How Cells Make ATP...  denovo pathways for bases and ribose sugars or salvage reactions.
 Salvage: by PHOSPHORYLATION... adds a phosphate to ADP
ADP   +   P    <------>    ATP     (recycle*)

  cell ratio of ATP/ADP is about 10:1 (a non-equilibrium state)
   How do cell's phosphorylate ADP to make ATP?  

a)  Substrate Level Phosphorylation...
                 where a substrate molecule ( X-P
) donates*  its P to ADP making ATP     fig 9.6
        b) Oxidative Phosphorylation...
involves capturing e-'s for use in Electron Transfer Chain...
1. food molecules donate e- & protons to acceptor molecules via oxidation NADH*
2. NADH passes on electrons & protons are pumped out of mitochondria...  
                                                (or the
chloroplast membranes in photosynthesis);
protons diffuse back into mitochondria thru an enzyme = ATP synthase,
ATP synthase enzyme makes ADP + -->  ATP                                figure
        c) Photophosphorylation...
e- of light energy, instead of food covalent bonds, are captured by chlorophylls
                    to make a
proton gradient
across the chloroplast membranes...                figure* 
66.gif (249 bytes)            protons move through a chloroplast ATP synthase enzyme to make ATP








1st -
Oxidative Metabolism...     (Cell Respiration in animal cells)
occurs in heterotrophic organisms that consume foods
                      ... we say organisms
oxidize (consume) foods (often glucose) to make energy
                                                        because it
removes & captures electrons...
                      ... where is energy in foods?     it's in the
covalent bonds (e-)
thus =  oxidative metabolism is...   cells capturing e- via REDOX reactions
        what is a REDOX  REACTION
*...       methane example - fig 9.2*
                      in step 6 of glycolysis  e- passed  from   PGAL --> to  diPGA*
                      energy is transferred via
a redox couple with NAD+ by e-

        OXIDATION     =    removal of electron &/or protons from food covalent bonds

        REDUCTION   =    gaining electron &/or protons; adds an electron to an
acceptor molecule

    Redox state* of molecules     -->     the more reduced  =  the more energy it holds 

 67.gif (173 bytes)        








a model redox reaction

      A-H   +  B-O       
<--->           A     +        B-O-H

         donor         acceptor          (:H)              acceptor                  donor

Let's look at a redox reaction in the Krebs cycle pathway using NAD+.

         isocitrate + NAD+  < --- >   alpha-ketoglutarate
*      +       NADH

              reducing    oxidizing                       becomes                 becomes
                agent         agent                          oxidized                  reduced

              a 2nd redox coenzyme (couple) of living cells is   FAD+  <-->  FADH2 
                                                                                                      acceptor       reduced

        Transition metals [metallic elements with unfilled valances that accept e's...
        Cu, Fe, Zn, Mn] and may have been involved in origin biological redox reactions
        and evolved into today's oxidoreductase (dehydrogenase) enzymes?
Structural similarity suggests an early origins of biological electron transfer* 

Thus : heterotrophic metabolism is the stepwise OXIDATION of molecules (foods)...
                    if AEROBIC - directly requires oxygen directly as the electron acceptor
                    if ANAEROBIC - requires no oxygen directly (uses other e- acceptors) 

   69.gif (256 bytes)       









  CELL RESPIRATION...  is the oxidation of food stuffs that produces ATP.

                                      Overview of Cellular Respiration


1.  oxidation of GLUCOSE  (6C)    -->    PYRUVATE   2 (3C)   -->   GLYCOLYSIS

.  oxidation of pyruvate --> CO2 + H2O     
                               --> KREBS CYCLE                           

      3.  reduction O2  to  H2O                              --> ELECTRON TRANSPORT CHAIN

C6H12O6 + 6O2   <---->   6 CO2 + 6 H20    + e- --->    36-38 ATP

ΔG  =  -686 Kc/mole                                              -270Kc  = 39%

                     called oxidation...  because e- are removed from glucose
                     called reduction...  because e- passed to O2 making water

      4.  phosphorylation of ADP  via  ATP synthase
                     thus the terminology... oxidative phosphorylation  [chemiosmosis]

 70.gif (264 bytes)                Cell Respiration is a multi-pathway process*...     









 a more complete description of Cell Respiration :
    -  series of enzyme rx's (in biochemical pathways) in the cytoplasm & mitochondria
                   (glycolysis, fermentations, pyruvate oxidation, Krebs cycle, electron transport),

