Video Transcript
A basic outline of the electron transport chain involved in oxidative phosphorylation
is provided, with the names of some key molecules removed. What molecules would correctly replace X and Y? (A) Water and carbon dioxide, (B) oxygen and water, (C) FAD and FADH2, (D) carbon and
methane.
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.
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. Meanwhile, the electrons in the electron transport chain terminate with oxygen to
form water. Oxygen is a strong electron acceptor and is a necessary part of the electron
transport chain. Without it, the electrons from NADH and FADH2 cannot move through the electron
transport chain and hydrogen ions cannot be pumped against their concentration
gradient to be used to make ATP.
Therefore, the molecules that correctly replace X and Y in the provided diagram are
given by answer choice (B), oxygen and water.