Let’s imagine that you are a cell. You’ve just been given a big, juicy glucose molecule, and you’d like to convert some of the energy in this glucose molecule into a more usable form, one that you can use to power your metabolic reactions. How can you go about this? What’s the best way for you to squeeze as much energy as possible out of that glucose molecule, and to capture this energy in a handy form?

Fortunately for us, our cells—and those of other living organisms—are excellent at harvesting energy from glucose and other organic molecules, such as fats and amino acids). Here, we’ll look at the electron transfer reactions (redox reactions) that are key to this process.

## Redox Reactions

Cellular respiration involves many reactions in which electrons are passed from one molecule to another. Reactions involving electron transfers are known as oxidation-reduction reactions (or redox reactions), and they play a central role in the metabolism of a cell. In a redox reaction, one of the reacting molecules loses electrons and is said to be oxidized, while another reacting molecule gains electrons (the ones lost by the first molecule) and is said to be reduced. You can remember what oxidation and reduction mean with the handy mnemonic “OIL RIG:  Oxidation Is Losing electrons; Reduction IGaining electrons.

## Energy in Redox Reactions

Click on the image for a larger view. Image based on similar diagram by Ryan Gutierrez.

In redox reactions, energy is released when an electron loses potential energy as a result of the transfer. Electrons have more potential energy when they are associated with less electronegative atoms (such as C or H), and less potential energy when they are associated with a more electronegative atom (such as O). Thus, a redox reaction that moves electrons or electron density from a less to a more electronegative atom will be spontaneous and release energy. For instance, the combustion of butane (C4H10) releases energy because there is a net shift of electron density away from carbon and hydrogen and onto oxygen. If you’ve heard it said that molecules like glucose have “high-energy” electrons, this is a reference to the relatively high potential energy of the electrons in their $\text{C}-\text{C}\\$ and $\text{C}-\text{H}$ bonds.

Quite a bit of energy can be released when electrons in $\text{C}-\text{C}$ and $\text{C}-\text{H}$ bonds are shifted to oxygen. In a cell, however, it’s not a great idea to release all that energy at once in a combustion reaction. Instead, cells harvest energy from glucose in a controlled fashion, capturing as much of it as possible in the form of ATP. This is accomplished by oxidizing glucose in a gradual, rather than an explosive, sort of way. There are two important ways in which this oxidation is gradual:

• Rather than pulling all the electrons off of glucose at the same time, cellular respiration strips them away in pairs. The redox reactions that remove electron pairs from glucose transfer them to small molecules called electron carriers.
• The electron carriers deposit their electrons in the electron transport chain, a series of proteins and organic molecules in the inner mitochondrial membrane. Electrons are passed from one component to the next in a series of energy-releasing steps, allowing energy to be captured in the form of an electrochemical gradient.

We’ll look at both redox carriers and the electron transport chain in more detail below.

The removal of an electron from a molecule, oxidizing it, results in a decrease in potential energy in the oxidized compound. The electron (sometimes as part of a hydrogen atom), does not remain unbonded, however, in the cytoplasm of a cell. Rather, the electron is shifted to a second compound, reducing the second compound. The shift of an electron from one compound to another removes some potential energy from the first compound (the oxidized compound) and increases the potential energy of the second compound (the reduced compound). The transfer of electrons between molecules is important because most of the energy stored in atoms and used to fuel cell functions is in the form of high-energy electrons. The transfer of energy in the form of electrons allows the cell to transfer and use energy in an incremental fashion—in small packages rather than in a single, destructive burst. This chapter focuses on the extraction of energy from food; you will see that as you track the path of the transfers, you are tracking the path of electrons moving through metabolic pathways.

## Electron Carriers

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+.

As shown in the image above, NAD+ is a small organic molecule whose structure includes the RNA nucleotide adenine. (FAD is a similar type of molecule, although its functional groups are different.) Both molecules are B vitamin derivatives, with NAD+ produced from niacin and FAD produced from riboflavin. NAD+  and FAD are coenzymes, organic molecules that serve as helpers during enzyme-catalyzed reactions, and they receive electrons and protons as part of these reactions. Specifically, both NAD+ start superscript, plus, end superscript and FAD serve as cofactors for enzymes called dehydrogenases, which remove one or more hydrogen atoms from their substrates.

## Overview of fuel breakdown pathways

The reactions that that allow energy to be extracted from molecules such as glucose, fats, and amino acids are called catabolic reactions, meaning that they involve breaking a larger molecule into smaller pieces. For example, when glucose is broken down in the presence of oxygen, it’s converted into six carbon dioxide molecules and six water molecules. The overall reaction for this process can be written as:

C6H12O6 + O2= 6CO2 + 6H20 + heat + ATP

This reaction, as written, is simply a combustion reaction, similar to what takes place when you burn a piece of wood in a fireplace or gasoline in an engine. Does this mean that glucose is continually combusting inside of your cells? Thankfully, not quite! The combustion reaction describes the overall process that takes place, but inside of a cell, this process is broken down into many smaller steps. Energy contained in the bonds of glucose is released during the reactions of glycolysis and citric acid cycle (Krebs)  So electron pairs are removed from the glucose molecule (oxidation) and are added to either NAD or FAD (reduction) to be used in the next reaction described below.

## The Electron Transport Chain

In their reduced forms, NADH and FADH2 carry electrons to the electron transport chain in the inner mitochondrial membrane. They deposit their electrons at or near the beginning of the transport chain, and the electrons are then passed along from one protein or organic molecule to the next in a predictable series of steps. Importantly, the movement of electrons through the transport chain is energetically “downhill,” such that energy is released at each step. In redox terms, this means that each member of the electron transport chain is more electronegative (electron-hungry) that the one before it, and less electronegative than the one after[1]. NAD+, which deposits its electrons at the beginning of the chain as NADH, is the least electronegative, while oxygen, which receives the electrons at the end of the chain (along with H+) to form water, is the most electronegative. As electrons trickle “downhill” through the transport chain, they release energy, and some of this energy is captured in the form of an electrochemical gradient and used to make ATP.   However, much of the energy from glucose is still lost as heat,

We can confirm this if we look at the actual electron shifts involved, as in the video below:

1. Ibid.