Diffusion through a selectively-permeable membrane: practical | Science

Starter quiz

What is the name of the structure labelled A in the diagram of an animal cell?
- cell membrane ✓
- cell wall
- cytoplasm
- nucleus
What is the job of the cell membrane in an animal cell?
- It controls what can enter and leave the cell. ✓
- It is a fluid that fills the cell.
- It is where photosynthesis takes place.
- It stores the cell’s DNA.
The cell membrane has lots of small ______ in it.
- 'holes' ✓
Who correctly explains how the particles of oxygen got into the cell?
- Jacob: “The cell breathed the oxygen particles in.”
- Aisha: “The particles diffused through the cell membrane.” ✓
- Lucas: “The cell sucked them in from the blood.”
- Sofia: “The cell is fully permeable to all particles, including oxygen.”
Particles of carbon dioxide …
- can move through the cell membrane in both directions. ✓
- cannot move through the cell membrane.
- will only move through the membrane from inside the cell to outside the cell.
- will only move through the membrane from outside the cell to inside the cell.
The ______ movement of carbon dioxide particles will be from inside to outside.
- 'net' ✓

Exit quiz

Particles of iodine can only move through holes in the membrane because they are ...
- concentrated.
- heated.
- large.
- small. ✓
Particles of starch are too ______ to move though holes in the membrane.
- 'large' ✓
Particles of iodine move through the membrane into the tubing due to diffusion down a ______ gradient.
- concentration ✓
- diffusion
- movement
- net
Because particles of iodine can move through the membrane but particles of starch cannot, the membrane is described as ______-permeable.
- 'selectively' ✓
The diagram shows the experiment at the start. Who correctly predicts what the liquid inside the tubing will look like after 10 minutes?
- Alex: “It will be yellow or pale brown.”
- Andeep: “It will still be colourless.”
- Sofia: “It will be blue/black.” ✓
The diagram shows the experiment after 10 minutes. Match each observation to the correct explanation for it.
- liquid inside the tubing is blue/black
  
  ⇔
  
  starch and iodine are both present ✓
- liquid outside the tubing is pale brown
  
  ⇔
  
  only iodine is present, no starch ✓
- full colour change took a few minutes
  
  ⇔
  
  iodine diffused into the tubing from the edges to the middle ✓

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Lesson Details

Key learning points

Particles of some substances are small enough to diffuse through holes in a selectively-permeable membrane.
Particles of these substances move through the selectively-permeable membrane in both directions.
Predict the direction of net movement of particles through a selectively-permeable membrane.
Observe the direction of net movement of a substance through a selectively-permeable membrane (e.g. Visking tubing).
Explain observations of the diffusion of a substance through a selectively-permeable membrane.

Common misconception

Particles only move through a selectively-permeable membrane in one direction.

The lesson explores the idea that particles move through a selectively-permeable membrane in both directions, but that net movement is down a concentration gradient.

Keywords

Selectively-permeable membrane - A membrane is selectively-permeable if some substances can move through it but others cannot.
Particles - All substances are made up of particles that are too small for us to see.
Diffusion - Diffusion is the net movement of particles of a substance down a concentration gradient.
Concentration gradient - A concentration gradient is a difference in the concentration of particles from one area to another.
Net movement - Net movement is the overall movement of particles from one area to another.