How does light intensity affect the rate of photosynthesis?

How does light intensity affect the rate of photosynthesis?

• A. Rate increases linearly with light intensity without limit.
• B. Rate is independent of light intensity.
• C. Rate increases with light intensity until other factors become limiting, then plateaus.
• D. Rate decreases as light intensity increases.

Answer: (C)

Light provides the energy that drives the light-dependent reactions, so as you increase light from very dim to moderate, the rate of photosynthesis climbs because more photons mean more electrons flowing and more ATP and NADPH available to power CO2 fixation. But this rise doesn’t go on forever. Once the system reaches a point where enzymes, CO2 supply, or other conditions can’t keep up, increasing light can’t speed things up any further, and the rate levels off into a plateau. That’s the situation described by the idea that the rate increases with light intensity until other factors become limiting, then plateaus. So the best answer reflects that—the rate grows with light until another factor becomes limiting, then it levels off. At very high light, the rate wouldn’t keep rising and, in extreme cases, could even drop due to damage, but under normal conditions the plateau is the key idea. In case you’re wondering about the other options: light is a necessary factor, so the rate isn’t independent of light; the rate doesn’t rise without limit as light increases; and the rate doesn’t decrease just from increasing light under typical conditions.

Light provides the energy that drives the light-dependent reactions, so as you increase light from very dim to moderate, the rate of photosynthesis climbs because more photons mean more electrons flowing and more ATP and NADPH available to power CO2 fixation. But this rise doesn’t go on forever. Once the system reaches a point where enzymes, CO2 supply, or other conditions can’t keep up, increasing light can’t speed things up any further, and the rate levels off into a plateau. That’s the situation described by the idea that the rate increases with light intensity until other factors become limiting, then plateaus.

So the best answer reflects that—the rate grows with light until another factor becomes limiting, then it levels off. At very high light, the rate wouldn’t keep rising and, in extreme cases, could even drop due to damage, but under normal conditions the plateau is the key idea.

In case you’re wondering about the other options: light is a necessary factor, so the rate isn’t independent of light; the rate doesn’t rise without limit as light increases; and the rate doesn’t decrease just from increasing light under typical conditions.