What is facilitated diffusion?

Facilitated diffusion is a specific type of passive transport that allows molecules to cross a cell membrane with the help of transport proteins. In this process, substances move from an area of higher concentration to an area of lower concentration, which is key to understanding why no energy is required. The transport proteins assist in the movement of larger or polar molecules that cannot easily pass through the lipid bilayer of the cell membrane. This process is vital for cells to take in essential nutrients and other molecules efficiently while maintaining their concentration gradients without expending energy.

In contrast, the other options reference concepts that do not apply to facilitated diffusion. Active transport, suggested by one option, involves the use of energy to move substances against their concentration gradient, which is fundamentally different from facilitated diffusion. Moving molecules "with energy" or the specific mention of nutrient absorption does not accurately describe the mechanism or purpose of facilitated diffusion as it is focused on the passive nature of the process, relying on the inherent concentrations of the molecules rather than energy expenditure.