Translocation Essays

Translocation Essays Translocation Paper Translocation Paper Translocation A. The Munch pressure flow model The Principal of Pressure-Flow Model of Phloem Transport The Munch pressure-flow model is an explanation for the movement of organic materials in phloem . By the Munch 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 that large amount of water in xylem enters the cells here. The hydrostatic pressure of the sucrose increase. The right-handed dialysis tubing represents the sinks regions, sites of assimilation such as the actively growing parts or the sites for storage . Here solutes are being used up constantly , or converted to insoluble forms for storage . In other word , they are being unloaded from the sieve tubes . This leads to higher water potential at the sink and subsequently less water enters the cells by osmosis. The hydrostatic pressure of these cells is thus lower than those at the sucrose. A hydrostatic pressure gradient is therefore built up between the sucrose(left-handed tubing) and the sink(right-handed tubing) . This leads to the mass flow of liquid through the phloem (glass tube) from the sucrose to the sink, and water is forced back to the xylem by hydrostatic pressure. The pressure gradient is maintained due to the continuous production and consumption of solutes. In plants, sieve tubes are analogous to the glass tube that connects the two dialysis tubings. Sieve tubes are composed of sieve-tube members, each of which has a companion cell. It is possible that the companion cells assist the sieve-tube members in some way. The sieve-tube members align end to end, and strands of plasmodesmata (cytoplasm) extend through sieve plates from one sieve-tube member to the other. Sieve tubes, therefore, form a continuous pathway for organic nutrient transport throughout a plant. An area where the sucrose is made is called a source. At the Source (e. g. , leaves). During the growing season, photosynthesizing leaves are producing sugar. Therefore, they are a source of sugar. This sugar is actively transported into phloem. Again, transport is dependent on an electrochemical gradient established by a proton pump, a form of active transport. Sugar is carried across the membrane in conjunction with hydrogen ions , which are moving down their concentration gradient . After sugar enters sieve tubes, this increases the solute concentration of the sieve tubes,so water passes into them passively by osmosis. In the Stem. The buildup of water within sieve tubes creates the positive pressure that accounts for the flow of phloem contents. An area where sucrose is delivered from the sieve tube is called a sink. Sinks include the roots and other regions of the plant that are not photosynthetic , such as young leaves and fruits. Water flowing into the phloem forces the sugary substance in the phloem to flow down the plant. The addition of water from he xylem causes pressure to build up inside the phloem and pushes the sugar down. At the Sink . The roots (and other growth areas) are a sink for sugar, meaning that they are removing sugar and using it for cellular respiration. After sugar is actively transported out of sieve tubes, water exits phloem passively by osmosis and is taken up by xylem, which transports water to leaves, where it is used for photosynthesis. Now, phloem contents continue to flow from the leaves (source) to the roots (sink). The pressure-flow model of phloem transport can account for any direction of flow in sieve tubes if we consider that the direction of flow is always from source to sink. Translocation is a passive process that does not require the expenditure of energy by the plant. The mass flow of materials transported in the phloem occurs because of water pressure, which develops as a result of osmosis. Discussion 20% sucrose solution has a lower water potential than that of 5% starch solution, more water molecules move into the sucrose dialysis tubing than that of the starch dialysis tubing. The rise in solution level in sucrose dialysis tubing will be much more significant when compare to that of starch dialysis tubing. In the experiment , the water in the left hand beaker turn from colorless to yellowish brown, this indicated that there is a movement of iodide ions and water molecules across the selectively permeable dialysis tubing. As the experiment proceeds , there is a rise of brown solution in the glass tubing at the right hand tubing. The solution flows through the glass tube slowly. This suggests that there is a net movement of water molecules into the dialysis tubing since the water potential of the sucrose solution is higher than that of pure water. The water level in the right hand dialysis tubing decrease over the time as the experiment is carried on. This is due to the hydrostatic pressure applied by the left-hand flowing solution on the right-hand dialysis tubing. The flow rate of the solution is not constant throughout the experiment, the flow rate increases at first then it slows down and eventually reaches a static static flow rate. This is because an equilibrium status has reached. At first , the water potential of sucrose solution is higher than that of water, therefore water molecules move into the tubing. The continuos influx of water molecules into the left -hand tubing lead to the sucrose-iodine solution move along the glass tube, and the flow rate However,this has generated the hydrostatic pressure towards the right-hand tubing, pushing water molecules in the rich-hand dialysis tubing out of the tube. The net movement of water molecules from the right-hand tubing to the beaker made the starch solution more and more concentrated. Therefore the water potential of the starch solution is lower , water molecules may start to move back to the tubbing. This explains why the flow rate slow down and eventually maintain at a static rate. Due to the hydrostatic pressure, the sucrose solution will be transferred to the right hand tubing which the starch remain in the same tubing. mrothery. co. uk/plants/planttransportnotes. htm http://en. wikipedia. org/wiki/Pressure_Flow_Hypothesis http://en. wikipedia. org/wiki/Dialysis_tubing http://vinzchamakh. wordpress. com/category/biology/chapter-8-transport/