This question is about the pressure-flow mechanism of phloem transport, and specifically whether the two end steps, loading at the source and unloading at the sink, need energy or not.
- Loading at source is active, unloading at sink is passive: at the source, sucrose is pumped into the phloem sieve tubes against its concentration gradient using ATP-driven proton pumps paired with sucrose transporters, building up a high sugar concentration in the phloem. At the sink, that already-high concentration pushes sugar out into the sink cells, which keep using up sugar, so this step needs no extra energy of its own.
- Loading at source is passive, unloading at sink is active: this reverses the real biology, since building a high sugar concentration in the phloem at the source cannot happen by passive diffusion alone, it needs the active pumping step.
- Both active: this makes unloading harder than it needs to be, since the sink can rely on the gradient set up during loading and does not always need its own active step.
- Both passive: this ignores that sugar must be moved into the phloem against a gradient at the source, which cannot happen without active transport.
Only the first statement matches the pressure-flow model, active loading builds the pressure, and passive unloading releases it at the sink.
Let's summarize:
- Source: active loading, using ATP-powered proton pumps and sucrose co-transporters.
- Sink: passive unloading, driven by the concentration gradient already created at the source.
So the correct answer is that loading at the source is active transport and unloading at the sink is passive transport.