Every living thing is a busy chemical workshop. A single cell may carry out thousands of chemical reactions each second, building some molecules, breaking down others and moving energy from one form to another. The sum of all these reactions is called metabolism: all the chemical reactions that take place in an organism.

Energy rules

Two basic laws of physics govern how living things use energy. First, energy is never created or destroyed, only changed from one form to another: light into chemical energy, chemical energy into motion and heat. Second, every time energy changes form, some of it escapes as heat that cannot easily be used again. That is why living things need a constant supply of energy. For almost every ecosystem on Earth, the original source is the sun. A notable exception was found in 1977, when scientists exploring deep-sea vents near the Galápagos Islands found communities of animals living off bacteria that draw energy from chemicals in the vent water rather than from light.

ATP: the cell's energy currency

Cells cannot spend sugar directly, any more than you can pay for a bus ride with a bar of gold. They first convert the energy in food into a small molecule called ATP (adenosine triphosphate). ATP works as the cell's short-term energy currency. When one of its three phosphate groups breaks off, energy is released to power muscle contraction, the building of new molecules and the pumping of substances across membranes. The cell then recharges the molecule and uses it again.

Enzymes

Most reactions in a cell would happen far too slowly on their own. Enzymes, which are usually proteins, speed them up. Every reaction needs an initial push, called activation energy, to get started. Enzymes lower the activation energy, so reactions can run quickly at body temperature. An enzyme is not used up; one molecule can work again and again.

Each enzyme has a particular shape, with a pocket called the active site that fits only certain molecules. In 1894 the German chemist Emil Fischer compared this to a lock and key; later work showed the fit is more flexible, with the enzyme shifting shape slightly as it binds. Because shape matters so much, high heat or extreme acidity can change an enzyme's shape so it stops working, a process called denaturing. This is one reason a very high fever is dangerous.

Photosynthesis

Plants, algae and some bacteria capture sunlight through photosynthesis, which takes place in chloroplasts. In summary: carbon dioxide + water + light energy → glucose (a sugar) + oxygen.

The green pigment chlorophyll absorbs mostly red and blue light and reflects green light, which is why leaves look green. In the first stage, light energy splits water molecules, and the oxygen released into the air comes from that water, not from carbon dioxide. Scientists confirmed this in 1941 by tracking a heavy form of oxygen through the process. In the second stage, known as the Calvin cycle after the chemist Melvin Calvin, who mapped it and won a Nobel prize in 1961, the plant uses the captured energy to build sugar from carbon dioxide.

Cellular respiration

Organisms then release the energy stored in sugar through cellular respiration, mostly in the mitochondria: glucose + oxygen → carbon dioxide + water + ATP. It is roughly photosynthesis in reverse. Plants do it too, day and night. Fully breaking down one glucose molecule with oxygen yields roughly 30 molecules of ATP.

When oxygen runs short, cells can fall back on fermentation, which produces only 2 ATP per glucose, far less than respiration with oxygen. Yeast ferment sugar into alcohol and carbon dioxide, which is how bread rises and beer gets its bubbles. Hard-working human muscle cells produce lactate instead.

Worked example: the journey of a carbon atom

Follow one carbon atom through a single year.

Spring. The atom is part of a carbon dioxide molecule drifting over a wheat field. It enters a leaf through a tiny pore.

Midday. In a chloroplast, sunlight splits water and the plant captures the energy. In the Calvin cycle, our carbon atom is built into a sugar molecule, later stored in the grain as starch. The oxygen the plant gives off came from water, not from the carbon dioxide our atom arrived in.

Harvest. The wheat is ground into flour and baked into bread. Yeast in the dough ferment some of the sugar, and the carbon dioxide they release makes the loaf rise. Our atom stays in the starch.

Lunch. You eat a sandwich. Digestive enzymes break the starch down into glucose, which your blood carries to a leg muscle as you climb stairs.

Afternoon. Inside a mitochondrion, cellular respiration breaks the glucose apart. Its energy is stored in ATP, which powers your muscle fibers. Our carbon atom leaves as part of a carbon dioxide molecule, travels to your lungs and is breathed out.

The atom has completed a circle: from air, through a plant and a person, and back to air. The matter cycled, while the energy, which began as sunlight, passed through and was finally lost as body heat. That pattern, matter cycling while energy flows, returns in the level on ecology.