The living cellular membrane is a structure with selective permeability. Small polar and uncharged molecules as well as, small hydrophobic molecules of solute can be transported unrestrainedly across the cell membrane. But if molecules have a charge, however small, their movement slows down and sometimes stops at all. For your diffusion and osmosis lab reports format, you should comprehend that all cell membranes can dynamically alter their protein and lipid contents in order to vary the size of membrane channels. Exclusively in this way cells regulate membrane permeability, allowing specific ions and molecules to be transported into as well as out of the cell. Particles jump through a semipermeable membrane according to the diffusion law, and if the chemical potentials are equal on both sides of the membrane, any movement of particles just ceases to occur. In the laboratory conditions, tube-shaped dialysis bags are commonly used – from a dissertation proposal to solid scientific investigations – as artificial membranes for separating differently sized molecules. Certainly, in those bags pore sizes are fixed, therefore biologists should use the bags only with specified substances that can pass through. That is to say, all molecules that are small enough to squeeze through pores move unrestrainedly across the membrane, being directed by the law of diffusion. As a good demonstration of diffusion, one may conduct an experiment with a blend of glucose and starch added inside a dialysis bag, whereas Lugol’s solution (I2KI) should be added outside the bag.
As it has been mentioned earlier, the phenomenon of osmosis is connected with the movement of water molecules through a membrane with selective permeability. Writing assignments about osmosis is an easily doable task, and the only thing you should keep straight is a definition of water potential. Strictly speaking, water potential (ψ) is the result from the mixed actions of pressure potential (ψp), which is a consequence from the exertion of tension in a medium, and osmotic potential (ψπ), which is connected with an amount of free energy in a medium. The connection between the aforementioned movement of water molecule and osmotic potential can be investigated in an experiment with sucrose. You may exploit the following scheme unless you need to strictly adhere to an assigned biology MLA essay outline. The experimenter will place dialysis bags with various sucrose concentrations into plastic containers filled with pure, distilled water. Hence, the water potential which is inside the bags will be negative, and as long as outside the bags ψ is zero, the water potential will be positive there. The only thing left to be predicted is which way water will be eager to move.