    -  removes
e- (oxidation) from covalent bonds of substrates (as glucose
), and
    -  pass e- to acceptor molecules [redox coenzymes] such as NAD+ & FAD*
                   which become reduced to 
    -  the reduces these coenzymes [ NADH & FADH2 ]  passing e- to other
e- acceptors...
                   a series of metallo
protein Electron Transport carriers, including cytochromes,
    -  the electron carriers (
cytochromes) pass e- to O
2  --- reducing it to --->   H2O
    -  while
pumping protons [H+]  out of mitochondria into the peri-mitochondrial space,
                    creating a proton gradient as an diffusive energy source

    -  the protons move back into mito thru a special enzyme
ATP synthase)  &  make ATP

71.gif (292
                         let's look at the details of each of these processes...                     






all the details  PATHWAYS of CELL RESPIRATION which make ATP...
           what should we know - what goes in,  what comes out, and where,
           also the types of chemical reactions that occur in the pathways.

  Glyco-lysis : 10 step pathway converts 1 glucose (C6) to 2 pyruvate (C3)
        products :  2 molecules of pyruvate,   2
NADH,   &   2 ATP (net)
        occurs in :  cytoplasm  
[and is anaerobic - does not require O2]
           may lead to:
   alcoholic fermentation     =   glucose --> alcohol
                lactic acid fermentation
   =   glucose --> lactic acid

  KREBS Cycle  :   oxidizes  2 pyruvates  from glycolysis  to   CO2  +  H2O
                              produces :         8 NADH,   2 GTP (ATP equivalent),    2 FADH2
                              releases :          6 CO
2   (thus called...  Cell Respiration)
                              occurs in the mitochondria 
[and is aerobic = directly requires O2]

  ETC - Electron Transport Chain  :   uses e- carrier proteins (including cytochromes, etc...) 
                   to pass  
e- & H+  from  NADH & FADH2  to   O2   making H2O   &
                   generates a
proton gradient
(chemiosmosis) across the inner mitochondria membrane

  ATP synthase :  a carrier enzyme in the inner mitochondrial membrane that
                            passively transports 
H+  back into mitoplasm & while making ATP directly
72.gif (228 bytes)



















KEY ENERGY REACTIONS of GLYCOLYSIS...     Animation of Glycolysis
                                            don't memorize the pathway, but learn the major reaction types...
substrate level phosphorylation* [occurs twice* in glycolysis]
 2.  redox reaction   step 6*  involving   NAD+    
3.  reaction paths -->    investment phase*  &  payoff phase*  -  Summary of glycolysis* 

FATES of the products:           
             if  anaerobic - involves alcoholic fermentation  &   lactic acid respiration
                  alcoholic fermentation*          "history of wines" 
                  lactic acid fermentation*
also called anaerobic respiration 
            if  aerobic - involves pyruvate dehydrogenase  +  Krebs Cycle  +  ETC 

2. cytosolic NADH... - it holds captured e- energy, but how some cells can use it?
         mito membrane is
impermeable to NADH -> cytoplasmic & mitoplasmic pools
        Shuttles*  malate shuttle        (liver, kidney, heart) =   NADHc --> NADHm 
                        glycerol-P shuttle   (muscle/brain)          =   NADHc --> FADH2m
        Purpose:  to move electrons captured in cytosolic NADHc into mitochondria


   77.gif (328 bytes) 







2 ATP to initiate
              -  2 substrate level phosphorylation steps  =  4 ATP gross (2 ATP net) and
 - 1 redox step
making NADH

                thus Glycolysis makes
ATP (net),
NADH, and
                                    remember the role of the ...  Fermentations &

               END OF MATERIAL FOR EXAM #3 (electron microscopy to here) 


      76.gif (316 bytes)                                                    the Glucose songlisten at home










Heterotrophic Metabolism (cell respiration) in Aerobic Organisms...
               if aerobic then the
Fate of Pyruvate
*  is oxidation in mitochondria

1.  PYRUVATE DEHYDROGENASE Reaction... [before Krebs cycle itself]
mitoplasm (Fig 9.10)*  oxidizes  PYR --> acetyl-CoA
            a multienzyme complex of 60 proteins and 5 coenzymes
CoASH*    --ADP
*-->  acetyl coenzyme-A [Fritz Lippman]
reactions:  1.   decarboxylation (-CO2),
                              2.   reduction of 2 NAD+ 
-->  2 NADH
                              3.   acylation
with synthesis of 2 Acetyl-coA  .

