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 passive net movement down a concentration gradient directly between phospholipids, without ATP or a transport protein.
Particles move randomly in both directions, but more leave the higher-concentration side, producing net movement until dynamic equilibrium is approached. Small non-polar oxygen and carbon dioxide dissolve in the lipid core and cross readily.
Diffusion is faster with a steeper gradient, larger surface area, shorter diffusion distance and greater particle motion at higher temperature, provided the molecule can enter the bilayer.
Oxygen can diffuse into a respiring cell while carbon dioxide diffuses out when their respective concentration gradients point in those directions.
The cell does not direct diffusion toward need. Each gas moves according to its own gradient, and movement continues both ways even when net movement is zero.
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.
