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Biology · Grades 8–11

Diffusion, osmosis and active transport

Use gradients and membrane properties to choose the process.

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Learn the idea

Diffusion

Random molecular motion can produce net movement down a concentration gradient.

Osmosis

Osmosis is net water movement across a selectively permeable membrane driven by a water-potential difference.

Active transport

Moving a solute against its relevant electrochemical gradient requires an energy-coupled process.

Equilibrium

Molecules continue moving even when there is no net transfer.

Worked examples

In simple diffusion, net movement is generally in which direction?

Random motion creates a net flux down the concentration gradient.

From higher toward lower concentration.

Osmosis specifically describes net movement of what in these examples?

The moving substance is water, and membrane selectivity matters.

Water across a selectively permeable membrane.

Check your understanding

  1. At diffusion equilibrium, do molecules stop moving?

    1. Only solute particles disappear.
    2. Yes, every particle stops.
    3. No.

  2. One side of a simplified model has 30 solute units in 10 volume units. Find its concentration.

  3. Why cannot active transport be identified from movement direction alone without knowing the relevant gradient?

Answers and marking notes

Answers and marking notes
  1. No.

    Random motion continues with no net concentration-driven transfer.

  2. 3

    30/10 = 3 solute units per volume unit.

  3. The concentrations, charges and membrane conditions determine whether movement is uphill.

    For ions, the electrical contribution may matter as well.

Further reading and review references

These are reference links, not copied exercises, institutional endorsement or proof of independent review.

Independent educational review required. Grade bands are approximate; these are not officially aligned curricula.