4. What process limits the rate of photosynthesis at low light levels?

Remember, in C3 and C4 plants, the light-dependent and the light-independent reactions operate in unison, as shown in the following diagram. The rate of the 'slower' reaction will determine ('limit') the overall rate of photosynthesis.

What factors determine the rates of metabolic processes, such as the light-dependent and light-independent reactions?

One factor is the maximum potential speed of physical processes (such as enzyme activity and electron transport) and the concentrations of the reactants (e.g., light, water, O2, CO2, ATP, ADP, NADP and NADPH) and temperature. Generally, physical processes only operate at their maximum potential rate when there is a large excess of the reactants. Under normal conditions, a shortage of one reactant or another usually creates the proverbial 'bottleneck' that slows the overall process. Under low light levels, the available light is insufficient to support the maximal potential rate of the light-dependent reactions, and thus limits the overall rate of photosynthesis.

Click here if you are unsure about the meaning of the terms 'reactant' and 'product'.

As light levels are increased, more ATP and NADPH are produced, and the overall rate of photosynthesis increases. This pattern will continue until some other reactant becomes limiting. This is what happens when the light response curve begins to level off at the "light saturation point."

Is water ever limiting for the light-dependent reactions? Probably not. In the aqueous environment inside a cell, availability of water is essentially limitless.


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