A. The Munch pressure flow model
The Principal of Pressure-Flow Model of Phloem Transport
The Münch pressure-flow model is an explanation for the movement of organic materials in phloem .By the Münch pressure-flow experiment, two dialysis tubings are connected by a glass tube. The dialysis tubings only permeable to water or particles which have smaller size than the pores of the tubing,but impermeable to the larger solutes.As larger molecules such as proteins and polysaccharides(starch) that have dimensions significantly greater than the pore diameter of the dialysis tubing can pass through the tubings and they are retained inside the tubings.Smaller molecules such as water molecules and iodide ions are small enough to pass through the pores. The left-handed dialysis tubing contains 20%sucrose and iodine solution .The right-handed dialysis tubing contained 5% starch solution . The two entire dialysis tubings are submerged in distilled water of two separated beakers.Distilled water flows into the left-handed dialysis tubing because it has the higher solute concentration than that of the right-handed one. The entrance of water creates a positive pressure,thus a higher hydrostatic pressure is developed in left-handed tubing .The higher hydrostatic pressure in left-handed dialysis tubing induces water to flow from left to right through the glass tube.Therefore,water flows toward the right-handed dialysis tubing. This flow not only drives water toward the right tubing, but it also provides enough force for water to move out from the membrane of the right-handed dialysis tubing—even though the right-handed tubing contains a higher concentration of solute than the distilled water. Eventually the system will come to equilibrium.
The left-handed dialysis tubing represents the sucrose regions, i.e. the photosynthetic tissues where sugars and other organic solutes are continuously synthesized. This results in a low water potential at the source so...