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Diffusion, Osmosis and Active transport

What you need to know

Reflections and Exam tips


Why do substances need to move in and out of cells?

  • To get raw materials for reactions, e.g. oxygen, water, glucose, sodium, chloride, potassium
  • To get rid of waste substances e.g. carbon dioxide, urea, toxins
  • To maintains constant conditions inside the cells- keep adequate levels of salts, water, sugar
  • To secrete useful substances e.g. Hormones

How do substances move?

The way a substance enters or leaves a cell depends on its properties and the properties of membranes:

Factors affecting movement

  1. Particle size (size of the atom, molecule or compound)
  2. Charge on the particle
  3. Membrane permeability and thickness

Cell walls are freely permeable which means that they do not block the movement of substances.

Cell membranes are partially permeable ( some people say semi-permeable)- this means that they only allow small substances through and prevent the passage of larger ones.


This is the movement of particles of a substance from high concentration (where they are crowded) to low concentration.

Diffusion is a passive process, which means that it doesn't need energy on the part of the cell. It relies only on the inbuilt energy of the particles (Brownian motion).

In most cases, only small molecules move by diffusion.

The concentration difference can also be called a gradient (slope).

So we can redefine diffusion as:

The movement of particles of a substance down a concentation gradient. (simply means down the slope)


Factors affecting rate (speed) of diffusion

  • Particle size- the smaller the particles, the more kinetic energy they have to spread
  • Diffusion distance- the thinner the surface through which diffusion occurs, the less time particles take to cross
  • Temperature- the higher the temperature, the more kinetic energy the particles have.
  • Diffusion gradient- the greater the diffusion gradient, the faster the diffusion

You can easily remember these using SAD TOES. Yes Sad toes.

Rate of diffusion = SAD/TOES i.e. Surface Area and Diffusion gradient (SAD) divided by the Thickness Of the Exchange Area.

So in general, the bigger SAD is and the smaller TOES is the faster diffusion will take place.

So what does it mean in terms of diffusion surfaces?

They have to be:

  • Thin- e.g. walls of the alveoli are one-cell thick, root hairs are very thin
  • Have a large surface area- normally folder or have many of them e.g. villi and micrivilli in the small intestines, many alveoli in the lungs, many root hairs on the roots of plants to absorb more nutrients and water.



Is the movement of water molecules from a weak solution (dilute solution) to a concentrated (strong) solution through a partially permeable membrane.

This is because the membrane only allows small (water) molecules through while stoping the large (salt/sugar) molecules which can't fit through the pores.


Active transport

This is the movement of substance molecules from low concentration to high concentration using energy from respiration.

You can see why it needs energy from the cell; the particles are moving up the slope.


Active transport is the movement of molecules up a concentration gradient using energy (ATP) from respiration.

For this reason, active transport can only happen in living cells which have many mitochondria for respiration.