Video Transcript
A basic outline of the electron transport chain involved in oxidative phosphorylation
is provided in the diagram below. What part of this process does phosphorylation refer to? (A) The movement of hydrogen ions from the matrix to the intermembrane space. (B) The combining of oxygen and hydrogen to make water. (C) The addition of a phosphate group to ADP to form ATP. (D) The conversion of NADH to NAD+.
This question is asking us about oxidative phosphorylation which, is a step in
cellular respiration. 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 a 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.
Now let’s turn our attention to the provided diagram which shows the basic outline of
oxidative phosphorylation. 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. To form ATP, ATP synthase joins a phosphate group, shown as Pi in the diagram, to
ADP.
Therefore, to return to our question, the option that correctly describes what
phosphorylation in oxidative phosphorylation refers to is given by answer choice
(C): the addition of a phosphate group to ADP to form ATP.
