Osmosis

A partially permeable membrane allows some substances through but not others. Imagine dilute sugar solution on one side of a membrane and concentrated sugar solution on the other. If water can cross but sugar cannot, water molecules move in both directions, but more move towards the concentrated solution. This overall movement is the net movement of water.

When the solutions balance, water continues to cross in both directions, but there is no net movement.

Connecting osmosis to changes in plant tissue

Plant cells have partially permeable cell membranes. If potato tissue is placed in a solution more dilute than its cell contents, water enters the cells by osmosis. The tissue gains mass, and its cells become firm as their vacuoles fill with water.

If the surrounding solution is more concentrated than the cell contents, water leaves the cells by osmosis. The tissue loses mass and becomes less firm. Measuring the change in tissue mass therefore provides evidence of net water uptake or loss.

Investigating solution concentration and tissue mass

The required practical investigates the effect of a range of salt or sugar solution concentrations on the mass of plant tissue. Potato cylinders provide a useful method.

You need potato tissue, a cork borer, a cutting tool and ruler, a balance, labelled test tubes or beakers, a measuring cylinder, a timer, paper towel and a range of salt or sugar solutions.

1. Prepare and label solutions of different concentrations, including water as the zero-concentration solution. Use the same solute throughout.
2. Use the cork borer to produce cylinders of the same diameter, then cut them to the same length. Keep the potato source consistent. Use cutting tools as directed by your teacher.
3. Gently blot each cylinder and measure its initial mass using the balance. Record which cylinder will go into each solution.
4. Measure equal volumes of solution into the containers and immerse the cylinders. Keep the temperature and immersion time the same for all concentrations.
5. After the chosen time, remove each cylinder and gently blot it using a consistent method. This removes surface solution so that it is not included in the mass measurement.
6. Measure and record each final mass. Calculate the percentage change in mass.
7. Repeat with several cylinders at each concentration and calculate a mean percentage change. Repeats reduce the influence of variation between individual samples.

The independent variable is solution concentration. The measured outcome is the change in tissue mass, usually expressed as a percentage. Cylinder dimensions, potato source, solution volume, temperature, immersion time and blotting method are controlled so that differences in mass change can be attributed to concentration.

Calculating percentage gain and loss

First calculate the change in mass:

$$\text{change in mass}=\text{final mass}-\text{initial mass}$$

Then express that change relative to the starting mass:

$$\text{percentage change in mass}=\frac{\text{final mass}-\text{initial mass}}{\text{initial mass}}\times100$$

For a cylinder with an initial mass of 5.0 g and a final mass of 5.5 g, the change is +0.5 g. Its percentage change is $\frac{0.5}{5.0}\times100=+10\%$. The positive result means a gain: water entered the tissue.

If another cylinder starts at 5.0 g and finishes at 4.5 g, its percentage change is $\frac{4.5-5.0}{5.0}\times100=-10\%$. The negative result means a loss: water left the tissue.

Percentages make comparisons fairer when initial masses differ. A 0.5 g gain represents a much larger relative change for a small cylinder than for a large one.

Plotting and interpreting an osmosis graph

Put solution concentration on the x-axis and mean percentage change in mass on the y-axis. Label both axes with units, choose even scales and plot the points accurately. Draw a suitable line or curve of best fit to show the trend rather than forcing the line through every point.

Data for How sugar concentration affects potato mass

This example shows percentage mass change falling as sugar concentration increases. Points above zero represent water entering the tissue; points below zero represent water leaving it.

The line crosses zero percentage change at 0.4 mol/dm³. This estimates the external concentration that balances the tissue’s cell contents, so there is no net movement of water. It does not mean water molecules have stopped moving. Actual tissue may give a different crossing point; use the experimental graph rather than memorising this value.

Measuring the rate of water uptake

Mass gain over a known time can estimate the average rate of net water uptake:

$$\text{average rate of mass gain}=\frac{\text{mass gained}}{\text{time taken}}$$

For example, if a cylinder gains 0.6 g during 30 minutes of immersion, its average rate is $0.6\div30=0.02\text{ g/min}$. This means an average net gain of 0.02 g per minute over that period, not necessarily the same gain in every minute. A larger mass gain does not automatically mean a faster rate if the immersion times differ.

Measure initial masses of equal-sized potato cylinders. Immerse them in a range of salt or sugar concentrations for the same time. Remove, blot consistently and measure final masses.

Control potato source, cylinder dimensions, solute type, solution volume, temperature and immersion time. Repeat each concentration and calculate mean percentage changes.

Calculations

$$\text{percentage mass change}=\frac{\text{final mass}-\text{initial mass}}{\text{initial mass}}\times100$$

Positive = gain; negative = loss.

$$\text{average uptake rate}=\frac{\text{mass gained}}{\text{time taken}}$$

Use a compound unit such as g/min.

Graph interpretation

• x-axis: solution concentration, with units.
• y-axis: mean percentage mass change (%).
• Above zero: net water uptake; below zero: net water loss.
• Zero crossing: estimated concentration that balances the tissue; no net water movement.