2. KREBS CYCLE [Sir Hans Krebs]   animation of cycle*  &   cycle summary* 
           Key Reactions   
             full cycle details
       &   enzymes involved     
            1.   NAD
+ is reduced  (6 NADH)   and   FAD is also reduced (2 FADH2)
            2.   substrate level phosphorylation occurs  (
2 GTP <--> 2 ATP)
            3.   decarboxylation occurs -4x [
* 2 acylation reactions via coenzyme-A  (forms 2 Succinyl-coA)  
  75.gif (329 bytes)   -->   how many actual ATPs per glucose are made so far?  only 4 (net) by SLP      







    the coupling of
oxidation of substrates (-e) to phosphorylation of ADP to make ATP

 remember, most of the energy of glucose's bonds is now carried in NADH & FADH2  
e- are passed from NADH/FADH2  to  O2 (aerobic   ETC animation*       
         these series of electron carrier proteins occur in 4 membrane subunits  fig 9.14*   
  NADH Reductase,                   II )    Succinate Dehydrogenase
Cytochrome Reductase,
         IV)    Cytochrome Oxidase

: hypothesis of e- transport createing a PROTON MOTIVE FORCE  -  figure
           e- carriers release protons into perimitochondrial space making a proton gradient.
           1978 Nobel Prize (Chemistry) to Peter Mitchell for Chemiosmotic Hypothesis.

           Can a hydrogen ion gradient (H+) created by  e- flow thru the ETC also make ATP ??? 
                experimental evidence of a   H+ gradient  making ATP came via  bacteriorhodopsin*
H+ diffuse back into mitoplasm thru ATP synthase  --->   ATP via a molecular motor*
               -  Boyer hypothesis*chemical to mechanical chemical energy (like a water wheel)*
  animation of ATP synthase action* 

74.gif (215 bytes) 









Overview of Cell Respiration
       A rule of thumb for the amount of ATP made per e- pair...                   
                  the Phosphate/Oxygen Ratio, or
(P/O ratio), refers to the amount of ATP produced
                  from the movement of 2 e's through the ETC to the reduction of an oxygen atom.    

                  Measured empirically it is  -->    NADH
 ≈  3 ATP     &      FADH2    2 ATP

        So How much ATP can be made per Glucose...
                  the ATP/O2 ratio...  
   based on 3 ATP per NADH  &  2 ATP per FADH2
                                                     idealized =
36 to 38 ATP      [ a yield table ]

        What can serve as substrates for aerobic metabolism?*
                carbohydrates, proteins, lipids... can serve as
        Aerobic metabolism intermediates are precursors for other molecules
        How is heterotrophic metabolism regulated?  
                1.  stiochiometric [
substrate concentration] levels is a main control mechanism
                2.  allosteric controls include:  +
AMP/ADP stimulate enzymes  &  +ATP
inhibits enzymes
                        Key mammalian allosteric enzyme :  
                                                                                       feedback inhibition
                                                                                       allosteric regulation
   74.gif (215 bytes)  






- Heterotrophic Metabolism - CELL RESPIRATION

    1. Substrates  =  sugars,   amino acids acids,   fatty acids

    2. Pathways  =  Glycolysis,  Krebs Cycle ETC,  &  ATP synthase are Universal to cells

    3. Products  =   C02, H20, and energy as  H+ gradient,   NADH,   FADH2,  &   ATP

    4. part of process is Anaerobic (doesn't require -02 ; Glycolysis)
                    & may include alcohol & lactate
fermentations    (...anaerobic respiration)
                    & part is 
Aerobic    (requires +02  ...electron transfer chain

 5. Reaction types include:
 oxidations,    reductions,    substrate level phosphorylations,
decarboxylations,    acylation,   oxidative phosphorylation,
hydrolysis  (or dephosphorylation)

72.gif (228










   6. Energy capture is via   electron transfers, 
proton pumps,  H+ gradients,  ATP synthase*

   7. Regulation is by:
                stiochiometry,  feedback inhibition  &  allosteric modulation 
                (key enzyme: 

   8. Intracellular compartmentation:      
glycolysis occurs in the cytosol
Krebs Cycle is mostly in the mitoplasm of mitochondria
ETC is in the cristae
membranes of mitochondria
                ATP synthase within the inner mitochondrial cristae membrane

   9. Cell Respiration is KEY and central to all of a cell's metabolic pathways
IUBMB-Nicholson Metabolic Pathways Chart

    back      next lecture* - photosynthesis                    a paradigmKey Concepts*

      copyright c2023     Last update -  March  2023
        Charles Mallery,    Biology 150, Department of Biology,   U. of Miami,  Coral Gables, FL 33124

















 If you have comments or suggestions, email me at

 oxidative phosphorylation animation

-   Individual reaction steps of glycolysis


    anim ATP synthase rotary mech - side  & top
ATP synthase & H-Gradient make ATP*view@home  &  BioVisions-Powering the cell*view@home      

     animation of ATP synthase action

      know what goes in & come out*

                                  Cancer reprograms energy metabolism

     "what are these carrier molecules*"