Explain why the rate of photosynthesis in a C3 plant does not continue to increase indefinitely as atmospheric CO₂ concentration rises, even when light intensity and temperature are kept constant at optimum levels.

In a C3 plant the first step of the Calvin cycle is the fixation of CO₂ by the enzyme RuBisCO, which combines CO₂ with the 5‑carbon sugar RuBP to give two molecules of the 3‑carbon product, glyceraldehyde‑3‑phosphate (G3P). When the atmospheric CO₂ concentration is increased, more CO₂ molecules are available for RuBisCO, so the rate of carboxylation rises until the enzyme’s active sites are saturated. At this point every RuBisCO molecule is busy and further increases in CO₂ cannot be used.

The regeneration of RuBP, which is required for the next carboxylation cycle, depends on ATP and NADPH produced in the light‑dependent reactions. With RuBisCO saturated, the supply of RuBP becomes the limiting factor; the plant can no longer fix CO₂ at a higher rate because it lacks the substrate (RuBP) to react with the enzyme.

Thus, even at optimum light and temperature, the rate of photosynthesis does not increase indefinitely with CO₂ because the process becomes limited by RuBisCO saturation and subsequently by RuBP regeneration rather than by CO₂ availability.