Cell Membranes and Diffusion

Understand the idea

Cell membranes regulate exchanges between a cell and its environment. Diffusion is net movement of particles down a concentration gradient due to random motion. At equilibrium, particles still move, but there is no net change in concentration from diffusion alone. Passive transport does not require cellular energy input to move material down its gradient; active transport uses energy to move substances against a gradient or power related processes.

Worked example

Imagine a membrane permeable to a small dissolved substance. Initially, its concentration is higher outside than inside. Particles move both ways, but the net movement is inward until other conditions or a dynamic equilibrium are reached. If a cell maintains a higher concentration inside by moving that substance against its gradient, a suitable active transport mechanism requires energy. This is a simplified model: membrane permeability and other forces also matter.

Try it yourself

1. What is the net diffusion direction if concentration is higher inside?
2. Do particles stop moving at equilibrium?
3. Why is membrane permeability important to the prediction?

Check your reasoning

1. Net movement is outward if the membrane permits passage and other relevant conditions are comparable.
2. No. Random movement continues; opposing flows can balance.
3. A substance that cannot cross the membrane will not diffuse through it simply because a concentration difference exists.

Make it work for your learner

For support, model the example one step at a time and ask the learner to explain each choice in their own words. Use a sketch, organizer or physical model where helpful. For a challenge, create a new example, test an alternative explanation or describe when the model would not apply.

Exit check

Use a model to explain diffusion and distinguish passive from active transport. Explain your reasoning with a specific example before moving on.