Simple Diffusion Moves Particles Down a Concentration Gradient Without Energy
Diffusion is the passive movement of particles caused by their constant random motion.
Particles spread from high to low concentration, giving a net movement down the concentration gradient.
Diffusion needs no energy, it is entirely passive.
Definition
Simple diffusion
Simple diffusion is the passive movement of molecules from an area of higher concentration to an area of lower concentration, down their concentration gradient, without requiring energy (ATP).
Only Small, Nonpolar Molecules Cross the Bilayer Directly
Simple diffusion happens when particles pass straight through the phospholipid bilayer.
Only molecules that can permeate the bilayer cross this way.
Nonpolar molecules such as oxygen and carbon dioxide diffuse easily.
Small polar molecules such as urea and ethanol diffuse more slowly.
The centre of the membrane is hydrophobic, so charged ions struggle to pass through.
Polar molecules with partial charges diffuse slowly because the hydrophobic core repels them.
Tip
Simple diffusion works best over short distances, such as across the thin membranes of alveoli or capillaries.
Over larger distances a transport system like the circulatory system is needed.
Diffusion Continues Until Concentrations Reach Equilibrium
No energy required: diffusion is passive and does not use ATP.
Selective: only certain molecules can pass directly through the membrane.
Hydrophobic molecules cross easily, while hydrophilic ones need help.
Driven by the gradient: molecules move from high to low concentration until equilibrium is reached.
Example
When aerobic respiration uses up oxygen inside a cell, oxygen there drops below the level outside.
This gradient drives oxygen to diffuse across the membrane into the cell.
Common Mistake: What Cannot Cross Freely
Common Mistake
Polar or charged molecules, like glucose and sodium ions, cannot cross freely.
They need specific transport proteins to move across the membrane.
Explore Simple Diffusion
The Cornea Relies on Oxygen Diffusion From the Air
The cornea of the eye has no blood supply, so its cells get oxygen by diffusion from the air.
Air holds a high concentration of oxygen.
Oxygen diffuses through the tear fluid and then through the plasma membrane of the surface cells.
Oxygen is lower inside the corneal cells because respiration keeps using it up.
This gradient keeps oxygen flowing into the cells, so they stay functional.
Hint
The cornea shows how diffusion alone can supply a cell when there is no blood supply.
Larger and Charged Molecules Need Other Transport Methods
Simple diffusion cannot move larger or charged molecules across the membrane.
These are moved by facilitated diffusion or active transport, which are covered in later subtopics.
Theory of Knowledge
How do concentration gradients in diffusion relate to patterns in natural systems, such as nutrient cycles in ecosystems?
Active recall
Why does simple diffusion need no energy?
Which types of molecule can cross the bilayer by simple diffusion?
What does it mean for diffusion to reach equilibrium?
Why does the cornea depend on diffusion for its oxygen?
What would happen to a cell if oxygen could no longer diffuse across its membrane?
