Chapter 3: Cellular Structure, Proteins, and Metabolic Pathways

The plasma membrane serves as a selectively permeable boundary composed of a phospholipid bilayer embedded with proteins, cholesterol, and carbohydrate moieties that collectively regulate molecular transport, intercellular communication, and cellular integrity. The chapter systematically examines organelles as specialized compartments with distinct functions: the nucleus houses genetic material and directs transcription, ribosomes catalyze protein synthesis through translation of messenger RNA, the endoplasmic reticulum processes newly synthesized proteins and lipids, the Golgi apparatus modifies and packages these molecules for transport, lysosomes and peroxisomes conduct intracellular degradation and detoxification, and mitochondria generate adenosine triphosphate through oxidative phosphorylation. The cytoskeleton provides structural support and enables cellular movement through three filament types: microfilaments, intermediate filaments, and microtubules. Protein structure and function are intimately connected through hierarchical organization from amino acid sequence through primary, secondary, tertiary, and quaternary configurations, determining how proteins perform their physiological roles. Enzymes accelerate biochemical reactions by lowering activation energy, with their activity regulated through temperature, pH alterations, cofactor availability, and inhibitor mechanisms including allosteric and covalent modifications alongside feedback inhibition loops. Cellular metabolism encompasses interconnected catabolic pathways that oxidize nutrients to harness energy and anabolic pathways that synthesize biological macromolecules. Glycolysis initiates glucose breakdown anaerobically in the cytoplasm, generating pyruvate, ATP, and reduced nicotinamide adenine dinucleotide, while the citric acid cycle in mitochondria completes oxidation and produces electron carriers that power oxidative phosphorylation. The electron transport chain coupled with chemiosmosis generates the majority of cellular ATP through proton gradient mechanisms. The chapter demonstrates how carbohydrate, lipid, and protein catabolism converges into shared metabolic intermediates, allowing metabolic flexibility based on energy demands. Clinical applications illustrate consequences of metabolic dysfunction, particularly mitochondrial disorders that compromise ATP generation and disrupt cellular homeostasis.