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
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.
| Factor | Effect on the rate of diffusion |
|---|---|
| Concentration gradient | The greater the difference in concentration between two regions, the faster the overall rate |
| Temperature | Higher temperature gives particles more kinetic energy, so they move faster and diffuse faster |
| Surface area | A larger surface area (or larger surface area to volume ratio) gives more room for particles to cross |
| Diffusion distance | A 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.
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.
- which gives a quantitative measurement that is easier to compare.
- 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.
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).
- so there was no concentration gradient.
Part 4 of 4Worked examples
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
1 cm cube: surface area 6 cm2, volume 1 cm3, ratio 6:1. 3 cm cube: surface area 54 cm2, volume 27 cm3, ratio 2:1. The 1 cm cube has the larger ratio so it turns colourless first.
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
(2.80 − 2.50) ÷ 2.50 × 100 = +12%.
Explain why a potato cylinder in 1 mol/dm3 sucrose loses mass. [3]3 marks
The solution has lower water potential than the cells, so water moves out by osmosis through the partially permeable membranes; the cells become flaccid.
Suggest one improvement to the agar-cube investigation. [1]1 mark
Measure the distance in mm the acid has diffused after a set time instead of judging the colour change time.
Why does a potato cylinder show no change in mass in one solution? [2]2 marks
The solution has the same concentration as the cell cytoplasm, so there is no concentration gradient and no net movement of water.
Key terms
| Term | Meaning |
|---|---|
| Diffusion | Net movement of particles from higher to lower concentration. |
| Osmosis | Net movement of water through a partially permeable membrane from higher to lower water potential. |
| Concentration gradient | A difference in concentration between two regions. |
| SA:V ratio | Surface area divided by volume; a larger ratio means faster exchange. |
| Plasmolysed | Describes a plant cell that has lost water so the membrane pulls away from the wall. |
| Flaccid | Describes a plant cell that has lost water and is soft. |
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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.