My course is designed to prepare students for the New York State Living Environment Regents exam. During our unit of study on diffusion and osmosis, my students learn that all cells contain membranes that are selectively permeable, allowing certain things to pass into and leave out of the cell. They learn that diffusion is the process in which molecules of a substance move from an area of high concentration to areas of low concentration.
Osmosis is the process in which water crosses membranes from regions of high water concentration to areas with low water concentration. While molecules in diffusion move down a concentration gradient, molecules during osmosis both move down a concentration gradient as well as across it. Both diffusion and osmosis are types of passive transport, which do not require help in the form of energy.
The cell membrane determines what substances can diffuse into a cell. This characteristic of a cell membrane is called its permeability. Many cells are selectively permeable – some substances can pass through the membrane but others cannot. Dialysis tubing is an artificial semi-permeable membrane with similar properties to the cell membrane.
The students were conducting an experiment to investigate the selective permeability of dialysis tubing. We tested the permeability of the tubing to glucose, starch and iodine (potassium iodide). The dialysis tubing was clipped to form a bag so that glucose and starch was fed into the bag through the other end, and was also clipped to avoid the seeping of the solution. Water with several drops of iodine added to it until it was visibly yellow-amber was added to a 400ml beaker. The bag was then placed in the beaker, which was stirred with a magnetic stirrer. It was left there for 30 minutes.
Students observed that the color of the solution in the bag changed to a blue-black color. This showed that iodine was able to pass through the membrane into the bag. The solution in the beaker became pale yellow-amber; this showed that starch didn’t pass through the membrane into the beaker. To confirm the presence of glucose in the beaker and also the bag, students performed a Benedict test on the solutions, including tap water, which was the control. The beaker solution turned a light brown color after Benedict solution was added to it and suspended in a water bath for 10 minutes.The bag solution also changed to a brown color, while the tap water remained blue. This experiment showed that dialysis tubing is selective in its permeability to molecules. It was permeable to glucose and iodine, but not starch.
The real world/life application that I wanted students to learn from this experiment is that living cells need to obtain nutrients from their environment and get rid of waste materials to their surroundings. This exchange of materials between the cell and its surroundings is crucial to its existence. Cells have membranes composed of a phospholipid bilayer embedded with proteins. This cell membrane can distinguish between different substances, slowing or hindering the movement of other substances and allowing others to pass through readily.
This property of the cell is known as selective permeability. Selective permeability is a property of a cell membrane that allows it to control which molecules can pass (moving into and out of the cell) through the pores of the membrane. A selective permeable membrane only allows small molecules, such as glucose or amino acids, to readily pass through, and it inhibits larger molecules like protein and starch from passing through it.
The dialysis tubing is a semi-permeable membrane tubing used in separation techniques and for demonstrating diffusion, osmosis, and the movement of molecules across a restrictive membrane. It separates dissolved substances of different molecular sizes in a solution, and some of the substances may readily pass through the pores of the membrane while others are excluded. The dialysis tubing is made up of cellulose fibers. This is shaped in a flat tube.
The students concluded that the dialysis tubing doesn’t allow all kinds of substances to pass readily through the pores of its membrane. This means that it is selective in its permeability to substances. The dialysis tubing was permeable to glucose and iodine, but not to starch. Starch was excluded because it has a larger molecular size than glucose and iodine.