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Electron transport system and chemiosmosis

Now finally we are about to obtain the ATP necessary for survival.  To do this it is necessary to use the reduced coenzymes, NADH and FADH2, which were formed in the citric acid cycle.  These go to the electron transport system in the mitochondrion where they pass their electrons to the carriers in the system.  The coenzymes become oxidized FAD and NAD and return to a citric acid cycle to be reduced again. 

NADH deposits its electrons to the first molecule in the carrier system.  The electrons lose energy as they pass down the electron transport system.  Some of this energy is used to pump protons (hydrogen ions) into the outer compartment of the mitochondrion.  In this way we build up a  proton gradient.  By now this situation should be familiar to you.  The protons pass through F1 particles and phosphorylate ADP to ATP.  For every two electrons passing through an F1 particle, one molecule of ATP is formed.   The animation shows that two protons are pumped at the beginning of the transport system, two more are pumped in the middle and a final two are pumped at the end.    For each NADH passing electrons to the electron transport system six protons are pumped into the outer compartment of the mitochondrion.   These six protons can generate three ATPs so each NADH is worth three ATP molecules formed by chemiosmosis

Now we are ready for the answer to the question asked earlier.  What is the function of oxygen in cells?  Watch the animation and you will see that it is oxygen that removes the electrons from the electron transport system.  Without oxygen to grab the electrons from the last carrier in the system, the electrons remain attached to the carrier.  The next electrons cannot reach the occupied carrier, so they are also stranded.  In this way the entire system backs up.  Finally there is no way for the NADH to hand over its electrons and it remains reduced instead of returning to the citric acid cycle.  The citric acid cycle will eventually stop operating and the entire ATP production in the mitochondrion will cease.  Without sufficient ATP, a cell depending upon mitochondrial ATP for energy  will die.   When enough cells die, the entire organism will die.  This is why active organisms such as human beings have evolved elaborate circulatory and respiratory systems to bring oxygen to their cells.  Your heart, blood vessels, red blood cells, lungs, and breathing apparatus all evolved to enable you to get oxygen to the mitochondria in your cells so that it could remove electrons from the mitochondrial electron transport systems. 

The next animation shows what happens when FADH2 brings its electrons to the electron transport system.  This coenzyme does not have enough energy to climb up the energy hill to the place in the system where the first two protons were pumped by electrons coming from NADH.  Instead it brings them to the place where the second pair of protons is pumped by NADH electrons.  As a result, only four protons are pumped into the outer compartment of the mitochondrion for each FADH2 reaching the electron transport system.  These four electrons can generate two ATPs so each FADH2 is worth two ATP molecules formed by chemiosmosis.  

Now lets do a little arithmetic and see how much ATP is generated, on the average, by each molecule of glucose aerobically oxidized. 

From glycolysis Protons pumped ATP
2 NADH 8-12* 4-6*
2 ATP (substrate level phosphorylation) 2

From bridge stage

2 NADH 12 6
From citric acid cycle
6 NADH 36 18
2 FADH2 8 4
2 ATP (substrate level phosphorylation) 2
TOTAL 36-38

    * The NADH that comes from glycolysis has to enter the mitochondrion in order to hand its electrons over to the electron transport system.  There is usually a loss of energy involved in doing this.  The cytoplasmic NADH usually hands its electrons to FAD inside the mitochondrion.  The FADH2 then hands the electrons to the transport system and only four electrons are pumped for each NADH that is produced by glycolysis.  So, NADH that is produced by glycolysis usually generates only 2 ATP by chemiosmosis. 

Remember back when you were learning about photosynthesis and we calculated the approximate number of ATPs necessary to make a molecule of glucose from carbon dioxide.   We saw that it took approximately 54 ATPs.  We get 36-38 ATPs back when we convert glucose back to carbon dioxide.  The second law of thermodynamics is operating again!  Nevertheless, this is a very efficient system.  No man made machine can operate anywhere near the 67% efficiency of a cell.  A car giving excellent mileage operates at about 15-20% efficiency, so cells do very well indeed.

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copyright June B. Steinberg, 2000