Glucose Metabolism - The breakdown of glucose takes place in the cytoplasm and within the mitochondria of the cell.  The purpose of this process is to provide the cell with additional ATP molecules for life activities.

Stage I  -  Glycolysis - occurs in the cytoplasm in three phases.

Fructose, 1-6 diphosphate
Text Box: Fructose, 1-6 diphosphate
Text Box: Glucose
    Phase 1: 


NET:  - 2ATP
Text Box: NET:  - 2ATP
This step costs the cell two molecules of ATP

   Phase 2:   


    2H  +  NAD+   
Text Box:     2H  +  NAD+    
2  Glyceraldehyde-3-phosphate (GAP)
Text Box: 2  Glyceraldehyde-3-phosphate (GAP)
Fructose, 1-6 diphosphate
Text Box: Fructose, 1-6 diphosphate

Text Box: PO4

2 PO4
Text Box: 2 PO4
Pyruvic acid
Text Box: Pyruvic acid
Text Box: GAP

NADH  +  H+
Text Box: NADH  +  H+

Phase 3:




Text Box: 2ADP


Text Box: 2ATP





The consequences of phase 3 are:

        1.  Each GAP molecule is oxidized by the loss of two electrons contained within the hydrogen atoms transferred to NAD+.

        2.  Each GAP molecule gains one phosphate then loses two phosphates.  This leads to the formation of two ATP molecules for each GAP molecule  metabolized.

            2 NADH + 2H+

Text Box: NET:  2 ATP
            2 NADH + 2H+
        3.  Remember that each glucose molecule produces two GAP.


Stage II  -  Krebs or Citric Acid Cycle

NADH  +  H+
Text Box: NADH  +  H+
2H  +  NAD+ 
Text Box: 2H  +  NAD+ 
Text Box: CO2
 Acetic acid
Text Box:  Acetic acid
    Each Pyruvic acid molecule must be processed into a substance which can enter the next series of reactions leading to the complete breakdown of glucose.  The reactions take place within the mitochondria.

 Pyruvic acid
Text Box:  Pyruvic acid







Acetic acid  reacts with coenzyme A to form Acetyl Co A.  Each Acetyl CoA enter the Kreb's cycle reactions.  Remember that each glucose produces 2 pyruvic acid and 2 acetic acid molecules.  The overall breakdown of acetyl CoA in the Krebs cycle can be summarized as follows:
                                 Acetyl Co A  +  3NAD+  +  1FAD


                      2 CO2   +   3 NADH   +   3H+   +   1 FADH2   +   1 ATP

NET:   4 ATP
           10 NADH
            2 FADH2

Text Box: NET:   4 ATP
           10 NADH
            2 FADH2




Keep in mind that two acetyl CoA molecules enter the Krebs cycle.  This doubles the products formed in the Krebs cycle.

     During glycolysis and the Krebs cycle reactions, hydrogen atoms are removed from glucose and its breakdown products.  These hydrogen atoms, along with their electrons, are received by two coenzymes, nicotinamide adenine dinucleotide (NAD+ ) and flavin adenine dinucleotide (FAD) .  When these molecules accept electrons they are said to be reduced and energized.  When they lose electrons they are oxidized and release energy.

Oxidation describes the loss of an electron by a molecule, atom or ion
Reduction describes the gain of an electron by a molecule, atom or ion
                               ("OIL RIG"—Oxidation Is Loss, Reduction Is Gain)







Stage III - The Electron Transport Chain

    During this series of reactions, each of the reduced coenzymes (NADH and FADH2) contributes a pair of electrons to a series of electron accepting compounds located on the inner membrane of the mitochondria.  NADH passes its electrons to three acceptor molecules in succession.  FADH2 only passes electrons to two acceptors.

From NADH:      

                                2H                        2H+   +   2e


    Each time the pair of electrons from NADH are released by an electron acceptor, energy is given off.  This energy is trapped in a molecule of ATP.  Since NADH passes electrons to three acceptors, 3 ATP are produced for each reduced NADH molecule.
For each reduced FADH2 molecule produced in the Krebs cycle, two ATP are produced.  The ten NADH molecules produced during glycolysis and the Krebs cycle give rise to 30 ATP in the electron transport chain.  The two FADH2 molecules give rise to 4 ATP. 
    The net gain of ATP from the breakdown of glucose is:

                                  Final Net:  4 ATP (direct)
                                                   30 ATP (from NADH)
                                               +   4ATP (from FADH2)

                                                    38 ATP (minus 2 ATP)  =  36 ATP net

     After being released by the last electron acceptor, the pair of electrons are accepted by oxygen atoms.  These atoms become charged (O2-).  The charged oxygen ions combine with the remaining hydrogen ions to form water.  The purpose of oxygen in the breakdown of glucose is to serve as the final acceptor of the electrons.