respiration go on, and they are the reason we need oxygen. The diffusion gradient pulls the oxygen in like a tractor beam. Inside the mitochondrion, after crossing yet another membrane, there are more molecular machines that break up food molecules, such as glucose, and use the energy stored in their chemical bonds to fuel life. The main purpose of these reactions is the production of a molecule called adenosine triphosphate, or ATP. ATP is the fuel for every cellular reaction that requires energy. It powers muscles, nerve cells, DNA copying, and everything else, in animals, plants, fungi, and bacteria. At any moment, a human body contains only about 10 grams of ATP, but we make and consume our own body weight of the molecule each day.

The oxygen molecule has come to the mitochondrion to pick up the trash. It reacts with the electrons and protons that these molecular machines produce in their fuel-generating work. In the process the molecule, which consists of two oxygen atoms, breaks apart. Each atom forms a new alliance with two hydrogen atoms, in a water molecule. Nothing is ever really destroyed. Each new water molecule will journey through my veins to my lungs, leave my body as vapor, and float into the atmosphere. Here it will become part of a cloud, then a raindrop, and perhaps eventually get taken up by a plant, where it will react with carbon dioxide to become carbohydrate, creating food for animals.

Aristotle thought that the function of breathing was to cool the blood. In fact, the opposite is true: By measuring the total amount of oxygen used by my body, Vasiliki Costarelli is measuring the rate at which I use energy. This is why I am lying on the couch—to have my energy consumption, or metabolic rate, measured.

The Torch of Prometheus

To be alive is to be using up and giving out energy. We understand this instinctively. Many cultures believe an energy field pervades the world and passes through living beings. Taoists call it chi; Hindus, prana. To say that living bodies burn fuel is not a figure of speech. The chemistry of respiration and that of combustion are identical, and the amount of energy released from food when you eat it is the same as when you burn it. Life is a slow fire: Every cell burns fuel to build molecules up

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