The complex organic compound that gets oxidized in the cell during respiration to release large amounts of energy is called respiratory substrate. Under normal condition, glucose is the most common respiratory substrate which is a carbohydrate along with six carbon atoms.
Carbohydrates, proteins, fats and organic acids are used as respiratory substrates and oxidation of these compounds releases energy in the cell. However, the energy released is not dissipated freely in the cell. In other words, it does not occur in one step. Instead, it is released in a series of slow step-wise reactions controlled by enzymes and is trapped in the form of ATP. This prevents the sudden increase in the temperature and avoids wastage of energy. This holds a lot of significance as ATP which stores the energy can be broken down whenever and wherever it is needed in the various energy-requiring processes of the organisms.
Schematic representation of Krebs’ cycle:
Schematic representation of glycolysis:
The metabolic pathway that uses the energy released by the oxidation of nutrients to produce adenosine triphosphate (ATP) is called oxidative phosphorylation. Almost all the forms of life on earth use a range of different nutrients to carry out oxidative phosphorylation to produce the molecule that supplies energy to metabolism i.e. ATP. This is a very efficient process of energy generation.
This process requires the presence of oxygen in the system. Oxygen drives the whole process as it removes hydrogen from the system and acts as the final hydrogen acceptor. During oxidative phosphorylation, electrons are transferred from electron donors like NADH2 to electron acceptors such as oxygen. These redox reactions release energy, which is used to form ATP. In eukaryotes, these redox reactions are carried out by a series of protein complexes within mitochondria, whereas, in prokaryotes, these proteins are located in the cells' inner membranes. These linked sets of proteins are called electron transport chains. It is the energy of the oxidation-reduction process that is used for the production of proton gradient required for phosphorylation and thus, this process is called oxidative phosphorylation.
Complete oxidation of substrates during aerobic respiration requires oxygen and apart from energy, carbon dioxide is produced as the by-product. The ratio of the volume of CO2 released to the volume of O2 consumed during complete oxidation of one molecule of a substrate in a given period of time at standard temperature and pressure is called the respiratory quotient (RQ).
For example, during aerobic respiration of one molecule of glucose, 6 molecules of CO2 are released and 6 molecules of O2 are consumed. Thus RQ for glucose is 1.
RQ value for fats: Fats need more oxygen molecule than carbohydrate (glucose) for complete oxidation through aerobic respiration, due to which the value of RQ for fat is always less than 1.
For example, when fatty acid tripalmitin is used as a substrate, 145 molecules of O2 are consumed whereas 102 molecules of CO2 are produced, the RQ value is 0.7.
The process of metabolism involves both anabolic and catabolic reactions. Anabolism is the synthesis of complex macromolecules like lipids and proteins from simple molecules like glycerol and amino acid respectively. On the other hand, catabolism includes the breakdown of macromolecules into simple molecules so that they can enter in the respiratory pathway as a substrate for the release of energy. If fatty acids are used as a respiratory substrate they are broken down to glycerol and acetyl CoA. Glycerol gets converted to 3-phosphoglyceraldehyde (PGAL) and enters in glycolysis while Acetyl CoA directly enters in Krebs’ cycle. However, when an organism needs to synthesize fatty acids, acetyl CoA is withdrawn from the above-said pathway and is made available for catabolic reaction. Similarly, when proteins are used as a substrate, they are first broken down to amino acid, which in turn, depending on their structure, gets converted into different intermediates of Krebs’ cycle. At the time of need, the same molecules are withdrawn to synthesize new proteins. Most of these reactions are reversible and depending on the requirement, the cell uses the respiratory substrate in the process of anabolism or catabolism. Thus, the respiratory pathway is known as the amphibolic pathway rather than only a catabolic pathway.
