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Respiration is inhaling oxygen from outside the body and using it to break down the food sources for cellular needs. Each cell requires energy to fulfil its daily functions. Where does a cell get energy from? Cellular respiration is the source of energy for the cells.
Cellular respiration is the process through which each cell can utilise energy from the glucose molecules broken down. Cellular respiration takes place in three different methods. All three methods follow different mechanisms, have different requirements and processes. Cellular respiration can occur in aerobic conditions (in the presence of oxygen) and anaerobic conditions (absence of oxygen or lack of oxygen).
Anaerobic conditions are those conditions where the air (oxygen) is completely absent, and the process carries spontaneously. It is also considered a normal part of cellular respiration. Generally, glucose is one of the reactants of cellular respiration, but glucose is the only reactant when it comes to anaerobic conditions.
Mechanism of cellular respiration
Cells use the nutrients derived from various foods to energise themselves and properly carry out various functions. The energy obtained by a diversity of organisms follows different strategies. Some organisms completely break down the glucose molecules into carbon dioxide and water using oxygen. Thus they produce ATP, the energy currency of cells. Others use mechanisms that don’t use oxygen. In all organisms, the first step remains the same: the breaking down of a six-carbon molecule (glucose) into a three-carbon molecule (pyruvate). This step occurs in the cytoplasm of all organisms.
Pyruvate is the further breaking of glucose into carbon dioxide, water, and energy (ATP) in the presence of oxygen in the mitochondria. It’s also known as aerobic respiration. The release of energy means the amount of ATP produced in aerobic respiration is greater than in anaerobic conditions. Stages of aerobic conditions include the Krebs cycle, oxidative phosphorylation, etc.
In Glycolysis two molecules of ATP are consumed during double phosphorylation of glucose to form fructose 1, 6-diphosphate. In return four molecules of ATP are produced by substrate level phosphorylation (conversion of 1, 3-diphosphoglycerate to 3-phosphoglycerate and phosphoenolpyruvate to pyruvate) and two molecule of NADH2 are formed at the time of oxidation of glyceraldehyde 3-phosphate to 1, 3-diphosphoglycerate.
The whole process may be expressed as under:
Glucose + 2 NAD+ + 2 ADP + 2H3PO4 → 2 Pyruvate + 2 NADH + 2H+ + 2ATP
Two molecules of NADH + H+ on oxidation produce 6 molecules of ATP. Therefore, a net gain of 8 ATP molecules occurs during glycolysis.
Methods of anaerobic conditions
1- Cellular respiration in yeasts
Cellular respiration in the yeast takes place in the absence of oxygen and is hence called anaerobic respiration. The pyruvate, which is synthesised in the cytoplasm, gets converted into ethanol and carbon dioxide. This process specifically takes place in yeast during fermentation. These organisms can continually convert energy without the need for oxygen. They undergo glycolysis, followed by the anaerobic fermentation process to make ATP.
Anaerobic respiration is different from fermentation as the final electron acceptor is different in both cases. In fermentation, the final electron acceptor is an organic molecule, while in anaerobic respiration, the final electron acceptor is an inorganic substance such as nitrate, sulphate but not oxygen. Many yeasts use alcoholic fermentation to produce ethanol. In yeast, pyruvate is decarboxylated to form acetaldehyde, which NADH then reduces to form ethanol. The products of anaerobic respiration in yeasts are ethanol, carbon dioxide, and ATP. The amount of ATP produced in anaerobic conditions is usually less (2 ATP) than aerobic respiration.
The energy released during cellular respiration is immediately used to synthesise ATP molecules, which fuel all other activities of the cells.
2- Cellular respiration in muscle cells
A different approach is taken to break the pyruvate down if there is a lack of oxygen in human muscle cells. In this case, the pyruvate is converted into lactic acid (a three-carbon molecule). This deposition of lactic acid in muscles during sudden activity causes cramps. This process of anaerobic respiration is also known as lactic acid fermentation.
RESPIRATORY SUBSTRATE
The organic substances which can be catabolized in the living cells to release energy are called respiratory substances. The most common respiratory substrate is glucose. It is formed from carbohydrate substrates like starch in most plants and glycogen in animals and fungi. Fats may be used as a respiratory substrate in some cases. Proteins are used in respiration rarely, as during germination of protein rich seeds and spores. In other organs, proteins are employed as respiratory substrate only when carbohydrates and fat reserves have been used up, as during long fasting.
Conclusion
Cellular respiration is a process where the molecules of glucose get converted into such forms that the cell can utilise energy. Cellular respiration can occur both in presence of oxygen (aerobic) and in the absence of oxygen (anaerobic). Glycolysis, the first step of cellular respiration, is considered an anaerobic process. Anaerobic respiration takes place in yeast and is referred to as alcoholic fermentation, and in muscle cells which is referred to as lactic acid fermentation. The product of anaerobic respiration is ethanol, carbon dioxide, ATP (yeasts), lactic acid, and ATP (muscle cells).
