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Biology 06103.1

Diffusion and osmosis

Passive movement down a concentration gradient, and the special case of water.

Review these first

Learning objectives

What you need to be able to do

Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.

  • 3.1.1Define diffusion and describe its importance in living organisms.
  • 3.1.2Define osmosis and describe the effects of osmosis on plant and animal cells.

6 minute read

Diffusion and osmosis

Substances must get in and out of cells. Two of the three ways cost the cell nothing at all.

Diffusion

Diffusion is the net movement of particles from a region of higher concentration to lower concentration, down a concentration gradient, as a result of their random movement. It is passive — no energy from respiration is required. Diffusion supplies oxygen to respiring cells and removes carbon dioxide from them. It is faster when the concentration gradient is steeper, the temperature is higher, the surface area is larger, or the distance is shorter.

Osmosis

Osmosis is the net movement of water molecules from a region of higher water potential (a dilute solution) to a region of lower water potential (a concentrated solution), through a partially permeable membrane. It is really just diffusion of water, but the membrane matters: it lets water through and holds the dissolved solute back, so only the water can move to even things out.

Effects on cells

In a dilute solution, water enters:

  • A plant cell swells, the vacuole pushes the membrane against the rigid cell wall, and the cell becomes turgid. The wall stops it bursting, and turgor is what holds a non-woody plant upright.
  • An animal cell has no wall, keeps swelling and can burst (lyse).

In a concentrated solution, water leaves:

  • A plant cell becomes flaccid, then plasmolysed as the membrane pulls away from the wall. The plant wilts.
  • An animal cell shrinks and crenates.

Think of it like this

Osmosis is a crowd squeezing through a turnstile that only people without luggage can pass. The people (water) redistribute freely; the luggage (solute) stays where it is, so the balancing happens entirely by people moving.

Worked examples

Method, step by step

A piece of potato is left in concentrated sugar solution for an hour. Predict what happens to its mass and explain why.

  1. 1The sugar solution has a lower water potential than the potato cells.
  2. 2Water therefore moves out of the cells by osmosis through their partially permeable membranes.
  3. 3Losing water means losing mass, and the cells become flaccid or plasmolysed.

The mass decreases, because water leaves the potato cells by osmosis into the more concentrated solution.

Common misconceptions

  • Saying osmosis is the movement of water "from high to low concentration" without saying *water* concentration or water potential — solute concentration runs the opposite way, so the sloppy version is exactly backwards.
  • Forgetting the phrase "partially permeable membrane" in the definition of osmosis, which is a required marking point.
  • Thinking plant cells burst in pure water. The cellulose cell wall resists the pressure — only animal cells lyse.

In the exam

  • Definitions here are marked almost word for word. Learn "net movement", "down a concentration gradient" and "partially permeable membrane" as fixed phrases.
  • For any osmosis question, state which side has the higher *water* potential first — it makes the direction of movement obvious and prevents the classic reversal error.