Living organisms need energy for their activities. This energy is provided within the cell by respiration. Respiration is a universal process. The breakdown of complex carbon compounds and the release of maximum usable energy within the cell is called respiration.
The term respiration is used in two ways in biology:
- The exchange of respiratory gases (CO2 and O2) between the organism and its environment is called external respiration. It is the most familiar term.
- The step-by-step breakdown of the C-chain molecules and the release of energy within the cell are called cellular respiration.
Respiration is a series of enzyme-controlled oxidation-reduction reactions during which carbohydrates (respiratory substrate) produced during photosynthesis are oxidized to carbon dioxide, and oxygen is reduced to water. Energy is released as a result of the bond by bond breakage of the respiratory substrate.
Much of this energy is stored in molecules of adenosine triphosphate (ATP). The process is also called tissue respiration since it takes place within cells. This process of acquiring oxygen or discharging carbon dioxide from/into the environment is called gas exchange or external respiration. The process can be summarized as:
Types of Respiration
Glucose is the most common fuel of the cell. It provides energy by cellular respiration. The metabolism of glucose depends on the availability of oxygen. The glucose molecule splits to form two molecules of pyruvic acid before entering the mitochondria. This reaction is called glycolysis. Glyco means glucose and lysis mean splitting. So, glycolysis literally means the splitting of sugar. It occurs in the cytosol.
This reaction occurs in all the cells. The biologists believe that an identical reaction may have occurred in the first cell that was formed on the earth.
The next steps in cellular respiration are different in different types of cells. The pyruvic acid is processed in three ways:
- Alcoholic fermentation
- Lactic acid fermentation
- Aerobic respiration
The first two reactions occur in the absence of oxygen. So these reactions are called anaerobic (without oxygen). The complete breakdown of the glucose molecule occurs only in the presence of oxygen i.e. in aerobic respiration. The glucose is oxidized to CO2 and water during respiration and energy is released.
Respiration requiring oxygen is referred to as aerobic respiration while t it occurs in the absence of, oxygen it is anaerobic respiration, however, it can occur in the presence of oxygen. Certain organisms, for example, the bacterium Clostridium sp respire only in the absence of oxygen. All plants and animals require oxygen and respire aerobically, therefore called aerobes. The end products of aerobic respiration are carbon dioxide and water.
1. Anaerobic Respiration
Anaerobic respiration occurs in the absence of oxygen. It has two types:
- Alcoholic Fermentation: It takes place in some primitive cells and some eukaryotic cells like yeasts. In this case, the pyruvic acid is broken down into alcohol and CO2 by alcoholic fermentation.
- Lactic Acid Fermentation: In this case, each pyruvic acid molecule is converted into lactic acid C3H6O3.
This anaerobic respiration occurs in the muscle cells of humans and in other animals. It takes place during extreme physical activities like sprinting. The oxygen cannot be transported to the cells during such extreme activities. So, pyruvic acid is changed into lactic acid.
Both alcoholic and lactic acid fermentation give a small amount of energy from the glucose molecules. Only 2% energy of glucose is converted into ATP (adenosine triphosphate).
2. Aerobic Respiration
It occurs in the presence of oxygen.
Significance of Respiration
The process of respiration is significant in that:
It is essentially an energy-providing mechanism. The stored energy of chemical compounds is converted into the usable energy of ATP.
Although the most important aspect of the respiratory pathway is to provide energy, many important metabolic intermediates produced during the glycolytic pathway and TCA cycle, are converted into important metabolites of the cell.
The metabolites derived from the respiratory pathway include amino acids. Pentose sugars are used in the cell wall and nucleotide synthesis, precursors of porphyrins, and acetyl CoA (coenzyme A) used in fatty acid, carotenoids, gibberellins, and abscisic acid synthesis.
The nature of the respiratory substrate in plant tissue can be determined by calculating the respiratory quotient of the tissue.
- If glucose (carbohydrate) is the respiratory substrate, the respiratory quotient would be:
RQ of glucose (carbohydrate) = 1
- The fats are poor in oxygen as these are not directly oxidized. These are first hydrolyzed to fatty acids and glycerol. A fraction of oxygen is used up in this process, therefore the fats require more oxygen for complete oxidation and their RQ is less than unity, i. e., 1
RO for fats = 0.7
- Like fats proteins also do not directly take part in respiration. These are first converted into amides or ammonia. Therefore, these require more oxygen for complete oxidation. The RO value ranges from 079 (in the case of amides) and 0.99 (in the case of ammonia).
