Video Transcript
In this video, we will learn about
oxidative phosphorylation as the final stage of cellular respiration. We will recall the key reactants
and products of oxidative phosphorylation and explain the role of the electron
transport chain. Finally, we will look at oxidative
phosphorylation in context with the rest of the respiration reactions.
The main purpose of cellular
respiration is to break down glucose in our cells to release energy. We can divide cellular respiration
into four main stages. First, glycolysis breaks down a
molecule of glucose into two molecules of pyruvate. Second, the link reaction converts
pyruvate into acetyl coenzyme A. Third, the Krebs cycle breaks down
and regenerates citric acid, producing multiple molecules of coenzymes in the
process. And finally, oxidative
phosphorylation uses these coenzymes to carry out a series of biochemical reactions
that generate many molecules of ATP.
So let’s learn more about this
final stage of respiration. Oxidative phosphorylation is an
aerobic process. This means that oxygen must be
present for it to take place. Oxidative phosphorylation takes
place within the mitochondria of most living cells. This is because oxidative
phosphorylation relies on something called the electron transport chain. The electron transport chain is the
collection of protein complexes that are specialized to transport, you guessed it,
electrons. These protein complexes are found
within the inner mitochondrial membrane shown here in this enlarged view. Let’s have a look at these proteins
in a little more detail.
Here’s a basic outline of the
protein complexes found within the electron transport chain. The e− symbol represents an
electron. We can see that an electron will
move through the different proteins, enzymes, and cytochromes before exiting the
electron transport chain. But where do these electrons come
from? Remember that in the Krebs cycle,
reduced NAD and reduced FAD were produced. Well, here they are. And these coenzymes are responsible
for providing the electron transport chain with a supply of electrons. The hydrogen atoms of NADH and
FADH2 are lost and split into hydrogen ions and electrons. The electrons are then donated to
the electron transport chain.
Now, let’s follow the journey of
these donated electrons. As the electrons move down the
electron transport chain, they lose energy. This energy is then used to
actively transport the hydrogen ions, we mentioned earlier, out of the mitochondrial
matrix and into the intermembrane space. The hydrogen ions then start to
accumulate in relatively high concentrations in the intermembrane space. You may remember from learning
about particles that they tend to move easily from an area of high concentration to
an area of low concentration. This is what happens with the
hydrogen ions. However, these ions cannot simply
move through the membrane. They have to use a special
channel. In this case, the hydrogen ions
move through the channel of the enzyme ATP synthase.
You may have already guessed what
the role of ATP synthase is from its name. ATP synthase is an enzyme
responsible for producing ATP. It does this by coupling the
movement of hydrogen ions through its channel to the phosphorylation of ADP. Remember that ADP, or adenosine
diphosphate, has two phosphate groups. During the phosphorylation
reaction, ADP gains another phosphate group to form ATP, or adenosine
triphosphate. By now, we are quite familiar with
ATP, which is the essential energy-carrying molecule of our cells.
The movement of hydrogen ions down
their concentration gradient is known as chemiosmosis. Because hydrogen ions are
electrically charged particles, we describe this particular concentration gradient
as an electrochemical gradient. We can say that ATP is produced by
chemiosmosis because ATP synthase uses this movement of ions to phosphorylate
ADP. You may have noticed that at this
point, both the electrons we discussed and the hydrogen ions have exited the
electron transport chain. So what happens to them? The electrons that leave the
electron transport chain are passed to oxygen molecules. Oxygen readily accepts these
electrons. This is why we refer to oxygen as
the final electron acceptor.
In fact, around 95 percent of the
oxygen taken in by your cells is used as the final electron acceptor in the electron
transport chain. This reaction also heavily
influences the entire sequence of reactions that precede oxidative
phosphorylation. If oxygen is not available,
electrons cannot move down the electron transport chain. So oxidative phosphorylation does
not take place. When oxygen is not present, the
only stage of cellular respiration that takes place is glycolysis. So the oxygen molecules accept the
electrons and combine with the hydrogen ions to form water. And there we have it! The final products of oxidative
phosphorylation are ATP and water.
