SPI Biology · Cell Biology

Diffusion and Osmosis

Osmosis is the movement of water through a selectively permeable membrane from the weaker solution to the stronger one.

Why this matters

Two questions in the bank, both EASY: what a cell does in a hypotonic solution and which way water moves in a hypertonic one. Both are won by one rule, water moves towards the stronger solution, and lost by mixing up the two words.

Concept 1 of 2: Diffusion: from more to less

Open a bottle of scent in one corner and soon the whole room smells of it. The scent spreads from where there is a lot of it to where there is little, until it is even everywhere. That spreading is diffusion.

Definition

Diffusion: substances move from a region of higher concentration to a region of lower concentration.
It needs no energy and goes on until the concentration is the same everywhere.
Oxygen and carbon dioxide enter and leave cells by diffusion.
Osmosis is diffusion of water alone, across a selectively permeable membrane.

Worked example

A drop of ink is put in a glass of still water. What happens, and why?

Practice this conceptself-check · 3 quick reps

Concept 2 of 2: Hypotonic, isotonic and hypertonic solutions

Water moves towards the side that has more dissolved substance, as if to dilute it. So first decide which side is stronger, the cell or the solution, and water goes that way.

Definition

Hypotonic solution: weaker (more dilute) than the cell. Water moves into the cell; the cell swells. An animal cell may burst; a plant cell becomes stiff (turgid), held by its wall.
Isotonic solution: same strength as the cell. No net movement; the cell stays the same.
Hypertonic solution: stronger than the cell. Water moves out of the cell; the cell shrinks.
A plant cell shrinking away from its wall in a hypertonic solution is called plasmolysis.

Solution outside	Compared with the cell	Water moves	The cell
Hypotonic (for example pure water)	Weaker	Into the cell	Swells
Isotonic	Same	No net movement	Stays the same
Hypertonic (for example strong salt or sugar solution)	Stronger	Out of the cell	Shrinks Hyper means more: more salt outside, so water leaves.

Water always moves towards the stronger solution.

Practice this conceptself-check · 5 quick reps

The same idea in a practice question:

Practice questionEasy

Example 2 · Cell Biology · Diffusion and Osmosis

What happens to a cell in a hypotonic solution?

Swapping hypo and hyper

Hypo means less, hyper means more. In a hypotonic solution the cell gains water and swells; in a hypertonic solution it loses water and shrinks.

Water following the weaker side

Water moves towards the stronger solution, the side with more dissolved substance, not away from it. Decide which side is stronger before you pick a direction.

Drill 1 more on hypotonic, isotonic and hypertonic solutions

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Reference tables (1)

Hypotonic, isotonic and hypertonic solutions3 rows

Solution outside	Compared with the cell	Water moves	The cell
Hypotonic (for example pure water)	Weaker	Into the cell	Swells
Isotonic	Same	No net movement	Stays the same
Hypertonic (for example strong salt or sugar solution)	Stronger	Out of the cell	Shrinks Hyper means more: more salt outside, so water leaves.

Water always moves towards the stronger solution.

Watch out for (2)

Swapping hypo and hyper
→ Hypotonic, isotonic and hypertonic solutions
Water following the weaker side
→ Hypotonic, isotonic and hypertonic solutions

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