K224 / K225 / 2027

Lesson 4 of 5 / Movement of substances

Investigating osmosis

What can a change in potato mass tell you about water movement?

I can calculate percentage mass change, interpret the evidence and plan a fair comparison.

  • I can define osmosis and investigate and describe its effects on plant and animal tissues.Syllabus

    Syllabus K224 / K225, Topic 2(b). Define osmosis and investigate and describe its effects on plant and animal tissues.

About 6 min

Make a guess. You are not marked.

Potato piece A goes from 2.0 g to 2.4 g. Piece B goes from 5.0 g to 5.5 g. Which gained more water for its size?

  1. They gained the same, as both got heavier.
  2. A, with a 20% gain against 10% for B.
  3. B, because it gained 0.5 g and A gained only 0.4 g.
Show the answer

A, with a 20% gain against 10% for B.

Percentage change divides by the starting mass. A gained 0.4 g out of 2.0 g; B gained 0.5 g out of 5.0 g.

The key idea

A 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

Illustrative potato mass change against sucrose concentrationThe invented values are: 0.0 mol per cubic decimetre, plus 20 percent; 0.2, plus 10 percent; 0.4, zero percent; 0.6, minus 10 percent; 0.8, minus 20 percent. The highlighted result is 0.4 mol per cubic decimetre and 0 percent mass change. All values also appear in the table.Mass change (%)+20+100-10-200.00.20.40.60.8
Sucrose concentration (mol/dm3)
All illustrative results
Sucrose
(mol/dm3)		Mass change
(%)
0.0+20
0.2+10
0.40Selected
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

Weigh each potato piece. Put the pieces in sugar (sucrose) solutions of different concentrations for the same length of time. Then take them out and weigh them again. Blot each piece gently in the same way before weighing, so that liquid on the outside does not add to the mass.

Work out the percentage change in mass: (final mass - starting mass) / starting mass x 100. A positive answer is a gain in mass; a negative answer is a loss. Percentage change lets you compare pieces that started at different masses. You should still use pieces of the same size and shape.

In this experiment, a change in mass is mainly caused by water moving. If a piece gains mass, water has entered its cells by osmosis, because the solution outside had a higher water potential. If it loses mass, water has left, because the solution outside had a lower water potential.

The concentration where the mass does not change is an estimate of where there is no net water movement. There, the solution and the potato cells have about the same water potential. It does not mean that water has stopped moving.

Change only the sugar concentration. Keep everything else the same: where the potato pieces come from, their size, the volume of solution, the temperature, the time and the way you blot them. Repeat each concentration, work out a mean, and check any result that does not fit. Testing more concentrations close to zero change gives a better estimate.

Step by step
  1. 1

    Calculate the change

    Subtract initial mass from final mass. Keep the positive or negative sign.

  2. 2

    Convert to a percentage

    Divide by the initial mass, then multiply by 100. Include the percent sign.

  3. 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.

Watch out for this

A student says: "Zero mass change means no water crossed the cell membranes." What is wrong with this?

Show the answer

Water can still move both ways. Zero change indicates approximately equal movement in and out overall, within the limits of the measurements.

Check your understanding

A potato piece changes from 4.00 g to 4.40 g. Which conclusion fits these measurements?

  1. It gained 10% because sucrose entered by osmosis.
  2. It gained 0.40%, so a little water entered.
  3. It gained 10%, consistent with net water entry by osmosis.
Show the answer

It gained 10%, consistent with net water entry by osmosis.

(4.40 - 4.00) / 4.00 x 100 = 10%. In this investigation, the gain is interpreted mainly as water entering the cells.

Quick recall

Card 1 of 3. Answer in your head, then check.

What can a change in potato mass tell you about water movement?

You can return to this lesson any time.