Chapter 15: The Plasma Membrane & Molecular Transport

The text details the fluid mosaic model, describing how lipids—including phosphatidylcholine, sphingomyelin, glycolipids, and cholesterol—are asymmetrically distributed between the inner and outer leaflets to form a viscous barrier, while integral and peripheral membrane proteins carry out specific functions like transport and cell signaling. The summary explores the structural complexity of the membrane, including the role of the glycocalyx in cell recognition and the organization of specialized domains such as lipid rafts and caveolae, which restrict protein mobility and facilitate signaling, alongside the distinct apical and basolateral domains found in polarized epithelial cells. A significant portion of the chapter is dedicated to the transport of small molecules, distinguishing between passive diffusion of hydrophobic molecules and facilitated diffusion mediated by carrier proteins, such as the glucose transporter (GLUT), and ion channels. The mechanics of ion channels are examined in depth, particularly their selectivity filters and gating mechanisms, which are crucial for generating membrane potentials and propagating action potentials in nerve and muscle cells as described by the Hodgkin-Huxley model. The text further elucidates active transport mechanisms that require energy, specifically the ATP-driven sodium-potassium pump (Na+/K+ ATPase) which maintains critical ion gradients and osmotic balance, and the ABC transporter family, which includes the multidrug resistance (MDR) proteins and the CFTR protein associated with cystic fibrosis. Additionally, secondary active transport is explained, illustrating how ion gradients drive the uptake of nutrients like glucose in intestinal cells. Finally, the chapter covers the uptake of macromolecules through endocytosis, detailing the actin-dependent process of phagocytosis for large particles and the highly specific mechanism of clathrin-mediated endocytosis, using the uptake of LDL cholesterol via the LDL receptor and its subsequent sorting in early endosomes as a primary example of receptor recycling and cargo degradation.