Diffusion, Osmosis and Active Transport

Rachel Hurst
Mar 30
4 min read

HSC Biology | Study Notes

This topic matters because cells must constantly move materials in and out across the cell membrane, and the syllabus specifically links this exchange to concentration gradients and the characteristics of the substances being moved.

In this lesson

what diffusion is
what osmosis is
how concentration gradients affect movement
what active transport is
common examples of each process in cells

Why transport matters in cells

Cells need to exchange materials with their surroundings in order to survive.

They need to:

take in useful substances such as oxygen, water and nutrients
remove wastes such as carbon dioxide
maintain stable internal conditions

Concentration gradients

A concentration gradient is the difference in concentration of a substance between two areas.

Why concentration gradients matter

Many transport processes depend on whether there is:

a high concentration in one place
a low concentration in another place

Substances often move from an area of higher concentration to an area of lower concentration. This is called moving down a concentration gradient.

If a substance moves from low concentration to high concentration, it is moving against the concentration gradient.

Diffusion

Diffusion is the net movement of particles from an area of high concentration to an area of low concentration.

Key features of diffusion

passive process
does not require energy from the cell
moves substances down a concentration gradient

What “net movement” means

Particles move randomly in all directions, but overall there is a greater movement from high concentration to low concentration until the concentrations become more evenly spread.

Examples of diffusion in cells

oxygen diffuses into cells where oxygen concentration is lower
carbon dioxide diffuses out of cells where carbon dioxide concentration is higher
small molecules may diffuse across the cell membrane if the conditions are suitable

Osmosis

Osmosis is the net movement of water through a selectively permeable membrane from a region of higher water concentration to a region of lower water concentration.

At this level, students often also describe this as water moving from a dilute solution to a more concentrated solution across a selectively permeable membrane.

Key features of osmosis

involves water only
requires a selectively permeable membrane
passive process
moves water down its concentration gradient

What makes osmosis different from diffusion

Osmosis is a special type of transport because:

it only refers to water
it must involve a partially or selectively permeable membrane

Examples of osmosis in cells

water enters plant root hair cells by osmosis
water moves into or out of animal cells depending on the surrounding solution
water entering plant cells helps maintain turgor

Active transport

Active transport is the movement of substances across a membrane against a concentration gradient, using energy from the cell.

Key features of active transport

requires energy, usually from ATP
moves substances from low concentration to high concentration
usually involves membrane proteins

Why cells use active transport

Sometimes a cell needs to absorb or keep a substance even when there is already a higher concentration inside the cell. Diffusion cannot do this, so the cell uses active transport.

Examples of active transport in cells

mineral ions are taken into plant root hair cells from the soil
some cells absorb useful substances even when the concentration outside the cell is lower than inside

The syllabus links active transport to membrane exchange, alongside diffusion and osmosis, as part of understanding how cells coordinate internal and external conditions.

Diffusion, osmosis and active transport compared

Process	What moves?	Direction of movement	Energy needed?
Diffusion	Particles	High to low concentration	No
Osmosis	Water	High water concentration to low water concentration	No
Active transport	Substances such as ions	Low to high concentration	Yes

Linking transport to concentration gradients

Down the gradient

Processes that move down a concentration gradient:

diffusion
osmosis

These are passive and do not require cellular energy.

Against the gradient

Processes that move against a concentration gradient:

active transport

This requires energy because the cell is pushing substances in the opposite direction to their natural movement.

How to decide which process is happening

Ask these questions:

Is it water only?

If yes, it is likely osmosis.

Is the substance moving from high concentration to low concentration?

If yes, it is likely diffusion.

Is the substance moving from low concentration to high concentration, using energy?

If yes, it is active transport.

Worked example

Exam-style question

A plant root hair cell takes in mineral ions from the soil even when the concentration of mineral ions is lower in the soil than inside the cell. Identify the transport process and explain why it is needed.

Worked answer

The process is active transport. This is needed because the mineral ions are moving from a lower concentration outside the cell to a higher concentration inside the cell, so they are moving against the concentration gradient. This requires energy.

Why this works

This answer:

identifies the correct process
refers to the concentration gradient
explains why energy is needed

Common mistakes

Saying osmosis is the movement of all particles, not just water.
Forgetting that osmosis needs a selectively permeable membrane.
Mixing up diffusion and active transport.
Saying active transport moves substances from high to low concentration.
Forgetting that active transport requires energy.

Quick quiz

What is a concentration gradient?
Define diffusion.
Why is osmosis different from diffusion?
Which transport process requires energy?
Give one example of active transport in cells.