Topic overview

Start with the big picture

Transport mechanisms differ in whether they require energy, carriers, or vesicles, and in the forces that drive movement. Passive diffusion moves suitable molecules down a concentration gradient, while facilitated diffusion uses specific carriers or channels without directly using energy. Active transport moves substances through ATP-powered pumps or by using an ion gradient established elsewhere. Water movement is shaped by osmolarity and tonicity; hydrostatic pressure drives bulk flow, and vesicles enable uptake, release, or passage across polarized cells. Tight junctions influence movement between cells, while electrochemical gradients combine chemical and electrical forces. The deeper lesson organizes these pathways and compares their properties, including carrier saturation and the distinctions between simple and facilitated diffusion.