Question

The pathway that converts glucose to acetyl-CoA is often referred to as an "aerobic oxidation pathway." (a) Is molecular oxygen involved in any of the steps of glycolysis? (b) Thinking back to Chapter 20 , where does molecular oxygen enter the picture?

Short answer

(a) No, oxygen is not involved in glycolysis. (b) Oxygen is involved in the electron transport chain.

Step-by-step solution

Understand the process of glycolysis

Glycolysis is the first step in the breakdown of glucose to extract energy for cellular metabolism. It occurs in the cytoplasm of the cell and consists of a series of ten enzymatic reactions that convert glucose into two molecules of pyruvate.

Determine if glycolysis involves oxygen

During glycolysis, glucose is converted to pyruvate through various enzymatic reactions. This process does not require oxygen and occurs under anaerobic conditions, i.e., it does not involve molecular oxygen.

Recall the involvement of oxygen in cellular respiration

In cellular respiration, molecular oxygen plays a critical role in the electron transport chain, which occurs after glycolysis. Oxygen acts as the final electron acceptor in the electron transport chain, a process that happens in the mitochondria and is essential for the conversion of pyruvate to acetyl-CoA through oxidative phosphorylation.

Identify where oxygen is involved post-glycolysis

Although oxygen is not used in glycolysis, it is essential for aerobic processes that follow glycolysis, particularly in the citric acid cycle and electron transport chain, during which acetyl-CoA is fully oxidized to carbon dioxide, producing ATP efficiently in the presence of oxygen.

Key concepts

Glycolysis

Glycolysis is the initial stage in the process of breaking down glucose to harness energy. It takes place in the cytoplasm, which is the jelly-like fluid inside the cell. Through a sequence of ten enzyme-driven reactions, one molecule of glucose, which is a 6-carbon sugar, is transformed into two molecules of pyruvate, each consisting of 3 carbons.
What is truly fascinating about glycolysis is that it does not need oxygen to occur, making it an anaerobic process. Even though glycolysis doesn't require oxygen, it is vital for the survival of cells as it provides a rapid supply of energy and reducing power, in the form of ATP and NADH molecules. These molecules serve as essential energy carriers and are used in subsequent reactions further down the cellular respiration pathway.

Cellular Respiration

Cellular respiration is a multi-step process that cells use to extract energy from food, notably glucose. This process not only includes glycolysis but also involves the transformation of pyruvate to acetyl-CoA, the citric acid cycle (or Krebs cycle), and the electron transport chain, where oxygen plays a crucial role. The entire process of cellular respiration results in a substantial production of ATP, the energy currency of the cell. It involves:

• Conversion of pyruvate to acetyl-CoA, which enters the citric acid cycle.
• Production of NADH and FADH₂, two crucial carriers of electrons and hydrogen ions.

In this aerobic phase of respiration, oxygen is fundamentally important as it is the terminal electron acceptor, enabling the release of energy stored in NADH and FADH₂.

Electron Transport Chain

The electron transport chain (ETC) is a series of protein complexes and other molecules that are located in the inner mitochondrial membrane. It is here that the electrons from NADH and FADH₂ are transferred through a series of steps, down the chain. The role of oxygen is pivotal in the electron transport chain, as it acts as the final recipient of electrons. This is crucial because without oxygen, the electron flow would cease, effectively halting ATP production. As electrons move through the chain:

• Protons are pumped across the mitochondrial membrane, creating a gradient.
• This gradient is vital for driving ATP synthesis, as protons flow back across the membrane through ATP synthase.

Understand that without this process, energy production would be significantly reduced, as the most ATP from glucose is obtained during this stage of cellular respiration.

Oxidative Phosphorylation

Oxidative phosphorylation is a critical step of cellular respiration closely tied to the electron transport chain. It is this process that captures the energy released from electrons to drive ATP synthesis. This occurs within the mitochondria and is dependent on the oxygen-driven flow of electrons. Through the action of ATP synthase, the enzyme that synthesizes ATP, the potential energy stored in the proton gradient established by the electron transport chain is converted into chemical energy in the form of ATP. The word "oxidative" refers to the role of oxygen as the final electron acceptor, which makes this process possible. Thanks to oxidative phosphorylation, a large amount of ATP is produced, making it a key driving force behind efficient energy utilization within aerobic organisms.