"

Chapter 12: Cellular Respiration

12.1. Redox Reactions and Coenzymes

Learning Objectives

By the end of this section, you will be able to:

  • Define the equation for the oxidation of glucose.
  • Define redox reactions.

We previously discussed ATP, the ‘energy currency the cell’, and how it acts as the ‘energy booster’ in coupled reactions (Chapter 7) .In this chapter we focus on the pathways that result in the synthesis of ATP.

High energy enters an organism’s body in one form and is converted into another form that can fuel the organism’s life functions. A series of metabolic pathways, collectively called cellular respiration, extracts the energy from the bonds in glucose and converts it into a form that the living cell can use.

The summary equation for the cellular respiration process is:

 C6H12O6 + 6O2 ⟶ 6CO2 + 6H2O + Energy (ATP)

Redox (oxidation-reduction) reactions

The equation above is an example of a redox reaction: glucose is oxidized to carbon dioxide (CO2) and oxygen (O2) is reduced to water (H2O).

An oxidation reaction strips an electron (and H+) from an atom in a high energy compound and the transfer of the electron (and H+) to a recipient compound is a reduction reaction. Oxidation and reduction occur in pairs, hence the name redox reactions. The redux reactions involving electron transfers/H transfers relate to energy levels. A mnemonic for redox reaction is OILRIG

  • Oxidation Is Loss  – of electrons, H+, Energy
  • Reduction Is Gain – of electrons, H+ Energy

Redox reactions in Coenzymes, the electron carriers in cellular respiration. Two coenzymes, both derived from the Vitamin B family play a crucial and direct role in enabling ATP synthesis in the final phase of cellular respiration. They are:

  • Nicotinamide adenine dinucleotide (NAD):  NAD+ is the oxidized low energy form; NADH is the reduced, high energy form of the coenzyme (Figure 12.1.1.).
  • Flavin adenine dinucleotide (FAD):  FAD is the oxidized low energy form; FADH2 is the reduced. high energy form.
This illustration shows the molecular structure of N A D superscript plus sign baseline and N A D H. Both compounds are composed of an adenine nucleotide and a nicotinamide nucleotide, which bond together to form a dinucleotide. The nicotinamide nucleotide is at the 5 prime end, and the adenine nucleotide is at the 3 prime end. Nicotinamide is a nitrogenous base, meaning it has nitrogen in a six-membered carbon ring. In N A D H, one extra hydrogen is associated with this ring, which is not found in N A D superscript plus sign baseline.
Figure 12.1.1: The oxidized form of the electron carrier (NAD+) is shown on the left, and the reduced form (NADH) is shown on the right. The nitrogenous base in NADH has one more hydrogen ion and two more electrons than in NAD+.

License and attributions:

  • Biology, Second edition, 2018, Clark, M.A. et al. License: CC BY 4.0. Located at https://openstax.org/books/biology-2e/pages/7-1-energy-in-living-systems

License

Icon for the Creative Commons Attribution 4.0 International License

BIO130: Introduction to Physiology Copyright © 2024 by Dinor Dhanabala; Sandra Fraley; and Gordon Lake is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

Share This Book