Which of the following best describes facilitated diffusion?

Facilitated diffusion is a transport process that allows substances to cross membranes with the help of specialized proteins. It does not require energy, as it relies on the natural tendency of molecules to move from an area of higher concentration to an area of lower concentration. This movement occurs through specific carrier proteins that assist in the passage of molecules across the cell membrane.

The key aspect of facilitated diffusion is that it utilizes these carrier proteins to facilitate the movement of substances that cannot easily pass through the lipid bilayer of the membrane due to their size, polarity, or charge. This process is vital for transporting various molecules, such as glucose and ions, which need assistance to cross the cell membrane effectively.

In contrast, options that mention energy input, movement against the concentration gradient, or classify it as a form of active transport do not align with the fundamental characteristics of facilitated diffusion. Active transport requires energy to move substances against their concentration gradient, while facilitated diffusion is a passive process.