explain mass flow in phloem sieve tubes down a hydrostatic pressure gradient from source to sink

Transport Mechanisms in Plants

Mass Flow in Phloem Sieve Tubes

Phloem is like a network of tubes that carries the “food” (sugar) produced in the leaves (the source) to all parts of the plant that need it (the sink – roots, fruits, growing leaves). The movement is driven by a hydrostatic pressure gradient:

$P_{\text{source}} > P_{\text{sink}}$

Because the pressure is higher at the source, water and sugars flow downhill, just like water flowing through a garden hose when the tap is turned on. The flow is continuous and can be visualised as a steady stream of “sugar‑laden water” moving from the leaves to the rest of the plant.

How the Pressure Gradient is Created

1. Loading: In the leaf, sugars are actively pumped into the sieve tubes, increasing the solute concentration.
2. Osmosis: The high solute concentration draws water in from the surrounding cells, raising the pressure inside the sieve tubes.
3. Pressure Drop: At the sink, sugars are unloaded, lowering the solute concentration and the pressure.
4. Flow: The pressure difference ($\Delta P$) drives the bulk flow of phloem sap from source to sink.

The relationship can be expressed as:

$$\Delta P = P_{\text{source}} - P_{\text{sink}}$$

Where a larger $\Delta P$ means a faster flow rate.

Key Points to Remember

• Phloem transport is a mass flow driven by pressure differences.
• Source = high sugar concentration, high pressure.
• Sink = low sugar concentration, low pressure.
• Water follows sugars by osmosis, creating the pressure gradient.
• Flow direction can reverse if a sink becomes a source (e.g., during fruit ripening).

Illustrative Table

Remember: the higher the pressure at the source, the stronger the “push” that moves sugars to the sink. This is the fundamental principle behind phloem transport in plants. 🚀💧