Professor Dave again, let's take a look at oxidative phosphorylation.
We've examined the first two steps of cellular respiration. These
are glycolysis and the citric acid cycle. As it turns out, neither process
generates much of an energy payoff, but the NADH and FADH2 that were generated
in the citric acid cycle move on to oxidative phosphorylation, which
generates by far the most ATP out of these pathways. This pathway utilizes an
electron transport chain, which is a series of mitochondrial membrane
proteins that sit in the inner membrane of the mitochondrion, and we refer to
these as protein complexes I-IV. These proteins bear a variety of
prosthetic groups, which are non-protein components that give the protein its
functionality, including flavin mononucleotides and cytochromes. There is
also one compound in the electron transport chain that is not a protein,
and that's ubiquinone, a small hydrophobic molecule that is mobile
within the membrane, and it is also known as coenzyme Q, or CoQ. Once NADH
feeds electrons into the first component of complex one,
these proteins facilitate a series of redox reactions, shuttling the electrons
downhill from one component to another, with each structure down the chain
having a higher affinity for electrons than the last.
This process does not generate ATP directly but a byproduct of this series
of electron transfers is the generation of a proton gradient across the membrane.
Protons accumulate outside of the inner mitochondrial membrane, which then go to
power another protein complex called ATP synthase. As you might guess from the name,
this is the component that synthesizes ATP. Because the proton concentration
becomes greater in the intermembrane space then inside the mitochondrial
matrix, the protons will spontaneously move with the proton
gradient to re-enter the mitochondrial matrix, and the only route available is
through ATP synthase. This process is called chemiosmosis, and because the
gradient is able to do work through chemiosmosis we also call this the
proton-motive force, and it is these protons that power ATP synthase in
phosphorylating ADP to generate ATP. ATP synthase has a fascinating structure
with a component that looks startlingly like a rotor, where individual protons
can bind and cause it to spin in such a way that catalyzes phosphorylation of
ADP, kind of like a stream of water turning a waterwheel, which can then
power some other process. With the NADH and FADH2 that is generated from a single
molecule of glucose, we can get around 26 or 28 ATP's from this activity, so this
is the pathway that generates the majority of cellular energy.
So in summary, your body uses a variety of metabolic pathways. Glycolysis, the most
ancient, produces two ATPs per glucose, but it also results in pyruvate, which
can enter the citric acid cycle. Here,
those two pyruvates generate another two ATP, but they also generate six NADH and
two FADH2 molecules that can go on to the electron transport chain, which gives us
our big ATP payout. Feel free to use this summary of the three steps in cellular
respiration if you need to remember the main points of each process, all starting
with just one molecule of glucose. Other sources of food like proteins, fats, and
other carbohydrates besides glucose, are initially broken down in unique ways, but
this will always result in some compound that is eventually fed into one of the
pathways we discussed, so all the food we eat eventually gets broken down this way.
This is why mitochondria are regarded as the engine of the cell, since most of the
energy needed for cellular activity comes from them. So the next time you're
walking home and you feel tired, just think of all those trillions of enzymes
slaving away to help you move your feet.
It might put a little pep in your step.
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