Electron Transport System
|In this section we'll focus on how the hydrogen and electrons combine with
oxygen to form water. This involves a series of oxidation-reduction reactions, in which
elctrons (and hydrogen) are tranferred from one chemical to another to another in what is
called the electron transport system. It comprises the lower left-hand
portion of this diagram.
Oxidation - Reduction
|Let's return to one of the reactions in which hydrogen atoms along with
their electrons are removed from a molecule (step 9 in the citric acid cycle). You should
know from your earlier work with oxidation-reduction reactions (in CH-105) that in a
reaction like this hydrogen and the electrons don't just simply leave, they have to be
removed. The reactant in this process is being oxidized. That doesn't happen by itself,
there has to be an oxidizing agent.
|Two oxidizing agents that operate in the citric acid cycle
are simply referred to as FAD and NAD. Each one is a fairly complicated molecule and each
is a nucleic acid derivative. (We will study nucleic acids in Lesson 9.) The structures of
these compounds can be found in your textbook or other references.
stands for flavin adenine dinucleotide and it is synthesized in our
bodies from the vitamin riboflavin. Its oxidized form is FAD
and its reduced form is FADH2. Note that FAD is reduced by
taking on two hydrogens and two electrons.
NAD stands for nicotinamide adenine dinucleotide and
it is synthesized in our bodies from the vitamin niacin. Its oxidized
form is NAD+ and its reduced form is NADH.
Note that NAD+ is reduced by taking on two electrons but only one hydrogen.
|NAD is the oxidizing agent that works with this reaction.
Two hydrogens and two electrons are removed from the reactant (malic acid) to oxidize it
into the the product (oxaloacetic acid). NAD takes both of the electrons, but it takes
only one of the hydrogens. The other hydrogen, without its electron, floats around in
solution as an H+ ion.
When we first dealt with oxidation-reduction reactions (in CH-105), we didn't deal with
electrons coming off along with hydrogen atoms. Instead, we dealt with electrons coming
off from simple metal atoms.
|At that time, we were able to develop an oxidation potential list that
rated these chemicals with regard to how easily the electrons could be pulled off. We were
even able to assign voltages to reflect how readily those electrons could be removed.
|We developed the idea that in an oxidation-reduction
reaction, not only did oxidation take place, but so did reduction.
|For example, a chemical such as a potassium atom would
lose electrons only if there were something available to take electrons, such as a sodium
ion or a magnesium ion. So in order for potassium reaction to
proceed to the right, a reaction further down on the table, such as sodium, has to proceed
to the left. In essence, the potassium atom (K in top box)
transfers an electron to a sodium ion (Na+ in top
box) and converts it to the sodium atom (Na in lower box).
The electron has been passed from potassium to sodium.
Na ¬ Na+
Mg « Mg2+
|K ® K+
Na ¬ Na+
Mg « Mg2+
|Once that has happened, the sodium in its reduced form with the electron (Na in new top box) can now react with something else that is lower
than it on this oxidation potential list, such as Mg2+
(in the same box). (Yes, that would actually have to happen twice in order for the
magnesium to become reduced because it requires two electrons.) The electron(s) has
been passed from sodium to magnesium.
|K « K+
Na ® Na+
Mg ¬ Mg2+
|K « K+
Na ® Na+
Mg ¬ Mg2+
|I hope you can visualize that if the process could be
controlled in just the right way, electrons could be passed stepwise from
potassium to sodium to magnesium (and right on down the line, if we had more redox
chemicals lined up). Each exchange is an electron transfer reaction, and the sequence of
reactions would be an electron transfer (or transport) system. A voltage can be calculated
for each step and that voltage is a measure of the amount of energy released
in each step.
|These same ideas apply to biochemical oxidation-reduction
reactions. Oxidation potential lists for biochemical compounds have been figured
(even with voltages, but we won't need those).
