Question

what is the principal means by which oxygen is taken from the air? what is the principal means by which it is restored to the air?

Explanation:

Oxygen is taken from the air mainly through respiration (both in animals and plants during their non - photosynthetic phases). Respiration is a process where organisms break down organic compounds (like glucose) in the presence of oxygen to release energy. The chemical equation for aerobic respiration is \(C_6H_{12}O_6+6O_2
ightarrow6CO_2 + 6H_2O+\text{Energy}\).

Oxygen is restored to the air mainly through photosynthesis. In photosynthesis, plants (and some other photosynthetic organisms like algae) use sunlight, carbon dioxide (\(CO_2\)) and water (\(H_2O\)) to produce glucose (\(C_6H_{12}O_6\)) and oxygen (\(O_2\)). The chemical equation for photosynthesis is \(6CO_2+6H_2O+\text{Sunlight}
ightarrow C_6H_{12}O_6 + 6O_2\).

Answer:

The principal means by which oxygen is taken from the air is respiration. The principal means by which it is restored to the air is photosynthesis.