Cellular respiration is an essential biological process by which organisms convert stored chemical energy into usable energy for cellular activities. It occurs in all living cells, including those of plants and animals, and is the key to living organisms' survival and growth. Cells use the chemical energy stored in glucose molecules to produce energy in the form of ATP (Adenosine Triphosphate). This energy is then used to power cellular activities such as protein synthesis, muscle contraction and membrane transport, and provide energy for other metabolic processes such as the Urea Cycle.
Normal cellular respiration is a complex set of metabolic processes that involve the breakdown of glucose molecules into smaller molecules known as pyruvate. This process begins with the process of glycolysis, where glucose molecules are broken down into two molecules of pyruvate and two molecules of ATP. This is followed by the Krebs cycle, where the pyruvate molecules are further broken down into carbon dioxide and water and also generate additional molecules of ATP. The final process is called the electron transport chain, where the released energy is used to produce additional molecules of ATP. In total, the process of cellular respiration produces 38 molecules of ATP from one molecule of glucose.
The first step of normal cellular respiration is glycolysis, a process that takes place in the cytoplasm of the cell. In glycolysis, glucose molecules are split into two molecules of pyruvate, with 2 molecules of ATP being produced in the process (-2 ATP + 4 ATP). This process is catalyzed by a set of enzymes and needs energy in the form of ATP molecules to get started (2 ATP). During the process, 2 molecules of NAD+ (Nicotinamide adenine dinucleotide) are also used up and converted into 2 molecules of NADH (Nicotinamide adenine dinucleotide).
The second step of cellular respiration is the Krebs cycle, also known as the citric acid cycle. This cycle is an aerobic process, meaning that it requires oxygen to take place. It begins with the conversion of pyruvate molecules into acetyl CoA (Acetyl-CoA), which is then used to enter the Krebs cycle. During the cycle, the acetyl CoA is further broken down into carbon dioxide, water and hydrogen atoms. The hydrogen atoms are used to generate additional molecules of NADH, which can then be used in the next step of cellular respiration.
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The third and last step of normal cellular respiration is the Electron Transport Chain. This process takes place on the inner membrane of the mitochondria and involves the transfer of electrons from NADH and FADH2 (Flavin adenine dinucleotide) to oxygen molecules. As the electrons are passed along the chain, they release energy, which is used to pump protons across the membrane. This creates a proton gradient, which is then used to generate molecules of ATP. The bulk of ATP is produced during this process (ETC).
Chain of reactions in glycolysis
1: Glucose (a 6-carbon molecule) is phosphorylated by the enzyme hexokinase (or glucokinase) to form glucose-6-phosphate (G6P). Enzyme: Hexokinase. Output: ATP.
2: G6P is then isomerized to form fructose-6-phosphate (F6P) by the enzyme phosphoglucose isomerase. Enzyme: Phosphoglucose isomerase. Output: No ATP.
3: F6P is then phosphorylated by the enzyme phosphofructokinase to form fructose 1,6-bisphosphate (F1,6BP). Enzyme: Phosphofructokinase. Output: ATP.
4: F1,6BP is cleaved into two 3-carbon molecules, dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (G3P) by the enzyme aldolase. Enzyme: Aldolase. Output: No ATP.
5: G3P is oxidized to form 1,3-bisphosphoglycerate (1,3-BPG) by the enzyme glyceraldehyde-3-phosphate dehydrogenase. Enzyme: Glyceraldehyde-3-phosphate dehydrogenase. Output: NADH and ATP.
6: 1,3-BPG is then phosphorylated to form 3-phosphoglycerate (3-PG) by the enzyme phosphoglycerate kinase. Enzyme: Phosphoglycerate kinase. Output: ATP.
7: 3-PG is then oxidized to form 2-phosphoglycerate (2-PG) by the enzyme phosphoglycerate mutase. Enzyme: Phosphoglycerate mutase. Output: No ATP.
8: 2-PG is then de-phosphorylated to form phosphoenolpyruvate (PEP) by the enzyme enolase. Enzyme: Enolase. Output: No ATP.
9: PEP is then phosphorylated to form pyruvate (3-carbon molecule) by the enzyme pyruvate kinase. Enzyme: Pyruvate kinase. Output: ATP.
10: In the presence of oxygen, pyruvate is oxidized to form acetyl-CoA and carbon dioxide by the enzyme pyruvate dehydrogenase. Enzyme: Pyruvate dehydrogenase. Output: NADH, FADH2, and CO2.
11: In the absence of oxygen, pyruvate is converted to lactate by the enzyme lactate dehydrogenase. Enzyme: Lactate dehydrogenase. Output: Lactate and NADH.
12: NADH is then oxidized to form NAD+ by the enzyme NAD+ oxidase. Enzyme: NAD+ oxidase. Output: NAD+.
13: NAD+ is then re-used in the first step of glycolysis by the enzyme hexokinase. Enzyme: Hexokinase. Output: ATP.
Total Output: 2 ATP, 2 NADH, and lactate (in the absence of oxygen, or in cancer cells even when there is an adequate amount of oxygen present).
Hexokinase is an enzyme that catalyzes the transfer of a phosphate group from ATP to glucose, forming glucose-6-phosphate, the first intermediate of glycolysis. This reaction is essential for glycolysis to begin, as it allows glucose molecules to be "trapped" in the cell, preventing them from diffusing out.
Phosphofructokinase is an enzyme that catalyzes the transfer of a phosphate group from ATP to fructose-6-phosphate, forming fructose-1,6-bisphosphate, the second intermediate of glycolysis. This reaction is the rate-limiting step of glycolysis, meaning that it determines the overall rate at which glycolysis occurs.
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