Energy is necessary to all the cells of the body to facilitate various metabolism, therefore every living organism involves cellular respiration to release energy, which is stored in the form of ATP. Food is moved into the stomach through the oesophagus after intake and it breaks into various smaller pieces including glucose by stomach acids and enzymes. Glucose is the most abundant monosaccharide and is an initial substrate for carbohydrate metabolism, and it is broken down to release energy, therefore glucose and ATP are the energy-carrying molecules.
Cellular respiration is the energy-producing process that occurs in all the cells of plants and animals (excluding RBCs). It is the splitting of glucose from food into carbon dioxide, water, and energy with or without oxygen. Therefore, it releases carbon dioxide as a waste product and liberates ATP (adenosine triphosphate).
| Respiration | Breathing |
| It is the physiological process of inhalation and exhalation. In cells, the chemical process of the breakdown of glucose from food liberates energy. | It is the moving of oxygen from the outside environment into the body and the release of carbon dioxide from the lungs into the outside environment. |
| It is classified into physiological respiration and cellular respiration. | Breathing is a type of respiration, therefore it is also called physiological respiration. |
| Cellular respiration occurs in cells, particularly, in cellular organelles such as cytosol and mitochondria. | It occurs in the respiratory organ- lungs. |
| Enzymes are used for various reactions in the cellular process. | There is no involvement of enzymes. |
| It produces ATP that is converted into energy. | It does not produce energy. |
It is the sequence of the breakdown of one glucose molecule into two pyruvate molecules with ATP production. It occurs in the cytoplasm of all cells of the body.
Hexokinase catalyses the addition of phosphate groups and the glucose converts into glucose 6-phosphate.
Glucose 6-phosphate converts to fructose 6-phosphate by phospho-hexose isomerase, which are the isomers of each other.
Phosphofructokinase catalyses the irreversible process of fructose 6-phosphate to fructose 1,6-bisphosphate, by phosphorylation.
The splitting of fructose 1,6 bisphosphate into glyceraldehyde 3-phosphate and dihydroxyacetone phosphate is catalysed by aldose.
Glyceraldehyde 3-phosphate and dihydroxyacetone phosphate are reversible interconverted compounds, it is carried out by phosphotriose isomerase.
Glyceraldehyde 3-phosphate dehydrogenase induces glyceraldehyde 3-phosphate to convert into 1,3-bisphosphoglycerate. In this stage, NADH + H+ is formed from nicotinamide adenine dinucleotide NAD+ therefore, it adds a phosphate group to glyceraldehyde 3-phosphate.
The enzyme phosphoglycerate kinase converts 1,3-bisphosphoglycerate to 3-phosphoglycerate with the synthesis of ATP.
3-phosphoglycerate is converted to 2-phosphoglycerate by phosphoglycerate mutase and these compounds are isomers.
2-phosphoglycerate is converted by enolase into the high-energy compound phosphoenolpyruvate with the removal of water.
Phosphoenol pyruvate is converted into pyruvate with the formation of ATP in the presence of pyruvate kinase.
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In glycolysis, the end products are 2 pyruvates, 2 NADH and 2 ATP molecules. Glucose is split into two pyruvates, therefore 8 ATP molecules are synthesised.
The Krebs cycle is a chain of chemical reactions that participate in converting acetyl CoA into carbon dioxide and water in a cyclic pathway. These reactions occur in the mitochondria of cells and the first formed product is citric acid, therefore called the citric acid cycle.
The enzyme pyruvate dehydrogenase catalyses oxidative decarboxylation to convert pyruvate to acetyl CoA. Oxidative decarboxylation is the removal of carboxylate groups to form carbon dioxide.
Citrate synthase condenses the acetyl CoA and oxaloacetate.
Citrate is catalysed to convert isocitrate by aconitase. Citrate and isocitrate are isomers and it produces an intermediate cis-aconitate along with dehydration following hydration.
The isocitrate is converted to oxalosuccinate (oxidative decarboxylation) by isocitrate dehydrogenase and then into -ketoglutarate.
The enzyme α-ketoglutarate dehydrogenase converts α-ketoglutarate to succinyl CoA by the removal of the carboxylate group and forming carbon dioxide.
The conversion of succinyl CoA to succinate is induced by succinate thiokinase. It is achieved by adding a phosphorylated group to GDP that forms GTP and the GTP converts to ATP by nucleoside diphosphate kinase.
