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Cellular Respiration:
Step 1: Glycolysis
Glycolysis is the first stage of cellular respiration. During this process, a six-carbon glucose molecule is broken down into two separate three-carbon molecules called pyruvate. These pyruvate molecules are carried into the cell's mitochondrion to be used as reactants in the Krebs cycle. Glycolysis uses energy from two ATP molecules to get started but ends up producing four ATP molecules. This means there is a net gain of two ATP molecules for each molecule of glucose that is broken down in this stage of cellular respiration. For each molecule of glucose processed in glycolysis, two pairs of high-energy electrons are released. Each NAD+ molecule accepts and electron pair, forming two NADH molecules. These electron carriers transport the high-energy electrons into the mitochondrion to be used in the third stage of cellular respiration, the electron transport chain.
Step 2: Krebs Cycle
If oxygen is present, each pyruvate molecule produced during glycolysis will enter the mitochondrion. This process does not need oxygen as a reactant, but it will occur without the presence of oxygen. First, the three-carbon pyruvate molecule is broken down into a two-carbon molecule. In the process, one carbon dioxide, (CO2) is released and one NADH molecule is formed. The two-carbon molecule bonds to coenzyme A, forming acetyl-CoA, in preparation of entering the Krebs Cycle. This equation summarizes the breakdown of two pyruvate molecules before entering the Krebs cycle:
2 pyruvate->2 NADH + 2 CO2 + 2 Acetyl-CoA
Each acetyl-CoA molecule enters the Krebs Cycle where is participates in a series of reactions with other organic compounds as it is broken down into carbon dioxide. In the first step of the Krebs Cycle, the two-carbon fragment from the acetyl-CoA bonds to oxaloacetate, a four-carbon molecule, to form a six-carbon molecule. When oxaloacetate accepts the two-carbon fragment at the beginning of the cycle, the six-carbon compound that is formed is called citric acid. This is the first compound formed in the Krebs cycle, which is why this cycle is sometimes called the citric acid cycle. Over the course of the cycle, two more carbon dioxide molecules are released. This CO2 gas diffuses out of the mitochondrion. This means that the six-carbon citric acid molecule is broken down to a four-carbon molecule. By the end of the cycle, the oxaloacetate molecule is re-formed. This means that it is available to start the Krebs cycle over again with a new acetyl-CoA molecule. As acetyl-CoA is broken down in the Krebs cycle, the reactions release energy. Some of that energy forms an ATP molecule, but the majority of the energy is stored in the electron-carrier molecules NADH and FADH2. These molecules carry the high-energy electrons to the third stage of cellular respiration, the electron transport chain, where they will be used to produce more ATP molecules.
Step 3: Electron Transport Chain
The electron transport chain is made up of three protein pumps embedded in the inner membrane of a mitochondrion. The NADH and FADH2 formed in the first two steps of cellular respiration transfer the high-energy electrons to these protein pumps. Energy is released as the electrons are transferred through the chain of proteins. That energy is used to move positive hydrogen ions (H+) from the mitochondrion's matrix (the space inside the inner membrane) to the intermembrane space between the two membranes. The greater concentration of hydrogen ions in the intermembrane space causes hydrogen ions to diffuse back into the matrix through a protein called ATP synthase. The ATP synthase is able to use the diffusion of hydrogen ions to build an ATP molecule. The flow of ions through the ATP synthase channel produces around 34 ATP molecules. At the end of the electron transport chain, the electrons combine with hydrogen ions and oxygen to form water molecules. The NAD+ and FAD molecules are sent back to the cytoplasm and mitochondrial matrix to participate in future rounds of glycolysis and the Krebs cycle.
There are three stages in cellular respiration. The reaction is:
C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + Energy (ATP and Heat)
In total, glycolysis, the Krebs cycle, and the electron transport chain provide the cell with more than 30 ATP molecules for each molecule of glucose processed. ATP is then used to power the cell. So for every bite of food you eat, your body takes the energy and multiplies in thirtyfold!
Explanation:
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