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This illustration shows the eight steps of the citric acid cycle. In the first step, the acetyl group from acetyl CoA is transferred to a four-carbon oxaloacetate molecule to form a six-carbon citrate molecule. In the second step, citrate is rearranged to form isocitrate. In the third step, isocitrate is oxidized to α-ketoglutarate. In the process, one NADH is formed from NAD^{+} and one carbon dioxide is released. In the fourth step, α-ketoglutarate is oxidized and CoA is added, forming succinyl CoA. In the process, another NADH is formed and another carbon dioxide is released. In the fifth step, CoA is released from succinyl CoA, forming succinate. In the process, one GTP is formed, which is later converted into ATP. In the sixth step, succinate is oxidized to fumarate, and one FAD is reduced to FADH_{2}. In the seventh step, fumarate is converted into malate. In the eighth step, malate is oxidized to oxaloacetate, and another NADH is formed.
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. In the process, three NAD + molecules are reduced to NADH, one FAD molecule is reduced to FADH 2 , and one ATP or GTP (depending on the cell type) is produced (by substrate-level phosphorylation). Because the final product of the citric acid cycle is also the first reactant, the cycle runs continuously in the presence of sufficient reactants. (credit: modification of work by “Yikrazuul”/Wikimedia Commons)

Steps in the citric acid cycle

Step 1. Prior to the start of the first step, a transitional phase occurs during which pyruvic acid is converted to acetyl CoA. Then, the first step of the cycle begins: This is a condensation step, combining the two-carbon acetyl group 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. In step two, citrate loses one water molecule and gains another as citrate is converted into its isomer, isocitrate.

Step 3. In step three, isocitrate is oxidized, producing a five-carbon molecule, α-ketoglutarate, together with a molecule of CO 2 and two electrons, which reduce NAD + to NADH. This step is also regulated by negative feedback from ATP and NADH, and a positive effect of ADP.

Steps 3 and 4. Steps three and four are both oxidation and decarboxylation steps, which release electrons that reduce NAD + to NADH and release carboxyl groups that form CO 2 molecules. α-Ketoglutarate is the product of step three, and a succinyl group is the product of step four. CoA binds the succinyl group to form succinyl CoA. The enzyme that catalyzes step four is regulated by feedback inhibition of ATP, succinyl CoA, and NADH.

Step 5. In step five, 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. GTP is energetically equivalent to ATP; however, its use is more restricted. In particular, protein synthesis primarily uses GTP.

Step 6. Step six is a dehydration process that converts succinate into fumarate. Two hydrogen atoms are transferred to FAD, producing FADH 2 . The energy contained in the electrons of these atoms is insufficient to reduce NAD + but adequate to reduce FAD. Unlike NADH, this carrier remains attached to the enzyme and transfers the electrons to the electron transport chain directly. This process is made possible by the localization of the enzyme catalyzing this step inside the inner membrane of the mitochondrion.

Step 7. Water is added to fumarate during step seven, and malate is produced. The last step in the citric acid cycle regenerates oxaloacetate by oxidizing malate. Another molecule of NADH is produced in the process.

Click through each step of the citric acid cycle here .

Products of the citric acid cycle

Two carbon atoms come into the citric acid cycle from each acetyl group, representing four out of the six carbons of one glucose molecule. Two carbon dioxide molecules are released on each turn of the cycle; however, these do not necessarily contain the most recently added carbon atoms. The two acetyl carbon atoms will eventually be released on later turns of the cycle; thus, all six carbon atoms from the original glucose molecule are eventually incorporated into carbon dioxide. Each turn of the cycle forms three NADH molecules and one FADH 2 molecule. These carriers will connect with the last portion of aerobic respiration to produce ATP molecules. One GTP or ATP is also made in each cycle. Several of the intermediate compounds in the citric acid cycle can be used in synthesizing non-essential amino acids; therefore, the cycle is amphibolic (both catabolic and anabolic).

