Chapter 11: Transmembrane Transport of Ions & Molecules

Transmembrane Transport of Ions & Molecules from Molecular Cell Biology (Ninth Edition) provides a comprehensive analysis of the molecular mechanisms governing the transmembrane transport of ions and small molecules, a fundamental process essential for maintaining cellular homeostasis and physiological function. It begins by establishing the properties of the phospholipid bilayer, which acts as a selective barrier permeable to gases and small uncharged polar molecules but impermeable to ions and larger polar substances, necessitating the use of specialized membrane transport proteins. The discussion distinguishes between three major classes of transport proteins: channels, which facilitate rapid, passive movement down electrochemical gradients; transporters (uniporters, symporters, and antiporters), which undergo conformational changes to move substrates; and ATP-powered pumps, which utilize energy from ATP hydrolysis to drive active transport against concentration gradients. Key examples of facilitated transport are examined, including the GLUT family of glucose uniporters, which function via an alternating access mechanism, and aquaporins, which selectively transport water molecules to regulate osmotic pressure. The chapter provides a detailed classification of ATP-powered pumps, specifically P-class pumps (such as the sodium-potassium ATPase and calcium ATPase), V-class proton pumps responsible for organelle acidification, and the ABC superfamily, which includes multidrug resistance proteins and the CFTR chloride channel involved in cystic fibrosis. Furthermore, the text explores the generation of the resting membrane potential, explaining how nongated potassium channels and the sodium-potassium pump establish an inside-negative voltage, calculated quantitatively using the Nernst equation. The structural basis of ion selectivity is detailed through the potassium channel's selectivity filter, which distinguishes ions based on hydration energy. Finally, the chapter integrates these concepts to explain transcellular transport in polarized epithelial cells, illustrating how the coordinated action of symporters, antiporters, and pumps facilitates complex physiological processes such as glucose absorption in the intestine, acid secretion in the stomach, and bone resorption by osteoclasts.