In the pressure‑flow model, high turgor pressure at the source region is caused primarily by

Question

BlackTom AI · Accepted Answer

Loading sucrose into the sieve tubes increases solute concentration in the phloem, lowering water potential so water enters by osmosis and raises turgor pressure at the source.

1. Relevant knowledge points and concepts
- Pressure‑flow (mass flow) model: phloem translocation is driven by differences in turgor (hydrostatic) pressure between source (high pressure) and sink (low pressure) regions.
- Source: tissue where sugars (mainly sucrose) are produced or released; Sink: tissue where sugars are consumed or stored.
- Active phloem loading: companion cells actively transport sucrose into sieve-tube elements, increasing phloem solute concentration.
- Osmosis: water moves from higher water potential (xylem/neighboring cells) to lower water potential (loaded phloem), generating turgor pressure.

2. Step‑by‑step reasoning process
- Step 1: Companion cells use ATP to load sucrose into sieve tubes at the source (active transport).
- Step 2: Sucrose accumulation raises solute concentration in sieve-tube sap → decreases water potential (more negative).
- Step 3: Water moves osmotically from adjacent xylem or cells into the phloem where water potential is lower.
- Step 4: The inward water flux increases hydrostatic (turgor) pressure in the source phloem.
- Step 5: This high pressure drives bulk flow of sap toward sinks, where sucrose is unloaded, reducing solute concentration and turgor, allowing water to exit.

3. Intermediate steps/details (no numerical calculation required)
- Molecular step: active transport (H+-sucrose symport or companion cell mechanisms) explains how sucrose concentration becomes higher in phloem than surrounding tissues.
- Physical step: osmotic water influx converts the chemical potential difference into a pressure gradient that powers mass flow.

4. Why the correct answer is correct
Active loading of sucrose is the proximate cause of the osmotic gradient that draws water into sieve elements and creates the high turgor pressure at sources; without active loading, the necessary solute gradient and resultant pressure would not be established.

5. Why other options are incorrect
- Evaporation from leaves (transpiration) primarily creates negative pressure (tension) in xylem, not high positive turgor in phloem.
- Root pressure is a xylem phenomenon generating modest positive pressure in roots; it does not primarily establish phloem source turgor or drive long‑distance phloem flow.
- Active water uptake from the soil increases xylem water availability but does not by itself create the phloem solute/osmotic gradient required to raise phloem turgor at sources.

6. Conclusion
Therefore, the high turgor pressure at the source region in the pressure‑flow model is caused primarily by loading of sucrose into the sieve tubes, which matches the provided correct answer.

In the pressure‑flow model, high turgor pressure at the source region is caused primarily by单项选择题

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