How do you do the osmosis required practical?

What does the osmosis practical investigate?

It tests how the concentration of the solution around a piece of plant tissue affects how much water moves in or out of it. You measure that as a change in mass. Potato is the usual tissue. The solutions are salt or sugar dissolved in water at a range of concentrations, with plain distilled water as the most dilute.

Osmosis is the diffusion of water from a dilute solution to a more concentrated one through a partially permeable membrane. If the solution outside is more dilute than the inside of the potato cells, water moves in and the potato gains mass. If it is more concentrated, water moves out and the potato loses mass.

It is one of AQA's required practicals in both GCSE Biology and Combined Science: Trilogy, in the cell biology topic.

What equipment do you need?

The key kit is a cork borer, which cuts potato cylinders of matching width, and a balance that reads to 0.01 g. You also need salt or sugar solutions at several concentrations.

• A potato
• A cork borer, to cut cylinders of the same width
• A scalpel, a white tile and a ruler, to trim them to the same length
• A balance that reads to 0.01 g
• Salt or sugar solutions at a range of concentrations, for example 0.2, 0.4, 0.6, 0.8 and 1.0 mol/dm³
• Distilled water
• Boiling tubes, a rack and labels
• A measuring cylinder
• Paper towels

What is the method?

Cut potato cylinders that are all the same size and weigh them. Leave each one in a different concentration for the same length of time, then blot them dry and weigh them again.

• Use a cork borer to cut potato cylinders, all the same width and with no skin on them.
• Trim them to the same length, for example 3 cm, with a scalpel on a white tile.
• Label a boiling tube for each concentration, plus one for distilled water. Add the same volume of solution to each, for example 10 cm³.
• Blot each cylinder gently with paper towel, weigh it and record its starting mass.
• Put one cylinder in each tube, fully covered by the solution. Leave them all for the same time, for example 24 hours.
• Take each cylinder out, blot it the same way as before, and weigh it again.
• Repeat the whole set, or use three cylinders for each concentration, so you can work out a mean.

What are the variables?

You change the concentration of the solution and measure the change in mass. Size is the control that needs most care: a bigger surface area lets water move in or out faster, so cut every cylinder to the same width and length.

How do you work out and present the results?

Work out the percentage change in mass for each cylinder: percentage change = (final mass − starting mass) ÷ starting mass × 100. A percentage lets you compare cylinders fairly, because they never start at exactly the same mass.

Worked example: a cylinder goes from 2.50 g to 2.80 g. The change is +0.30 g, and 0.30 ÷ 2.50 × 100 = +12%. Another goes from 2.40 g to 2.04 g. That change is −0.36 g, and −0.36 ÷ 2.40 × 100 = −15%. Keep the minus sign: it tells you the potato lost water.

Plot concentration on the x-axis and percentage change in mass on the y-axis. Some values are negative, so the y-axis has to go below zero. Draw a line or curve of best fit. Where it crosses zero, the potato neither gained nor lost mass, so that concentration is an estimate of the concentration inside the potato cells.

How do you make it accurate and safe?

Blotting matters more than it looks. Water left on the surface adds mass that never went into the potato, so blot every cylinder the same way, before and after. Use a balance that reads to 0.01 g, and check each cylinder stays fully covered. Repeats let you spot an odd result and work out a mean.

The main hazard is the cutting. Cut down onto a tile, away from your fingers, and press the cork borer into potato resting on the tile, never towards your hand. The solutions are low hazard, but wipe up spills so nobody slips.

What do exam questions ask about it?

Questions usually give you a results table to work from. You might calculate a percentage change, or read off the concentration where the line of best fit crosses zero. If one result doesn't fit the pattern, the likely cause is a cylinder that was blotted less than the others, or one that was not fully covered. Expect to explain a gain or loss in mass using the word osmosis, and to say why percentage change is used rather than change in mass. The 6-mark version asks you to describe the method.

An exam-style question we wrote: "A potato cylinder had a mass of 3.20 g. After a day in sugar solution its mass was 2.88 g. Calculate the percentage change in mass." The change is −0.32 g, and −0.32 ÷ 3.20 × 100 = −10%.

Common mistakes

• Dividing by the final mass. Always divide the change by the starting mass.
• Dropping the minus sign on a loss of mass. A negative result is meaningful: it shows water left the cells.
• Saying "the sugar moves into the potato". In osmosis, it is the water that moves.
• Leaving out the partially permeable membrane when you explain osmosis. Water moves through it from the dilute solution to the more concentrated one.
• Blotting some cylinders harder than others, so the masses aren't comparable.