Leaf Gas and Water Uptake Mechanisms

Steps:

• Identify entry points: CO2 enters through stomata via gas exchange; water enters roots but moves up to leaves.
• Recall processes: CO2 moves by diffusion down concentration gradient; water pulled by transpiration.
• Match choices: Diffusion fits CO2; transpiration pull fits water ascent in xylem.
• Eliminate mismatches: Osmosis is for solutes, active transport requires energy not primary here.

Why B is correct:

• CO2 diffuses passively through stomata (Fick's law: rate proportional to concentration gradient); transpiration pull creates negative pressure in xylem, drawing water upward (cohesion-tension theory).

Why the others are wrong:

• A: Active transport uses ATP for ions, not primary for CO2 or bulk water.
• C: Osmosis moves water across membranes via solute gradients, but leaf entry relies on transpiration, not osmosis directly.
• D: Osmosis doesn't apply to CO2 gas entry; transpiration pull is for water, not osmosis.

Final answer: B