Diffusion, Osmosis & Cell Transport
Movement across the cell membrane: diffusion moves solutes high-to-low concentration without energy, osmosis is the diffusion of water toward higher solute concentration, and active transport pumps substances against the gradient using ATP.
Passive transport (diffusion, osmosis, facilitated diffusion through channel proteins) needs no energy; active transport and bulk transport (endocytosis, exocytosis) cost ATP. The membrane’s selective permeability is what makes a gradient possible at all.