The pressure gradient is created through the active transport of sucrose into the phloem sieve tubes at the source. This increases the solute concentration inside the sieve tubes, causing water to flow in from the surrounding tissues by osmosis. The influx of water generates a high turgor pressure within the sieve tubes. At the sink, sucrose is actively or passively transported out of the phloem, lowering the solute concentration and allowing water to exit the phloem, thereby reducing the turgor pressure. This pressure differential drives the bulk flow of the phloem sap from source to sink.
