Connect sugar loading to pressure-driven source–sink transport

1. Orient: Connect sugar loading to pressure-driven source–sink transport

Phloem transports organic solutes among sources and sinks. A mature photosynthetic leaf may be a source, while growing roots, fruits or storage tissues may be sinks. A storage organ can later become a source. These roles describe net loading and unloading, not permanent top and bottom positions.

2. Connect plant structure and mechanism

Sugar loading can lower water potential near a source, promoting water entry and raising pressure. Unloading and water exchange at sinks help maintain a pressure difference. Bulk flow then moves solution through sieve pathways; living sieve elements and associated cells support the transport system. Loading mechanisms vary and are not equivalent to an active pump at every point along a tube.

The ideal packet uses Q = (Psource−Psink)/R with R = 2 MPa·h/mL and solution sugar concentration 0.5 mmol/mL. Sugar flux equals concentration times solution flow. Equal source and sink pressure yields zero modeled bulk flow even if sugar is present.

Different sieve tubes can carry material in different directions, so a whole plant is not represented by one universal downward arrow. Xylem water transport and phloem source–sink transport use different tissue organization and driving conditions, although water exchange couples them. Real viscosity, unloading, network geometry and metabolism are omitted.

3. Work a complete evidence example

With source pressure 1 MPa and sink pressure 0.2 MPa, Q = 0.8/2 = 0.4 mL/h. Sugar flux is 0.5 × 0.4 = 0.2 mmol/h. A concentration alone cannot specify that transport rate without flow.

4. Predict and investigate

Predict first: Would doubling solution sugar concentration necessarily double pressure-driven flow under the explicitly fixed pressures?

Compare source pressures 0.4 and 1 MPa at sink 0.2, then set equal source/sink pressures. Keep solution flow and transported sugar flux separate and explain how an actual loading change could alter the supposedly fixed pressures.

Retain the original first prediction, stated controls, complete ledger and a conflicting or null comparison. Original supplied calculations are not measured plant specimens or completed practical work.

5. Transfer to a changed plant case

Apply the mechanism to a changed case

For source pressure 0.8 MPa and sink 0.4 MPa, find solution and sugar flux.

Retain your calculation, units and assumptions in the lesson notebook before opening this public practice response.

Compare a worked response

Q = 0.4/2 = 0.2 mL/h; sugar flux = 0.5 × 0.2 = 0.1 mmol/h. The calculation assumes the stated concentration and resistance remain fixed.

6. Reflect and defend a bounded conclusion

Connect a tissue or developmental mechanism to the evidence, identify an omitted alternative and propose an independent discriminating observation. Preserve a first report and human critique in the General Biology II workspace. Public practice does not establish independently assessed practical skill, and these conceptual models do not prescribe hormone, fertilizer or field interventions.

Supporting scientific reference, checked 30 October 2026. Original prose, numerical packets and diagrams; no source exercise or figure reproduced. Actual calibrated plant transport or specimen work requires separate supervised evidence.