IB Biology SL B3.2 Transport questions

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Question 1

Multiple choiceMedium1 mark

In a plant, the rate of water loss from leaf-cell walls increases. Which change in the xylem and role of cohesion are expected?

Show answer and explanation

Answer: B. Water loss from leaf-cell walls draws water out of the xylem through the cell walls, increasing tension (making the pressure potential more negative). Cohesion keeps water molecules connected in a continuous column, allowing this tension to draw water upward.

A: reverses the direction of water movement, so it incorrectly predicts reduced tension.
C: has the correct movement and tension but reverses the role of cohesion.
D: gets the water movement right but reverses the pressure change; tension is negative pressure potential.

Remember: transpiration creates tension, while cohesion maintains the continuous water column.

Question 2

Paper 2Hard16 marks

Six similar, well-watered potted plants were exposed for 2 h to air at either 40% or 90% relative humidity (RH), with six plants in each condition. Light intensity, temperature and air movement were kept the same. Leaf water loss and xylem pressure potential in the upper stem were measured. Figure 1 shows the experimental arrangement and the individual measurements.

A Experimental arrangementair chamber; RH set to 40% or 90%leaf tissuestem xylemupper-stem xylempressure-potential sensorroots; water suppliedB Leaf water lossleaf water loss / g per 2 h1.51.41.31.21.11.00.90.80.70.60.50.40.30.20.10.040% RH90% RHrelative humidity / %C Upper-stem xylem pressure potentialxylem pressure potential / MPa0.0−0.1−0.2−0.3−0.4−0.5−0.6−0.7−0.8−0.9−1.040% RH90% RHrelative humidity / %
(a)

Using the six measurements in Figure 1B, calculate the mean leaf water loss under each RH condition and the percentage decrease from 40% RH to 90% RH.

[3]
(b)

Using Figure 1C, compare the mean upper-stem xylem pressure potential between the two RH conditions, and relate this comparison to leaf water loss.

[2]
(c)

Explain how water loss from cell walls in the leaf leads to movement of water from roots to leaves in the plant.

[5]
(d)

A student concludes: “Figure 1 proves that xylem tension alone is sufficient to maintain upward water transport; cohesion is unnecessary.”

Evaluate this conclusion using Figure 1 and your knowledge of water transport. Include one limitation of the evidence.

[6]

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