What is Diffusion?

The net movement of molecules moving from a region of higher concentration to a region of lower concentration until they reach equilibrium.

2 types of equilibrium:

  • Static equilibrium occurs when there is no action taking place.
  • Dynamic equilibrium occurs when two opposing actions occur at the same rate.


Where – Occurs in all 3 states of matter, but most evidently in gas and liquids.

Why – Occurs as a result of intrinsic thermal energy (heat) found in all matter at temperatures above absolute zero.


  1. When a lump of sugar is dropped into a beaker of water, the sugar molecules will move in a net direction from a region of higher concentration to a region of lower concentration
  2. This occurs until the sugar molecules are evenly spread throughout the beaker of water.
  3. (concentration of sugar molecules in the beaker reaches equilibrium)
  4. —Diffusion stops here—
  5. However, the molecules are still in constant random motion.

Passive Transport

Passive transport refers to the movement of molecules across a membrane where energy is not necessary.

It is called passive because it is spontaneous.

Simple diffusion

The process where a substance passes through a membrane without an intermediary.

Substances: Gases (e.g. oxygen, carbon dioxide)

Facilitated Diffusion animation

A special carrier protein with a central channel acts as a selective corridor to transport some molecules past a membrane.

Osmosis animation

The net movement of water molecules from a region of higher water potential to a region of lower water potential across a selectively permeable membrane

Water potential –> the concentration of free water molecules. It measures the tendency of these water molecules to diffuse to another area.

Why – Although water molecules are polar, they are small enough to pass through a membrane compared to other polar molecules


  1. Sugar is added to the right side of the membrane
  2. The sugar molecules cannot diffuse to the other side as they are large and polar
  3. The polar water molecules will cluster around the sugar molecules (solute)
  4. There are less free water molecules able to diffuse
  5. Thus the free water molecules from the left side (higher water potential) will diffuse to the right side (lower water potential) in a net direction
  6. Osmosis takes place until both sides have the same amount of free water molecules.
  7. —Osmosis stops—

Difference between osmosis and diffusion



Input of energy

Not necessary

Membrane? Not necessary Selectively permeable membrane needed
Type of molecule transported Any type Water molecules
Movement of molecules From a region of higher concentration to a region of lower concentration From a region with less solute to a region with more solute
Equilibrium of concentration Reached

Factor affecting direction of diffusion

Concentration gradient

  • the change in concentration of a type of molecule across a distance/ when molecules exist in different concentrations across a membrane
  • each kind of molecule has its own concentration gradient
  • (A) If the molecules were free to move, they would spontaneously diffuse down the concentration gradient (higher concentration –> lower concentration)
  • (B) Molecules need energy to move up the concentration gradient (active transport)

Factors that affect the rate of diffusion



Chemical driving force
–  The net force driving molecules down a concentration gradient–  The greater the gradient, the greater the chemical driving force, the greater the speed of the molecules
Temperature The greater the temperature, the greater the rate of molecular movement
Pressure The greater the pressure, the greater the speed of the  molecules
Size of molecules The smaller the molecule size, the greater the net flux of molecules
Membrane surface area The greater the membrane surface area, the greater the net flux of molecules
Membrane permeability –  Permeability depends on the membrane’s properties that affect the molecule’s ability to cross it.-  The greater the membrane permeability, the greater the net flux of molecules

 More info

Osmosis in plant and animal cells

Case in point:


The right side has a higher concentration of solute and a lower water potential than the left side.

Water molecules will move into the right side.


Both sides contain the same concentration of solute.

Water molecules flow equally between both sides.


The left side has a lower concentration of solute and a higher water potential than the right side.

Water molecules will move out of the left side.

Diffusion and osmosis in plant and animal cells

Plant cells

Osmosis – Plasmolysis in a Hydrilla leaf

Before /\                                                                                    After /\

In the second slide, the Hydrilla leaf was soaked in salt solution.

As the salt solution is hypertonic (had a lower water potential than the cells), the vacuole shrinks and cytoplasm pulls away from the cell wall.

As the cell wall is freely permeable, salt solution enters the cell wall and takes up the space once occupied by the cytoplasm.

Animal cells

Diffusion – Alveoli (air sacs in lungs)

Oxygen diffuses from the alveoli and into the bloodstream, while carbon dioxide diffuses from the bloodstream and into the alveoli.

Importance of diffusion and osmosis

Life processes to maintain the balance between the cell and the environment