All human beings eat food and make use of the chemical energy in it, so do all other animals. Perhaps you wonder where all that chemical energy comes from. Why doesn’t the food all get used up?

    The answer is that new food is being grown as fast as old food is used to. It is the green plants that form the new food. Animals either eat the plants or eat other animals that have eaten plants.

    The green substance of plants is chlorophyll. Chlorophyll can absorb sunlight. When it does so, it changes the energy of the sun into chemical energy. The chemical energy present in sunlit chlorophyll is used to combine dioxide in the air with water from the soil. Starch and other complicated compounds are formed. These are high in chemical energy obtained from the sunlit chlorophyll.

    They make up the food on which mankind and all other animals live. In the process of forming this food, some oxygen atoms are leftover. These are given off into the air by the plants. The whole process is called photosynthesis.

    Thus, plants use sunlight to form food and oxygen to form carbon dioxide and water again. Plants change the sun’s energy into chemical energy. And animals change the animal energy into kinetic and heat energy.

56. The green substance of plants is chlorophyll. The underlined word in the above is closest in meaning to . . . .