ATP Synthase MiraCosta Biology

Uploaded by MeerdesIrrtums on 17.11.2011

I’m hoping that at this point you’re saying to yourself, “Gee, that’s great—now
I understand why I have to breathe all the time—it’s because my cells need ATP energy
and the oxygen goes into getting all of the energy out of the food molecules that my body
is processing for energy. I also have to get rid of all the CO2 that my cells have left
over after oxygen degrades the food.”
But as I mentioned at the end of the previous video, we have covered only half of what goes
on inside of the mitochondrion.  The other half is even more spectacularly cool, and
it basically is about how the mitochondrion drives ATP synthesis by using an electrical
gradient--that's right, just like a battery.  Every mitochondrion in your body is like a
rechargeable battery that powers the phosphorylation of ADP—which is the same thing as making
ATP). A little bit of the mitochondrial battery’s electrical potential is used every time an
ATP forms, while simultaneously that electrical potential is being re-charged from the energy
that we talked about previously—the energy we get from the oxygens pecking away at the
food fragments that are coming into the mitochondria from the cytosol. 
It works like this. The Krebs cycle works together with another set of proteins called
the electron transport chain, and these are in the inner membrane of the mitochondrion.
Hydrogen ions, shown here as H+, get hauled out of the inner part of the mitochondrion
and stuffed into the space in between the inner and outer membranes--notice how mitochondria
are always drawn showing two membranes?  Well we actually call those the inner membrane
and the outer membrane. The movement of all these positive charged hydrogen ions from
the inner space into the space between the membranes is enough to create a pretty significant
voltage across the mitochondria’s inner membrane—“voltage” here refers to the
difference in overall charge between two sided of a membrane. The mitochondrion’s inner
area--called the inner matrix, if you want to use the proper term—is quite negative
compared with the pretty strong positive charge in the space between the two membranes--which
we'll call the "intermembrane space."
This charge difference is compounded with a chemical gradient because of the higher
and lower concentration of hydrogen ions, and this combination of a chemical and an
electrical gradient represents a large amount of potential energy. Note how the mitochondrial
inner membrane is the boundary that separates the two sides of this double-gradient. On
one side you have the inner matrix with a low concentration of hydrogen ion and a negative
charge, while on the other you have the intermembrane space with its positive charge and high concentration
of hydrogen ion. This is our cellular “battery” in a literal and completely non-metaphorical
sense. At this point you might want to review what this means with respect to the pH on
either side of the inner mitochondrial membrane.
But we can’t really use a literal battery to meet our cells’ energy needs—it’s
not like they’re cheap electronic doo-dads from overseas markets—you need energy in
the form of ATP. Well in the inner mitochondrial membrane there's a fabulously complex protein
called ATP synthase that first attaches to ADP and phosphate. Then it allows a tiny bit
of the mitochondrial battery’s electrical potential to discharge, and it uses the current
(or amperage) that results to reorganize the bonds so that we now have an ATP.
Now realize that there is a lot of ATP that is forming at any given time through this
process—based on this calculation, there’s an average of ten million of molecules of
ATP used and produced every second in the human body—and this on its own would result
in the mitochondrial batteries going dead very quickly once all of the energy potential
got used up by the ATP synthase as it cranks out the ATP.
Well, this doesn’t ever really happen because there’s a continual re-charging of the mitochondrion’s
electrical potential by the activity of the Krebs cycle and the electron transport chain,
and these processes depend on a steady inflow of pyruvic acids and oxygen.
The textbook will likely be giving you a “bottom line” analysis for the aerobic metabolism
of glucose, in which you’d be keeping track of this many ATPs forming here and here, and
another big wad of ATPs forming over here---we aren’t going to bother with that approach.
I would actually prefer for you to carry away a mental image of the mitochondrion as this
sort of battery that is constantly subject to two continual forces:
A) a draining of the battery’s energy by the production of ATP by ATP synthase; and
B) a recharging of the battery’s electrical potential from the energy released as oxygen
pecks away at all the energy contained in the parts of glucose that it gets from the
If you ever get around to taking higher-level biology classes I’m sure you’ll get around
to the step-wise analysis of these fueling reactions. For now though, we’ll leave things
here and start talking about ways in which these metabolic processes have direct relevance
to our lives as non-biologists.