Which type of transport causes long-distance water movement in the xylem?

Question

BlackTom AI · Accepted Answer

Bulk flow moves large volumes of water under a pressure gradient through xylem vessels.

1) Relevant knowledge points and concepts
- Bulk flow: mass movement of fluid driven by pressure differences, not by individual molecule diffusion. In plants this occurs in xylem conduits.
- Cohesion–tension mechanism: transpiration at leaf stomata generates a negative pressure (tension) that pulls a continuous water column upward; cohesion between water molecules and adhesion to xylem walls transmit this tension.
- Water potential and hydrostatic pressure: differences in pressure potential (Ψp) drive mass flow in the xylem; the driving force is a gradient of negative pressure from root (less negative) to leaf (more negative).
- Distinction from diffusion/osmosis/active transport: those operate at cellular or short distances and involve solute gradients or membrane transport and are much slower than bulk flow for long-distance movement.

2) Step-by-step reasoning
- Evaporation of water from mesophyll cell walls at the leaf surface reduces leaf water potential and creates tension in the apoplast.
- Because water molecules are cohesive, this tension is transmitted down the continuous xylem water column to the roots.
- A pressure gradient (more negative at the leaf, less negative at the root) is established, producing bulk flow of water up xylem vessels.
- Xylem vessel structure (wide lumen, continuous conduits) allows rapid, long-distance movement driven by that pressure difference.

3) Calculations
- No numerical calculation is required here; the mechanism is qualitative (pressure-driven mass flow). If pressures were given, flow (Q) could be related to pressure difference (ΔP) and resistance (R) via Q = ΔP/R (Poiseuille-type relation for conduits).

4) Why the correct answer is correct
- Bulk flow specifically denotes pressure-driven mass movement appropriate for long-distance transport in xylem; the cohesion–tension system produces the necessary pressure gradient and continuous columns for bulk flow.

5) Why other options are incorrect (brief)
- Diffusion/osmosis: operate via concentration or water potential gradients across membranes and are effective only over short distances, too slow for whole-plant water transport.
- Active transport: requires metabolic energy and moves solutes across membranes, not bulk water through xylem.
- Root pressure: can push water upward but is generally insufficient for tall plants and is not the primary long-distance mechanism.

6) Conclusion
The long-distance movement of water in the xylem is caused by bulk flow; this matches the provided correct answer, "bulk flow."

Which type of transport causes long-distance water movement in the xylem?单项选择题

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