- In succulents where malic acid is the sole respiratory substrate, the respiratory quotient is:
Any organic plant constituent that is partially or completely oxidized to CO2 and water during the process of respiration is called the respiratory substrate. Carbohydrates are the principal respiratory substrates in cells of higher plants.
The most important are sucrose and starch. Sucrose along with fructose and glucose are the principal soluble sugars in plant cells. Similarly, sucrose is the main carbohydrate translocated within the plant body Starch is the chief reserve in plants.
In addition to carbohydrates, other substances sometimes serve as respiratory substrates, e. g., castor-oil seeds are rich in fat reserves stored in endosperm tissue. During germination of these seeds, the fats are converted to sucrose which is respired by the growing embryo.
In some tissues, organic acids may be utilized as respiratory substrates. For example, 4-C malic acid accumulates in the leaves of succulents (plants with flesh, lees belonging to Crassulaceae) during the night and is oxidized to CO2 and water. Similarly, glycolic acid, a 2-C organic acid produced in illuminated leaves of higher plants, is also used as a respiratory substrate.
Proteins are seldom respired except in special cases. In detached leaves, protein degradation has been observed. In seeds high in protein reserves, proteins serve as respiratory substrates during the early stages of germination. The proteins are first degraded to amino acids which in turn are converted into intermediates that are oxidized to CO2 and water.
Role of Mitochondria in Respiration
Mitochondria are large granular or filamentous organelles. These are distributed throughout the cytoplasm of animals and plant cells. Each mitochondrion is made up of two membranes. The outer is an enclosing membrane. The inner membrane has elaborate folds or cristae. These cristae extend into the interior of the mitochondria.
Mitochondria are involved in cellular respiration. They transfer the energy of the organic molecules into chemical energy in the form of ATP. Mitochondria have a large battery of enzymes and coenzymes. These enzymes and coenzymes slowly release energy from the glucose molecule. This energy is used for many cellular functions. Thus mitochondria are the “powerhouse” of the cell.
Adenosine Triphosphate (ATP) and its Importance
Adenosine triphosphate is generally abbreviated as ATP. It is found in every living cell. ATP is an essential chemical of life. It plays a key role in most biological energy transformations.
Conventionally the “P” stands for the entire phosphate group. The second and third phosphate molecules have high energy bonds. On hydrolysis, these bonds release more energy than the other bonds of the ATP molecule. The breaking of the terminal phosphate molecule of ATP releases about 7.3 Kcal of energy. So the cell can store a large amount of energy in a very small space. This energy in the form of ATP is always ready for use.
The cells use the energy of the ATP molecule for various functions. Some of these functions are a synthesis of complex compounds, active transport across the cell membrane, contraction of muscles and nerve conduction, etc.
Living organisms require a continuous supply of free energy. This energy is derived from different oxidation-reduction reactions. The photosynthetic and some bacterial chemosynthetic processes are themselves oxidation reactions. So they do not require free energy. All other reactions require free energy.
This energy comes from oxidation reductions in the respiratory processes. In some cases, biological oxidation involves the removal of hydrogen. This reaction is catalyzed by dehydrogenase enzymes. The dehydrogenases are linked to some other specific enzymes. Cellular respiration is an oxidation process.
MECHANISM OF RESPIRATION
Respiration suggests that respiration is a multi-step process in which glucose is oxidized during a series of reactions. These reactions can be subdivided into three stages:
It is carried out by a group of soluble enzymes located in the cytosol (the liquid part of the cytoplasm). Chemically the glucose undergoes a limited amount of oxidation to produce two molecules of pyruvate (a 3C compound), ATP, and reduced nucleotide NADH.
ii. The Tricaboxylic Cycle (TCA) or Krebs cycle
The cycle brings about the complete oxidation of pyruvate to CO2 and water. During the cycle, about 10 reduced nucleotides (NADH) are generated The TCA cycle operates in a matrix of mitochondrion where soluble enzymes for the cycle are present. This phase is also called oxidative decarboxylation since carbon atoms are oxidized to carbon dioxide during the phase.
iii. The Electron Transport Chain
It consists of a collection of electron carriers bound to the proteins of mitochondrial membranes. The system transfers electrons from NADH produced during glycolysis and the TCA cycle, to oxygen. The electron transfer releases a large amount of energy. Much of which is stored in ATP produced from ADP and Pi.