Final answer:
Facilitated transport, or facilitated diffusion, allows substances like glucose and Na+ ions to diffuse through the plasma membrane with the help of membrane proteins without requiring energy. Glucose transporters are an example, helping glucose move down its concentration gradient into cells.
Step-by-step explanation:
Facilitated transport, also known as facilitated diffusion, is a process where materials move across the plasma membrane without the use of cellular energy. This movement occurs down a concentration gradient with the assistance of membrane proteins. A quintessential example involves the movement of glucose into cells, which is too polar to pass through the lipid bilayer of the cell membrane unaided. Glucose transporters, which are specific protein channels, facilitate this process, allowing glucose to move from an area of higher concentration outside the cell to a lower concentration inside the cell.
Another example is the transport of Na+ ions in certain situations, which similarly occurs down a concentration gradient through specific channel proteins without the expenditure of energy. Some of these transport mechanisms involve the simultaneous movement of two different substances either in the same direction, known as symport, or in opposite directions, referred to as antiport.
Facilitated transport is distinguished from active transport, in which substances are moved against their concentration gradient at the expense of energy in the form of ATP. Integral proteins in the membrane form channels to enable facilitated transport, particularly for water-soluble substances like ions or polar molecules that do not efficiently diffuse through the membrane's lipid bilayer.