A cell is placed in concentrated salt solution. Which way does the water move?

Outside the cell there is more solute and less water. Water moves towards the higher solute concentration. That is outwards. Out. The cell shrinks

What happens to a red blood cell in distilled water, and to a plant cell?

In both, water moves inwards. The red blood cell has no wall to stop the swelling. The plant cell has a rigid cell wall. The red cell bursts; the plant cell becomes firm and upright

A root absorbs potassium from soil where it is less concentrated than inside the cell. Which mechanism?

The direction is low to high. Diffusion only runs the other way. So energy has to be spent. Active transport

Cell Structure and Function · Grade 10

What is the difference between diffusion, osmosis and active transport?

Diffusion is movement from high concentration to low, happening by itself at no cost. Osmosis is the same thing for water across a selective membrane. Active transport pushes a substance against the concentration gradient, and so it costs energy. The distinction explains why a cell in salt water shrinks and in distilled water swells.

Learning objectives

Predict which way water will move between a cell and the solution around it
Distinguish simple diffusion, facilitated diffusion and active transport by direction and by energy cost
Explain what happens to a plant cell and to a red blood cell placed in pure water
Explain why a cell that runs out of usable energy loses control of its contents

What moves by itself

Particles in constant motion spread out on their own, from where they are crowded to where they are sparse. That is diffusion, and it costs nothing — it is simply what happens.

If a molecule is small and uncharged it passes straight through the fat. If it is charged or large it needs a protein, but the direction is still high to low — that is facilitated diffusion, and it is still free.

Osmosis: the water moves

When a solute cannot cross the membrane, the water moves instead. It travels towards the side with more solute — which is to say, towards the side with less water.

A simple rule that always works: water follows the salt. A cell in a concentrated solution loses water and shrinks; a cell in distilled water takes water in and swells.

What happens next depends on the wall. A red blood cell has none, and in distilled water it swells until it bursts. A plant cell has a rigid wall, so it merely becomes firm and upright — and that is exactly what holds a leaf up.

When you have to push against it

Sometimes the cell needs a substance that is less concentrated outside than in. Diffusion would carry it precisely the wrong way, so a pump is needed — a protein that spends energy to push against the gradient.

That is active transport, and it is what lets a root absorb minerals from poor soil and a nerve cell hold a charge difference. It is also why a cell that runs out of available energy loses control of its contents: the pumps stop and the concentrations even out.

Worked examples

A cell is placed in concentrated salt solution. Which way does the water move?
1. Outside the cell there is more solute and less water
2. Water moves towards the higher solute concentration
3. That is outwards
Answer: Out. The cell shrinks
What happens to a red blood cell in distilled water, and to a plant cell?
1. In both, water moves inwards
2. The red blood cell has no wall to stop the swelling
3. The plant cell has a rigid cell wall
Answer: The red cell bursts; the plant cell becomes firm and upright
A root absorbs potassium from soil where it is less concentrated than inside the cell. Which mechanism?
1. The direction is low to high
2. Diffusion only runs the other way
3. So energy has to be spent
Answer: Active transport

Common mistakes

Saying water moves towards more water: It moves towards more solute, which is the same statement inverted. Confusing the two reverses every osmosis answer.
Thinking facilitated diffusion costs energy: It uses a protein but runs in the natural direction, high to low. The cost only appears when you push against the gradient.
Assuming a plant cell bursts in water: The cell wall limits the swelling. It becomes firm — a desirable state, since a plant that has lost it wilts.

What to remember

Diffusion: high to low, free.
Osmosis: water follows the salt.
Active transport: against the gradient, at a cost.
A cell wall is the difference between firm and burst.