Which mechanism involves the spontaneous passive transport of molecules through membrane proteins?

• A. Endocytosis
• B. Co-transport
• C. Facilitated diffusion
• D. Active transport

Answer: (C)

Facilitated diffusion is the mechanism that involves the spontaneous passive transport of molecules across cell membranes through specific membrane proteins. This process does not require energy input from the cell because it occurs down a concentration gradient, meaning that molecules move from an area of higher concentration to an area of lower concentration.

The proteins involved are known as transport proteins, which can be channel proteins that form pores through the membrane or carrier proteins that undergo conformational changes to shuttle molecules across. This system is crucial for the movement of polar or charged molecules, such as glucose or ions, that cannot easily pass through the lipid bilayer of the membrane on their own due to their hydrophilic nature.

In comparison, endocytosis involves the engulfing of molecules into the cell by the membrane wrapping around them, which is an active process that requires energy. Co-transport refers to a mechanism where two substances are simultaneously moved across a membrane, either together in the same direction or in opposite directions, usually driven by the gradient of one of the substances, which is also not a simple passive transport mechanism. Active transport, on the other hand, involves the movement of molecules against their concentration gradient and requires energy, typically in the form of ATP.

Thus, facilitated diffusion uniquely captures the