The incredibly thin, yet sturdy, plasma membrane that encloses every cell is vitally important in maintaining cellular integrity. It is a permeability barrier that separates the interior from the external environment of the cell, regulates the vital flow of molecular traffic into and out of the cell, and provides many of the unique functional properties of specialized cells. Membranes inside the cell surround a variety of organelles. A plasma membrane acts as a selective gatekeeper for the entrance and exit of the many substances involved in cell metabolism. Because conditions outside the cell are different from and more variable than conditions within the cell, it is necessary that the passage of substances across the membrane be rigorously controlled. Intracellular organelles are also constrained by a membrane which separates them from the cytoplasm of the cell. The main constituents of the plasma membrane are phospholipids, proteins and carbohydrates. The phospholipids are regularly organized in the membrane according to their polarity: two layers of phospholipids form the lipid bilayer with the polar part of the phospholipids pointing to the exterior of the layer and the non-polar phospholipid chains in the interior. Proteins can be found embedded in the lipid bilayer and there are also some carbohydrates bound to proteins and to phospholipids in the outer face of the membrane. Channel proteins provide hydrophilic pathways which regulate the movement of water molecules and small ions in and out of the cell.
Diffusion: The spontaneous movement of molecules or particles in solution along a concentration gradient (i.e. from areas of high concentration to a low concentration) until there is an equilibrium.
Osmosis: The diffusion of water molecules through a selectively permeable membrane from a region of low solute concentration to a region of high solute concentration.
• Demonstrate diffusion across a semi-permeable membrane
•Measure the effects of various concentrations of solute in the process of osmosis
• Examine the effects of osmosis on non-living model using dialysis tubing.
Method: Refer to for full details to USQ, Biology 1, Kennedy and Withers practical manual 2013. The dialysis tubing serves as the permeable membrane with which diffusion will occur. Since cells are so small in size, this experiment poses as a model in which diffusion can be witnessed and recorded, to begin the experiment 30cm dialysis tubing was used and tied off at the bottom and filled with a water for control, .5Mand 1M sucrose and salt solution . The bag was filled with one solution at a time and a straw was inserted to record rate of osmosis of each solution inside the dialysis tubing and were then placed in jar filled with water to see if there will be any diffusion across the membrane. Once all the bags were placed in water jar, the height of solution level on straw were recorded every five minutes interval from the initial starting time to calculate the rate of osmosis.
This experiment was performed to demonstrate the process of osmosis and to show visible as well as quantitative evidence proving that osmosis occurred. The intent of the experiment was to study osmosis. To that effect, the experiment was successful. Osmosis occurred—there was a change in level of solution in straw, which was placed in dialysis tubing filled with different concentration of sucrose and salt solution. This indicates that water diffused across the semi-permeable dialysis tubing.
Hypothesize that the more concentrated sucrose solutions will cause a greater amount of osmosis. From the results taken, the more concentrated solutions of 1M sucrose seemed to draw the water towards it.