Refer to the diagram of a plant stem below to finish the following statement:
On a sunny day, there will most likely be…

Question

BlackTom AI · Accepted Answer

Mass flow in the phloem is driven by pressure differences created when sugars produced in the leaves (the source at C) are loaded into sieve-tube elements and then unloaded at sinks (the roots at G). This is the pressure‑flow (mass flow) mechanism, so mass flow from C to G is the expected outcome on a sunny day.

Relevant concepts:
- Phloem translocation (pressure‑flow hypothesis): active loading of sucrose at a source raises solute concentration in phloem, lowering water potential and drawing water in from the xylem; the resulting high hydrostatic pressure pushes phloem sap toward sinks where sugars are removed.
- Source vs sink: leaves (C) are sources when photosynthesizing; roots (G) are common sinks for transported sugars.
- Water movement between xylem and phloem: water enters phloem at the source and often returns to xylem at the sink.

Step-by-step reasoning:
1. On a sunny day photosynthesis in the leaf (location C) is high, increasing sucrose concentration in phloem sieve tubes at C (phloem loading).
2. Higher solute concentration at C lowers the water potential in phloem there relative to adjacent xylem, so water moves from xylem into phloem by osmosis.
3. The influx of water raises hydrostatic (turgor) pressure at C.
4. At the sink (G, root tissue), sucrose is actively or passively unloaded, lowering solute concentration and hydrostatic pressure in phloem at G.
5. The pressure gradient (high at C → low at G) drives bulk flow (mass flow) of phloem sap from C to G, delivering sugars to the roots; water may then move back into xylem.

Why the other options are incorrect:
- “A higher water potential at C compared to B.” Water potential relationships depend on solute loading and transpiration; the key predictable process is pressure-driven flow from source to sink, not a simple assertion that C > B.
- “Transpiration from G to C, and ultimately D.” Transpiration is loss of water from leaves and drives xylem flow upward from roots to leaves, not a flow from root phloem (G) to leaf phloem (C).
- “Cohesion and adhesion in phloem that prevents water loss.” Cohesion-tension applies to xylem water transport; cohesion/adhesion in phloem is not the mechanism preventing water loss and does not explain bulk sugar movement.

Therefore, the correct choice is mass flow from C to G, which matches the provided answer.

Refer to the diagram of a plant stem below to finish the following statement: On a sunny day, there will most likely be…单项选择题

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