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Structures and Functions in Living Organisms

Movement of substances into and out of cells — Edexcel Linear - OL Revision Notes & Topical Questions

5 min read 11 parts Lesson 11 of 25 Unit quiz

📚 Edexcel Linear - OL Hub / Topical Revision
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Lesson summary

Diffusion and Osmosis: Factors and Practicals — EDEXCEL LINEAR - OL notes: Explain what affects the rate of diffusion and carry out and evaluate the agar-cube and potato-cylinder practicals.Diffusion is the net movement of particles from a region of higher concentration to a regi…

  • Evaluation
  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Worked example 4
  • Worked example 5

Part 1 of 4Diffusion and the factors that affect its rate

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient. It is passive and happens because particles move randomly: on the side with the higher concentration more random collisions occur against the membrane, so more particles cross from that side.

FactorEffect on the rate of diffusion
Concentration gradientThe greater the difference in concentration between two regions, the faster the overall rate
TemperatureHigher temperature gives particles more kinetic energy, so they move faster and diffuse faster
Surface areaA larger surface area (or larger surface area to volume ratio) gives more room for particles to cross
Diffusion distanceA thinner membrane or shorter distance gives a faster rate

Part 2 of 4Practical: diffusion in agar cubes

Agar prepared with sodium hydroxide and phenolphthalein indicator is pink. Acid diffuses into the cube and reacts with the sodium hydroxide, so the pink indicator turns colourless.

1
Use a scalpel to cut cubes of pink agar on a white tile to different dimensions (for example 1 cm, 2 cm and 3 cm sides).
2
Place each cube in the same volume of hydrochloric acid and start a timer.
3
Time how long each cube takes to turn colourless.
4
Calculate surface area, volume and the SA:V ratio for each cube and compare it with the time.

Result smaller cubes have a larger surface area to volume ratio (SA:V), so the acid has more area to enter relative to the cube’s size and they turn colourless faster.

Evaluation judging the endpoint (when the cube is completely colourless) is difficult and introduces human error.

Practical: diffusion in agar cubes
An improvement is to measure the distance in mm that the acid has diffused into the cubes after a set time,
  • which gives a quantitative measurement that is easier to compare.
Another limitation is that it is hard to cut cubes exactly the same size,
  • and small differences in side length affect the SA:V and the rate of diffusion.

Part 3 of 4Practical: osmosis in potato cylinders

Osmosis is the net movement of water molecules from a region of higher water potential (more dilute solution) to a region of lower water potential (more concentrated solution) through a partially permeable membrane.

1
Prepare sucrose solutions from 0 mol/dm3 (distilled water) to 1 mol/dm3.
2
Set up 6 labelled test tubes, each containing 10 cm3 of one of the solutions.
3
Cut potato cylinders of the same size, blot dry and weigh each, then place one in each tube.
4
After a set time remove, blot dry and re-weigh each cylinder.
5
Calculate the percentage change in mass = (final mass − initial mass) ÷ initial mass × 100.

The potato in the strongest sucrose solution loses the most mass because water moves out of the cells by osmosis, making them flaccid and the potato soft (the cells become plasmolysed).

Practical: osmosis in potato cylinders
A cylinder that has neither increased nor decreased in mass showed no net movement of water; the surrounding solution had the same concentration as the cytoplasm of the potato cells,
  • so there was no concentration gradient.
Slight differences in cylinders (size, surface area) may make results unreliable, so use identical cylinders and repeat.

Part 4 of 4Worked examples

  1. A 1 cm agar cube and a 3 cm agar cube are placed in acid. Calculate the SA:V ratio for each and say which turns colourless first. [4]4 marks

  2. A potato cylinder has a mass of 2.50 g before and 2.80 g after being placed in a solution. Calculate the percentage change. [2]2 marks

  3. Explain why a potato cylinder in 1 mol/dm3 sucrose loses mass. [3]3 marks

  4. Suggest one improvement to the agar-cube investigation. [1]1 mark

  5. Why does a potato cylinder show no change in mass in one solution? [2]2 marks

Key terms

TermMeaning
DiffusionNet movement of particles from higher to lower concentration.
OsmosisNet movement of water through a partially permeable membrane from higher to lower water potential.
Concentration gradientA difference in concentration between two regions.
SA:V ratioSurface area divided by volume; a larger ratio means faster exchange.
PlasmolysedDescribes a plant cell that has lost water so the membrane pulls away from the wall.
FlaccidDescribes a plant cell that has lost water and is soft.

Avoid these mistakes

1
Saying diffusion needs energy from respiration — it is passive.
2
Saying osmosis is the movement of solute — it is the movement of water.
3
Not blotting potato cylinders dry before weighing.
4
Forgetting to calculate a percentage change relative to the initial mass.
5
Ignoring SA:V — smaller cubes diffuse faster because the ratio is larger.

Last updated:

Cambridge 9700 AS Biology Core Formula

Magnification Triangle & Unit Conversion

Formula: M = I / A  ⇔  A = I / M  ⇔  I = M × A

Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.

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