E. coli and elephants run their cells on the same chemical cycle
A single-celled gut bacterium and a huge elephant look like they have nothing in common, yet deep inside their cells the same set of chemical reactions is turning over. The citric acid cycle's intermediates turn up in every known organism, a sign these pathways appeared very early in evolution and worked too well to abandon.
Metabolism, from a Greek word for change, is the web of chemical reactions that keeps an organism alive. It has three main jobs: converting the energy in food into a form cells can use, turning food into the building blocks of proteins, fats, nucleic acids and some carbohydrates, and getting rid of waste. Reactions come in two directions. Catabolism breaks molecules down, as when glucose is split into pyruvate, and usually releases energy; anabolism builds molecules up and usually consumes it.
These reactions are organised into pathways, chains of steps in which one chemical is gradually transformed into another, each step handled by a dedicated enzyme. Enzymes speed reactions up and let cells regulate them in response to their surroundings or to signals from other cells. Crucially, they allow reactions that would never happen on their own by coupling them to others that release energy. The total energy consumed by all this activity is the basal metabolic rate.
Life's structures rest on four classes of molecules: amino acids, carbohydrates, nucleic acids and lipids. Proteins, chains of amino acids, include most enzymes and also form the cell's internal scaffolding and take part in signalling and immunity; when glucose runs short, amino acids can be fed into the citric acid cycle for energy. Lipids are the most varied group, forming cell membranes and storing energy; a glycerol attached to three fatty acids makes a triglyceride.
What counts as food or poison depends on an organism's metabolism. Some bacteria feed on hydrogen sulfide, a gas toxic to some animals, yet humans make tiny amounts of it naturally to use as a chemical signal. Metabolism goes awry in type 2 diabetes, metabolic syndrome and cancer, and the distinctive metabolism of cancer cells offers potential targets for treatment.
Source: Metabolism
