Explain why the rate of photosynthesis in a plant increases as light intensity increases, but only up to a certain point.

Light is a reactant that supplies the energy for photosynthesis. As light intensity rises, more photons reach the chloroplasts, so more ATP and NADPH are produced, increasing the rate of photosynthesis.
The rate rises only up to a point because, after a certain intensity, the other components of the reaction become limiting. When the light‑dependent reactions are saturated, the Calvin cycle cannot use the extra energy because the supply of CO₂, the temperature, or the availability of enzymes is insufficient. Thus the rate levels off or stops increasing.
In short, the rate increases with light until another factor (e.g. CO₂ concentration or temperature) limits the process.