Section summary

In the presence of oxygen, pyruvate is transformed into an acetyl group attached to a carrier molecule of coenzyme A. The resulting acetyl CoA can enter several pathways, but most often, the acetyl group is delivered to the citric acid cycle for further catabolism. During the conversion of pyruvate into the acetyl group, a molecule of carbon dioxide and two high-energy electrons are removed. The carbon dioxide accounts for two (conversion of two pyruvate molecules) of the six carbons of the original glucose molecule. The electrons are picked up by NAD + , and the NADH carries the electrons to a later pathway for ATP production. At this point, the glucose molecule that originally entered cellular respiration has been completely oxidized. Chemical potential energy stored within the glucose molecule has been transferred to electron carriers or has been used to synthesize a few ATPs.

The citric acid cycle is a series of redox and decarboxylation reactions that remove high-energy electrons and carbon dioxide. The electrons temporarily stored in molecules of NADH and FADH 2 are used to generate ATP in a subsequent pathway. One molecule of either GTP or ATP is produced by substrate-level phosphorylation on each turn of the cycle. There is no comparison of the cyclic pathway with a linear one.

Questions & Answers

what is nutrition
Joseph Reply
the process of providing or obtaining the food necessary for health and growth.
fatimah
what are micro-organism
Bukiwe Reply
what is immunity
Moriba Reply
What is cross pollination
Moriba
what is pollution
Georgiana Reply
what is pollution
Georgiana
list four type of biological association
Georgiana
Pollution is the release of harmful substances in the atmosphere.
Immanuel
Four biological associations are 1) commensalism 2)symbiosis 3)mutualism 4) parasitism
Immanuel
What is the function of the hormones
Immanuel Reply
What is the other name of filament of a flower
Immanuel
oxygen
Georgiana
list three bone disorders and explain them
Blessings Reply
oxygen
Georgiana
what is self pollination?
Kelvin Reply
well, self-pollination occurs in a single flower which has the organs of both genders. the pollen grains from the anther shall be captured by the stigma of the flower, when released. and the pollen grain shall start grow pollen tube which grows down in the style, to the ovary, ovules and the
Izzati
insides of the ovules. so pretty much the pollen grains being pollinated from the anther to the stigma of the same flower
Izzati
I think
Izzati
the pollen grains being pollinated from the anther to the stigma of the same flower
Izzati
what is animal science
Rose Reply
Animal science is described as "studying the biology of animals that are under the control of humankind".
AMIIRA
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Izzati
what is self pollinating
JOSHUA Reply
what is cleistogamy
JOSHUA
Self pollination occurs when the pollen from the anther is deposited on the stigma of the same flower, or another flower on the same plant.
Franklina
Cleistogamy is a type of automatic self pollination of certain plants that can propagate by using non opening, self pollinating flowers
Franklina
Self pollination is the transfer of pollen grain from the anther to the stigma
Immanuel
who is the father of biology
Sean Reply
Aristotle
Lubera
Aristotle
estella
yes Aristotle looking at classification
John
Aristotle
JOSHUA
Aristotle
Grace
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Sean
Aristotle
Rose
Aristotle
Immanuel
what are prokaryotic cells
Kusubira Reply
having cell that lack membrane - bound nuclei.
Lubera
Pronkayotic cells are cells that lacks nuclear membrane
Immanuel
Please each person should suggest ur answers
Franklina Reply
differentiate between autelogical and synecology
Christiana Reply
differentiate between autelogical and synecology
Christiana Reply
answer please
Edmond
Autecology is the study of individual organism or individual species. It is also known as population ecology. Synecology is the study of group of organisms of different species which are associated together as a unit in form of a community.
Daniel
Autecology is the study of the environment in relation to only one species in contrast while synecology is the study of the environment affecting groups of species coexisting in an area.
Franklina
Describe the factors affecting homotasis
Grace Reply
Fluids and electrolytes Energy and nutrition Immune response mediators
Franklina

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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