membrane transport theories

Membrane transport theories describe how substances move across cell membranes. They include passive transport, where molecules move naturally from high to low concentration (like diffusion) or with help from channels or carriers, needing no energy. Active transport requires energy to move molecules against their concentration gradient, often via transport proteins. Facilitated diffusion is a passive process using specific proteins, while active transport and endocytosis involve energy and cellular effort. Overall, these theories explain how cells control access to nutrients, remove waste, and maintain internal balance.