Mass Flow Model

The mass flow hypothesis explains translocation: sucrose solution moves through the phloem from a source, where sucrose is made and loaded, to a sink, where it is used or stored, because of a difference in hydrostatic pressure. Mass flow is the bulk movement of a substance through a given channel or area in a specified time.A model of the mass flow hypothesis can be set up as in the diagram below:Provided sucrose is continually being produced at the source and continually being removed at the sink, the mass flow of sucrose from A to B continues.In the model, two bulbs made of partially permeable membrane are joined by a tube. Bulb A contains a concentrated sucrose solution and represents the source. Bulb B contains a dilute sucrose solution and represents the sink. The tube represents the sieve tube elements, and the beakers of water represent the xylem and surrounding cells. The membranes let water through but not sucrose.Bulb A has the lower water potential, so water enters it by osmosis. This raises the hydrostatic pressure in A, which pushes the sucrose solution along the tube by mass flow towards B. The rising pressure in B forces water out of B into its beaker.The model has a limitation: in a plant, sucrose is actively loaded at the source and removed at the sink, but in the model the flow stops once the concentrations in A and B become equal.