Cellular respiration may be defined as a set of metabolic processes by which cells generate energy in the form of ATP (adenosine triphosphate) from the food molecules and release waste products. In cellular respiration, oxygen is required to generate large amount of energy but cells can also generate a small amount of energy without oxygen under several conditions, including in hypoxic state, shock, and during exercise or in cells that lack mitochondria.
Purpose of cellular respiration:
Purpose of cellular respiration is energy generation in the form of ATP. Generated energy through cellular respiration utilize the cells for energy consuming activities including active transport, synthesis, nerve impulse conduction, muscle contraction, and so on.
Stages of cellular respiration:
Energy generation through cellular respiration occurs in three stages: first stage, second stage, and third stage. First stage of cellular respiration takes place in the cytoplasm, second stage carried out in the mitochondrial matrix, and third stage occurs in the inner mitochondrial membrane of a cell.
First stage: Glucose, amino acid, and fatty acid can all be used as fuel molecules in first stage of cellular respiration, but glucose is most commonly used. Therefore, the principal process of first stage of cellular respiration is glycolysis. In glycolytic pathway of cellular respiration, simple molecules of glucose is degraded into Acetyl Co-A and generates energy in the form of ATP. Usually oxygen is required in this stage. But during this stage of cellular respiration, cell can also generate energy without utilization of oxygen via anaerobic glycolysis under several conditions, including in cells deprived of sufficient oxygen such as in hypoxic state, shock, and during heavy exercise or in cells that lack mitochondria, such as mature red blood cells, leukocytes, lens and cornea of the eyes, retina, renal medulla, and testes. Only a small amount of energy is generated as ATP in glycolytic pathway of cellular respiration.
Metabolic fuels fatty acid and amino acid both are not produced any ATP in first stage of cellular respiration. Read details…….
Second stage: Second stage of cellular respiration carried out through the Citric acid cycle/Krebs cycle/TCA cycle. Acetyl Co-A from first stage of cellular respiration enters into the citric acid cycle and further broken down into even smaller molecules by utilization of oxygen for complete oxidation to carbon dioxide and water, and reduces co-enzymes, NADH and FADH2. These reduced co-enzymes are re-oxidized via the electron transport chain, linked to the production of ATP. In second stage of cellular respiration, large amount of energy is generated as ATP by oxidation of Acetyl Co-A.
Third stage: Third stage of cellular respiration occurs through the electron transport chain. It is the final stage of cellular respiration and the final common pathway by which electrons arise from different fuels of body flow to oxygen. In the electron transport chain, NADH and FADH2are oxidized to generate ATP as electrons are passed from one carrier to another. Eventually the electrons combine with oxygen and hydrogen ions to form water. Greater amount of energy is generated in third stage of cellular respiration.
Equation of cellular respiration:
C6H12O6 (glucose) + 6O2 (oxygen) ⟹ 6CO2 (carbon dioxide) + 6H2O (water) + 38 ATP (energy).
Therefore, cellular respiration generates total 38 molecules of ATP from one molecule of glucose. Cells store this energy and utilize in appropriate amounts when needed.