Fill in the blank. ADP is phosphorylated to form ATP, and this reaction is initiated by the movement of
hydrogen ions through blank.
This question is asking us about a step during oxidative phosphorylation, which is a
part of 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 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 contains the intermembrane space, as well as the matrix, which is
contained within the inner membrane. This diagram shows a 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.
Therefore, in the statement “ADP is phosphorylated to form ATP, and this reaction is
initiated by the movement of hydrogen ions through blank,” the correct response to
fill in the blank is ATP synthase.