Chemical Reactions & Energy — Why Chemistry Happens

A chemical reaction transforms one set of substances (reactants) into another (products) by breaking old chemical bonds and forming new ones. Atoms are rearranged — never created or destroyed (the Law of Conservation of Mass). Reactions are written as equations: Reactants → Products The arrow means "yields" or "produces." In reversible reactions (common in biochemistry), a double arrow (⇌) shows the reaction can go in either direction. Why do reactions happen? Two factors drive reactions: 1. Energy: Reactions tend to move toward lower energy states (exothermic reactions release energy — energetically "downhill") 2. Entropy: Reactions tend to move toward greater disorder (higher entropy) The combination of these factors is captured in Gibbs free energy (ΔG). If ΔG is negative, the reaction is spontaneous (will happen on its own, given the right conditions). Most metabolic reactions in the body have a negative ΔG — they are thermodynamically favourable.

Exothermic reactions release energy (usually as heat) to the surroundings. Products have less energy than reactants — energy is released as bonds form. ΔH (enthalpy change) is negative. Examples: Combustion of glucose in cellular respiration releases energy used to make ATP. The burning sensation of a fire. The warmth from your body. Endothermic reactions absorb energy from the surroundings. Products have more energy than reactants — energy is needed to break bonds. ΔH is positive. Examples: Photosynthesis (absorbs light energy to build glucose). The feeling of cold from an ice pack (endothermic dissolving of ammonium nitrate absorbs heat from the surroundings). In medicine — exothermic vs endothermic matters in: - Fever: Infection triggers increased metabolic rate → more exothermic reactions → increased heat production → body temperature rises → fever. Paracetamol/ibuprofen work partly by reducing prostaglandins that raise the hypothalamic temperature set-point. - Thermogenesis: Brown adipose tissue (brown fat) in newborns and cold-adapted adults contains uncoupling protein (UCP1) that "short-circuits" the electron transport chain — protons flow back without making ATP, generating heat instead. This is deliberate, non-shivering thermogenesis.

Even exothermic reactions (thermodynamically favourable) don't always happen spontaneously — they need an initial push of energy to get started. This is called the activation energy (Ea): the minimum energy needed to break the reactants' bonds so the reaction can proceed. Think of it like pushing a boulder over a hill: the boulder wants to roll down the other side (exothermic — lower energy state), but you first have to push it to the top of the hill (activation energy). Once over the peak, it rolls on its own. Catalysts lower the activation energy without being consumed in the reaction. They provide an alternative reaction pathway with a lower energy "hill." This makes reactions happen faster — dramatically faster — without altering the products or the overall energy change. Enzymes are biological catalysts — protein molecules that lower activation energies for specific metabolic reactions. Without enzymes, most biochemical reactions would be too slow to sustain life. With enzymes, reactions that would take millions of years can happen in milliseconds. Enzyme mechanisms: - The enzyme has an active site — a precisely shaped pocket that binds the specific substrate(s) - Induced fit: The active site changes shape slightly to grip the substrate - Once bound, the enzyme stresses the substrate's bonds → lowers activation energy → reaction proceeds → products released → enzyme is free to repeat Temperature and pH affect enzyme activity: Each enzyme has an optimal temperature and pH. Denaturation (unfolding) by excess heat or extreme pH permanently destroys function — which is why a high fever is dangerous.

Oxidation and reduction always occur together (redox reactions) — when one substance is oxidised, another is reduced. OIL RIG: Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons). Alternatively: LEO the lion says GER — Loses Electrons = Oxidised; Gains Electrons = Reduced. Why this matters biologically: Cellular respiration is fundamentally a series of redox reactions. Glucose is oxidised (loses electrons/hydrogen) step by step — in glycolysis, the Krebs cycle, and the electron transport chain. The released electrons are captured by NAD⁺ (which is reduced to NADH) and FAD (reduced to FADH₂). At the electron transport chain, these electrons are passed along protein complexes, ultimately reducing oxygen (O₂ + 4e⁻ + 4H⁺ → 2H₂O). The energy released drives ATP synthesis. Oxidative stress in medicine: - Free radicals are highly reactive molecules with unpaired electrons — they oxidise (damage) DNA, proteins, and cell membranes. - Antioxidants (vitamins C, E; glutathione) neutralise free radicals by donating electrons. - Oxidative stress is implicated in ageing, atherosclerosis, cancer, and neurodegenerative diseases. Drugs and redox: - Metronidazole (antibiotic/antiprotozoal) — reduced by anaerobic bacteria and protozoa to a toxic free radical that damages their DNA. It only works in low-oxygen environments (hence its use for anaerobic infections). - Vitamin K — essential for the carboxylation (oxidation-requiring reaction) of clotting factors. Warfarin works by blocking vitamin K recycling → clotting factors inactive → anticoagulation.