Carbon dioxide is a very busy chemical, constantly being made, broken apart and remade. If this chemical cycle stopped, life as we know it would come to an end. For the carbon dioxide in the air plays a vital role of give and take between plant and animal life. Yet the proportion of this busy gas in the air is very small.
We breathe in oxygen and, as everyone knows, we breathe out carbon dioxide. From this bald statement, you might think that the air we exhale is stripped of its oxygen and teeming with carbon dioxide. But this is not so. Ordinary air is 21% oxygen and the air we exhale is about 16% oxygen. Less than 5% of the oxygen atoms combine with atoms of carbon and become molecules of carbon dioxide.
But all over the world millions of us are producing small amounts of carbon dioxide. More is produced by the respiration of animals and plants, by decay, by fires and furnaces, by active volcanoes and by the breakdown of minerals such ^s limestone. These small amounts add up to countless tons of carbon dioxide yet this gas is only .03% of the atmosphere. In 10,000 gallons of ordinary air, there are only three gallons of carbon dioxide.
The carbon dioxide we exhale spread s out as far as possible and mixes evenly with the other gases of the air. It spreads through the atmosphere and comes in contact with the green plant world. Now the useless waste gas becomes useful,, for green plants need carbon dioxide to carry on photosynthesis, the process by which they make their basic food.
Plants produce some carbon dioxide during respiration, but not enough for their needs in photosynthesis. They need the waste carbon dioxide exhaled by us and the animal world. The process of photosynthesis is still not fully understood, but we know that energy from sunlight is used to reassemble the atoms in molecules of water from the soil and carbon dioxide from the air into molecules of sugar. In this complex remaking of chemicals, some oxygen atoms are left over and they are returned to the air. The plant world needs our waste carbon dioxide to keep going. In return, it supplies us with fresh oxygen which we use to keep producing more carbon dioxide to supply the plant world.
A molecule of carbon dioxide is a unit of one atom of carbon and one atoms of oxygen. Our respiration process uses this trick to get rid of the body waste chemicals. Oxygen combines with carbon and we exhale waste carbon dioxide gas which in turn is used by the plant world. This give and take makes the plant and animal kingdoms dependent upon each other.