Respiration provides O2 to cells for energy production and removes CO2 from cells.
In animals, respiration involves:
Organs of respiration in fish are gills. In humans - nostrils, trachea, bronchi, lungs.
Breathing happens by change in position of ribs and diaphragm. Air moves from region of its high pressure to its low pressure.
Blood vessels carry oxygen from alveoli to tissues. Cells of tissues take up the oxygen. In the mitochondria of the cells, oxygen acts upon glucose to yield energy and release carbon dioxide. This is called cellular respiration.
O2 enters blood and reaches cells. CO2 produced by cells goes out into blood and from lungs out of body. Exchange of O2 and CO2 between alveoli and blood. In the cells, energy production by oxidation of glucose occurs in mitochondria.
Gas Exchange between alveoli and blood capillaries
In plants, oxygen enters through stomata of leaves by diffusion. CO2 diffuses out of cells, also through stomata.