Welcome to You Ask Andy

Mary Pauline Bizil, age 10, of Cleveland, Ohio, for her question:

Why do plants need air?

The air, as we know, is a mixture of different gases. We need its oxygen to keep going, as do other animals and the plants. But the leafy green plant world also needs to take carbon dioxide from the air. This is the waste gas we breathe out and we have no use for it. We need air for one good reason    its oxygen. The plants need air for two good reasons    oxygen and carbon dioxide.

All the earth's plants and animals use up oxygen day and night. All the earth's plants and animals pour out waste carbon dioxide, day and night. Those that live on the land take their oxygen from the air: those that live in the sea, lakes and streams take dissolved oxygen from the water. You might wonder why the world does not run out of oxygen. There are two reasons why this does not happen. Green plants need carbon dioxide as well as oxygen    and nature knew about recycling things long before we did.

Nowadays, we gather up empty bottles so that they can be spruced up and refilled.. This is recycling. We learned about it when we started to remodel and reuse our trashy pollution. Nature recycles the oxygen and carbon dioxide for the plants, animals and people. All their living cells need oxygen as fuel to do their work.. The oxygen they use up is changed into carbon dioxide.

We breathe oxygen through our lungs and breathe out our used carbon dioxide. Plants let air seep through pores and spread among the living cells inside. Their used carbon dioxide seeps out through the same pores. This goes on all the time, so plants need oxygen from the air day and night. But during the daytime, the greenery also does something extra. This second operation needs carbon dioxide. So from sunrise to sunset the green plants need both carbon dioxide and oxygen.

Green chlorophyll in the leaves uses the energy of sunlight to make plant food from carbon dioxide in the air and water from the soil. This sunlight recipe is photosynthesis. It needs the used carbon dioxide that both plants and animals put into the air. Photosynthesis also gives off a waste gas as it works. And, of all things, this gas is oxygen. Day and night, the forests take oxygen from the air and pour out carbon dioxide. All day long their greenery also takes in carbon dioxide  ¬and pours out enough fresh oxygen for all the plants, animals and people.

This is nature's recycling on a grand global scale. It explains why the air never runs out of oxygen and also why used carbon dioxide does not crowd out all its other gases.

The great recycling system works because animals need one gas and plants need two gases from the air. Both use up oxygen and give off carbon dioxide. The plants need all this used gas and change it back again to fresh oxygen. The two gases change back and forth in cycles. This recycling will go on as long as plants and animals share the air.

 

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