Read this article to learn about the definition, types and efficiency of respiration.
Definition:
The term respiration (L. respirare = to breath) was first used to describe the breathing i.e. exchange of gases between the organism and the environment.
Subsequently the term respiration was used in a wider sense including breathing, transport of gases as well as the oxidation of food leading to liberation of CO2 and energy.
It is now agreed that respiration has two fundamental processes-external respiration and internal respiration. The external respiration or breathing is a physical process of exchange of respiratory gases (O2 and CO2) between the organisms and the surroundings. Lavoisier (1789) first studied external respiration in animals. He observed that animals take in oxygen and give out CO2 + H2O. Higher animal possesses special organs for gas exchange but plants have stomata and lenticels for this purpose. This is also facilitated by air spaces present in the parenchyma.
Internal respiration, on the other hand, is a chemical process of food oxidation that occurs within the cell to liberate free energy, CO2 and H2O. Internal respiration takes place in the cytosol and mitochondria of eukaryotic cells. Hence, it is also known as cellular or tissue respiration.
Types of Respiration:
Depending on the availability of oxygen, Sachs (1890) classified cellular respiration into two major types: aerobic and anaerobic.
(i) Aerobic Respiration:
It uses oxygen and completely oxidizes the organic food to carbon dioxide and water. It, therefore, releases the entire energy available in glucose. It occurs in most plants and animals. The organisms which carry on this type of respiration are called aerobes.
686 kcal or 2870 kJ of energy is liberated per mole of glucose. The value was previously calculated to be 673 Kcal. One Kcal is equal to 1000 calories. It is that amount of energy (as heat) which can raise the temperature of one litre of water 1o C.
(ii) Anaerobic Respiration:
It does not use molecular oxygen and incompletely oxidizes the organic food with or without production of carbon dioxide. It, therefore, releases a small amount of energy. It occurs in yeasts, certain bacteria and some parasitic worms (e.g. Ascaris, Taenia). The organisms which carry on anaerobic respiration are termed anaerobes. Anaerobes may be either facultative or obligate.
(a) Facultative anaerobes:
Organisms which normally require oxygen but which can live anaerobically when grown on suitable media e.g., Butyric acid bacteria, Lactic acid bacteria, Bacillus phosphoresces.
(b) Obligate anaerobes:
They live in the absence of oxygen or in the presence of negligible concentration of this gas. The common products of anaerobic respiration are CO2 ethyl alcohol and lactic acid.
Anaerobic respiration is the only mode of respiration in some micro-organisms. In higher organisms it occurs as a temporary measure.
Anaerobic respiration cannot continue for long in higher organisms because:
(i) It yields little energy;
(ii) More substrate is decomposed so that little is left for growth and repair;
(iii) Some of the end products and intermediates of anaerobic respiration are toxic in higher concentration.
Protoplasmic and Floating Respiration:
Respiration which utilizes proteins as substrate is called protoplasmic respiration, whereas that uses carbohydrates or fats is termed floating respiration. Protoplasmic respiration cannot be continued for long as it depletes protoplasm of structural and functional proteins as well as liberates toxic ammonia.
Gaseous Exchange:
Lavosier observed that in respiration of animals, oxygen is taken in from the air. In return they give out carbon dioxide and water. There is a regular exchange of O2 → CO2 in most of the organisms. Some organisms, especially micro-organisms, do not require oxygen for their respiration. Some of them give out CO2 while a few do not do so. In these organisms there is no gaseous exchange.
Gaseous exchange occurs not only between the organisms and its environment but also between every cell and its surrounding environment. An animal cell exchanges gases with the extracellular fluid while a plant cell does so with the air present in intercellular spaces. An organism shows exchange of gases in a liquid or gaseous environment depending upon the habitat.
Efficiency of Respiration:
During respiration whole energy contained in the respiratory substrates is not released in a single step or released free into the cell, otherwise the cell would be burnt. Rather energy is released slowly in a series of stepwise reactions controlled by enzymes, and is trapped as chemical energy in the form of ATE At each step considerable amount of energy is lost in the form of heat. Hence, it can be said that in biological systems energy efficiency is not cent percent.
Efficiency of respiration is the percentage of total free energy captured in ATP during complete oxidation of a mole of the respiratory substrate.
Mathematically it is:
Respiration Efficiency = Energy captured in ATP per mole of substrate oxidised/Total energy released per mole of substrate oxidized x 100
The energy content of respiratory substrate is expressed in terms of a measure of heal energy because heat is the ultimate form of all energy. This is often referred to as calorie (cal) or joule (j). Now physiologists commonly use kilocalorie (kcal) as a unit of energy measure (1 kcal = 1000 calories; 1 kcal = 4.184 kJ.). One kcal is the amount of energy required to raise the temperature of 1 liter of water through Io C.
The complete combustion of 1 mole of glucose in a bomb calorimeter yield about 686 kcal or 2870 kilojoule (kJ). It means, total energy contained in glucose is 686 kcal/ mole. The complete oxidation of 1 mole of glucose in a cell produces 38 ATP.
Therefore, the actual amount of free energy captured in ATP per mole of glucose oxidized is 38 x 8.15 = 309.7 kcal which is just 45% of 686 kcal (309.7/686 X 100 = 45). Thus, in biological systems the free energy capturing efficiency during complete oxidation of glucose is 45% and the remaining 55% of the energy stored in glucose is lost as heat. This heat is useful for homoeothermic animals to maintain their body temperature.