Final answer:
Facilitated diffusion is a passive transport method utilizing channel or transport proteins to move molecules across a cell's plasma membrane, along the concentration gradient without energy expenditure.
Step-by-step explanation:
Facilitated diffusion is the passive movement of molecules across the membrane via the aid of a transport or channel protein. Hydrophilic molecules, charged ions, and larger molecules such as glucose typically require the assistance of transport proteins, because they cannot easily cross the cell membrane due to its hydrophobic nature. There are mainly two types of proteins involved in this process: channel proteins, which are less selective, and carrier proteins, which are more selective and often only allow a particular type of molecule to cross.
Channel proteins mildly discriminate between their cargo based on size and charge, while carrier proteins are more specific. In both cases, the molecules move along the concentration gradient, meaning from an area of high concentration to an area of low concentration without the use of cellular energy. The rate of facilitated diffusion is directly proportional to the number of transport proteins in the membrane, which can be saturated at a maximum rate, analogous to Vmax in enzyme kinetics.