Cells and diffusion
Connect cell structures with the movement of small particles.
Before you begin
- Particle motion
- Concentration as amount per volume
Relate cell structures to functions
Plant and animal cells have a cell membrane that controls movement of substances and cytoplasm where many reactions occur. Typical examples have a nucleus containing genetic information. Plant cells also have a cellulose cell wall and usually a large permanent vacuole; photosynthetic cells contain chloroplasts. Not every specialised cell has every structure shown in a simplified diagram.
Explain net movement by diffusion
Particles move randomly. When their concentration differs between two regions, this movement produces a net transfer from higher to lower concentration. At equilibrium, particles still move but there is no overall directional transfer. Diffusion does not require a cell to supply metabolic energy. A short diffusion distance and a larger concentration difference generally increase the rate.
Worked example
Oxygen concentration is greater outside a cell than inside. Explain its net movement through a permeable membrane.
Show the worked solution
- Oxygen molecules move randomly on both sides.
- More cross from the higher-concentration side per unit time than in the reverse direction.
- There is a net movement into the cell down the concentration gradient.
Oxygen diffuses into the cell.
Try it yourself
At diffusion equilibrium, what happens to particles?
Common mistakes
- Diffusion and active transport have different mechanisms.
What can you explain now?
Why can a thinner membrane or surface make diffusion faster?
Compare with the explanation
It reduces the distance particles must cross.
A shorter diffusion path allows more particles to complete the crossing in a given time, when other conditions are comparable.
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