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Selectively Permeable Membrane


The cell membrane is composed of a phospholipid by-layer that selectively allows substances in and out of the cell. The movement of substances in and out of the cell is facilitated through a couple of transport mechanisms. The type of transport employed is determined by the type of substance transported (Chiras, 2010). It is as a consequence of this molecular movement that osmosis and diffusion processes take place (Audesirk, Audesirk & Byers, 2008).

Diffusion process occurs when molecules move from a region of high concentration to a region of less concentration until equilibrium is reached. On the other hand, osmosis occurs when water molecules move through a semi-permeable membrane from an area of elevated water concentration to an area of lower water concentration. The cell membrane acts as the semi-permeable membrane as it allows only the small water molecules to move across it while preventing movement of the solutes that are big.

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The process of osmosis does not need energy to take place. It is the difference in concentration of water across a semi-permeable membrane drives the process. The process of osmosis is regulated by osmotic pressure that is defined as the force per unit area needed to stop the absolute movement of pure water into aqueous solution through a semi-permeable membrane (Chiras, 2010).

In this experiment, a dialysis membrane was prepared and then used to measure diffusion and osmosis process when suspended in a beaker containing concentrated solution and results recorded. Molecules whose size is smaller than the pores of the membrane prepared will go through the membrane and can be determined on the other end by carrying out experiments and determining the actual constituents (Smallwood, 1981).

Materials and Method

The 9 oz cups provided were filled with water to up to two thirds full and then test for presence of starch. This was followed by adding 20 drops of iodine potassium iodide solution to the cup. Approximately 15 cm of dialysis tubing was cut and a knot tied and one end. After tying a knot, 10ml of soluble starch solution and 10ml of glucose solution was pipette and then added in the dialysis tubing and then the remaining end where we were using to add these solutions was tied. The contents of this dialysis membrane were thoroughly mixed and the final colour recorded (Smallwood, 1981).

The exterior of the bag was washed with running water. The colour of the solution in the beaker was recorded before carefully placing the bag inside the beaker containing iodine potassium iodine. This was then kept warm for 10 minutes and then the colour of the solution was tabulated. The bag was then removed and 3 ml of the solution was taken and put in a test tube. The solution in the beaker was mixed and 3 ml taken from and put in the test tube for analysis. To each test tube, 3 ml of Benedict’s reagent was and then heated for 15 minutes and any observable change was noted (Smallwood, 1981).


Before inserting the bag in the beaker we see the membrane was clear while the content of the beaker was orange in colour. After 10minutes and testing, the solution in the bag gave a dark blue solution which is an indication of starch in the bag. This means starch remained in the bag. This is because the size of their molecules is big and therefore could not pass through the semi-permeable membrane. On the other hand, glucose left the bag and entered the beaker as confirmed by dark purple colour in both the beaker and dialysis tubing. This confirmed presence glucose both in the beaker and the membrane.

Glucose molecules are smaller in size than the pores of the membrane prepared and therefore went through the semi-permeable membrane and were determined outside the dialysis tubing i.e. inside the cup. We can also explain this movement of glucose from dialysis tubing into the cup in terms of concentration gradient. Glucose is in high concentration in the dialysis tubing and therefore moved from this region of high concentration to the region low concentration across the semi-permeable membrane (Smallwood, 1981).


Osmosis is a “cell membrane transport mechanism that entails movement of water molecules from a region of high water concentration to a region of low water concentration through a semi-permeable membrane”( Chiras, 2010, p.15).

In this experiment, osmosis process occurred when water molecules moved from the cup into the dialysis tubing where there was low concentration of water. Selective permeability is a process whereby a membrane allows only particles whose size is smaller than the size of its pores. This experiment has shown selective movement of particles across a semi-permeable membrane where glucose molecules whose size is smaller than the pores of the semi-permeable membrane prepared went through the semi-permeable membrane. For starch whose size molecules is bigger than the size of the pores of semi-permeable membrane did not cross the membrane. It was selectively blocked from passing.


Smallwood, W. (1981). Laboratory investigations in biology record book: Teacher’s edition. Morristown, New Jersey, NJ: Silver Burdett Company.

Audesirk, T., Audesirk, G., & Byers, E.B. (2008). Biology with physiology: Life on earth. 5th edn. San Francisco, CA: Benjamin Cummings.

Chiras, D.D. (2010). Human biology. Sudbury, MA: Jones & Bartlett Learning.