Read it through once

This is more easily seen by tracing the probable history of an atom of carbon. We will suppose that it enters a grass leaf as carbonic acid gas and becomes starch: next evening it will become sugar and may pass from cell to cell up the stem to where the fruit or grain is ripening. It will be stored up as starch in the grain. This grass will become hay and in due course be eaten by a bullock. The starch is changed and may be stored up in the fat of the animal's body. When this is eaten at somebody's dinner, the fat will most probably be consumed or broken up; this breaking up may be compared to a fire, for heat is given off, and the heat in this case will keep up the body-temperature of the person. The carbon atom will again become carbonic acid gas, for it will take part of the oxygen breathed in, and be returned to the atmosphere as carbonic acid gas when the person is breathing.