamp is 10 cm and 20 cm away from a piece of pondweed.

Step 1: Calculate intensity at 10 cm.

• $\text{Intensity} = 1 \div 10^2 = 1 \div 100 = 0.01 \text{ a.u.}$

Step 2: Calculate intensity at 20 cm.

• $\text{Intensity} = 1 \div 20^2 = 1 \div 400 = 0.0025 \text{ a.u.}$

Step 3: Compare the results.

• As the distance doubled from 10 cm to 20 cm, the light intensity decreased by four times ($0.01 \div 0.0025 = 4$).

Chlorophyll Levels

Chlorophyll is essential for absorbing light energy. If a plant has less chlorophyll, it will produce less glucose, leaving it with less energy for protein synthesis, which leads to stunted growth.

Factors that reduce chlorophyll include:

1. Nutrition: A lack of magnesium in the soil prevents the plant from making chlorophyll, causing a yellowing of the leaves called chlorosis.
2. Disease: Pathogens like the Tobacco Mosaic Virus (TMV) cause a mosaic discoloration, destroying chlorophyll.
3. Genetics: Some plants have naturally variegated leaves with white patches containing no chlorophyll.