Chapter 11: Small-Molecule Transport and Electrical Properties of Membranes

Channel proteins and transporters are introduced as two major classes of membrane proteins that mediate facilitated diffusion and active transport. Transporters, including uniporters, symporters, and antiporters, are covered in detail, especially in the context of ATP-driven pumps like the Na⁺/K⁺-ATPase and Ca²⁺-ATPase, which establish ion gradients essential for cellular homeostasis. The chapter explains how electrochemical gradients combine concentration and voltage differences, and how these gradients drive secondary active transport. Channel proteins, including voltage-gated, ligand-gated, and mechanically gated channels, are described for their roles in rapid ion flux across membranes. Aquaporins are introduced as water-selective channels critical for osmoregulation. The concept of membrane potential is explored through the Goldman and Nernst equations, and the resting membrane potential is explained as a balance of passive ion movement, especially of K⁺, across selective leak channels. The electrical excitability of neurons is highlighted as a case study in membrane transport, with a focus on action potentials, depolarization, and the role of voltage-gated Na⁺ and K⁺ channels. Patch-clamp techniques are introduced as powerful tools for measuring ion channel activity. The chapter concludes by reinforcing that membrane transport and electrical signaling are tightly linked processes fundamental to nerve transmission, muscle contraction, and cellular signaling across a variety of tissues.