Many assumptions have been made in order to calculate the net gain of ATP from one molecule of glucose. This is required as the cellular system is very complex where numerous biochemical reactions take place simultaneously. The assumptions are as follows:
(a)
|
Aerobic respiration |
Anaerobic respiration |
1 |
It occurs only in the presence of molecular oxygen. |
It occurs in the absence of molecular oxygen. |
2. |
It is highly efficient and produces 38 ATP molecules. |
It is less efficient and generates only 2 ATP molecules. |
3. |
It takes place in both cytoplasm and mitochondria. |
It takes place in the cytoplasm only. |
4. |
It is a multistep process having glycolysis, Krebs’ cycle, and ETS. |
It is not a multistep process. |
5. |
It produces carbon dioxide and water as the by-product. |
It produces ethyl alcohol and carbon dioxide as a by-product. |
(b)
|
Glycolysis |
Fermentation |
1. |
It is a common step in both aerobic and anaerobic respiration. |
It is strictly an anaerobic mode of respiration. |
2. |
It results in the production of pyruvic acid. |
It produces ethyl alcohol. |
3. |
Net gain is 8 ATP molecules. |
Net gain is 2 ATP molecules.
|
4. |
The product of glycolysis is used as an intermediate in Krebs’ cycle. |
The product of fermentation (ethyl alcohol) is not used by cells further. |
(c)
|
Glycolysis |
Citric acid Cycle |
1. |
It occurs in the cytoplasm. |
It occurs in the mitochondria. |
2. |
It is a non-cyclic process. |
It is a cyclic process. |
3. |
It is common in both aerobic and anaerobic respiration. |
It takes place only in aerobic respiration. |
4. |
It produces 8 ATP molecules from one molecule of glucose. |
It produces 15 ATP molecule from one molecule of Acetyl CoA. |
The electrons removed from the substrates of glycolysis and the Krebs’ cycle are stored in the reduction equivalents, namely NADH2 and FADH2. This energy is released when NADH2 and FADH2 are oxidized by passing their electrons to a chain of electrons carrier complex called Electron transport system, present in the inner membrane of mitochondria. These complexes transfer the electron through a series of redox reactions with high energy electrons entering the system and low-energy electrons leaving the system. The energy released through this process is utilized to pump out protons which develop a proton gradient (Proton motive force) across the inner membrane. This proton motive force is utilized by ATP synthase to generate high energy ATP molecules at 3 different sites.
Process: The NADH2 produced during the citric acid cycle are oxidized by an NADH dehydrogenase (complex I), and electrons are then transferred to ubiquinone which gets reduced. Ubiquinone also receives reducing equivalents via FADH2 (complex II). The reduced ubiquinone (ubiquinol) is then re-oxidized by transferring its electrons to cytochrome c via cytochrome bc1 complex (complex III). The electron is transferred from complex III to complex IV through cytochrome C which is a mobile carrier present in the inner membrane. Complex IV is called cytochrome C oxidase complex and consists of cytochromes a-a3, and two copper centers.
The four main steps of aerobic respiration are as follows:
S.No |
Steps of aerobic respiration |
Site of occurrence in the cell |
1. |
Glycolysis |
Cytoplasm |
2. |
Krebs’ Cycle |
Mitochondrial matrix |
3. |
Electron transport chain |
Inner membrane of mitochondria |
4. |
Oxidative phosphorylation |
F0-F1 particle of cristae present in the inner membrane of mitochondria |
(a)
|
Respiration |
Combustion |
1. |
It occurs within living cells only. |
It does not occur inside a living system. |
2. |
It requires enzymes. |
It does not require enzymes.
|
3. |
It occurs in a highly regulated mode under controlled condition. |
It is non-regulated and uncontrolled. |
4. |
It produces energy equivalents in the form of high energy ATP molecules. |
It produces energy in terms of heat and light only. |
(b)
|
Glycolysis |
Krebs’ Cycle |
1. |
It occurs in the cytoplasm. |
It occurs in the mitochondria. |
2. |
It is a non-cyclic process. |
It is a cyclic process. |
3. |
It is common in both aerobic and anaerobic respiration. |
It takes place only in aerobic respiration. |
4. |
Less productive in terms of ATP and NADP generation. Produces 8 ATP molecules from one glucose molecule |
Produces 15 ATP molecules from one molecule of Acetyl CoA |
(c)
|
Aerobic respiration |
Fermentation |
1. |
It occurs in the presence of molecular oxygen only. |
It does not require molecular oxygen. |
2. |
It takes place in both cytoplasm and mitochondria. |
It takes place only in the cytoplasm. |
3. |
It is highly efficient and produces 38 ATP molecules per molecule of glucose. |
It is non-economical, produces only 2 molecules of ATP per molecule of glucose |
4. |
The final products formed from one glucose molecule are carbon dioxide and water. |
The final products formed from one glucose molecule are ethyl alcohol and carbon dioxide |
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