IB DP Biology- B3.2 Transport- IB Style Questions For HL Paper 2 -FA 2025

(a)

1. Empty lumen: Xylem vessels have no cytoplasm or internal cell contents, forming an open tube that allows water to move freely with very little resistance.
2. Lignified walls: Their thick, lignin-reinforced walls strengthen the vessel and prevent collapse even when water is under tension during upward movement.
(Other valid features include: perforated or missing end walls forming continuous columns, or pits that permit sideways flow between vessels.)

(b)

When humidity is high and transpiration slows down, plants rely on root pressure to push water upward.
1. Root cells actively pump mineral ions into the xylem.
2. This increases the solute concentration in the xylem, making it hypertonic relative to the soil.
3. Water enters the xylem from the soil by osmosis.
4. The resulting buildup of water creates positive pressure in the root, pushing water up the xylem to the leaves even without transpiration pull.