The Latin word ‘respiare’ from where the term respiration has come which means breathing. Respiration means inhalation of oxygen rich air and exhalation of carbon dioxide rich air from the lungs. The entry and exit of air into and from the lungs are called inspiration and expiration respectively. Breathing is merely came mechanical process of inspiration and expiation, whereas the process of respiration is a wider phenomenon that includes breathing, transport and exchange of gases between lungs and tissues as well as the chemical process of oxidation of food which leads to liberation of carbon dioxide and energy. So we can say breathing is simply and external manifestation of respiration and it is seen only in higher animals possessing typical lungs.
The process of respiration in unicellular organism is simple. In this case this cell directly takes in oxygen from the environment by a process of diffusion, oxidises the organic matters present within it to liberate carbon dioxide and energy and gives out the carbon dioxide to the environment by diffusion. Here the process of respiration occurs throughout the whole cell surface.
On the other hand in multicellular animals, as all cells are not in direct contact with the environment, special arrangement for transportation of gases between the environment and the individual sales have evolved. Various types of respiratory organs have developed for this purpose in different animal species. But the basic plan of these organs is to provide a large surface area of membranes through which the body fluids comes in contact with the environment for the exchange of free gases. The plants of higher group in general have no such definite respiratory organs. Here the oxygen from the air enters stomata and plenty lenticrls and defuses through the continuous intercellular space system to reach the living cells where it oxidises the food and release carbon dioxide and energy.
The process of respiration has been divided into two phases those are external respiration and internal respiration.
What is external respiration?
External respiration is a physical process of respiration where the gaseous exchange between the organism and its environment occurs by diffusion and the organism takes up oxygen and gives out carbon dioxide. Site and mode of external respiration varies in different living organism depending upon they are complexity and habitat. In unicellular organisms the gaseous exchange occurs directly between the environment and the cell through the whole cell membrane, on the other hand in higher animals who have initialised respiratory organs, exchange of gases take place at two sites, first at the respiratory organs and then at the tissues.
What is internal respiration?
The internal respiration is a chemical process of oxidation occurring between the cells as a result of which the food is oxidisation to liberate energy and carbon dioxide. So this process of respiration (internal) is also called cellular respiration or tissue respiration. Internal respiration takes place in the cytosol and mitochondria of the cells. In all living beings process of internal respiration is basically same; it is because of the involvement of common biochemical pathways. But there are some specialities in certain cells. So we can say that internal respiration is the actual respiration, on the other hand the external respiration is simply a manifestation of the internal respiration.