What type of molecules are selectively moved across membranes during facilitated diffusion?

Facilitated diffusion is a process that enables specific molecules to cross cell membranes with the help of transport proteins, which are embedded in the lipid bilayer. This approach is necessary because the substances being transported are often unable to pass through the hydrophobic core of the membrane unaided.

The term "specific molecules" refers to the selectivity of transport proteins, which usually have a particular affinity for certain substances, such as glucose or ions. This specificity means that not just any molecule can pass through; only those that can fit and interact properly with the transport proteins will be effectively facilitated across the membrane.

For example, glucose transporters are designed to transport glucose specifically, and ion channels only permit the movement of charged particles like sodium or potassium ions. This selective transport is crucial for maintaining the cell's internal environment and ensuring that essential nutrients and ions enter the cell while waste products are removed.