Direct Answer
Simple diffusion and facilitated diffusion are passive processes: particles move down a concentration gradient without energy from ATP. Active transport uses energy from ATP to move specific particles against a concentration gradient. This is SL and HL content in B2.1 Membranes and membrane transport.
How the Processes Differ
A concentration gradient exists when a substance has different concentrations in two regions. Random particle movement produces a net movement from higher to lower concentration during diffusion.
| Process | Route and direction | Energy and specificity |
|---|---|---|
| Simple diffusion | Particles move directly between phospholipids, down the concentration gradient | No ATP; permeability depends on particle size and whether it is hydrophobic or hydrophilic |
| Facilitated diffusion | Particles move through specific membrane proteins, down the concentration gradient | No ATP; channel proteins allow particular ions or polar particles to cross |
| Active transport | Pump proteins transfer particles across the membrane, usually against the concentration gradient | Requires energy from ATP; each pump transports specific particles |
Small, non-polar molecules such as oxygen and carbon dioxide cross the hydrophobic core of the phospholipid bilayer by simple diffusion. Ions and many polar particles cannot readily cross this core, so they require membrane proteins.
In facilitated diffusion, an appropriate channel must be open, but particles still move down their concentration gradient. In active transport, ATP provides energy for a pump protein to change conformation and transfer particles against the gradient, allowing cells to establish concentration differences across membranes.
A common misconception is that all movement through proteins requires ATP. This is incorrect: facilitated diffusion uses proteins but remains passive because movement is down the concentration gradient.