Membrane Transport

Carefully controlled solute movement into and out of cells is an essential feature of life. There are many ways solutes are transported across the thin (~40 Å) hydrophobic barrier. Transport is divided into passive diffusion and active transport. A biological membrane is semi-permeable, being permeable to some molecules, most notably water, while being very impermeable to most solutes that require some form of transporter. Passive diffusion (simple and facilitated) only requires the energy inherent in the solute’s electrochemical gradient and results in equilibrium across the membrane. In contrast, active transport requires additional energy (i.e. ATP), and results in a non-equilibrium, net accumulation of the solute. Passive transport can involve simple diffusion or facilitated carriers including ionophores and channels. Active transport comes in many, often complex forms. Examples of active transport include primary active transport (uniport), secondary active transport (cotransport, antiport) and group translocation. Besides the multitude of transport systems, transport can be accomplished by Gap Junctions, receptor-mediated endocytosis, phagocytosis, pinocytosis, exocytosis and apoptotic membrane blebbing.