Living organisms need a constant source of energy to drive the metabolic reactions that takes place inside their cells. In other words, the maintenance of individual life requires energy for performing various activities. This energy is released by the breakdown of the food stuffs during the process of cellular respiration. This respiration is a fundamental process of energy release in our body. It occurs in all living cells. The energy- rich food stuffs such as glucose and fructose called respiratory substances are changed to water and carbon dioxide during respiration, accompanied with the release of usable energy (ATTP). Respiration involves an exchange of gases, i.e., intake of oxygen and release of carbon dioxide. This process is commonly called breathing.

During respiration, most organisms get oxygen from the environment (i.e. air or water) through their specialized breathing organs such as lungs, gills or skin. The release of energy during respiration takes place only with the help of various chemical reactions, generally catalysed by the enzymes. These reactions collectively are called cellular respiration and occur in all living organisms from bacteria to humans. This exhibits a functional? (Physiological) diversity in living organisms. The process of respiration is similar in all living organisms and results in the release of energy. It is a cellular process and occurs in cytoplasm and mitochondria. Mitochondria are sites of curobic respiration in the cell. They contain certain enzymes which are involved in utilization of oxygen gas and production of energy rich ATP molecules. Mitochondria are called power plants of the cell.

A sugar (glucose/fructose) is a substrate for the respiration. In the process of cellular respiration. In the process of cellular respiration, the glucose molecule breaks down and utitises oxygen to release energy. Carbon dioxide and water are formed during this process.

Respiration in plants

All the plants cells need a supply of oxygen because they carry out respiration all the time. The plants normally do not have any organ for breathing. But in them exchange of gases, i.e. is, oxygen and carbon dioxide, is performed by a simple diffusing process. Oxygen enters root tissue by diffusion from air spaces below that in the soil, o maintaining a diffusion gradient. The root system is extensive, giving a large surface area/ volume ratio and the thin root hairs provide little barrier to diffusion.

In fact most gas exchanges in plants occur in the leaves. Leaves are thin and have a large surface area/ volume ratio. They also have an extensive internal system of air spaces. In the leaves, the cells of spongy mesophyll are able to diffuse in and out of these air spaces- through tiny pores called stomata (singular stoma). Stomata are found both in upper and lower epidermis of the leaves, but they are more abundant in lower epidermis. They allow exchange of gases to occur between the air and the internal tissue of the leaf. Stomata with nearby cells are known as Stomatal apparatus.

Stomatal Apparatus: - The stomatal apparatus includes an opening, called stoma, surrounded by two guard cells and several subsidiary cells. The expansion and shrinkage of the guard cells due to flow of water (ormosis) from surrounding cells results in the opening and closing of stomata. The stomata tend to open during the day and close at night.