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Respiration
1. Exchange of Gases:
Almost all the living organisms require oxygen (O 2 ) to break down various food substances like carbohydrates, fats and proteins to derive energy for performing various functions. During this oxidation of nutrients, a harmful gas carbon dioxide (CO 2 ) is also released. Therefore, an adequate supply of O 2 must be continuously provided to the cells and CO 2 produced by the cells must be eliminated out. This process of exchange of O 2 from the atmosphere with CO 2 produced by the cells is called breathing. In general, overall respiration involves the following steps:
- Breathing or gaseous exchange by which atmospheric air (O 2 ) is drawn in and CO 2 rich alveolar air is released out.
- Diffusion of gases (O 2 and CO 2 ) across alveolar membrane.
- Transport of gases by the blood.
- Diffusion of O 2 and CO 2 between blood and tissues.
- Cellular respiration that utilizes O 2 for the breakdown of glucose to oxidize it to CO 2 and H 2 O, and produces energy for daily life functioning
2. Oxygen Transport through blood:
Oxygen once reached to the lung through breathing, it diffuses across the alveolar membrane and is further transported via blood to the different cells of the body where it is consumed. Approximately 1.5% of the oxygen transported by the blood is dissolved directly in the blood plasma. However, most of the oxygen (~ 98.5) is transported from the lungs to the tissues by binding with a protein called haemoglobin (Hb). Haemoglobin is a globular protein molecule and its primary function is to transport oxygen from lung to tissues and to carry CO 2 from tissues to the lung. Haemoglobin is found in red blood cells (RBC) and is made up of four protein chains (globin): two α-subunits (141 amino acids) and two
β-subunits (146 amino acids). Each polypeptide chain is attached to a prosthetic group “heme”, which is an iron (Fe2+)-porphyrin compound that binds to the one oxygen molecule. Therefore, each haemoglobin molecule can carry maximum four oxygen molecules. Fig. Structure of Haemoglobin The binding of oxygen and its dissociation from Haemoglobin is principally governed by the partial pressure of oxygen. Pressure contributed by an individual gas in a mixture of gases is called partial pressure and is represented as P O 2 for oxygen and P CO 2 for carbon dioxide. The higher the P O 2 greater is the binding. Fig. Gaseous exchange in lung
transition of Hb protein, further facilitates the binding of successive oxygen molecules until all four heme groups are occupied. Cooperative behaviour is also observed in dissociation: as the first oxygen molecule dissociates and is released at the tissues, the next oxygen molecule dissociates more readily. When all four heme sites are occupied, the haemoglobin is said to be saturated. In fully deoxygenated haemoglobin, the molecule's quaternary structure is described as the ‘T’ or ‘tense’ form in which the crevices are small, making it difficult for oxygen to gain access to the heme. As each successive oxygen binds to the molecule, the structural changes described above result in the molecule relaxing, enlarging the crevice on adjacent globin chains and increasing their oxygen affinity. When fully oxygenated with four oxygen molecules, the haemoglobin achieves its ‘R’ or ‘relaxed’ quaternary structure. Fig. Cooperative binding of oxygen to haemoglobin Image credit: https://academic.oup.com/bjaed/article/12/5/251/
2.2 Oxygen-haemoglobin Dissociation curve:
An oxygen–haemoglobin dissociation curve is a graph that describes the relationship of partial pressure to the binding of oxygen to heme and its subsequent dissociation from heme. This graph of percent saturation of haemoglobin as a function of P O 2 is sigmoid-shaped, which is due to cooperative binding of oxygen to the haemoglobin. At very low partial pressure when most of the haemoglobin is in deoxyhaemoglobin state (tense form), the affinity for oxygen is the lowest. This results in a small increase in percent saturation. With increasing P O 2 most of the haemoglobin acquires at least one oxygen molecule. This causes haemoglobin to undergo conformational changes such that, each subsequent oxygen molecule binds to haemoglobin more easily. Hence, cooperativity leads to steep rise in curve gradient ( P O 2 from ~15 to 7 0 mm Hg). With further increase in P O 2 (Above 70 mm Hg), O 2 binds to haemoglobin and only fewer binding sites become available so the curve starts to levels off again. 100 percent O 2 saturation of haemoglobin means that every heme unit in all RBCs of body is bound to oxygen. In a healthy
individual, at 100 mm Hg the haemoglobin saturation generally ranges from 95 percent to 98 percent. Fig. Oxygen-haemoglobin Dissociation curve Image credit: https://opentextbc.ca/anatomyandphysiology/chapter/22- 5 - transport-of-gases/
3. Myoglobin: Like haemoglobin, myoglobin is also a globular protein which is composed of single -helical protein chain that is attached to a prosthetic group heme moiety. Due to presence of only one binding site (heme group), myoglobin can carry just one oxygen molecule. Heme consists of a protoporphyrin ring co-ordinated to central iron ion in Fe2+^ state. Fig. Structure of myoglobin
3.1 Comparison of Haemoglobin and Myoglobin:
HAEMOGLOBIN MYOGLOBIN
- Haemoglobin = Heme: Blood Globular protein found in blood (water soluble) Myoglobin = Myo: Muscle Globular protein exists in muscle cells (water soluble).
