Simple diffusion is passive net movement down a concentration gradient, allowing small non-polar molecules to cross membranes without ATP; This process depends on random motion, permeability, and differences between internal and external concentrations.
Coverage 2021–2025 · Updated 15 Jul 2026
Simple diffusion is net movement from higher concentration to lower concentration directly through the membrane, without ATP or a transport protein.
Random molecular motion is present in both directions, but more particles leave the high-concentration side, producing net movement down the gradient until equilibrium is approached.
Check: concentration gradient exists; molecule can cross the lipid core; no energy input or protein is required.
If oxygen is higher outside a cell than inside, oxygen molecules cross in both directions but net oxygen entry occurs until the gradient shrinks.
Diffusion is not movement toward ‘where the cell needs it’. The gradient and membrane permeability determine net direction.
This objective is assessed through structured response, commonly using Compare / Outline / Explain.
Compare / Outline / Explain / Identify
Build the answer around this relationship: Simple diffusion is passive movement from higher to lower concentration.
Adding channel proteins or ATP to simple diffusion when the row requires direct passive movement.
Representative question
Explain how the rate of diffusion of oxygen into a cell is affected by the concentration of oxygen outside of the cell.
The SL membrane model is a decision system. The bilayer forms because phospholipids are amphipathic, and the hydrophobic core creates selective permeability. Small non-polar molecules diffuse directly; water moves by osmosis and often through aquaporins; ions and polar molecules use channels or transporters; pumps use ATP for movement against gradients. Proteins and glycocalyx components add transport, recognition, and model evidence.
