Short answers, pulled from the story.
Metabolism is the set of life-sustaining chemical reactions that occur within living organisms. Its three main functions are converting energy in food into a usable form, converting food into building blocks for macromolecules such as proteins, lipids, nucleic acids, and some carbohydrates, and excreting metabolic wastes. The word comes from the Greek metabole, meaning "change."
Catabolism breaks down compounds, such as glucose into pyruvate, and usually releases energy. Anabolism builds compounds like proteins, carbohydrates, lipids, and nucleic acids, and usually consumes energy. ATP bridges the two, since catabolic reactions generate it and anabolic reactions consume it.
ATP, or adenosine triphosphate, is the energy currency of cells because it transfers chemical energy between different reactions. There is only a small amount of ATP in cells, but because it is continuously regenerated, the human body can use about its own weight in ATP per day.
Enzymes catalyze the chemical reactions of metabolism, allowing reactions to proceed more rapidly and letting organisms drive energy-requiring reactions by coupling them to spontaneous reactions that release energy. Metabolic reactions are organized into pathways, where each step is facilitated by a specific enzyme. Enzymes also allow the rate of a reaction to be regulated in response to the cell's environment.
Santorio Santorio published the first controlled experiments in human metabolism in 1614. Louis Pasteur studied fermentation by yeast in the 19th century, Friedrich Wohler synthesized urea in 1828, Eduard Buchner discovered enzymes at the start of the 20th century, and Hans Krebs discovered the urea cycle, the citric acid cycle, and the glyoxylate cycle.
Basic metabolic pathways are remarkably similar among vastly different species because they appeared early in evolutionary history and were retained for their efficacy. The carboxylic acids known as citric acid cycle intermediates appear in all known organisms, from the bacterium E. coli to elephants. Central pathways like glycolysis and the citric acid cycle were present in the last universal common ancestor.