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Phloem moves organic solutes from sources to sinks. Explain how local loading and unloading can establish a pressure gradient, then evaluate what experiments actually show.
What you'll be able to do
A source releases assimilates such as sucrose into the transport system; a photosynthesising leaf is a common example. A sink uses or stores assimilates, as in growing roots, fruits or storage organs. A storage organ may later act as a source when reserves are mobilised, so roles depend on developmental and seasonal conditions.
Phloem contains living sieve-tube elements associated with companion cells. Sieve plates connect the transport pathway, while companion cells support metabolic functions. A plant can transport assimilates in different directions in different tubes, depending on source–sink relationships; a universal “phloem always goes downward” rule is incorrect.
Loading sucrose at a source lowers the water potential in the sieve-tube region. Water enters from nearby xylem by osmosis, increasing hydrostatic pressure. At sinks, removal and use or storage of sucrose can raise local water potential, allowing water to leave and reducing pressure.
The resulting pressure difference drives bulk flow of phloem sap from source towards sink. Active loading or unloading can require ATP in associated cells, but the long-distance bulk flow is not a succession of sucrose molecules each being pumped through every sieve plate. The hypothesis links solute handling, osmosis and pressure in a causal chain.
A radioactive carbon tracer supplied as carbon dioxide can become incorporated into photosynthetic products. Tracking its later distribution shows where labelled assimilates move, but the label’s presence alone does not prove every mechanistic step of pressure flow.
Removing a ring of bark containing phloem can lead to accumulation of sugars above the ring and reduced delivery to sinks below, while xylem may initially remain functional. This supports phloem’s role, but tissue damage can have additional effects. Use controls and ask which tissue was removed; no single experiment automatically proves every assumption of a transport hypothesis.
Try each question before opening the worked response. Use precise biological terms, connect each cause to its effect, and check units and assumptions when interpreting data.
Common mistakes to avoid
Key takeaways
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.