Slow Fire: How Scientists Learned That Living Things Burn Their Food

Slow Fire

[1]Hold your hand a safe distance from a candle flame and you feel warmth. Watch for an hour and the candle is shorter. The wax has not vanished. It has reacted with oxygen from the air and left as two invisible gases, carbon dioxide and water vapor. A burning candle is a chemical reaction: a fuel and oxygen go in, new molecules come out, and energy leaves as heat and light. People long suspected that a living body's warmth came from a fire inside it, but nobody could test the idea until chemists learned to measure gases and heat.

[2]The test came in the winter of 1782 to 1783, in Paris. The chemist Antoine Lavoisier and the mathematician Pierre-Simon Laplace built an ice calorimeter: a chamber surrounded by a jacket packed with ice. Anything warm inside melted some ice, and the water that dripped out could be weighed to measure the heat. They kept a guinea pig in the chamber for about ten hours and collected the melted ice and the carbon dioxide it breathed out. Then they burned charcoal until it had produced the same amount of carbon dioxide, and measured the heat from that. The two amounts of heat were close. Lavoisier concluded that breathing animals carry out a slow form of combustion, a burning without flame.

[3]Why no flame? In a candle, the reaction runs fast and hot in one place. In a living thing, the same overall change, food plus oxygen becoming carbon dioxide and water, happens in many small steps at body temperature, inside cells. Energy is released a little at a time. The cell captures part of it to power its work, such as building new molecules or moving, and the rest spreads out as heat, which is why a guinea pig, a person and even a pile of sprouting seeds feel warm. One detail matters for later classes: the reaction releases energy because the molecules it forms, carbon dioxide and water, hold less chemical energy than the sugar and oxygen that went in. Energy is not created in the cell; it is converted from one form to another.

[4]Where inside the cell does this happen? In 1890 the German scientist Richard Altmann described tiny grains in stained cells and guessed they were important to the cell's life. In 1898 another German scientist, Carl Benda, named them mitochondria, from Greek words meaning thread and grain. During the twentieth century, researchers showed that most of the steps of cellular respiration take place inside these structures. Mitochondria are found in plant cells, animal cells and many single-celled organisms such as amoebas. Not every cell has them: your mature red blood cells do not. A heart muscle cell, which contracts every second of your life, is packed with them.

[5]Here is the part that surprises many students: the slow fire burns in plants too. A seed sprouting in dark soil, a root growing underground, and a leaf at midnight are all releasing energy from sugar in their mitochondria, using oxygen and giving off carbon dioxide. A bag of sprouting seeds in a closed, insulated container warms up by several degrees. Plants have no lungs or gills. In an animal, breathing moves air or water across a surface where oxygen enters the blood and carbon dioxide leaves; that is a transport job, not the reaction itself. Plant cells get oxygen from the air through tiny openings in their leaves and from air pockets in the soil. When a field stays flooded for days, those air pockets fill with water and the roots can die for lack of oxygen.

[6]Lavoisier's guinea pig and the candle on your table run the same overall reaction at different speeds. In both, the atoms are conserved. The carbon in the gas the guinea pig breathed out had arrived in its body as carbon in its food; the carbon in the gas above the candle had been part of the wax. More than two hundred years later, the central idea has not changed: living things get the energy they need by combining the molecules in their food with oxygen, a controlled burn that happens, in tiny steps, inside almost every cell.