All living things need food, water and air to survive. The oxygen provided by respiration is used to convert food into energy. While humans use their respiratory organs to take in oxygen. Plants do not have any such respiratory organs. Plants respire through small openings called the stomata on leaves and lenticels on stems and roots. Plants respire at a slower rate as compared to animals. In the woody stems, the living cells are present close to the surface and aid in the diffusion of gases. Diffusion of gases is also facilitated by the presence of loosely arranged parenchyma cells that provide air spaces.
The process of breathing is connected to the oxidation of food inside the cells. While breathing involves gas exchange between organisms and the environment, respiration involves the breakdown of the C-C bonds of complex compounds of food using oxygen to release energy inside the cells. In eukaryotes, it takes place in three distinct stages, namely glycolysis in cytoplasm, Krebs’ cycle and the Electron Transport Chain in the mitochondria. Since it takes place inside the cells, it is called ‘cellular respiration’. The oxidised compounds are called respiratory substrates. Carbohydrates are the usual substrates for cellular respiration.
During the oxidation of respiratory substrates, energy is not released directly in the cell; instead, it is released slowly in a series of steps controlled by several enzymes.
It is used for synthesising ATP, it is utilised for carrying out all the activities. Compounds like NADH formed during the oxidation of respiratory substrates are utilised in the ETC to derive energy in the form of ATP. The by- products of aerobic respiration are water and carbon dioxide. This type of respiration takes place in humans, plants and some bacteria. When the oxidation of food substances takes place in the absence of oxygen, it is called anaerobic respiration. It yields alcohol and carbon dioxide as the by-products. This type of respiration takes place in anaerobes and archaea. Anaerobes are divided into facultative anaerobes and obligatory anaerobes such as Escherichia coli that can survive in the presence as well as absence of oxygen.