The Intuition: A Garden Hose and a Squeeze
Imagine you have a long, flexible garden hose filled with water. If you squeeze one end of the hose, water doesn't just dribble out the other end — it shoots out. Why? Because the pressure you apply at one end pushes the entire column of water forward. The water itself doesn't move because it's "trying" to go somewhere; it moves because there's a pressure difference between the two ends.
Now replace the hose with a living plant. The plant needs to move sugar (made in the leaves) down to the roots, or up to developing fruits. It can't pump sugar actively along the entire length of the plant — that would cost too much energy. Instead, it uses a clever trick: it creates a pressure difference at the two ends of the phloem, and lets the whole liquid column flow downhill in pressure.
That is the core idea of the Pressure Flow Hypothesis.
The Precise Statement
The Pressure Flow Hypothesis (also called the Mass Flow Hypothesis, proposed by Ernst Münch in 1930) states that sugars are translocated through the phloem from a source to a sink by a bulk flow of sap driven by a hydrostatic pressure gradient. The gradient is created by the active loading of sucrose at the source (which draws water into the phloem by osmosis) and the unloading of sucrose at the sink (which causes water to leave the phloem).
Pressure gradient=Source turgor−Sink turgor>0
The flow is passive once the gradient is established — the plant only spends energy to load and unload the sugar.
Step-by-Step Mechanism
1. Source: Loading Sugar Creates High Turgor
At the source (e.g., a mature leaf), photosynthesis produces sucrose. This sucrose is actively transported into the sieve tube elements of the phloem. Active transport means the plant uses ATP to pump sucrose against its concentration gradient into the phloem.
The result: the concentration of solutes inside the sieve tube becomes very high. Water from the adjacent xylem (which is always under tension) moves into the phloem by osmosis. As water enters, the volume inside the sieve tube increases, and because the tube walls are rigid (they have cell walls), the pressure rises. This is called turgor pressure.
The phloem is a living tissue, but the sieve tube elements lose their nuclei and most organelles. They rely on companion cells to do the active transport for them.
2. The Pressure Gradient Drives Flow
The high turgor at the source (say, 1.5 MPa) pushes the sap — a solution of sucrose, amino acids, and other solutes — toward regions of lower pressure. The sap moves as a single, continuous column through the sieve tubes. This is mass flow or bulk flow: everything moves together, not molecule by molecule.
3. Sink: Unloading Sugar Lowers Turgor
At the sink (e.g., a growing root, a developing fruit, or a storage tuber), sucrose is unloaded from the phloem. This can happen by diffusion (if the sink cell has lower sucrose concentration) or by active transport (if the sink cell needs to concentrate sugar). As sucrose leaves the sieve tube, the solute concentration inside drops. Water follows by osmosis, leaving the phloem and moving into the surrounding tissues or back into the xylem.
The result: turgor pressure at the sink falls (say, to 0.5 MPa). The pressure difference between source and sink (1.5 − 0.5 = 1.0 MPa) drives the continuous flow of sap.
A common mistake is to think the sap flows because the plant "pushes" it from the source. The flow is driven by the difference in pressure, not by the absolute pressure at either end. If the sink pressure were equal to the source pressure, flow would stop.
Why This Works: The Key Properties
- The phloem is a continuous, living tube — the sieve plates (porous end walls) allow sap to flow freely between cells.
- Water moves by osmosis — the phloem is surrounded by water-conducting xylem, so water can enter and leave easily. …