|In the reaction that we were just looking at, the starting compound lost
two hydrogens along with its electrons to form a double bond and released two hydrogens
and electrons. Essentially, the hydrogens and their electrons have been transferred to the
|In turn, the NAD will transfer those hydrogens and electrons to another
chemical (FAD) and as soon as it has given those electrons to another chemical, it's free
to react with another molecule and take its hydrogens and electrons. The FADH2,
in turn, will pass those hydrogens and electrons on to other biochemical redox compounds.
|The sequence of chemicals (in part "disguised" here as X and Y)
which transfer electrons and hydrogen atoms from one to the next to the next to the next
is referred to as the electron transport system. Ultimately, those
electrons and the hydrogen atoms are transferred to oxygen to make water.
|Just as with the inorganic oxidation-reduction reactions,
every time electrons are transferred some voltage (or energy) is provided. With an SOP
list you are able to calculate the voltage. Similarly, in the electron transport system,
as the electrons and hydrogen atoms are transferred from one chemical to another, some
energy is provided at each step along the way. That is the energy that living things use
to carry on their activities.
"Storing" Energy in ATP
|One of the things for which that energy is used is to convert a chemical
known as ADP (adenosine diphosphate) into a molecule of ATP
(adenosine triphosphate) by adding a phosphate group to it.
|The reaction can be reversed and the process of converting ATP
back to ADP can release energy. ATP is quite often represented
as A-P-P~P where the "~" is characterized as a
"high energy bond" which can be broken and release energy that is needed for
biological processes. There is a problem with this kind of representation.
energy + A-P-P + P ®
A-P-P~P ® A-P-P + P +
The problem here is that this implies that the formation of the last phosphate bond
requires energy, and that breaking that bond releases energy. Now if you think about that
from a chemical point of view, it shouldn't make any sense at all. That is because the formation
of bonds releases energy and breaking bonds requires
energy. Just the opposite of what is implied here. The resolution of that dilemma involves
taking a closer look at the molecules that are involved in this reaction.
|Shown here are the structural formulas for ADP and ATP and phosphoric
acid. In biochemistry, phosphoric acid is quite often represented by Pi
to mean inorganic phosphate. I have chosen to use molecular formulas in this equation even
though, at biological pH, these chemicals will be partially ionized.
|Notice that when the hydrogens and oxygens are taken into
account, the nature of this reaction looks a bit different. In order to change from ADP
to ATP, a dehydration reaction occurs in which an -H
and an -OH are taken from the ADP and the Pi
to form a water molecule.
|In summary, bonds need to be broken and formed in order
for this reaction to occur. This reaction does require energy and it is endothermic.
However, the endothermic portion of the reaction is in breaking
bonds (indicated by / in the diagram), not in forming the new "high
In a biochemical situation where energy is required, an ATP molecule
can be hydrolyzed to form ADP and the inorganic phosphate. This reaction
will require breaking the "high energy" bond and also a bond in the water
molecule. But two new bonds will be formed (in the ADP and in the Pi), which
apparently are stronger than the bonds that were broken. For that reason,
this reaction is exothermic and this process will release energy.
That released energy can be used by enzymes to force reactions to take place that are
necessary for a living system to carry out the functions that it needs to carry out.
Electron Transport System in Perspective
Let's again place the electron transport system in the overall context of the the
oxidation of fats. It is the portion of the process in which the hydrogen and electrons
that have been stripped from the fat molecules are shuffled along to an eventual
combination with oxygen to make water molecules. The energy released in that process is
temporarily stored (in ATP and other "high energy" compounds) for use by enzymes
to catalyze endothermic biochemical reactions.
|This diagram shows that process in the lower left hand portion. That,
however, is a gross oversimplification because hydrogen and electrons are removed from the
breakdown products of fat molecules at many steps throughout this overall process.
Electrons and hydrogen from all of those steps are run through the electron transport
system to ultimately combine with oxygen and become water.
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