Succinate dehydrogenase catalyses succinate to fumarate by addition of oxygen and produces FADH2
The conversion of fumarate to malate is catalysed by fumarase by adding water.
Malate is oxidised to oxaloacetate by malate dehydrogenase and produces NADH. The oxaloacetate is reused by combining with other molecules of acetyl CoA and the cycle is continued.
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In the Krebs cycle, 2 CO2, 3 NADH, and 1 FADH2 are produced and the results of 12 ATP are generated.
Electron Transport Chain or Terminal Oxidation or Oxidative Phosphorylation
The electron transport chain (ETC) is a sequence of proteins that move electrons through a mitochondrial membrane and it produces a proton gradient to drive the synthesis of ATP. A chain of enzyme complexes in ETC:
NADH-ubiquinone reductase - Complex I
Succinate CoQ reductase - Complex II
Ubiquinone-cytochrome c oxidoreductase - Complex III
Cytochrome oxidase - Complex IV
ATP synthase - Complex V
These catalyses the electron transfer through electron carriers such as flavoproteins, cytochromes, coenzyme Q, nicotinamide nucleotides, and iron-sulphur proteins.
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Electrons are transferred from the one end of the metabolic substrate to the other end of oxygen by a series of electron carriers and that forms water. The passing of electrons through ETC leads to a loss of energy, therefore, the energy is utilised for the generation of ATP to ADP by the ATP synthase complex and the reaction involves oxidative phosphorylation. There are 32 ATP molecules produced in ETC and oxidative phosphorylation.
| Glycolysis | Krebs cycle |
| A stage of aerobic and anaerobic respiration. | A stage of aerobic respiration. |
| Glucose is a substrate material. | Acetyl CoA is a substrate. |
| Occurs inside of the cytoplasm. | Occurs in the mitochondria. |
| It consumes 2 molecules of ATP. | It does not consume ATP. |
| Carbon dioxide is released in glycolysis. | Carbon dioxide is not produced in the Krebs cycle. |
| It is a linear enzymatic reaction. | It is a non-linear pathway. |
| It occurs in both eukaryotes and prokaryotes. | It occurs in eukaryotes. |
| Aerobic respiration | Anaerobic respiration |
| It is the breakdown of food with oxygen and releasing energy. | It is the breakdown of food without oxygen and releasing of energy. |
| The substrate glucose is completely oxidised. | The substrate glucose is not completely broken down. |
| It occurs in plants and aerobic organisms. | It occurs in microbes such as anaerobic bacteria. |
| The end products are CO2 and H2 O. | The end product is ethyl alcohol. |
| It liberates an enormous amount of energy. | It liberates energy less than aerobic respiration. |
Respiration is the gaseous exchange and it is categorised into breathing and cellular respiration. Cellular respiration is essential for animals and plants to obtain energy that plays a crucial role in participating in various functions. It can be aerobic or anaerobic respiration, to break down the substrate for producing energy and end products. However, aerobic respiration produces more energy than anaerobic respiration. The metabolic pathways involve various metabolic reactions to convert the substrate into a product resulting in the liberation of ATP. Therefore, ATP breaks down to supply energy to the cell.
1. What is the role of electron carriers?
The metabolite is present at one end and oxygen is at the other end, therefore the electrons are carried by a series of proteins.
2. What are the energy-requiring steps in glucose to pyruvate conversion?
Glyceraldehyde 3-phosphate and dihydroxyacetone phosphate are obtained from glucose by involving intermediate reactions with enzymes. These are energy-requiring processes.
3. Define oxidative phosphorylation.
The oxidation of reactions adds the phosphate group by utilising energy that forms ATP from ADP and it occurs in the mitochondria is called oxidative phosphorylation.
4. How much ATP is produced during glycolysis?
Glucose is split into two molecules, therefore 8 ATP molecules are produced during glycolysis.
5. Why cellular respiration is important for living organisms?
Cellular respiration liberates energy that stimulates several activities in the body. Therefore, it is important for living beings to survive.
Aerobic vs Anaerobic Respiration - Difference and Comparison | Diffen. Diffen.com. (2022). Retrieved 31 May 2022, from https://www.diffen.com
cellular respiration | Definition, Equation, Cycle, Process, Reactants, & Products. Encyclopedia Britannica. (2022). Retrieved 31 May 2022, from https://www.britannica.com
Krebs Cycle - Definition, Products and Location | Biology Dictionary. Biology Dictionary. (2022). Retrieved 31 May 2022, from https://biologydictionary.net
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