Video Transcript
Why is oxidative phosphorylation considered an aerobic stage of respiration? (A) Each stage of the reaction produces an oxidized molecule. (B) It can only happen in the presence of carbon dioxide. (C) It requires oxygen to be able to take place. (D) It produces oxygen as a by-product.
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 molecules of NADH and FADH2. It is during oxidative phosphorylation that 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, like all the previous
steps of cellular respiration after glycolysis, which takes place in the
cytoplasm. In the mitochondrial inner membrane are a number of proteins that are specialized in
harvesting energy from the high-energy electrons stored in 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 matrix into the intermembrane space. This concentration gradient 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. This requirement for oxygen to act as the terminal electron acceptor in the electron
transport chain is what makes oxidative phosphorylation aerobic.
Therefore, the option that best describes why oxidative phosphorylation is aerobic is
given by answer choice (C). It requires oxygen to be able to take place.