Outline of Campbell Biology Chapter 9

III. Glycolysis

  1. Introduction
    1. Glycolysis = (Glyco-sweet, sugar; lysis=to split) Metabolic pathway where six-carbon glucose is split into two three-carbon sugars which are then oxidized and rearranged by a step-wise process producing two pyruvic acid molecules.
      1. Occurs in all cells.
      2. As the two intermediate three-carbon sugars are converted to pyruvic acid:
        1. Two molecules of NAD+ are reduced to NADH.
        2. There is a net production of two ATPs by substrate-level phosphorylation.
      3. The ten steps of glycolysis occur in the cytoplasm and are catalyzed by specific enzymes, dissolved in the cytosol. These steps can be grouped into two segments:
        1. Preparatory steps.
        2. Oxidative steps.
  2. The Steps in Glycolysis: You do not have to memorize these reactions; but, you must understand the overall reaction.
    1. The first phase includes five preparatory steps where glucose is split in two. This process actually consumes ATP.
    2. Step 1: Glucose enters the cell and is phosphorylated on the number six carbon. This ATP coupled reaction:
      1. Is catalyzed by hexokinase. (Kinase is an enzyme involved in phosphate transfer.)
      2. Requires an initial investment of ATP.
      3. Makes glucose more chemically reactive.
      4. Produces glucose-6-phosphate. Since the plasma membrane is relatively impermeable to ions, the addition of an electrically charged phosphate group traps the sugar in the cell
    3. Step 2: An isomerase catalyzes the rearrangement of glucose-6-phosphate to its isomer, fructose-6-phosphate.
    4. Step 3: Carbon one of fructose-6-phosphate is phosphorylated. This reaction:
      1. Required an investment of still another ATP.
      2. Is catalyzed by phosphofructokinase, an allosteric enzyme that controls the rate of glycolysis.
    5. Step 4: Aldolase cleaves the six-carbon sugar into two isomeric three-carbon sugars.
      1. This is the reaction for which glycolysis is named.
      2. For each glucose molecule that begins glycolysis, there are two product molecules for this and each succeeding step.
    6. Step 5: An isomerase catalyzes the reversible conversion between the two three-carbon sugars. This reaction:
      1. Never reaches equilibrium because only one isomer, glyceraldehyde phosphate, is used in the next step of glycolysis.
      2. Is thus pulled towards the direction of glyceraldehyde phosphate, which is removed as fast as it forms.
      3. Results in the net effect that, for each glucose molecule, two molecules of glyceraldehyde phosphate progress through glycolysis.
    7. The second segment of reactions is where oxidation of the sugar occurs. These last five steps produce ATP and NADH.
    8. Step 6: An enzyme catalyzes two sequential reactions:
      1. Glyceraldehyde phosphate is oxidized and NAD+ is reduced to NADH + H+.
        1. This reaction is very exergonic (Delta G = -10.3 kcal/mol) and is coupled to the endergonic phosphorylation phase.
        2. For every glucose molecule, 2 NADH are produced.
      2. Glyceraldehyde phosphate is phosphorylated on carbon number one.
        1. The phosphate source is inorganic phosphate, which is always present in the cytosol.
        2. The new phosphate bond is a high energy bond at least as energetic as the phosphate bonds of ATP.
    9. Step 7: ATP is produced by substrate level phosphorylation.
      1. In a very exergonic reaction, the phosphate group with the high energy bond is transferred from 1, 3-diphosphoglyceric acid to ADP.
      2. For each glucose molecule, two ATP molecules are produced. The ATP ledger now stands at zero as the initial debt of two ATP from steps one and three is repaid.
    10. Step 8:
      1. In preparation for the next reaction, a phosphate group on carbon three is enzymatically transferred to carbon two.
    11. Step 9: Enzymatic removal of a water molecule:
      1. Creates a double bond between carbons one and two of the substrate.
      2. Rearranges the substrate's electrons, which transforms the remaining phosphate bond into an unstable high energy bond.
    12. Step 10: In this last step of glycolysis, ATP is produced by substrate level phosphorylation.
      1. In a highly exergonic reaction, a phosphate group is transferred from PEP to ADP.
      2. For each glucose molecule, this step produces two ATP.
    13. Summary of Glycolysis:

      The summary equation for glycolysis is:

      C6H12O6 + 2 NAD+ + 2 ADP + 2 P -->
      2 C3H4O3 (Pyruvic Acid) + 2 NADH + 2 H+ + 2 ATP + 2 H2O

       

      1. Glucose has been broken down and oxidized into two pyruvic acid molecules.
      2. The process is exergonic (Delta-G = -140 kcal/mol), and most of the energy made available is conserved in the high-energy electrons of NADH and the phosphate bonds of ATP.
      3. The first segment of glycolysis uses two ATP per glucose molecule.
      4. The second segment of glycolysis produces four ATP per glucose molecule, which is a net gain of two ATP per glucose. These ATP are produced by substrate-level phosphorylation.
      5. Glycolysis produces two molecules of NADH per glucose. Energy conserved in the high-energy electrons of NADH can be used later to make ATP by oxidative phosphorylation.
      6. Most of the chemical energy originally stored in glucose still resides in the two pyruvic acid molecules produced by glycolysis.