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Xylem and the transpiration stream: the pull comes from the top questions
Root hair cells and the uptake of water and mineral ions, the apoplast and symplast routes across the cortex, the Casparian strip at the endodermis, the structure of xylem vessels and the reason for each feature, the cohesion-tension theory of water movement and the evidence for it, the limited part played by root pressure, and the use of a potometer including the distinction between uptake and transpiration.
7 original questions · 21 marks · the xylem and the transpiration stream: the pull comes from the top notes · Plant transport and mineral nutrition
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Water reaches the leaves of a fifty-metre tree even though atmospheric pressure could support a column of water only about ten metres high. Explain how, using the cohesion-tension theory.
Mark scheme
- B1 water evaporates from the wet cellulose walls of the mesophyll cells into the leaf air spaces and diffuses out through the stomata
- B1 the water potential of that mesophyll cell falls, so it draws water osmotically from the cell next to it, and so on back to the xylem in the vein
- B1 water is therefore pulled out of the xylem vessel, and the column of water in the vessel is put under tension rather than being pushed from below
- B1 cohesion, from hydrogen bonding between water molecules, stops the column pulling apart, so removing a molecule at the leaf draws one in at the root
- A1 adhesion, from hydrogen bonding between water and the lignified wall, holds the column against the wall of the narrow vessel
The capillary tube of a potometer has an internal diameter of 0.80 mm. The air bubble travels 62 mm in 4.0 minutes. Calculate the rate of water uptake by the shoot in mm³ min⁻¹.
Mark scheme
- M1 radius is half the diameter, so r = 0.40 mm
- M1 cross-sectional area = πr² = π × 0.40² = 0.50 mm²
- M1 volume swept out = 0.50 × 62 = 31 mm³
- A1 rate = 31.2 ÷ 4.0 = 7.8 mm³ min⁻¹, accepting 7.79 with the unit given
Describe the apoplast and symplast routes taken by water across the root cortex, including the point on each route at which water has to cross a plasma membrane.
Mark scheme
- B1 on the apoplast route water travels through the cellulose cell walls and the spaces between them, without entering the cytoplasm of any cell
- B1 on the symplast route water travels through the cytoplasm, passing from one cell to the next through plasmodesmata
- A1 the symplast route crosses a plasma membrane at the root hair cell, while the apoplast route crosses none until the Casparian strip forces water into the cytoplasm of an endodermal cell
A student records a potometer reading and writes it down as the rate of transpiration. Explain why this will not score, and explain whether the true rate of transpiration was above or below the figure she recorded.
Air enters one xylem vessel of a transpiring tree and the water column in that vessel breaks, yet the leaves above it go on receiving water. Suggest how.
A potted plant shows yellowing between the veins of its oldest leaves, while the youngest leaves stay green. Identify the mineral ion most likely to be in short supply, and account for the older leaves being affected first.
Name the band of waterproof material in the walls of the endodermal cells that blocks the apoplast route into the xylem.
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