Question

What happens during the citric acid cycle?

Answer 1

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for eukaryotes citric acid cycle is done in the mitochondria, just like the conversion of pyruvate to acetyl CoA. In prokaryotes, citric acid cycle take place in the cytoplasm. The citric acid cycle is a closed loop; the last part of the pathway reforms the molecule used in the first step. The cycle includes eight major steps.

This steps are showed in above diagram. First, acetyl CoA combines with oxaloacetate, a four-carbon molecule, losing the CoA group and forming the six-carbon molecule citrate. After citrate undergoes a rearrangement step, it undergoes an oxidation reaction, transferring electrons to NAD+ to form NADH and releasing a molecule of carbon dioxide. The five-carbon molecule left behind then undergoes a second, similar reaction, transferring electrons to NAD+ to form NADH and releasing a carbon dioxide molecule. The four-carbon molecule remaining then undergoes a series of transformations, in the course of which GDP and inorganic phosphate are converted into GTP—or, in some organisms, ADP and inorganic phosphate are converted into ATP—an FAD molecule is reduced to FADH2, and another NAD+ is reduced to NADH. At the end of this series of reactions, the four-carbon starting molecule, oxaloacetate, is regenerated, allowing the cycle to begin again.

In the first step of the cycle, acetyl CoA combines with a four-carbon acceptor molecule, oxaloacetate, to form a six-carbon molecule called citrate. After a quick rearrangement, this six-carbon molecule releases two of its carbons as carbon dioxide molecules in a pair of similar reactions, producing a molecule NADH. The enzymes that catalyze these reactions are key regulators of the citric acid cycle, speeding it up or slowing it down based on the cell’s energy needs.

The remaining four-carbon molecule undergoes a series of additional reactions, first making an ATP molecule—or, in some cells, a similar molecule called GTP—then reducing the electron carrier FAD to 2FADH2​, and finally generating another NADH. This set of reactions regenerates the starting molecule, oxaloacetate, so the cycle can repeat.

Overall, one turn of the citric acid cycle releases two carbon dioxide molecules and produces three NADH, oneFADH2​, and one ATP or GTP . The citric acid cycle goes around twice for each molecule of glucose that enters cellular respiration because there are two pyruvates—and thus, two acetyl CoA—made per glucose.