Lesson 6 of 6 / Movement of substances
Investigating osmosis
What can a change in potato mass tell you about water movement?
In this lesson: Calculate percentage mass change, interpret the evidence and plan a fair comparison.
About 6 min
The key ideaA gain in potato mass suggests net water entry; a loss suggests net water loss. Use percentage change to compare pieces with different starting masses.
Explore the idea
What does zero mass change tell you about the potato?
Illustrative results: invented for this activity
| Sucrose (mol/dm3) | Mass change (%) |
|---|---|
| 0.0 | +20 |
| 0.2 | +10 |
| 0.4 | 0Selected |
| 0.6 | -10 |
| 0.8 | -20 |
0.4 mol/dm3: 0% mass change.No net mass change. These results suggest that the solution and potato tissue had approximately equal water potentials during this test. Water still moved across the cell membranes in both directions.
- An illustrative result
- Selected result
- Line joining these illustrative results
Percentage mass change(final mass - initial mass) / initial mass x 100
The zero crossing at 0.4 mol/dm3 applies only to these invented results. Real potatoes can give different results. Compare equal-sized pieces under the same temperature and time, and blot their surfaces consistently before weighing. Repeat measurements rather than assuming a perfectly straight line.
Explanation
Measure the initial mass of each potato piece, place the pieces in different sucrose concentrations for the same time, then measure their final masses. Blot the surfaces gently and consistently before weighing so liquid clinging to the outside does not distort the result.
Calculate percentage change in mass as (final mass - initial mass) divided by initial mass, multiplied by 100. A positive result is a gain; a negative result is a loss. Percentage change accounts for different starting masses, but does not replace controlling the size and shape of the pieces.
For this investigation, interpret mass changes mainly as water movement. If a piece gains mass, water has entered its cells by osmosis from a higher water potential outside. If it loses mass, water has left towards a lower water potential outside.
A concentration giving zero percentage change estimates where there is no net water movement under the experimental conditions. The outside solution and the tissue have approximately equal water potentials. It does not prove that water has stopped moving or that every cell has the same solute concentration.
Change the sucrose concentration and measure percentage mass change. Keep the source and dimensions of potato pieces, solution volume, temperature, immersion time and blotting method consistent. Repeat each concentration, calculate a mean, and investigate anomalous results. Use more concentrations near zero change to improve the estimate.
Step by step
- 1
Calculate the change
Subtract initial mass from final mass. Keep the positive or negative sign.
- 2
Convert to a percentage
Divide by the initial mass, then multiply by 100. Include the percent sign.
- 3
Explain the biology
Connect the mass change to net water movement, the water potential difference and the partially permeable cell membrane.
Worked example
From a measurement to an explanation
A potato piece has an initial mass of 2.50 g and a final mass of 2.25 g. Calculate its percentage mass change and explain the result.
One way to explain it
(2.25 - 2.50) / 2.50 x 100 = -10%. The potato lost mass because water left its cells by osmosis, from higher water potential inside to lower water potential in the surrounding solution, through partially permeable cell membranes.
Why this answer works
- Use initial mass as the denominator.
- Keep the negative sign to show a loss.
- Explain the cause of the mass loss, rather than repeating the observation.
Is this true? "Zero mass change means no water crossed the cell membranes."
Water can still move both ways. Zero change indicates approximately equal movement in and out overall, within the limits of the measurements.