Dividing the process of respiration into two categories, internal and external respiration, can help clarify some of the complexities of this biologically important process. Together, the different parts of the respiration process allow organisms to exchange gases with their environments and convert energy into a form cells can use.
All living organisms perform some form of respiration to make energy and exchange gases.
External respiration in most organisms is inhalation and exhalation, or breathing.
Internal respiration has two parts: gas exchange and cellular respiration, a chemical reaction where sugar is converted into chemical energy.
Gas exchange, or the "exchange" of carbon dioxide and oxygen, occurs in animals across a respiratory surface. In humans this surface is the fine, thin capillaries in our lungs. In fish, it occurs across the gills, and some organisms exchange gases through the skin. In plants, gas exchange occurs through pores in the leaves called stomata.
Cellular respiration, the second phase of internal respiration, is a chemical reaction that requires oxygen to convert sugar into chemical energy cells need to function. Carbon dioxide is produced during this conversion and is then exhaled.
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