- Binds to the oxygen in lungs and releases to the tissues (including muscles). Binds and releases oxygen to the muscle cells.
- Haemoglobin has 4 polypeptide chains of two different types- 2 alpha and 2 beta It contains single polypeptide chain.
- It consists of 4 heme units each of which can bind to one oxygen molecule. Therefore, one molecule of Hb can carry up to 4 oxygen molecules. It comprises of one polypeptide chain attached to single heme group. One molecule of myoglobin carries one oxygen molecule.
- Quaternary protein structure Quaternary protein structure
- Haemoglobin has lower oxygen binding affinity towards oxygen than myoglobin. Myoglobin binds to O2, tightly and firmly. (grabs oxygen from Hb in tissues and releases to the muscle cells).
- Oxygen dissociation curve is Sigmoid Oxygen dissociation curve is hyperbolic
- It requires more pressure of oxygen to get saturated. It becomes saturated at higher partial pressure.
- It is a transport protein which supplies the oxygen in whole body Myoglobin is a storage protein which supplies oxygen to the muscles only during starving time of oxygen.
4. Cellular Respiration: Our body is composed of several types of cells that are organized into tissues and organs that perform specific functions. Living cells require a constant source of energy in the form of ATP (adenosine triphosphate) for the life processes. Cellular respiration is the process by which cells regenerate the energy (ATP) by the oxidation of foods (eg. glucose). This process occurs in three stages: glycolysis, the Krebs cycle, and electron transport chain. The latter two stages i.e. krebs cycle and electron transport demands oxygen. However, cellular respiration can occur both in presence of oxygen (aerobically), or without oxygen (anaerobically). 4.1 Aerobic Respiration: ❖ Occurs in the presence of oxygen in most of the eukaryotes and prokaryotes. ❖ Takes place in specialized organelle mitochondria of the cell. ❖ During aerobic cellular respiration glucose is completely oxidised to liberate energy (in the form of ATP). CO 2 and H 2 O are formed as byproducts. ❖ Oxidation of 1 glucose molecule produces 38 ATP. ❖ Involves three steps – ❖ Overall Reaction: – 4.2 Anaerobic Respiration: Some organisms have potential to generate energy even in the absence of oxygen, which is also known as fermentation. During the process glucose undergoes glycolysis, followed by further anaerobic process of fermentation to yield 2 molecules of ATP. This can be of two types: 4.2.1 Alcoholic Fermentation: This occurs in yeasts and some bacteria. Partial oxidation of glucose in absence of oxygen produces ethyl alcohol and 2 ATP molecules. This process finds its commercial application in baking and brewing industry. Alcoholic fermentation in yeasts releases CO 2 gas that makes dough rise to give bread its holes.
5. Respiratory Quotient: Respiratory quotient, also known as the respiratory ratio (R.Q.), is defined as the volume of carbon dioxide released over the volume of oxygen absorbed during respiration. The volume of carbon dioxide released, and the volume of oxygen consumed by the cell during respiration depends upon the kind of respiratory substrate utilized. Therefore, RQ is a measure of efficiency of combustion of food substrate in the process of respiration to release energy. It is calculated for a particular substrate i.e., carbohydrates, organic acid, fat, and protein. Carbohydrates are oxidized through aerobic respiration resulting in an equal ratio of CO 2 release and O 2 consumption. Therefore, for carbohydrates RQ is always 1. Subsequently, the R.Q. for fat and protein is 0.7 and 0.8, respectively. In anaerobic respiration, since oxygen is not involved, hence R.Q. is infinity. The cells of the body consume an average 250 ml of oxygen per minute and produce about 200 ml of carbon dioxide per minute. Hence, the average human respiratory quotient is 0.8.
