Common Pathways of Cellular Metabolism - 149 



mainly heme compounds, in which an atom oxygen per minute, which represents the 



of iron (Fe) occupies an important position How of some 2.8 X 10- 2 electrons in the given 



near the center of the molecule. In accept- time. However, the voltage drop between 



ing an electron, the iron is shifted to the the primary acceptors and oxygen is small 



reduced (ferrous) state, Fe 2 +; whereas in (about 1.2 volts). Accordingly, it may be cal- 



yielding up an electron it returns to the oxi- culated that, on an average, the oxidative 



dized (ferric) state, Fe 3 + . Thus we can repre- metabolism of a man generates energy at the 



sent the essential feature of these reversible rate of about 90 watts. This rate is roughly 



oxidation-reduction reactions quite simply: equivalent to the consumption of lamp bulbs 



commonly employed for household lighting. 



Fe ;i + + le- 



Fe 2 " 



The cytochromes change color as they al- 

 ternately undergo reduction and oxidation. 

 This is analogous to the darkening of the red 

 blood pigment, hemoglobin, when it is re- 

 duced by lack of oxygen, as it is in venous 

 blood. The intracellular color changes of the 

 cytochromes are so faint, however, that their 

 detection requires a spectroscope. If a spec- 

 troscope is directed upon a tissue that has 

 been deprived of oxygen, the spectra of the 

 reduced pigments are revealed. Then, when 

 adequate oxygen is restored, the character- 

 istic spectra of the oxidized cytochromes can 

 be detected. 



Although the quantity of primary acceptor 

 compounds and cytochromes in a cell is 

 small, the rapidity with which these agents 

 keep shifting back and forth between the 

 oxidized and reduced states provides for the 

 delivery of considerable energy. An average 

 man, for example, consumes about 260 cc of 



OTHER METABOLIC PROCESSES 



Oxidations and reductions, of course, do 

 not by any means account for the complete 

 metabolism of organic substrates in the cell. 

 Now, therefore, we must consider some other 

 important processes. 



Phosphorylation and Related Processes. 

 Although glucose is an excellent protoplas- 

 mic fuel, the glucose molecule must be acti- 

 vated before it is launched upon the meta- 

 bolic stream. First it receives energy from 

 ATP, undergoing phosphorylation, in the 

 presence of the enzyme glucokinase, as may 

 be indicated as follows: 



glucose -f- ATP 



glucokinase 



glucose-6-phosphate -j- AMP 



Fig. 8-5. (facing). Principal pathways of cellular metabolism. The path from glycogen (or glucose) to pyruvic 

 acid (or lactic acid) is generally called glycolysis; whereas the similar route to ethyl alcohol is fermentation— 

 both anaerobic processes. Note that the acetyl CoA complex stands at the crossroads where carbohydrate, 

 protein, and fat metabolism interconnect with one another. Also note that free energy liberated by oxidative 

 metabolism, mediated collectively by the enzymes of the Krebs cycle, the primary acceptors, and the cyto- 

 chrome system, is conserved by a build-up in the high energy phosphate (ATP) reserves of the cell. In short, 

 these phases of metabolism accomplish oxidative phosphorylation. Some intermediary steps, now well known, 

 have been omitted from the diagram. Below the level of triose phosphate, only one of the two molecules de- 

 rived from a glucose molecule has been followed. Actually, of course, two acetyl units are derived from each 

 original molecule of glucose. Three formulas not given above are: 



(1) Citric acid: CH..-COOH 



COHCOOH 



(2) Oxaloacetic acid: COH-COOH (3) Triose phosphate: H„C-H.,P0 4 



CHCOOH 



HCOH 



CH..COOH 



HC=0 



