Redox Reactions & Electron Carriers in Cellular Respiration: Definitions and Examples

Chemical Reaction for Cellular Respiration

We have learned previously that energy production is an extremely important cellular task. Failure to produce enough energy in the form of ATP can result in fatigue, among other things. It's the difference in being the runner whose body is still performing at a high level and the runner who's gassed. Let's explore how our cells try to keep our bodies from running out of energy by learning about the chemical reactions that contribute to making ATP.

Cellular respiration is a biological process in which organic compounds are converted into energy. During cellular respiration, oxygen reacts with an organic compound to produce carbon dioxide, water, and energy. This seems like a pretty generalized formula for chemical reactions, so let's see if we can make it a little more specific and chemical-like.

Organic compound + O2 --> CO2 + H2O + Energy

The sugar glucose is the main fuel source for cellular respiration. So let's replace 'organic compound' with the chemical formula for glucose, which is C6H12O6.

C6H12O6 + O2 --> CO2 + H2O + Energy

If you recall, we also know that ATP is the molecule that is the 'currency' for energy in cells. Cellular respiration converts ADP into ATP, so let's add that to our cellular respiration equation as well.

C6H12O6 + O2 + ADP --> CO2 + H2O + ATP

This is the basic formula that describes cellular respiration.

We shall soon see how the cell uses a series of redox reactions to break down glucose to release energy. That energy is used to change ADP into ATP that can be used to power biological processes throughout the cell.

Electron Carrier

Recall that a 'redox reaction' is simply shorthand for an oxidation-reduction reaction. That means that during cellular respiration, some molecules in our cellular respiration chemical reaction will be oxidized and some will be reduced.

What exactly does that mean? Remember our mnemonic aid, 'LEO the lion says GER.' A molecule that is oxidized loses electrons, and a molecule that is reduced gains electrons.

Lose

Electrons

Oxidation

Gain

Electrons

Reduction

Well, that's a super mnemonic reminder, but what does it mean in terms of cellular respiration? How is a redox reaction going to help make ATP?

There's a lot of energy stored in the bonds between the carbon and hydrogen atoms in glucose. During cellular respiration, redox reactions basically transfer this bond energy in the form of electrons from glucose to molecules called electron carriers. So an electron carrier is basically a molecule that transports electrons during cellular respiration. By using electron carriers, energy harvested from glucose can be temporarily stored until the cell can convert the energy into ATP.

Equation for the NAD+ and NADH reaction during cellular respiration

NAD+ and NADH

Two molecules that serve this role are NAD and FAD. NAD stands for nicotinamide adenine dinucleotide. It is one of the major energy carriers during cellular respiration.

Let's see how NAD is able to store energy for a cell during cellular respiration. Recall that we are going store energy in NAD by adding electrons to it. That means that the NAD molecule can exist in either an oxidized or a reduced form.

NAD+ is the oxidized form of NAD. When NAD+ reacts with two hydrogen atoms, two electrons can be added to the NAD+ molecule, resulting in a NADH molecule and a proton, or H+.

This equation may seem a little confusing, but let's break it down further to see if it makes more sense. A hydrogen atom consists of a proton and an electron, so we can rewrite the two hydrogen atoms as 2H+ and 2e-. In the reduction reaction, NAD+ accepts the two electrons and one of the protons to form a neutral NADH molecule. That leaves a free proton as the second product of the reaction.

FAD and FADH2

FAD is a second electron carrier used by a cell during cellular respiration. It stands for flavin adenine dinucleotide. Like NAD, FAD can temporarily store energy during cellular respiration via a reduction reaction. When FAD reacts with two hydrogen atoms, it can form FADH2.

Important NAD and FAD reactions within the cellular respiration process

Significance of NAD and FAD

Both NAD and FAD play a crucial role in cellular respiration to temporarily store energy as it's released from glucose. Transferring the electrons NAD and FAD are carrying during an oxidation reaction releases the stored energy that was harvested from glucose. NADH is oxidized back to NAD+, and FADH2 is oxidized back to FAD. This stored energy can then be used to generate ATP, the basic energy currency of a cell. As we learn more detail about the cellular respiration process, we will see specifically how and where these reactions take place within a cell.

Lesson Summary

In summary, cellular respiration is the biological process in which organic compounds are converted into energy. A redox reaction is shorthand for an oxidation-reduction reaction and is a chemical reaction in which one molecule loses electrons while another molecule gains electrons.

An electron carrier is a molecule that transports electrons during cellular respiration. NAD is an electron carrier used to temporarily store energy during cellular respiration. This energy is stored via the reduction reaction NAD+ + 2H --> NADH + H+. FAD is another electron carrier used to temporarily store energy during cellular respiration. This energy is stored via the reduction reaction FAD + 2H --> FADH2. The energy stored in NADH + H+ and FADH2 will be released later in cellular respiration to generate ATP, which can be used as an energy source throughout the cell.

Learning Outcome

At the end of this lesson, you will be able to explain the components of the chemical reaction that produces cellular respiration.