Required practical: osmosis in plant tissue

Run the potato practical, step by step

Think of each boiling tube as a question you're asking the potato: is the liquid outside more dilute, more concentrated, or the same as inside? Step through to see how the method gets a clear answer.

1. 1Cut matching cylindersUse a cork borer to cut five potato cylinders, all the same diameter. The only thing you want to differ between the tubes is the solution each cylinder sits in.+
2. 2Trim off skin, trim to lengthTrim any skin off each cylinder with the knife — skin affects the movement of water, and water movement is exactly what you're measuring. Then trim every cylinder to the same length.+
3. 3Measure before you startMeasure the mass and the length of each cylinder and record them in a table. The balance must read to 0.01 g — that's what lets you see a measurable change in a short time.+
4. 4Set up five labelled tubesMeasure 10 cm³ of each sugar (or salt) solution into its own boiling tube, and 10 cm³ of distilled water into the fifth. Label every tube clearly.+
5. 5SoakAdd one cylinder to each tube and leave them for a chosen amount of time. Optional upgrade: a water bath at 30 °C for 30 minutes gives measurable changes in mass and length, and lets you control the temperature.+
6. 6Remove and blot dryTake each cylinder out of its tube and carefully blot it dry with paper towel before it goes back on the balance.+
7. 7Measure again and compareMeasure the new mass and length of each cylinder and record them for each concentration. Now work out the change and the percentage change for every cylinder — that's where the evidence comes from.+

AQA GCSE Biology: this is Required practical activity 3 — investigate the effect of a range of concentrations of salt or sugar solutions on the mass of plant tissue.

Reading the evidence

Which way did the water go?

Pick a result or condition, then choose what the water did.

7 of 7 still to sort.

Osmosis is the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane. Use that one sentence to sort every result.

Predict, then check

Picture the class graph: concentration of the solution along the bottom, change in mass up the side — above zero is a gain, below zero is a loss.

Where on that graph do you read the sugar concentration inside the potato?

What you need to know

• Osmosis is the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.
• The method: five same-diameter cylinders, skin removed, trimmed to the same length, weighed and measured, soaked in 10 cm³ of solution or distilled water, blotted dry, then measured again.
• Change = final − initial; percentage change = change ÷ initial × 100; rate = change ÷ time.
• Gain in mass → water moved in; loss → water moved out; no change → equal concentrations and no net movement.

The big picture

Potato cylinders are soaked in a range of sugar or salt solutions and in distilled water, and their mass and length are measured before and after. A gain means water moved in by osmosis; a loss means water moved out; the point of no change on the graph gives the potato's own sugar concentration.

⚠ Watch out

Stopping at 'the cylinder gained mass' or 'lost mass'. That's the evidence, not the conclusion — say which way the water moved by osmosis and why: in, because the solution outside was more dilute; out, because it was more concentrated.

Memory hook

Heavier: water came in. Lighter: water went out. No change: you've found the potato's own concentration.

Check yourself

Close the page and talk through the practical in order, from the cork borer to the final weighing. At which step does the skin come off, and when do you blot the cylinders dry?