What is the mechanism behind sap flow in phloem?

Sap flow in phloem is driven by the pressure-flow hypothesis, involving active transport and osmotic pressure.

The pressure-flow hypothesis, also known as the mass flow hypothesis, is the primary mechanism behind sap flow in phloem. This process involves the active transport of sugars into and out of the phloem, creating a pressure gradient that drives the flow of sap.

In the leaves of a plant, where photosynthesis occurs, sugars are produced. These sugars, primarily sucrose, are actively transported into the phloem cells. This active transport requires energy in the form of ATP. As the concentration of sugars in the phloem increases, water follows by osmosis from the adjacent xylem vessels, which are part of the plant's water transport system. Osmosis is the movement of water from an area of lower solute concentration to an area of higher solute concentration. This influx of water increases the pressure in the phloem at the source (the leaves).

At the same time, sugars are being actively transported out of the phloem into the plant's roots and other non-photosynthesising parts, known as sinks. This decreases the sugar concentration in the phloem at the sink, causing water to move out of the phloem and back into the xylem by osmosis, reducing the pressure at the sink.

The difference in pressure between the source and the sink drives the flow of sap along the phloem from the source to the sink. This is a bulk flow mechanism, meaning it involves the movement of water and solutes together.

In summary, the pressure-flow hypothesis explains sap flow in phloem as a process of active transport of sugars and passive transport of water, creating a pressure gradient that drives the flow from areas of high pressure (source) to areas of low pressure (sink).