The first is called substrate-level phosphorylation. This term tells you exactly what it’s about. In the diagram on the left, you can see an enzyme (1) transfer a phosphate from some organic substrate (2) to ADP (3), creating ATP (5). “Substrate-level” means that the process is catalyzed by an enzyme, working on a substrate that’s losing its phosphate group, and passing that phosphate to a second substrate, ADP. “Phosphorylation” means that ADP is gaining a phosphate, becoming ATP. And, just in case you were worried about what “4” is, it’s one of the two products of this reaction (the other one being ATP).

During cellular respiration, substrate-level phosphorylations occur in the cell’s cytoplasm, and the mitochondrial matrix, generating a small amount of ATP. But we’ll see that the majority of the ATP is made through a less direct process called oxidative phosphorylation. Oxidative phosphorylation belongs to a class of chemical reactions called oxidation/reduction reactions, and we’ll see how these work below.

Also, remember that ATP production is also a part of photosynthesis (which we’ll cover in the next AP Bio Topic).

5. Oxidation and reduction reactions are how most ATP is made during cellular respiration

As you read above, substrate-level phosphorylations account for only a small amount of the ATP generated during cellular respiration (about an eighth of the ATP produced). Much more ATP is produced by oxidizing glucose and using the energy harvested from that oxidation to set up the conditions to generate ATP from ADP and phosphate. So what is oxidation?

Oxidation means the loss of electrons. Oxidation is paired with a complementary process called reduction, which means the gain of electrons. Oxidation-reduction reactions happen all the time. Every time you drive your car, light a match or operate a gas stove, you’re involved with an oxidation-reduction reaction.

In all of the processes listed above, an energy-rich fuel (gasoline, wood, or natural gas, respectively) gets oxidized, losing its energy-rich electrons and hydrogen atoms. At the same time, oxygen gets reduced, gaining electrons and hydrogen atoms.

Here’s the equation for what happens when methane, CH₄ (commonly called “natural gas”) is burned:

CH₄ + 2O₂→ CO₂ + 2H₂O + energy (in the form of heat and light)

Methane, an energy-rich fuel, is oxidized. The result is energy (heat and light), along with carbon dioxide and water (two exhaust products)

From a biology student’s perspective, it’s useful to roughly equate oxidation with the release of energy (powering work), and reduction with storing energy. In other words, the oxidation of a fuel like methane is exergonic: it releases energy, making work possible. Oxidized substances (like carbon dioxide) don’t have energy for accomplishing cellular work. That’s why carbon dioxide is an exhaust product. Think of this oxidized compound as being exhausted: unable to do work. By contrast, substances that are reduced (like methane) have available free energy that can be harvested to do work.

What are some other examples of reduced substances?

• gasoline
• peanut butter
• alcohol
• spaghetti
• meat
• sugar
• wood

Note that foods are all reduced substances. They’re full of high-energy electrons that cells can use to power the work organisms need to do to stay alive, grow and reproduce. When cells need energy, they take a reduced fuel (which means foods like glucose) and oxidize it. The result, just like in the combustion of methane shown above, is the release of energy, and the production of the waste products carbon dioxide and water. Here, once again, is the overall chemical equation for the oxidation of glucose that happens in your cells.

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (in the form of ATP)

But while the chemical equations for the oxidation of methane during combustion and the oxidation of glucose during cellular respiration are similar, the details couldn’t be more different. But before going into these details, let’s make sure you’ve mastered the overall equation for cellular respiration.

6. The oxidations and reductions in cellular respiration involve electron carriers

When glucose (or other foods) get oxidized to create ATP, the oxidation happens in many small steps. During these steps, glucose and the intermediate products that it gets broken down into as it slowly gets transformed into carbon dioxide and water are oxidized. As this oxidation occurs, electrons and hydrogen atoms get transferred to two mobile electron carriers: NAD⁺, and FAD. When NAD⁺ and FAD accept these electrons, they become reduced. NAD⁺ becomes NADH. FAD becomes FADH₂.

NADH and FADH₂: Mobile Electron carriers

It’s useful to think of NADH and FADH₂ as charged-up, rechargeable batteries. When you place a charged battery into a device like a radio, the battery can create an electrical current. An electrical current is a flow of electrons that can be used to do some work (like taking a radio signal, amplifying it, and converting it into sound). NADH and FADH₂ do a similar thing. They carry electron energy from either the cytoplasm or the mitochondrial matrix and bring it to the inner mitochondrial membrane. Once there, they release their electrons to a series of membrane-embedded electron carriers called the electron transport chain. The electron transport chain uses this electron energy to create ATP.

Just think of it this way: If cellular respiration were a game of football and energetic electrons were the ball, then the end zone would be the inner membrane of the mitochondria, where the electron transport chain is found. NADH and FADH₂ are like fullbacks: they carry the ball (the electrons) to the end zone.

We’ll learn the astounding details of how this ATP is generated in a later tutorial, but for now, study this diagram.

“1” represents the source of all the energy: food. A series of oxidations take the energy in glucose and transfer it to NAD⁺ and FAD, creating NADH and FADH₂ (shown at “2”). NADH and FADH₂ are mobile electron carriers: they bring these energetic electrons from the cell’s cytoplasm or the mitochondrial matrix to the electron transport chain, which is located along the inner membrane of the mitochondria. There, the proteins that make up the electron transport chain use the energy in these electrons to synthesize ATP. At the end of the process, these electrons flow to oxygen, which is the final acceptor in the chain. The multiple -ΔGs indicate that each of the many steps in the electron transportation is exergonic, and can be used to power cellular work.

7. Cellular respiration occurs in four phases

Let’s take stock of what we’ve covered so far

1. The purpose of cellular respiration is to convert the potential chemical energy in food into the potential chemical energy in ATP, life’s moment-to-moment energy currency.
2. During cellular respiration, a small amount of ATP is generated through substrate-level phosphorylations that occur in the cell’s cytoplasm and the mitochondrial matrix.
3. Much more ATP is generated through a several-step process involving:
   1. The step-by-step oxidation of glucose
   2. The reduction of the mobile electron carriers NAD⁺ and FAD to NADH and FADH₂, respectively.
   3. The transport of energetic electrons by NADH and FADH₂ to the electron transport chain in the inner membrane of the mitochondria, where most of the ATP generated during cellular respiration is synthesized.

This brings us almost to the end of this overview of cellular respiration. The last things to learn are the names of the four phases of cellular respiration. Don’t let the diagram below panic you. Soon, it will be as familiar as an old friend. Let’s look at each phase.

1. Phase 1 is glycolysis. It produces ATP and NADH. Glucose is split apart and oxidized, resulting in a three-carbon molecule that still has lots of potential energy. This molecule is called pyruvic acid, or pyruvate. Note that there are two molecules of pyruvate produced for every glucose that enters glycolysis.
2. Pyruvate then diffuses into the mitochondrial matrix. There, in the link reaction, pyruvate is oxidized, producing more NADH. In this process, a carboxyl group (COO^–) is removed, leaving a two-carbon molecule called acetyl Co-A.
3. This two-carbon acetyl Co-A still has plenty of potential energy. In the Krebs cycle, this energy is harvested to generate ATP, NADH, and FADH₂.
4. In the electron transport chain, the cell harvests the electron energy it has stored away in the reduced mobile electron carriers NADH and FADH₂. The majority of the ATP generated during cellular respiration is synthesized in this final step.

8. Cellular Respiration Overview: Checking Understanding

Before exploring each phase of cellular respiration in more detail, let’s consolidate our understanding with the following quiz.