Author:
Tina B. Jones
Subject:
Applied Science, Life Science, Biology
Material Type:
Module
Level:
Community College / Lower Division
Provider:
Rice University
Provider Set:
OpenStax College
Tags:
  • Acetyl CoA
  • Cellular Respiration
  • Circular Pathway
  • Citric Acid Cycle
  • Coenzyme a
  • Fumarate
  • Gtp
  • Krebs Cycle
  • Linear Pathway
  • Pyruvate Oxidation
  • Succinyl CoA
  • Tca Cycle
  • Tina B. Jones
    License:
    Creative Commons Attribution Non-Commercial Share Alike
    Language:
    English

    Oxidation of Pyruvate and the Citric Acid Cycle

    Oxidation of Pyruvate and the Citric Acid Cycle

    Overview

    By the end of this section, you will be able to:

    • Explain how a circular pathway, such as the citric acid cycle, fundamentally differs from a linear pathway, such as glycolysis
    • Describe how pyruvate, the product of glycolysis, is prepared for entry into the citric acid cycle

    Oxidation of Pyruvate Occurs During the Transition Reaction

    If oxygen is available, aerobic respiration will go forward. In eukaryotic cells, the pyruvate molecules produced at the end of glycolysis are transported into intermembrane space of the mitochondria, the organelle that carryies out cellular respiration, and broken down further in a process often referred to as the transition reaction or prep reaction. Once in the intermembrane space, the three-carbon pyruvate will be transformed into a two-carbon acetyl group and one molecule of CO2 gas which will diffuse back out into the body.  The acetyl group will be picked up and activated by a carrier compound called coenzyme A (CoA). The resulting compound is called acetyl CoA (made from vitamin B5).  Acetyl CoA can be used in a variety of ways by the cell, but its major function is to deliver the acetyl group derived from pyruvate to the next stage of the pathway in glucose catabolism.  Since two molecules of pyruvate travel from the cytosol into the intermembrane space, for every glucose metabolized this process produces two acetyl CoA, two CO2, and two NADH.  No ATP is produced during this process.

     

    A graphic shows pyruvate becoming a two-carbon acetyl group by removing one molecule of carbon dioxide. The two-carbon acetyl group is picked up by coenzyme A to become acetyl CoA. The acetyl CoA then enters the citric acid cycle. Three NADH, one FADH2, one ATP, and two carbon dioxide molecules are produced during this cycle.

    Figure 4.14 During the transition reaction, three-carbon pyruvate is converted into two-carbon acetyl-CoA before entering the citric acid cycle.  One molecule of CO2 and one molecule of NADH are also produced. 

     

    The Citric Acid Cycle Carries Out the Final Oxidation of Food Substrates

    The citric acid cycle in eukaryotic cells takes place in the matrix of the mitochondria. Unlike glycolysis, the citric acid cycle is a closed loop: The last part of the pathway regenerates the compound used in the first step. The eight steps of the cycle are a series of chemical reactions that produces two carbon dioxide molecules, one ATP molecule (or an equivalent), and reduced forms (NADH and FADH2) of NAD+ and FAD+, important coenzymes in the cell. Part of this is considered an aerobic pathway (oxygen-requiring) because the NADH and FADH2 produced must transfer their electrons to the next pathway in the system, which will use oxygen. If oxygen is not present, this transfer does not occur.

    Two carbon atoms come into the citric acid cycle from each acetyl group. Two carbon dioxide molecules are released on each turn of the cycle; however, these do not contain the same carbon atoms contributed by the acetyl group on that turn of the pathway. The two acetyl-carbon atoms will eventually be released on later turns of the cycle; in this way, all six carbon atoms from the original glucose molecule will be eventually released as carbon dioxide. It takes two turns of the cycle to process the equivalent of one glucose molecule. Each turn of the cycle forms three high-energy NADH molecules and one high-energy FADH2 molecule. These high-energy carriers will connect with the last portion of aerobic respiration to produce ATP molecules. One ATP (or an equivalent) 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 both anabolic and catabolic.

    A graphic shows pyruvate becoming a two-carbon acetyl group by removing one molecule of carbon dioxide. The two-carbon acetyl group is picked up by coenzyme A to become acetyl CoA. The acetyl CoA then enters the citric acid cycle. Three NADH, one FADH2, one ATP, and two carbon dioxide molecules are produced during this cycle.

    Figure 4.14  The right half of the diagram shows the products of one turn of the citric acid cycle:  one ATP, three NADH, one FADH2, and two CO2.  The citric acid cycle must turn twice for every glucose metabolized.   

     

    Link to Learning

    QR Code representing a URL

    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 FADH2 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 FADH2 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.

    Review Questions

    What is removed from pyruvate during its conversion into an acetyl group?

    A.  oxygen

    B.  ATP

    C.  B vitamin

    D.  carbon dioxide

    Hint:

    D

    What do the electrons added to NAD+ do?

    A.  They become part of a fermentation pathway.

    B.  They go to another pathway for ATP production.

    C.  They energize the entry of the acetyl group into the citric acid cycle.

    D.  They are converted to NADP.

    Hint:

    B

    Per glucose molecule taken in by the cell, how many molecules of  ATP are produced during the citric acid cycle?

    A.  1

    B.  2

    C.  3

    D.  4

    Hint:

    B

    How many NADH molecules are produced on each turn of the citric acid cycle?

    A.  one

    B.  two

    C.  three

    D.  four

    Hint:

    C

    Free Response

    What is the primary difference between a circular pathway and a linear pathway?

    Hint:

    In a circular pathway, the final product of the reaction is also the initial reactant. The pathway is self-perpetuating, as long as any of the intermediates of the pathway are supplied. Circular pathways are able to accommodate multiple entry and exit points, thus being particularly well suited for amphibolic pathways. In a linear pathway, one trip through the pathway completes the pathway, and a second trip would be an independent event.