Transport mechanisms — practice questions

1. Uptake: water enters the root hair cell by osmosis, down a water potential gradient, because the soil solution has a higher (less negative) water potential than the cell sap. Root hairs provide a large surface area for this. [1]
2. Apoplast pathway: some water travels through the cellulose cell walls and intercellular spaces of the cortex, without crossing membranes; this route carries most of the water when transpiration is rapid. [1]
3. Symplast pathway: some water moves through the cytoplasm and through the plasmodesmata from cell to cell, by osmosis down the water potential gradient maintained by the concentrated sap of the inner cells. [1]
4. Endodermis: the Casparian strip of suberin in the endodermal cell walls is impermeable to water, so the apoplast route is blocked and all water must cross the cell surface membrane into the symplast; this lets the plant control which ions reach the xylem. [1]
5. Into the xylem: water passes from the endodermis into the xylem vessels through pits in their lignified walls, from where transpiration pull draws it upwards. [1]

Examiner's note: the marks sit on the named pathways, osmosis, suberin/Casparian strip and cell surface membrane. "Water travels to the xylem" with no mechanism earns none of them.

1. Evaporation creates tension: water evaporates from the walls of mesophyll cells and diffuses out through the stomata, so water is drawn out of the xylem in the leaf and the hydrostatic pressure at the top of the vessel falls below that at the bottom — the transpiration pull. [1]
2. Cohesion: water molecules are held together by hydrogen bonds, so the tension at the top is transmitted down the whole column and the water moves up as a continuous transpiration stream rather than breaking. [1]
3. Adhesion: water molecules are attracted to the cellulose in the xylem walls, which stops the column pulling away from the walls and helps water rise in the narrow vessels. [1]
4. Together: this is the cohesion-tension mechanism; because the pull comes from the leaf, no energy is spent by the plant — it is a passive process, and the lignified xylem walls stop the vessels collapsing under the tension. [1]

Examiner's note: both hydrogen bonds and the two named properties must appear. Candidates who write only "water is pulled up by transpiration" score one mark of the four.

1. Proton pumps: companion cells use ATP to actively transport hydrogen ions out of their cytoplasm into the cell wall, building a high concentration of H⁺ outside the cell. [1]
2. Cotransport: H⁺ ions diffuse back into the companion cell down their concentration gradient through a cotransporter protein that carries sucrose in with them, so sucrose enters against its own concentration gradient (secondary active transport). [1]
3. Into the sieve tube: sucrose passes from the companion cell to the sieve tube element through plasmodesmata. [1]
4. Water follows: the sucrose lowers the water potential of the sieve tube, so water enters by osmosis from the xylem and surrounding cells, raising the hydrostatic pressure at the source. [1]
5. At the sink: sucrose is unloaded and used or stored as starch, the water potential of the sieve tube rises, water leaves by osmosis and the hydrostatic pressure there is low. [1]
6. Mass flow: the sap moves along the sieve tube down the hydrostatic pressure gradient from source to sink, carrying the assimilates with it. [1]

Examiner's note: a 6-mark question needs both halves — the loading (steps 1–3) and the flow (steps 4–6). The commonest lost mark is omitting that water enters by osmosis at the source: without it there is no pressure gradient to explain.