The last time we were talking we were
discussing the Krebs cycle and we had generated a ton of NADH and FADH2
molecules, and I promised we would explain how those molecules are used to
generate ATP which is a form of energy that the cell can use in all the
chemical reactions that it needs to undergo. So we want to do that today and
we also want to discuss an alternate way that cells can get energy from
carbohydrates and that's called fermentation. So let's get to it! The electron transport chain is a series of proteins that are embedded in the plasma
membrane of bacteria, and they're in the inner mitochondrial membrane of the
mitochondria. And these proteins they do exactly what the title says - they
transport electrons and these electrons come from NADH and FADH2. And as the
electrons travel down the chain and it's shown here as a set of stairs - in
reality these proteins are just lined up next to each other in the membrane. But
this is a nice illustration because it shows how the proteins in the beginning
of the electron transport chain, the electrons are at a higher energy level
and as they go down the electron transport chain they go to to
successively lower energy levels until they ultimately land on oxygen which is
called the ultimate electron acceptor because it's going to accept the
electrons that were originally from NADH and FADH2. And it's going to form
water when this happens so that's one of the
byproducts of respiration. If you exhale and, you know, take your
glasses and breathe on them, and get layer of moisture and that moisture
comes from the water - that's part of your respiratory process. Now as these
electrons travel down these transporters they're releasing energy, and the cell is
coupling that energy released to the production of ATP, and that's what the
electron transport chain is all about. It's about taking the energy that was
saved up in these electron carriers NADH and FADH2 and converting it into ATP in the
form of cellular energy that the cell can use to do the work that it needs to
do. This is a picture of the electron
transport chain - it's a little bit more true to life. Here's our
complexes and so we're not going to memorize the names of these but there
are some big picture things you should know. This is really the top of the
stairs, so this has the higher - the electrons in this complex are at the
highest energy state, and then as they travel through complex 2 3 & 4 they
gradually are lowering their potential energy until they ultimately land on
oxygen to form water. Now as we go down the steps or these
these electron transporters we have to capture the energy somehow that they're
releasing. The cell does that by using that energy to force protons into the
extra-cytoplasmic space of a bacteria or the inner membrane space of the
mitochondria. Remember when we talked about active transport? We said that when
you try to move molecules from low concentration to high concentration it
the cell has to use ATP to do that. Well this is similar to that except we're not
using ATP, we're using the energy that was in those electrons. So as they travel
down the electron transport chain the energy that was in those electrons is
used to force protons into this inner membrane space. So you end up with a high
concentration of protons here and a low concentration of protons in the inside
of the mitochondria or the inside of the bacterial cell. And once you have a
gradient like this, this is a source of potential energy and so the cell can
then use this gradient to make ATP. So this is another picture here so just
showing NADH and FADH2 and how they are releasing their electrons, and these
electrons are transported (the red arrows showing you how they get transported)
through these electron transporters and the result of the release of energy is
the pumping of protons, and again in bacteria this is outside the cytoplasmic
membrane. This is actually a picture from your book. I do want to say one more
thing that's actually illustrated on both the last slide let's go back for a
sec here. Notice that NADH puts its electrons
here in Complex I, and FADH2 deposits its electrons in Complex II - so this is
higher up on the stairs, if we think of this as a set of stairs, than the FADH2
and this would make you think that NADH has more potential energy than FADH2 and
you'd be right! For every NADH molecule, the cell generates about 3 ATP. For every
FADH2 molecule, the cell generates about 2 ATP. And you can see the same idea here,
that the NADH enters the electron transport chain earlier than FADH2 and
if we think of this as a set of stairs, NADH is higher up on the stairs than FADH2.
The higher up you are, the more potential energy you have, right? If you're standing
on a cliff, the taller it is the more potential energy - the harder
you'll smash if you land. Don't do that! But you get the idea, so FADH2 has less
potential energy than NADH. Regardless each one of these three
carriers here is able to pump out protons. It's not all of them some of the
intermediates don't, but these three do, and by the time the NADH and FADH2
electrons have deposited on oxygen to form water, we have lots of these protons
in this - forming this gradient again in a bacterial cell it would be outside the
cytoplasmic membrane and then underneath the cell the cell wall. And now the cell
can control this gradient, because these protons can't just get across the
bilayer by themselves they need a transporter protein. But this isn't any
random transporter protein, this is a special molecule called ATP synthase and
it regulates allowing these protons to go back through from high concentration
to low concentration. That's a spontaneous process that
releases energy. The ATP synthase allows the protons to go through but it couples
the release of energy to the production of ATP. So as these protons go from high
to low, and release energy, that energy is used to form a high high-energy
phosphate bond from ADP forming ATP. So ADP plus
inorganic phosphate plus energy can form ATP. And then that energy is stored in
that bond and the cell can use it to do other work that it needs to do. This is
another picture - I like this picture because it really shows how there's a
very low concentration of protons on the inside of the membrane, much higher on
the outside and it also shows a little bit more of the detail of ATP synthase
which is a really interesting molecule. It's actually a molecular motor, so it
rotates as the protons go through the protein the this molecular motor rotates
and that's part of how the ATP is formed. This is the catalytic head which where
the actual enzymatic reaction is occurring. Alright, so as long as you have
NADH and FADH2 being produced, and as long as you have oxygen to accept
those electrons, then you're going to be pumping protons into this inner membrane
space and allowing the ATP synthase to make ATP. Now if I go back a slide or two -
You know what happens if you don't have oxygen? I mean if you hold your breath
for a few minutes, it gets unpleasant really quickly and the reason is because
you're shutting down this process. Without oxygen, there's nothing for these
electrons to land on and it shuts down the ability of your electron transport
chain to create the gradient and ultimately if there's no gradient no ATP
gets made so that's why you need oxygen to breathe. Pretty interesting. What I'm
going to do now is post two animations for you. The first one is very simple I
encourage you to watch that one first. The level that you need to understand
this is really very well documented in the first animation. The second animation
is was put together by Harvard and it's exquisite. It's absolutely beautiful - the
graphics are phenomenal, and I'd really love to have you watch it too, and maybe
for the second one don't take notes just let their animations help you see the
big picture. There's a lot of detail here that you don't need to know, so I
don't want to stress you out but I do want to give you the opportunity to see
the animation. I think if you're able to watch it and see these proteins in
action the way they do it might actually help your overall understanding of the
process and get to sort of a deeper level of understanding. And deeper levels
of understanding are always really good for doing well on a test.
So enjoy these two animations and I'll be back in a few minutes.