At what sunlight levels does photosynthesis reach maximum rate?

• A. Full sunlight
• B. Complete darkness
• C. >1/3 full sunshine
• D. <1/3 full shade

Answer: (C)

Light provides the energy for photosynthesis, and as light increases, the rate of photosynthesis rises as well—until the photosynthetic machinery becomes saturated and cannot work any faster. In complete darkness there’s no photosynthesis, and with very dim light the rate is low because the plants can’t drive all the available reactions efficiently. Once light is above a modest threshold, the system can operate at its maximum capacity, so the rate is at its peak when light levels are enough to saturate the process. This makes the statement about more than a third of full sunshine the best fit, because it identifies the minimal light level above which the maximum rate is reached.