We now understand more about
oxidative phosphorylation and the electron transport chain, but we need to revisit
the other reactions of cellular respiration to fully appreciate the importance of
this stage. Let’s start with the overall
chemical equation for aerobic cellular respiration. Glucose plus oxygen yields carbon
dioxide plus water. Remember, energy is released during
this process in the form of ATP. One molecule of glucose will enter
glycolysis. For every molecule of glucose, the
net yield of ATP is two molecules, and the net yield of reduced NAD is two
molecules.
The final products of glycolysis
are two molecules of pyruvate. Each molecule of pyruvate now acts
as a reactant in the link reaction. So for each molecule of glucose,
two link reactions take place. The net yield of reduced NAD for
each link reaction is one molecule, and the final product of each link reaction is
one molecule of acetyl coenzyme A. This acetyl coenzyme A now acts as
a primary reactant of the Krebs cycle. During one turn of the Krebs cycle,
one molecule of ATP, one molecule of FADH2, and three molecules of NADH are
produced. These molecules of reduced NAD and
FAD and the ones produced in earlier reactions now enter oxidative
phosphorylation.
For every molecule of reduced NAD
that enters oxidative phosphorylation, typically three molecules of ATP are
produced. And for every molecule of reduced
FAD that enters oxidative phosphorylation, typically two molecules of ATP are
produced. Per molecule of glucose, oxidative
phosphorylation can produce between 26 and 32 molecules of ATP. Taking into consideration all
stages of cellular respiration, the total ATP yield is 30 to 38 molecules of
ATP. Now, we can see why the final stage
of cellular respiration is so important. It’s where the vast majority of the
energy-carrying molecule ATP is produced.
Now that we’ve learned more about
oxidative phosphorylation and its role in cellular respiration, let’s try a practice
question.
What is the role of reduced NAD and
FAD in the electron transport chain? (A) To act as the final electron
acceptor. (B) To actively transport hydrogen
ions across the mitochondrial membrane. (C) To provide the energy to
phosphorylate ADP to form ATP. (D) To provide electrons for the
electron transport chain.
The electron transport chain refers
to a series of protein complexes including cytochromes, enzymes, and coenzymes that
are specialized to transport electrons. The electron transport chain is
found within the inner membrane of mitochondria. Let’s have a look at the electron
transport chain in a bit more detail. We can see in the diagram that
electrons move between protein complexes in the chain before eventually leaving and
joining with hydrogen ions and oxygen to form water. As electrons move through the
electron transport chain, free energy is released. This energy is used to actively
transport hydrogen ions across the membrane, out of the matrix, and into the
intermembrane space.
But what about reduced NAD and
reduced FAD? Where do they come into this? In chemistry, when we say that
something is reduced, we mean it has gained electrons. NAD and FAD become reduced in the
preceding stage of respiration, the Krebs cycle. When they enter oxidative
phosphorylation, reduced NAD and FAD donate their electrons to the electron
transport chain.
Let’s go through these reactions
together now. During the reactions, their
hydrogen atoms disassociate. This means that the atoms split
into positively charged ions and negatively charged electrons. The electrons now move along the
electron transport chain, and hydrogen ions are actively transported across the
inner mitochondrial membrane. Due to the electrochemical gradient
produced, the enzyme ATP synthase can use the movement of hydrogen ions back into
the matrix to produce ATP.
So now we can accurately describe
the role of reduced NAD and reduced FAD in the electron transport chain. Our correct answer is (D) to
provide electrons for the electron transport chain.
Let’s summarize what we’ve learned
about oxidative phosphorylation with some key points. Cellular respiration is the process
by which living organisms break down glucose and other substrates to release
energy. Oxidative phosphorylation is the
final stage of cellular respiration. Oxidative phosphorylation involves
the movement of electrons through the electron transport chain in the
mitochondria. The movement of electrons provides
the energy to move hydrogen ions across the inner membrane of the mitochondria. ATP is produced by chemiosmosis as
ATP synthase couples the movement of hydrogen ions to the phosphorylation of
ADP. Oxygen acts as the final electron
acceptor.
