What happens when stomata are opened during photosynthesis?

• A. It blocks CO2 entry and reduces water loss.
• B. It opens to allow CO2 entry for the Calvin cycle and increases water loss via transpiration.
• C. It reduces transpiration but increases CO2 uptake.
• D. It prevents the Calvin cycle from functioning.

Answer: (B)

Opening stomata allows CO2 to diffuse into the leaf, supplying the Calvin cycle with the carbon it needs to produce sugars. At the same time, those openings let water vapor escape, so transpiration increases. This combination—CO2 intake for photosynthesis paired with greater water loss—is the key idea. If CO2 couldn’t get in, or if transpiration didn’t occur, photosynthesis wouldn’t proceed as efficiently; and stomata don’t prevent the Calvin cycle from functioning—rather, they enable it to proceed when CO2 is available.