Questions: Transpiration

Question 1

What drives water movement upward from the roots to the leaves of a plant?

• A. A pump in the roots pushes water up
• B. Gravity pulls water up from the ground
• C. Water evaporating from the leaves through transpiration creates a pulling force that draws water up
• D. The leaves squeeze water up through the stem

Answer: (C)

Transpiration — the evaporation of water from leaf surfaces — creates a continuous pulling force. As water molecules evaporate from the stomata, they pull on the water molecules below them (because water molecules stick to each other), drawing water up through the stem from the roots. There is no pump — the evaporation at the top drives the entire flow.

Question 2

Stomata can be permanently open without any cost to the plant.

• T. True
• F. False

Answer: False

When stomata are open, the plant can take in CO₂ for photosynthesis, but it also loses water through transpiration. If stomata stay open too wide or too long — especially in hot, dry conditions — the plant can lose water faster than roots can absorb it, leading to wilting. Plants regulate their stomata, opening them when CO₂ is needed and closing them to conserve water.

Question 3

Explain the tradeoff plants face with their stomata during photosynthesis.

Model answer: Plants need to open their stomata to let carbon dioxide in for photosynthesis, but when the stomata are open, water vapor escapes through transpiration. In dry or hot conditions, plants may close their stomata to conserve water, but this limits photosynthesis because CO₂ cannot enter. Plants must balance the need for CO₂ with the need to prevent excessive water loss.

This tradeoff is a central challenge in plant biology. Desert plants have evolved special adaptations (thick waxy coatings, opening stomata only at night, storing CO₂ as acid) to minimize water loss while maintaining photosynthesis. The stomatal tradeoff also explains why plants wilt on hot, dry days.