Factors Affecting the Rate of Photosynthesis
Light Intensity: Light provides the energy required to drive the endothermic reactions of photosynthesis. As light intensity increases, the rate of photosynthesis increases proportionally until the chloroplasts are working at their maximum capacity or another factor becomes scarce.
Carbon Dioxide Concentration: is a fundamental raw material used to synthesize glucose. Increasing its concentration increases the frequency of collisions between molecules and the enzymes in the chloroplasts, speeding up the reaction until a plateau is reached.
Both factors follow a similar graphical pattern: a steep initial rise where the factor itself is limiting, followed by a horizontal plateau where the rate remains constant because a different factor (like temperature) has become the bottleneck.
Chlorophyll is the pigment responsible for absorbing light energy. Any condition that reduces the amount of chlorophyll in a leaf will directly decrease the maximum possible rate of photosynthesis.
Magnesium deficiency is a common cause of reduced chlorophyll, as magnesium is a central component of the chlorophyll molecule. Without it, leaves turn yellow (chlorosis) and cannot capture light efficiently.
Other factors affecting chlorophyll include diseases (like Tobacco Mosaic Virus), which destroy chloroplasts, and environmental stress that leads to the loss of leaves.
| Factor | Graphical Shape | Reason for Plateau/Decline |
|---|---|---|
| Light Intensity | Saturation Curve | Another factor (e.g., ) is in short supply. |
| Conc. | Saturation Curve | Another factor (e.g., Light) is in short supply. |
| Temperature | Bell Curve | Enzymes denature at high temperatures. |
It is vital to distinguish between raw materials (water and ) and energy sources (light). While light is required, it is not a physical substance consumed in the same way as .
Note that while water is a reactant, it is almost never the limiting factor for the chemical process itself; rather, a lack of water causes the plant to wilt or close its stomata, which then limits intake.
Graph Analysis: When looking at a photosynthesis graph, identify the 'rising' part of the curve. In this section, the variable on the x-axis is the limiting factor. If the line is flat, the limiting factor is something not on the x-axis.
Identify the 'Why': If an exam asks why the rate decreases at high temperatures, always use the term denature and explain that the active site has changed shape.
Magnesium Connection: Always link magnesium ions to chlorophyll production. If a plant lacks magnesium, it cannot make chlorophyll, which limits light absorption and thus photosynthesis.
Check Units: Ensure you distinguish between 'rate' (how fast) and 'total volume' (how much) when interpreting experimental data.