Why does krebs cycle happen twice

The citric acid cycle : In the citric acid cycle, the acetyl group from acetyl CoA is attached to a four-carbon oxaloacetate molecule to form a six-carbon citrate molecule. Through a series of steps, citrate is oxidized, releasing two carbon dioxide molecules for each acetyl group fed into the cycle. Because the final product of the citric acid cycle is also the first reactant, the cycle runs continuously in the presence of sufficient reactants.

The first step is a condensation step, combining the two-carbon acetyl group from acetyl CoA with a four-carbon oxaloacetate molecule to form a six-carbon molecule of citrate. CoA is bound to a sulfhydryl group -SH and diffuses away to eventually combine with another acetyl group. This step is irreversible because it is highly exergonic.

The rate of this reaction is controlled by negative feedback and the amount of ATP available. If ATP levels increase, the rate of this reaction decreases. If ATP is in short supply, the rate increases. Citrate loses one water molecule and gains another as citrate is converted into its isomer, isocitrate.

Steps 3 and 4. CoA binds the succinyl group to form succinyl CoA. Step 5. A phosphate group is substituted for coenzyme A, and a high- energy bond is formed. This energy is used in substrate-level phosphorylation during the conversion of the succinyl group to succinate to form either guanine triphosphate GTP or ATP.

There are two forms of the enzyme, called isoenzymes, for this step, depending upon the type of animal tissue in which they are found. One form is found in tissues that use large amounts of ATP, such as heart and skeletal muscle.

This form produces ATP. The second form of the enzyme is found in tissues that have a high number of anabolic pathways, such as liver. This form produces GTP. Climate Change 5: Evolution 1. Evolution Evidence 2. Natural Selection 3. Classification 4. Cladistics 6: Human Physiology 1.

Digestion 2. The Blood System 3. Disease Defences 4. Gas Exchange 5. Homeostasis Higher Level 7: Nucleic Acids 1. DNA Structure 2. Transcription 3. Translation 8: Metabolism 1. The resulting compound is acetyl coenzyme A.

Acetyl - which some chemists nowadays call ethanoyl - is a fairly simple group containing only 2 carbon atoms: CH 3 CO- , and coenzyme A merely functions as a carrier - it does not add any more carbon atoms, as it is released to repeat the process. The link reaction is distinct from glycolysis and the following cycle. Through a series of steps, citrate is oxidized, releasing two carbon dioxide molecules for each acetyl group fed into the cycle. Because the final product of the citric acid cycle is also the first reactant, the cycle runs continuously in the presence of sufficient reactants.

Step 1. The first step is a condensation step, combining the two-carbon acetyl group from acetyl CoA with a four-carbon oxaloacetate molecule to form a six-carbon molecule of citrate. CoA is bound to a sulfhydryl group -SH and diffuses away to eventually combine with another acetyl group. This step is irreversible because it is highly exergonic. The rate of this reaction is controlled by negative feedback and the amount of ATP available. If ATP levels increase, the rate of this reaction decreases.

If ATP is in short supply, the rate increases. Step 2. Citrate loses one water molecule and gains another as citrate is converted into its isomer, isocitrate. Steps 3 and 4. CoA binds the succinyl group to form succinyl CoA. Step 5. A phosphate group is substituted for coenzyme A, and a high- energy bond is formed. This energy is used in substrate-level phosphorylation during the conversion of the succinyl group to succinate to form either guanine triphosphate GTP or ATP. There are two forms of the enzyme, called isoenzymes, for this step, depending upon the type of animal tissue in which they are found.

One form is found in tissues that use large amounts of ATP, such as heart and skeletal muscle. This form produces ATP. The second form of the enzyme is found in tissues that have a high number of anabolic pathways, such as liver. This form produces GTP.