Cambridge IGCSE Biology 0610 - 3.2 Osmosis

Objective: Investigate osmosis using materials such as dialysis tubing.

What is Osmosis?

Osmosis is the net movement of solvent molecules (usually water) across a semi-permeable membrane from a region of high solvent concentration to a region of low solvent concentration.

A semi-permeable membrane allows some molecules to pass through but blocks others. Dialysis tubing is a common example of a semi-permeable membrane.

Factors Affecting Osmosis

The rate of osmosis is affected by several factors:

• Concentration Gradient: The greater the difference in concentration of solutes on either side of the membrane, the faster the rate of osmosis.
• Temperature: Higher temperatures generally increase the rate of osmosis.
• Nature of Solute: The size and charge of the solute molecules can affect the rate of osmosis.
• Surface Area: A larger surface area allows for a faster rate of osmosis.
• Thickness of Membrane: A thinner membrane allows for a faster rate of osmosis.

Investigating Osmosis with Dialysis Tubing

Dialysis tubing is often used to model a cell membrane. We can place a substance inside the tubing and observe the movement of water across the membrane.

A common experiment involves placing dialysis tubing containing a solute (like sugar or starch) in a beaker of pure water.

Over time, water will move through the dialysis tubing from the outside (where the water concentration is higher) to the inside (where the solute concentration is higher). This is because the water is trying to dilute the solute inside the tubing.

Experimental Procedure (Example Experiment)

1. Cut a piece of dialysis tubing and tie off both ends to create a bag.
2. Add a known amount of a solute (e.g., sugar) to the dialysis bag.
3. Place the bag in a beaker of pure water.
4. Observe the bag over a period of time (e.g., 30-60 minutes).
5. Measure the change in mass of the bag. The increase in mass is due to the uptake of water.

Data Analysis and Calculations

The amount of water taken up by the dialysis bag can be used to calculate the osmotic pressure.

The osmotic pressure ($P$) can be calculated using the following formula:

$$P = iMRT$$ Where:

• $i$ = van't Hoff factor (1 for non-electrolyte like sugar)
• $M$ = molar concentration of the solute
• $R$ = gas constant ($0.0821 \, \text{dm}^3 \text{kPa}^{-1} \text{K}^{-1}$)
• $T$ = temperature in Kelvin

Alternatively, the change in mass of the dialysis bag can be used to estimate the amount of water taken up.

The amount of water taken up is related to the osmotic pressure and the volume of the dialysis bag.

Table: Example Data and Calculations

Variable	Value	Units
Mass of empty dialysis bag	2.00	g
Mass of dialysis bag + solute	2.00 + 5.00	g
Mass of dialysis bag + solute + water	2.00 + 5.00 + 15.00	g
Volume of dialysis bag	5.00	cm3
Concentration of solute (mol/dm3)	0.10	mol/dm3
Temperature (°C)	25	°C

Conclusion

The results of the experiment will demonstrate that water moves across the semi-permeable membrane from the region of lower solute concentration (pure water) to the region of higher solute concentration (sugar solution). This is due to the process of osmosis.