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Sequence of Events in Cellular Respiration

Cellular respiration is the process by which the body's cells break down food to use for energy. This glucose, which stores the energy, is used to produce adenosine triphosphate, or ATP, which, in turn, powers the body's cells. Cellular respiration, therefore, is the production of ATP for cellular fuel. All plants and animals carry out cellular respiration, which is essential to sustaining life. Cellular respiration has three main steps, and each has sub-steps.

  1. Glycolysis

    • The first step of cellular respiration is glycolysis, in which glucose from food carbohydrates is broken down, or split, into molecules of pyruvic acid. Energy from sugars such as fructose is gathered in glycolysis. This step occurs in the cell's cytoplasm and requires no oxygen: It occurs in both aerobic (with oxygen) and anaerobic (without oxygen) reactions. Once the glucose has been broken down by various enzymes, the result is the pyruvic acid, or pyruvate, and a gain of two ATP molecules and two NADH molecules. NADH is nicotinamide adenine dinucleotide, an energy-rich molecule. The pyruvate then moves on to the next step.

    Krebs Cycle

    • The Krebs cycle is a complex string of chemical reactions in the cell that produces more energy. In this cycle (an aerobic process), the pyruvate moves from the cell's cytoplasm (the part of the cell inside the membrane), into the mitochondrion, a membrane-enclosed organelle found inside the cell. Again, enzymes go to work on these molecules. One carbon and two oxygen atoms are removed from each, and a two-carbon sugar is produced, called Acetyl CoA, or Acetyl coenzyme A. This sugar goes through the Krebs cycle repeatedly until it is completely oxidized. The result of this cycle is a gain of eight NADH molecules, two FADH2 molecules, four carbon dioxide molecules and two more ATP molecules. NADH and FADH2 are energy-producing molecules, and FADH2 stands for flavin adenine dinucleotide.

    Electron Transport Chain

    • Most of the ATP energy produced in cellular respiration happens in the electron transport chain, which is a sequence of proteins that are in the mitochondrion. These proteins produce water by passing electrons down the chain. When the hydrogen in the water passes along the chain, it comes in contact with the membrane of the mitochondrion; this friction produces ATP. This step produces about 34 molecules of ATP. Some energy is used in this process at the same time, so the net gain of ATP in cellular respiration is 36 molecules of ATP per molecule of glucose. By the end of cellular respiration, then, the cell gains 36 molecules of ATP per molecule of glucose to use for energy.

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