Medical Biochemistry, Genetics, and OmpathStudy

Medical Biochemistry, Genetics, and Molecular Biology This article provides a concise overview of key concepts in medical biochemistry, genetics, and molecular biology, essential for Year 2 MBChB students. It covers fundamental processes of energy metabolism, principles of inheritance and chromosomal disorders, and the core mechanisms of gene expression. Energy Metabolism Metabolism encompasses all chemical reactions occurring within a living organism, broadly categorised into two main processes: anabolism and catabolism. Anabolism: These are biosynthetic pathways where small, simple precursor molecules are assembled into larger, more complex organic molecules. Anabolic reactions typically require an input of energy (endergonic), often supplied by ATP. Examples include protein synthesis from amino acids, or glycogen synthesis from glucose. Catabolism: These are degradative pathways where larger, complex organic molecules are broken down into smaller, simpler ones. Catabolic reactions typically release energy (exergonic), which is captured in molecules like ATP. Examples include the breakdown of glucose during glycolysis or the oxidation of fatty acids. ATP Production: Oxidative vs. Substrate-Level Phosphorylation The primary energy currency of the cell is adenosine triphosphate (ATP). Its synthesis occurs through two main mechanisms: Oxidative Phosphorylation: This is the major pathway for ATP synthesis in aerobic organisms. As electrons flow through the electron transport chain (ETC) , much of their free energy is conserved in the form of ATP. This process occurs on the inner mitochondrial membrane in eukaryotes (or plasma membrane in prokaryotes). Electrons from NADH and FADH2 are passed along a series of protein complexes, creating a proton gradient across the membrane. The potential energy stored in this gradient is then harnessed by ATP synthase to produce ATP. Substrate-Level Phosphorylation: In this process, ATP is generated by the direct transfer of a phosphate group from a high-energy phosphorylated intermediate molecule to ADP. This occurs independently of the electron transport chain and does not require oxygen directly. Key examples include steps in glycolysis (e.g., the conversion of 1,3-bisphosphoglycerate to 3-phosphoglycerate) and the citric acid cycle (e.g., the conversion of succinyl CoA to succinate). Glucose Metabolism: Glycolysis and Fermentation Glycolysis is the metabolic pathway that breaks down glucose into pyruvate, generating ATP and NADH. The most common pathway is the Embden-Meyerhof pathway . An alternative pathway, the Entner-Doudoroff pathway , is found in some bacteria, yielding different amounts of ATP and NADH. Fermentation is an anaerobic process that allows for the regeneration of NAD+ from NADH, enabling glycolysis to continue in the absence of oxygen. The acquisition of energy by glucose fermentation requires substrate-level phosphorylation as its primary means of ATP generation, as it does not involve an electron transport chain or an external terminal electron acceptor. Respiration: Aerobic vs. Anaerobic Respiration involves the transfer of electrons from fuel molecules to a terminal electron acceptor, releasing energy for ATP synthesis. Aerobic Respiration: In aerobic respiration, the terminal electron acceptor is oxygen (O2) . This process yields a large amount of ATP through oxidative phosphorylation. Anaerobic Respiration: In anaerobic respiration, various inorganic compounds serve as terminal electron acceptors instead of oxygen. Common electron acceptors include nitrate, sulfate, and fumarate . Hydrogen sulfide (H2S) is typically a product of sulfate reduction, not an electron acceptor itself. Carbon Dioxide Fixation and Photosynthesis Carbon dioxide fixation is the process by which inorganic carbon (CO2) is converted into organic compounds. The Calvin Cycle is the most common pathway for CO2 fixation in chemolithotrophs (organisms that obtain energy from the oxidation of inorganic compounds) and photoautotrophs (organisms that use light energy). Anoxygenic photosynthesis is a form of photosynthesis that does not produce oxygen. It is carried out by certain bacteria (e.g., purple and green sulfur bacteria). In these organisms, bacteriochlorophyll uses light energy to energize an electron, initiating electron flow. Unlike oxygenic photosynthesis (which uses water as an electron donor), anoxygenic photosynthetic bacteria oxidize electron donors other than water, such as sulfide (H2S) , elemental sulfur, or ferrous iron. Plasma Membrane Functions The plasma membrane is a vital component of all cells, performing multiple critical functions: Selective Permeability: Regulates the passage of substances into and out of the cell, maintaining intracellular homeostasis. This includes passive diffusion (driven by concentration difference), facilitated diffusion, and active transport. Responding to External Stimuli: Contains receptors that bind to signaling molecules, initi