What is carrier-mediated membrane transport?

For non-lipid soluble molecules, rapid, active transport (if against gradient) or facilitated (if with gradient) Active (needs energy, has saturation, selectivity and competitiveness/ occurs at specific places like BBB neurons and renal tubules and liver) Facilitated (No energy needed, just a carrier, usually for endogenous products but can be used by drugs (glucose by RBC/ Choline by neurons to make Ach)