Overview of Metabolic Reactions

Core Concepts

Catabolism and anabolism: two directions of one process

Catabolism breaks molecules apart: glycogen → glucose, triglycerides → fatty acids + glycerol, proteins → amino acids. Digestion (Chapter 23) is catabolism, and so is the burning of fuels inside cells. These reactions are generally exergonic — they release energy, much of which is captured rather than lost as heat. Anabolism builds molecules up: glucose → glycogen, amino acids → proteins, fatty acids → triglycerides. These reactions are endergonic — they require energy input. The trick of metabolism is coupling: exergonic catabolism drives endergonic anabolism, with ATP as the transfer agent. When you lift weights and then eat, the catabolism of food energy pays for the anabolic building of muscle protein.

ATP: the energy currency

ATP (adenosine triphosphate) is a nucleotide with three phosphate groups. Hydrolysis of the terminal phosphate converts ATP to ADP + inorganic phosphate and releases energy that cells use for work — muscle contraction, active transport, synthesis reactions. Because cells keep only a small pool of ATP, it must be regenerated constantly: ADP + phosphate + energy → ATP, powered by the catabolic pathways. The commonly taught analogy is a rechargeable battery: ATP is the charged state, ADP is the discharged state, and metabolism is the charger. The takeaway: ATP hydrolysis delivers usable energy, and ATP must be continuously regenerated.

Redox reactions and electron carriers

Energy extraction from food is mostly a matter of moving electrons. Oxidation is the loss of electrons (frequently along with hydrogen atoms); reduction is the gain of electrons; the two always occur together. When a fuel is oxidized, electrons are handed to carriers: NAD+ becomes NADH, and FAD becomes FADH2. These reduced carriers then deliver electrons to the electron transport chain, where the energy released as electrons cascade down the chain is used to make ATP. The carriers are the shuttle system that connects fuel breakdown to the ATP-producing machinery.

Glucose catabolism in three stages

Glucose is the model fuel, and its oxidation proceeds in three stages:

1. Glycolysis (cytosol): glucose (6 carbons) is split into two pyruvate molecules (3 carbons each). The commonly taught net yield is 2 ATP and 2 NADH per glucose. Oxygen is not required — glycolysis runs anaerobically.
2. Krebs cycle (mitochondrial matrix): pyruvate is converted to acetyl-CoA, which enters the cycle; the carbons are progressively released as CO2. The commonly taught yield per glucose is 2 ATP (as GTP), 6 NADH, and 2 FADH2.
3. Electron transport chain and oxidative phosphorylation (inner mitochondrial membrane): NADH and FADH2 donate electrons; the energy drives protons across the membrane, building a gradient; ATP synthase uses the flow of protons back across the membrane to make ATP (chemiosmosis). Oxygen is the final electron acceptor — this is where most ATP is made, commonly taught as roughly 28–34 ATP per glucose depending on the shuttle system used.

When oxygen is limited — during intense exercise, for example — the electron transport chain backs up, NADH cannot be unloaded, and glycolysis would stall. The cell solves this by converting pyruvate to lactate (fermentation), regenerating NAD+ so glycolysis can continue. Lactate is not waste; the liver recycles it back to glucose (the Cori cycle, Topic 2).

Acetyl-CoA: the metabolic hub

Carbohydrates, fats, and proteins all converge on acetyl-CoA. Glucose gives acetyl-CoA via pyruvate; fatty acids give acetyl-CoA via beta-oxidation (Topic 3); many amino acids give acetyl-CoA after deamination (Topic 4). Acetyl-CoA then enters the Krebs cycle — or, when the cycle is overwhelmed, as in prolonged fasting, the liver converts excess acetyl-CoA into ketone bodies, alternative fuels discussed in Topic 3. The hub concept explains why fuels are partly interchangeable: the body can burn whatever it has, but the routes to the hub differ.

Regulation of metabolism

Pathways are not constant; they are switched on and off. The key control points are enzymes that catalyze slow, irreversible steps, regulated by feedback inhibition (the end product inhibits an early enzyme), allosteric effectors, and hormones — insulin generally promotes anabolic (storage) reactions in the fed state, while glucagon, epinephrine, and cortisol promote catabolic (mobilization) reactions during fasting or stress. The overall pace is the metabolic rate — total energy expended per day — which varies with activity, body size, age, and thyroid status (commonly taught concepts; see Topic 6).

Common Confusions