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18.3C: Citric Acid (Krebs) Cycle - Biology


Learning Objectives

  • State two other names for the citric acid cycle.
  • Briefly describethe function of the citric acid cycle during aerobic respiration and indicate the reactants and products.
  • Compare where the citric acid cycle occurs in prokaryotic cells and in eukaryotic cells.
  • State the total number of ATP produced by substrate-level phosphorylation for each acetyl-CoA that enters the citric acid cycle.
  • State the total number of NADH and FADH2 produced for each acetyl-CoA that enters the citric acid cycle.
  • During aerobic respiration, state what happens to the NADH and the FADH2 produced during the citric acid cycle.

The citric acid cycle, also known as the tricarboxylic acid cycle and the Krebs cycle, completes the oxidation of glucose by taking the pyruvates from glycolysis (and other pathways), by way of the transition reaction mentioned previously, and completely breaking them down into (CO_2) molecules, (H_2O) molecules, and generating additional ATP by oxidative phosphorylation. In prokaryotic cells, the citric acid cycle occurs in the cytoplasm; in eukaryotic cells the citric acid cycle takes place in the matrix of the mitochondria.

The overall reaction for the citric acid cycle is:

[ ext{2 acetyl groups} + 6 NAD^+ + 2 FAD + 2 ADP + 2 P_i]

[ ightarrow 4 CO_2 + 6 NADH + 6 H^+ + 2 FADH_2 + 2 ATP]

The citric acid cycle (Figure Figure (PageIndex{1})) provides a series of intermediate compounds that donate protons and electrons to the electron transport chain by way of the reduced coenzymes (NADH) and (FADH_2). The electron transport chain then generates additional ATPs by oxidative phosphorylation. The citric acid cycle also produces 2 ATP by substrate phosphorylation and plays an important role in the flow of carbon through the cell by supplying precursor metabolites for various biosynthetic pathways.

The citric acid cycle involves 8 distinct steps, each catalyzed by a unique enzyme. You are not responsible for knowing the chemical structures or enzymes involved in the steps below. They are included to help illustrate how the molecules in the pathway are manipulated by the enzymes in order to to achieve the required products.

Step 1: The citric acid cycle begins when Coenzyme A transfers its 2-carbon acetyl group to the 4-carbon compound oxaloacetate to form the 6-carbon molecule citrate (Figure Figure (PageIndex{2})).

Step 2: The citrate is rearranged to form an isomeric form, isocitrate (Figure (PageIndex{3})).

Step 3: The 6-carbon isocitrate is oxidized and a molecule of carbon dioxide is removed producing the 5-carbon molecule alpha-ketoglutarate. During this oxidation, (NAD^+) is reduced to (NADH) and (H^+) (Figure (PageIndex{4})).

Step 4: Alpha-ketoglutarate is oxidized, carbon dioxide is removed, and coenzyme A is added to form the 4-carbon compound succinyl-CoA. During this oxidation, NAD+ is reduced to NADH + H+ (Figure (PageIndex{5})).

Step 5: CoA is removed from succinyl-CoA to produce succinate. The energy released is used to make guanosine triphosphate (GTP) from guanosine diphosphate (GDP) and Pi by substrate-level phosphorylation. GTP can then be used to make ATP (Figure (PageIndex{6})).

Step 6: Succinate is oxidized to fumarate. During this oxidation, (FAD) is reduced to (FADH_2) (Figure (PageIndex{7})).

Step 7: Water is added to fumarate to form malate (Figure (PageIndex{8})).

Step 8: Malate is oxidized to produce oxaloacetate, the starting compound of the citric acid cycle. During this oxidation, NAD+ is reduced to NADH + H+ (Figure (PageIndex{9})).

The NADH + H+ and FADH2 carry protons and electrons to the electron transport chain to generate additional ATP by oxidative phosphorylation.

Summary

  1. Aerobic respiration involves four stages: glycolysis, a transition reaction that forms acetyl coenzyme A, the citric acid (Krebs) cycle, and an electron transport chain and chemiosmosis.
  2. The citric acid cycle, also known as the tricarboxylic acid cycle and the Krebs cycle, completes the oxidation of glucose by taking the pyruvates from glycolysis, by way of the transition reaction, and completely breaking them down into CO2 molecules, H2O molecules, and generating additional ATP by oxidative phosphorylation.
  3. The citric acid cycle provides a series of intermediate compounds that donate protons and electrons to the electron transport chain by way of the reduced coenzymes NADH and FADH2. The citric acid cycle also produces 2 ATP by substrate phosphorylation.
  4. The overall reaction for the citric acid cycle is:[ 2 acetyl groups + 6 NAD^+ + 2 FAD + 2 ADP + 2 P_i yields 4 CO_2 + 6 NADH + 6 H^+ + 2 FADH_2 + 2 ATP.]
  5. The citric acid cycle also plays an important role in the flow of carbon through the cell by supplying precursor metabolites for various biosynthetic pathways.

How Is Atp Produced In Krebs Cycle

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How Many Atp Is Formed In Krebs Cycle

The krebs cycle also known as the citric acid cycle produces 2 atp molecules 10 carrier molecules and co 2 from each glucose molecule. The citric acid cycle cac also known as the tca cycle tricarboxylic acid cycle or the krebs cycle is a series of chemical reactions used by all aerobic organisms to release stored energy through the oxidation of acetyl coa derived from carbohydrates fats and proteins into adenosine triphosphate atp and carbon dioxide.

how many atp is formed in krebs cycle is important information accompanied by photo and HD pictures sourced from all websites in the world. Download this image for free in High-Definition resolution the choice "download button" below. If you do not find the exact resolution you are looking for, then go for a native or higher resolution.
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The bulk of a cells atp must be produced in the mitochondria.

How many atp is formed in krebs cycle. Two molecules of acetyl coa are produced in glycolysis so the total number of molecules produced in the citric acid cycle is doubled 2 atp 6 nadh 2 fadh2 4 co2 and 6 h. The electron transport chain then produces 34 atp molecules and h 2o from the carrier molecules. The krebs cycle produces two molecules of atp for every molecule of glucose.

Now the molecule is ready to accept another acetyl coa molecule to begin another turn of the cycle. How many atp are produced in the krebs cycle. Since two atp are used to start the cycle there are 36 atp produced net.

In eukaryotes the krebs cycle uses a molecule of acetyl coa to generate 1 atp 3 nadh 1 fadh2 2 co2 and 3 h. Technically there is no atp formed in the krebs cycle from glycolysis products. The krebs cycle also produces eight molecules of nadh and two molecules of fadh2 per molecule of glucose.

All told the krebs cycle forms per two molecules of pyruvic acid two atp molecules ten nadh molecules and two fadh2 molecules. A total of 38 atp are produced during the krebs cycle. A total of 38 atp are produced during the krebs cycle.

There are two gtp formed which are atp equivalents and the rest are electron carriers that bring energy to electron transport to produce atp. The nadh and the fadh2 will be used in the electron transport system. Since two atp are used to start the cycle there are 36 atp produced net.

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How Is Atp Produced By Substrate Level Phosphorylation

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