Transport in Cells » Osmosis

What is osmosis?

Cells are surrounded by a cell membrane. Water can move across this membrane, and the way it does it is called osmosis.

Osmosis is a special case of diffusion, so it follows the same idea of particles spreading out. The difference is that only water is moving, and it must cross a membrane.

Osmosis is the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.

Key terms:

• Osmosis: the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.
• Partially permeable membrane: a membrane with tiny holes that lets small particles such as water through, but not larger particles such as sugar.
• Solute: a substance dissolved in a liquid, such as sugar or salt.
• Dilute solution: a solution with a lot of water and only a little solute.
• Concentrated solution: a solution with a lot of solute and less water.

Which way does the water go?

Think of a cell sitting in a sugar solution. The membrane lets water through but not the sugar. Water particles are free to move both ways, but there are more water particles on the dilute side. So more water moves from the dilute solution into the concentrated solution than the other way round.

There is a net movement of water from the dilute side to the concentrated side. It carries on until the two sides are the same concentration.

Osmosis in animal cells

An animal cell has a cell membrane but no cell wall, so its size depends on how much water is inside it.

• In a more dilute solution than the cell: water moves into the cell by osmosis. The cell swells and may burst.
• In a more concentrated solution than the cell: water moves out of the cell by osmosis. The cell shrinks and shrivels.
• In a solution of the same concentration: there is no net movement of water. The cell stays the same size.

Osmosis in plant cells

Plant cells have a cell wall as well as a membrane, and the wall is strong. This changes what happens.

• In a dilute solution or pure water: water moves into the cell by osmosis and the vacuole swells. The cell wall stops the cell bursting, and the cell becomes firm and turgid. This is how plant cells help to support a plant.
• In a concentrated solution: water moves out of the cell by osmosis and the cell shrinks and goes soft and limp.

Plant tissue, such as a piece of potato, behaves in the same way. It gains mass in a dilute solution and loses mass in a concentrated one. The change in mass is a measure of how much water has moved.

Percentage gain and loss of mass

Two pieces of tissue rarely start at exactly the same mass. To compare fairly, we use a percentage change.

Percentage change in mass = (change in mass ÷ starting mass) × 100

The change in mass is the final mass minus the starting mass. A gain gives a positive answer. A loss gives a negative answer.

Notice that both cylinders changed by 0.6 g, but the percentage changes are different because they started at different masses. That is why percentages make the comparison fair.

Rate of water uptake

A rate tells you how fast something happens. For osmosis in plant tissue, the rate of water uptake is the gain in mass for each unit of time. It is a simple compound measure, because its unit combines two units: grams and minutes.

Rate of water uptake = change in mass ÷ time

If the mass was measured in grams and the time in minutes, the unit is g/min.

Plotting and interpreting a graph

Students often investigate a range of concentrations of salt or sugar solution and measure the percentage change in mass of plant tissue in each. The full method is in the lesson Required Practical: Osmosis in Plant Tissue. Here we focus on the results.

Sugar solution (mol/dm3)	Percentage change in mass (%)
0.0	+14
0.2	+6
0.4	−2
0.6	−9
0.8	−15

How to plot it:

• Put the concentration, which you chose, on the x-axis. Put the percentage change in mass on the y-axis.
• Choose scales that use most of the graph paper, and include the negative values on the y-axis.
• Label both axes with their units.
• Plot each point accurately and draw a smooth line of best fit.

How to interpret it:

• The line goes downwards: as the concentration of the solution increases, the percentage change in mass falls.
• Where the line crosses 0% on the y-axis, there is no change in mass. Here the solution is the same concentration as the inside of the plant cells, so there is no net movement of water. For this data it is about 0.35 mol/dm³.
• Where the line is above 0% the tissue gains mass, because water moved in. Where it is below 0% the tissue loses mass, because water moved out.

Exam-style question

A student places a piece of potato in a concentrated salt solution.

(a) Describe what osmosis is. [2 marks]

(b) The potato had a starting mass of 8.0 g and a final mass of 6.8 g. Calculate the percentage change in mass. [2 marks]

(c) Explain why the mass of the potato changed. [2 marks]

(d) A human cell is placed in pure water. Explain what happens to it. [2 marks]