Why doesn't the air just run out, no matter how hard you scrub it?
We are always breathing and puffing air. The amount of air we breathe in and out is almost the same as the amount of air we breathe out, so the volume of air itself does not decrease even if there are many people in the room.
However, when humans and animals breathe air, the oxygen in the air is taken into their bodies. The expelled air contains less oxygen and more carbon dioxide instead. If this were not the case, the oxygen in the air would continue to decrease and the carbon dioxide would continue to increase.
The reason this does not happen, however, is because of plants. Plants do exactly the opposite of humans and animals. Plants use sunlight as energy to produce starch from water and carbon dioxide. This is called photosynthesis, and in the process of photosynthesis, they release oxygen. In other words, plants convert carbon dioxide released by humans and animals back into oxygen.
Humans and animals remove carbon dioxide from oxygen, and plants remove oxygen from carbon dioxide. Animals and plants help each other to survive.
If plants were to disappear from the earth, it would be a disaster. The oxygen supply would gradually decrease, and eventually all humans and animals on the earth would die. Thus, plants are important for all animals.
