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Cellular respiration

Cellular Respiration is a series or reactions and processes in the cells of an organism that convert energy in the form of nutrients to ATP (adenosine triphosphate), as well as releasing waste products. The first cycle of reactions is called Glycolysis, which is followed by the Citric Acid Cyle (or the Krebs Cycle), and finally the Electron Transport Chain.

Glycolysis is an anaerobic process, meaning that it can occur without oxygen presentses the molecule glucose, a six-carbon ring molecule; ADP (adenosine diphosphate), an ATP that has lost lost a phosphate group; and the NAD+ coenzyme, an electron transporter. The cycle begins when glucose is broken up into two pyruvates, which are 3-carbon molecules (6 carbons total which is equivalent to the 6-carbon ring in glucose). One ATP is used to add a phosphate group to the glucose molecule in order to create glucose-6-phosphate. This molecule is then rearranged to become fructose-6-phosphate, and another ATP is used to add another phosphate group to the fructose. The molecule is then split into two even halves to form 2 G3P molecules (glyceraldehyde-3-phosphate), they each gain an inorganic phosphorous to become 3-biphosphoglycerates, and they each contribute a hydrogen ion and two electrons to 2 NAD+ molecules in order to form 2 NADH molecules. Finally, the 2 phosphates of each biphosphoglycerate are transferred to 4 ADP molecules in order to elevate them to 4 ATP molecules, changing the biphosphoglycerates to pyruvates. This way, by the end of Glycolosis, there is a net yield of 2 pyruvate molecules, 2 ATPs, and 2 NADH molecules.

The next cycle in Cellular Respiration is the Citric Acid Cycle, which takes place in the mitochondria. Prior to the actual beginning of the cycle, each pyruvate left from glycolysis is split apart and then combined with coenzyme A (CoA) to form acetyl-CoA, a 2-carbon molecule. The third carbon from the original pyruvate is combined with oxygen to form CO2, which is released as waste, and high energy electrons are released and caught to form NADH molecules. The Citric Acid Cycle begins when acetyl-CoA combines with OAA (oxaloacetate), a 4-carbon molecule, to produce a 6-carbon molecule called citrate. This molecule then loses 2 carbons to form 2 CO2 molecules as waste. The citrate goes through many forms throughout the cycle; isocitrate, a-ketoglutarate, succinyl CoA, succinate, fumarate, malarate, and finally oxaloacetate. The end product of the Citric Acid Cycle (oxaloacetate) is also required for the cycle to begin, and thus the cycle essentially keeps itself going. At the end of one cycle, one ATP, one FADH2, and 3 NADH molecules are produced. Two molecules of CO2 are also generated as waste. Since one glucose molecule provides two pyruvates, the cycle can happen twice per glucose, so one glucose yields 2 ATPs, 2 FADH2 molecules, and 6 NADH molecules.
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The final and most energy-productive process in Cellular Respiration is the Electron Transport Chain (ETC). In this process, electrons are donated by NADH and FADH2 to the multiple complexes in the chain, which then move through each complex through a series of redox (reduction-oxidation) reactions. The electrons flow from higher to lower energy levels as they move through the complexes, thus giving off energy. Most of that energy is used to pump H+ ions into the intermembrane space of the mitochondria, which creates a proton gradient. The ions that were pumped into the intermembrane space only have one channel to escape from, which forces them to move through ATP Synthase, a protein that catalyzes the phosphorylation of ADP through use of the energy created by the proton gradient, which makes ATP for use throughout the cell.Image result for electron transport chain steps