Characteristics of solute transport

1. When phloem is cut, contents move out. Thus, phloem is under pressure.
2. Phloem sap is rich in solutes (sucrose and other sugars, amino acids)
3. There is a gradient in solute transport - high at source, low at sink
4. Movement of solute is more rapid than diffusion
5. Transport requires energy (blocked if respiration is inhibited)

The Münch hypothesis for phloem transport

Solutes like sucrose are "pumped" into phloem at the source.
The increased solute decreases phloem water potential and water enters by osmosis
The increased water movement increases the pressure of the system.
At the sink, solutes are removed from the phloem.
Removal of solutes increases the water potential of the phloem cells relative to surrounding cells and water exits by osmosis

This inward flux of water at the source increases pressure enough that the phloem sap is pushed along the phloem towards the sink where the pressure is being reduced by removal of solutes and loss of water. The pressure gradient results in "bulk flow" of the solution so that water and solutes can move at rates substantially higher than by diffusion alone.

A simple model for demonstrating the Münch hypothesis for phloem transport

The diagram shows two water-filled compartments connected by a small tube. Another U-shaped tube with bags made of semipermeable membranes are attached to the ends. At the start, the tube and the bag on the left are filled with water while the bag on the right is filled with a concentrated sugar solution. The membrane that the bags are made from allow water but not sugar to diffuse across. Due to the difference in water potential between the contents of the bag on the left and the water it is immersed in, osmosis will occur and water will begin to accumulate in the bag. As this happens, pressure will build and a flow will develop that pushes the sugar solution through the tube to the other bag. This system will only function for a short time since the sugar will eventually be distributed evenly throughout the bag and tube.
In the phloem, there is a continuous input of solute from source tissues and a continuous efflux at the sink. This input and output at the two ends will maintain a pressure differential that will keep liquid flowing. Thus, the driving force for solute transport is a pressure gradient between the source and sink regions.

It takes energy to maintain the pressure gradient. Respiration provides the ATP but the immediate energy source is the electrochemical gradient across the plasma membrane. The electrochemical gradient is generated by ATPase enzymes in the plasm membrane that transport protons from inside the cell into the cell wall using energy derived from ATP hydrolysis.

By increasing the H⁺ ion concentration in the cell wall and lowering the H⁺ concentration in the cytoplasm, a gradient in pH and charge (from the H⁺) is generated. Other proteins in the plasma membrane use this pH an electrical gradient as an energy source for transporting other molecules, like sucrose across the plasma membrane.

The plasma membranes of phloem companion cells are loaded with ATPases and sucrose carriers. In source regions, the sucrose carrier proteins use the H⁺ gradient as an energy source to load sucrose into the companion and and phloem sieve elements. At the sink, sucrose carriers are present that function in the other direction to move sucrose out of the phloem.

Thus, the “electrochemical” pumping of H⁺ causes sucrose loading and unloading of phloem. This in turn maintains a pressure gradient that drives the mass flow of phloem sap.