Simple diffusion occurs across a permeable membrane, meaning all of the molecules, such as water, oxygen, or carbon dioxide, diffuse through without the help of transport proteins. Osmosis occurs over a selectively permeable membrane. The membrane is selectively permeable to water molecules more than to the solute molecules in solution with the water.
Passive transport is a movement ofbiochemicals and other atomic or molecularsubstances across cell membranes without need of energy input. Unlike active transport, it does not require an input of cellular energy because it is instead driven by the tendency of the system to grow in entropy. The rate of passive transport depends on thepermeability of the cell membrane, which, in turn, depends on the organization and characteristics of the membrane lipids andproteins. The four main kinds of passive transport are simple diffusion, facilitated diffusion, filtration and osmosis.
