The Osmosis Lab
Potato cylinders, sugar solutions and a balance: master the three ways substances cross a membrane, and the one calculation examiners love.
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The Osmosis Lab
A cell has to let some things in and keep others out. Oxygen in, waste out, water balanced, minerals pulled in even when they are scarce. There are **three** ways substances cross a membrane: **diffusion**, **osmosis** and **active transport**. Tell them apart fluently and the marks are yours.
Diffusion and osmosis
Both are **passive**: no energy from respiration is needed, and both run **down** a concentration gradient. Four differences separate them, and examiners test every one:
Which one is it?
A visking tubing bag of concentrated sugar solution is tied shut and left in a beaker of pure water. After an hour the bag is noticeably heavier, and the water in the beaker is still not sweet. What has happened?
- Water has moved into the bag by osmosis, and the sugar could not move out because the tubing is partially permeable
- Sugar has diffused out of the bag and water has diffused in
- The bag has used active transport to pull water in
- Nothing has moved: the bag is heavier because the sugar has expanded
Pin down the definition
Osmosis is the movement of _____ across a _____ membrane, from a _____ solution to a more concentrated one. Like diffusion it is _____, so it needs no energy from respiration.
Active transport: uphill
Sometimes a cell needs to move a substance the **wrong** way: from a **low** concentration to a **higher** one, **against** the gradient. Diffusion will not do that, any more than water runs uphill. **Active transport** does it, and it **needs energy** released by **respiration**. That is the whole distinction, and it is why cells doing a lot of it are stuffed with mitochondria. Two examples worth knowing: **root hair cells** absorb mineral ions from soil water far more dilute than the cell itself, and the **gut** absorbs glucose into blood that already contains more of it.
Which process, and why?
- Oxygen crossing from an alveolus into the blood
- Water entering a root hair cell from damp soil
- Nitrate ions entering a root hair cell from very dilute soil water
- Glucose absorbed from the gut when the blood already holds more of it
- Down the gradient, passive, and open to any small particle
- Down the gradient, passive, but water only and through a partially permeable membrane
- Up the gradient, so it costs energy from respiration. Without it the plant would starve of minerals
- Up the gradient again, this time so that a meal can be absorbed to the last of it
RP3: the potato experiment
**Required practical 3** measures how the concentration of a sugar solution affects a plant tissue. You cut equal-sized **potato cylinders**, blot and weigh each one, then leave each in a different sugar concentration for the same length of time. If water moves **in** by osmosis, the cylinder **gains** mass. If water moves **out**, it **loses** mass. The result is recorded as the **percentage change in mass**, not the raw change in grams, and the reason is worth understanding rather than memorising: no two cylinders start at exactly the same mass, and a 0.6 g gain means something quite different to a 5 g cylinder than to a 50 g one.
RP3: get the method right
Put the steps of the potato osmosis practical into the correct order.
- Cut potato cylinders of equal size
- Blot dry and weigh each cylinder to find its starting mass
- Place each cylinder in a different sugar concentration
- Leave them for a set time
- Remove, blot dry and reweigh each cylinder
- Calculate the percentage change in mass for each
Why blot the cylinders?
Why must each potato cylinder be **blotted dry** before it is weighed?
- Surface water would add to the mass and give a false reading
- To clean sugar off so it tastes better
- To remove the water from inside the cells
- To make the osmosis happen faster
Percentage change: a gain
A potato cylinder started at 5.0 g and ended at 5.6 g after soaking. Calculate the percentage change in mass. (Use: change ÷ starting mass × 100.)
Percentage change: a loss
A second cylinder from the same potato started at 4.0 g and ended at 3.4 g in a stronger sugar solution. Calculate the percentage change in mass. Give the sign as well as the number.
Plot the results
Six potato cylinders gave these results. The x-axis is sugar concentration in units of 0.1 mol/dm³ (so x = 3 means 0.3 mol/dm³); the y-axis is the percentage change in mass. Plot all six points: 0.0 → +12, 0.1 → +8, 0.2 → +4, 0.3 → 0, 0.4 → −4, 0.5 → −8.
Reading the graph
On a graph of **percentage change in mass** (y-axis) against **sugar concentration** (x-axis), the line crosses **zero** at 0.3 mol/dm³. What does that crossing point tell you?
- The cell sap concentration equals 0.3 mol/dm³, so there is no net osmosis
- The solution there is pure water
- The potato cells have died
- Osmosis is happening fastest there
Defend your method
Your RP3 results are challenged. Four questions, the kind a six-mark evaluation asks.
- Why does every cylinder have to be cut to the same size?
- A partner suggests taking one cylinder out after ten minutes and another after an hour, to save time. What is wrong with that?
- The experiment is repeated with a potato from a different farm, and this time the line crosses zero at 0.25 mol/dm³ instead of 0.3. Is that a mistake?
- One cylinder gives a percentage change far off the line the others make. What is the right response?
Transport summary
Diffusion and osmosis are both _____, so they need no energy, while _____ transport works against the concentration gradient and needs energy from respiration. Osmosis moves _____ only, across a partially permeable membrane. In RP3 the results are recorded as the _____ change in mass, so that cylinders with different starting masses can be compared.