Gas Exchange and Water Uptake in Leaves

Steps:

• Identify entry points: CO2 enters through stomata via air spaces in the leaf.
• Recall CO2 movement: As a gas, CO2 moves down its concentration gradient from high outside to low inside.
• Determine process for CO2: This passive movement is diffusion, requiring no energy.
• Examine water entry: Water vapor exits via transpiration, creating pull that draws water up from roots through xylem.

Why B is correct:

• Diffusion drives CO2 across the moist stomatal membrane per Fick's law of diffusion, while transpiration pull (cohesion-tension theory) moves water upward against gravity.

Why the others are wrong:

• A: Water uses transpiration pull, not active transport, as it's passive flow in xylem.
• C: CO2 is diffusion, not osmosis (which is solvent movement across membranes); water isn't active transport.
• D: CO2 is diffusion, not osmosis.

Final answer: B