Video Transcript
Which of the following best describes oxidative phosphorylation? (A) The process by which electrons move down an electron transport chain, resulting
in the formation of ATP from ADP. (B) The process by which electrons are released from reduced coenzymes in the
mitochondrial matrix. (C) The active transport of hydrogen ions from the intermembrane space into the
mitochondrial matrix. (D) The formation of ATP via the phosphorylation of coenzymes NAD and FAD.
Cellular respiration is an important process in all living organisms. It’s the process where glucose is broken down to release energy that is stored in
molecules of ATP. There are four main steps in cellular respiration: glycolysis; the link reaction; the
Krebs cycle, also known as the citric acid cycle; and oxidative phosphorylation,
also known as the electron transport chain.
Throughout glycolysis, the link reaction, and the Krebs cycle, molecules of ATP are
formed in addition to the coenzymes NADH and FADH2. During oxidative phosphorylation, NADH and FADH2 are used to produce even more
ATP. In fact, oxidative phosphorylation is the step where the most ATP is produced.
Oxidative phosphorylation takes place in the mitochondria. Here you can see a diagram of the mitochondrion with its outer membrane and inner
membrane, which contain the intermembrane space, as well as the matrix, which is
contained within the inner membrane. In the mitochondrial inner membrane are a number of proteins that are specialized in
harvesting energy from the high-energy electrons stored in the coenzymes NADH and
FADH2. You can follow the journey of an electron from the left.
There is a lot of stored energy in these electrons. And throughout the electron transport chain, the electrons lose some of this energy
in little steps. This release of energy can be used to actively transport hydrogen ions against its
concentration gradient from the mitochondrial matrix into the intermembrane
space. This concentration gradient represents a form of stored energy and can be used to
make ATP as we can see on the right. This specialized protein complex, called ATP synthase, couples the movement of
hydrogen ions down their concentration gradient to the synthesis of ATP.
We should now have enough information to answer our question. So let’s go through the answer choices to determine which option describes oxidative
phosphorylation best.
In (A), “The process by which electrons move down an electron transport chain,
resulting in the formation of ATP from ADP” sounds like a good choice, because this
describes the overall process of oxidative phosphorylation. But let’s first compare the other answer choices to be sure.
In (B), “The process by which electrons are released from reduced coenzymes in the
mitochondrial matrix” isn’t true, as these electrons are released to different
proteins embedded in the inner membrane of the mitochondria, not in the matrix.
In (C), “The active transport of hydrogen ions from the intermembrane space into the
mitochondrial matrix” is not true, since active transport of hydrogen ions occurs
between the matrix and the intermembrane space, not the other way around.
In (D), “The formation of ATP via the phosphorylation of coenzymes NAD and FAD” is
not true, because this isn’t what happens. ATP is created by ATP synthase using the energy stored in the concentration gradient
of hydrogen ions.
Therefore, the best description of oxidative phosphorylation is given by answer
choice (A), the process by which electrons move down an electron transport chain,
resulting in the formation of ATP from ADP.
