Respiration is one of the vital processes in our body that supports life. It is a process through which energy stored in the food is converted into chemical energy which is a source of fuel for the body and provides energy to the body. Respiration at the cellular level is called cellular respiration. Cellular respiration is a type of catabolic reaction in which large molecules are converted into smaller ones and provide energy for maintaining activities at the cellular level.
Cellular respiration involves a series of metabolic reactions in which biochemical energy is produced from glucose in the form of ATP and then this energy currency is stored and used when cells require the energy.
In cellular respiration, oxygen is used. It is a highly electronegative molecule that attracts electrons towards itself and energy stored in the chemical bonds is released. Oxygen and the energy stored in the food (potential energy) react together and produce carbon dioxide, water, and energy as byproducts. This released energy is used in the formation of ATP.
It is performed by both prokaryotic organisms and eukaryotic organisms.
In prokaryotes, it takes place in the cytoplasm and in eukaryotes, it takes place in the cytosol and mitochondria.
There are 4 stages of cellular respiration in Eukaryotes-glycolysis, pyruvate oxidation, citric acid cycle (also known as Krebs cycle), and oxidative phosphorylation which occurs due to electron transport.
Cellular respiration operates in both aerobic and anaerobic conditions. Cellular respiration is essentially an aerobic process since the cell accepts molecular oxygen and releases carbon dioxide in return. It is aerobic when the final electron acceptor is oxygen and when the oxygen is not the final electron acceptor, then it is called an anaerobic process. Some anaerobic bacteria carry out anaerobic respiration.
Anaerobic respiration is of the following types −
Lactic acid fermentation − In this process, ATP is formed from a sixcarbon compound i.e., glucose and lactate is produced as a by-product. It occurs in eukaryotes and in some prokaryotes. In human muscle cells, it occurs during vigorous exercise and lactic acid gets stored in our muscles and we feel pain.
Alcoholic fermentation − In this process, ATP is not produced from the sugar, instead ethyl alcohol is produced. This process is also known as Ethanol fermentation. It is mainly involved in the production of alcoholic drinks.
Methanogenesis − In this process, methane is produced and is performed by only anaerobic bacteria. Example: Methanococcales, Methanogens, Methanopyrales, etc.
This process is of primary importance for both prokaryotes and eukaryotes as this provides fuel to perform various body functions.
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The process of cellular respiration starts with glycolysis and ends with oxidative phosphorylation.
Glycolysis involves the conversion of one molecule of glucose into 2 molecules of pyruvate.
The location of this process is the same both in prokaryotes and eukaryotes; i.e., Cytosol.
The enzymes involved in glycolysis are also present in the cytosol
It is also known as the Embden-Meyerhof pathway.
Before glycolysis starts, glucose must be carried inside the cell and should be phosphorylated.
Glycolysis is divided into 2 phases -The preparatory phase and other is pay off phase. In the first phase, ATP is utilized for phosphorylation whereas in the second phase it is produced again.
The net yield in glycolysis when carried out in the presence of oxygen- 7 ATP and in the absence of oxygen- 2 ATP.
The location of this process differs in both prokaryotes and eukaryotes. In eukaryotes, mitochondrial matrix is involved and cytosol in the case of Prokaryotes.
In this process, pyruvate undergoes oxidation to form acetyl-CoA, and the reaction is an oxidative decarboxylation process.
Pyruvate dehydrogenase complex is involved in this reaction. Three enzymes are present in this complex: Pyruvate dehydrogenase (E1), Dihydrolipoyl transacetylase (E2), and Dihydrolipoyl dehydrogenase (E3).
In this process, one molecule of NADH is produced.
The location of the process differs in both prokaryotes and eukaryotes. In eukaryotes, the mitochondrial matrix is involved and in prokaryotes, the cytosol is involved.
It is also known by other names- tricarboxylic acid cycle or Krebs cycle.
H A Krebs discovered the Krebs cycle.
The byproducts of the Krebs cycle are 2 moles of carbon dioxide, 3 moles of NADH, 1 mole of FADH2, and 1 mole of GTP.
The location of this process also differs in both prokaryotes and eukaryotes. The inner mitochondrial matrix is involved in eukaryotes and the plasma membrane is involved in prokaryotes.
In this process, the energy is stored in the reduced NADH and FADH which was released during the oxidation of glucose to carbon dioxide. NADH and FADH are coenzymes that are produced during glycolysis and the citric acid cycle.
In this process, electrons are passed from NADH2\FADH to oxygen with the help of electron carriers which are present in the inner mitochondrial membrane (Eukaryotes) and in the plasma membrane (Prokaryotes).
During this transport of electrons, a proton gradient is produced which is responsible for the synthesis of ATP. Chemiosmotic theory explains the whole phenomenon of ATP formation through the proton gradient produced.
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Cellular respiration refers to respiration at the cellular level. Cellular respiration involves various biochemical reactions in which the biochemical energy is produced in the form of ATP and it acts as a fuel for performing various body functions. Cellular respiration involves a series of biochemical reactions that occur in different organelles in the case of prokaryotes and eukaryotes.
Q1. Name the enzymes of glycolysis that catalyze the irreversible steps of glycolysis.
Ans. Hexokinase, phosphofructokinase-1, pyruvate kinase are the enzymes of glycolysis. Since they are irreversible, these enzymes regulate the rate of glycolysis.
Q2. Name the cofactors of the pyruvate dehydrogenase complex.
Ans. Cofactors of the pyruvate dehydrogenase complex are as follows −
TPP - Pyruvate dehydrogenase
Lipoic acid and CoA - Dihydrolipolyl transacetylase
FAD and NAD+ - Dihydrolipolyl dehydrogenase.
Q3. Name the regulatory enzymes of the citric acid cycle.
Ans. Alpha-ketoglutarate dehydrogenase, citrate synthetase, isocitrate dehydrogenase are the enzymes of citric acid cycle. Since these enzymes regulate the irreversible steps of the citric acid cycle, thus they regulate the citric acid cycle.
Q4. Name the four electron transport complexes.
Ans. The four electron transport complexes include −
Complex I - NADH dehydrogenase
Complex II - Succinate coenzyme Q reductase
Complex III - Coenzyme q-cytochrome c reductase
Complex IV - Cytochrome c oxidase.
Q5. Name some electron transport chain inhibitors.
Ans. Some of the electron transport chain inhibitors include −
Antimycin A - Inhibits electron transport of complex III
Amobarbital - Inhibits electron transport of complex I
Piericidin A - Inhibits electron transport of complex I
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