Respiration in Plants Class 11
◊ The term respiration is derived from a Latin word Respirare which b – means to breathe. Dutrochet introduced the term “respiration” and studied it in plants.
◊ Light energy is converted into chemical energy that is stored in the bonds of complex organic molecules of carbohydrates like glucose and starch. The breaking of C–C bonds of such compounds through oxidation releases a considerable amount of energy for utilization by plants. This phenomenon of the release of energy by oxidation of various organic molecules, for cellular use, is known as respiration. The compounds that are oxidized during this process are known as respiratory substrates, e.g. Carbohydrate, fat, organic acid or protein. Of these, carbohydrate is the most preferred substrate for biological oxidation.
◊ Respiration may also be defined as an oxidative, catabolic and exothermic process in which large molecules of food materials are broken down into the smaller ones with the release of energy in usable form.
◊The whole of energy contained in respiratory substrates is not released all at once. It is released slowly in stepwise series of reactions controlled by enzymes.
◊ Respiration is independent of light, i.e., it occurs throughout day and night and hence also called dark respiration. It decreases with increasing age of the cell.
Type of Respiration
◊ Blackman suggested two types of respiration on the basis of the type of respiratory substrate
♦ Protoplasmic respiration– Respiration involving proteins as respiratory substrate, and
♦ Floating respiration – Respiration, which uses carbohydrates and fats.
Protoplasmic respiration cannot be continued for long as it depletes protein as well as liberates toxic ammonia. It occurs during starvation when carbohydrates and fats have been exhausted. It occurs in starving leaves, germination of protein-rich seeds and spores.
◊ Depending upon the availability of O2, respiration is classified into two major groups – aerobic respiration and anaerobic respiration.
♦ Aerobic Respiration – It is that type of respiration in which organic food is completely oxidized with the help of oxygen, and releases CO2, water and a large amount of energy. It is the most common in higher organisms.
♦ Anaerobic Respiration – This type of respiration takes place in the complete absence of O2. It generally occurs in lower organisms, such as bacteria and fungi. It also occurs in higher plants and animals under a certain condition, particularly when O2 is limiting.
In anaerobic respiration, the carbohydrate is incompletely oxidized into some carbonic compounds, such as ethyl alcohol or acetic acid or lactic acid and CO2, and the amount of energy released is also much less as compared to aerobic respiration. This process can be shown by the following equation.
This process of oxidation in microbes is known as fermentation, which is very much similar to that of anaerobic respiration in the case of higher plants.
◊ The ratio of the volume of CO2evolved to the volume of O2 consumed in respiration is called respiratory quotient R.Q or respiratory ratio.
◊It is due to the ratio of C: O in the respiratory material. It depends on nature of the respiratory substrate, amount of O2 present in the respiratory substrate, the extent to which substrate is broken down, and interconversion of one substrate into another in the cell. This is different for different substrates. The value of R.Q. may be one, zero, less than one or more than one.
♦ Carbohydrates: When carbohydrates are used as a substrate and are completely oxidized, the R.Q. will be one, because equal amounts of CO2 and O2 are evolved and consumed, respectively.