Chapter 7: Membrane Structure and Function

The phospholipid bilayer, formed by amphipathic molecules with hydrophobic tails and hydrophilic heads, creates a selectively permeable barrier that regulates what enters and exits the cell. Integral proteins span the membrane completely and often function in transport or signaling, while peripheral proteins attach to membrane surfaces and provide structural support. Membrane fluidity, determined by temperature and the saturation level of fatty acids within the phospholipid chains, affects the membrane's flexibility and protein mobility. The chapter explores how cells maintain concentration gradients and electrochemical gradients through multiple transport mechanisms. Simple diffusion and osmosis move substances along concentration gradients without energy input, while facilitated diffusion uses channel and carrier proteins to transport molecules down their gradients. Active transport, exemplified by the sodium-potassium pump, consumes ATP to move substances against concentration gradients, establishing and maintaining the electrical and chemical potential differences critical for cellular function. Bulk transport processes enable movement of large particles and volumes: exocytosis expels materials from the cell, while endocytosis internalizes them through mechanisms including phagocytosis, pinocytosis, and receptor-mediated endocytosis. The chapter emphasizes how tonicity—the relative solute concentration between the cell and its environment—determines water movement and cell volume changes in hypertonic, hypotonic, and isotonic solutions. By integrating membrane structure with transport function, the chapter illustrates how membrane dynamics support cellular metabolism, facilitate communication with other cells, generate membrane potential for electrical signaling, and establish the regulated internal environment necessary for survival and coordinated cellular activities.