Cellular Respiration

The mitochondria is the powerhouse of the cell... pretty sure everyone has heard that somewhere. It's probably the only thing many adults remember from their biology classes - so much so that it has turned into a rather depressing catchphrase, almost more depressing than Robert Pattinson ruining the Twilight saga. As someone who read and adored the Twilight series (a literary masterpiece), I was extremely distraught to discover that its last remnants are terrifying pictures of the most goofy romance scenes known to mankind, and that the series has been reduced to a meme.

But no matter. The reason the mitochondria got its infamous name is because of the process, cellular respiration, that occurs inside it - allowing it to produce 95% of the ATP required by a cell. Cellular respiration is the complex process in which cells create adenosine triphosphate (ATP) by breaking down organic compounds. Without this incredibly complex process, our bodies would not be able to function and we would be even more useless than we are right now.

Let's start our exploration with an overview. The equation for cellular respiration is:

This equation depicts a Redox reaction - which is not as cool as the name implies. I was a little disappointed to. Redox reactions track the movement of electrons - one material loses electrons (oxidation) while the other gains it (reduction). I honestly think it was stupid to name a gain in electrons a "reduction," but scientists don't seem to care, so a better way to remember a redox reaction is with the phrase OIL RIG. Or:

Oxidation
I
Lose

Reduction
I
Gain

If this doesn't help, remember the phrase LEO goes GER:

Losing
Electrons
Oxidation

Gaining
Electrons
Reduction

In cellular respiration, glucose is oxidized - it loses electrons and hydrogen atoms to become CO2. Oxygen, meanwhile, is reduced. It gains electrons, or hydrogen atoms, to become H2O, as shown in the picture below.

Electrons and hydrogen atoms are rather important in the process of cellular respiration - hence there are two molecules: NAD+ and FADH+, that are electron carries. NAD+ and FADH+ accept electrons and are reduced to become NADH AND FADH2. NAD+ and FADH+ are low-energy versions of NADH and FADH2. Also, ATP, or adenosine triphosphate, is the high-energy form of ADP (adenosine diphosphate), which is created by removing a phosphate group to release energy. They are vital to cellular respiration. Lastly, before I explain the first step of cellular respiration, remember these facts:

Anaerobic respiration: does not involve oxygen.

Aerobic respiration: involves oxygen.

Step 1: Glycolysis

Omg... do y'all like this new formatting? I feel like I just yassified the entire website 😋

But anyway. Glycolysis is the only anaerobic step in cellular respiration. It also doesn't happen in the mitochondria - it actually occurs in the cytosol. In this step, glucose (6-carbon) is broken down into Glyceraidehyde Triphosphate (G3P), a 3-carbon sugar, which is further converted into pyruvic acid - two pyruvate molecules. First, two ATP are converted into ADP, which helps in the creation of two G3P molecules. Then, 2 NAD+ are converted into NADH and 4 ADP are converted into ATP, helping create two 3-carbon pyruvate molecules, 2 ATP, and 2 NADH as the net yields for this step.

The next step on this process depends entirely on the presence of oxygen. If oxygen is present, cellular respiration continues onto the Prep Step. If it is not, the process moves to fermentation.

Step 2 (oxygen is present): Pyruvate Oxidation

When oxygen is present, the process continues onto Pyruvate Oxidation, or the Prep Step, which occurs in the mitochondria. In this step, one NAD+ is converted into a NADH, and an enzyme called Coenzyme A (CoA) binds to the pyruvate, creating Acetyl CoA. CO2 is released as a byproduct. However, since this step happens twice (there are two pyruvates per glucose molecules), 2 NADH and 2 Acetyl CoA are yielded from this step. From this step, the cycle moves onto step 3: the Citric Acid Cycle, or the Krebs Cycle.

Step 2 (oxygen is not present): Fermentation

In this step, 2 NADH are converted into 2 NAD+ to convert the pyruvates into 2 lactates, or 2 NADH are converted into 2 NAD+ to convert the pyruvates into 2 ethenol molecules with a byproduct of CO2. The NADH used are regenerated in glycolysis (when they are converted back into 2 NADH from NAD+), thus resulting in the net yield being 2 ATP from this step. This creates two different types of fermentation: lactic acid and alcoholic fermentation. Lactic acid fermentation mostly happens in muscle cells and it gives the body a short, about 30 second burst of energy. It is also used to produce milk and cheese. Alcoholic fermentation produces CO2 and also is used to produce beer and bread. This step is anerobic and occurs only in the absence of oxygen.

Image credit

Step 3: The Citric Acid Cycle

Now the Acetyl CoA molecules enter the Citric Acid Cycle, or the Krebs Cycle. In this cycle, 3 NAD+ are converted into NADH, and 1 FADH+ is converted into FADH2. 1 ADP is converted into ATP, and 2 CO2 molecules are released. This cycle yields a 4-carbon molecule called oxaloacetate, which, frankly, is not too important right now. This cycle also occurs in the mitochondria and happens twice, because there are two Acetyl CoA molecules per glucose molecule. In total, this step yields 2 ATP, 6 NADH and 2 FADH2 and releases 4 CO2 as a byproduct. All these steps release many excess electrons, which become relevant in step 4, oxidative phosphorylation.

The process of Oxidative Phosphorylation.

An ATP synthase enzyme complex.

Step 4: Oxidative Phosphorylation

Oxidative phosphorylation is a step divided into two parts: The Electron Transport Chain and Chemiosmosis. In the electron transport chain, the excess electrons I mentioned earlier are utilized. They are taken, using the electron carriers, down the electron transport chain. These electrons are traded from carrier to carrier to reach the inner mitochondrial membrane, which generates enough energy to volley H+ across the membrane (at certain locations within the membrane) and into the intermembrane space. Since there is a large negative charge in the intermembrane and a large positive charge in the intermembrane space, a concentration gradient (an unequal distribution of charge across a membrane) is developed and the H+ ions immediately wish to diffuse back across the membrane. This is where chemiosmosis begins. In order to diffuse through the membrane, the H+ ions flow through an enzyme complex called an ATP synthase, whose molecular structure allows for the synthesis of ATP. The H+ ions flow into the synthase like water through a waterwheel, generating around 28 ATP molecules - much more than all the other steps combined. On the other side of the synthase, oxygen molecules take two H+ molecules and two electrons each and bind with them, creating a byproduct of water. In total, one glucose molecule can produce around 32 ATP molecules. Below is a picture of the full process of cellular respiration.

Cellular respiration is truly an incredible process that generates a large amount of the energy needed by a cell to function. I hope you admire the process as much as I do after learning about it. It's truly fascinating and incredible. In the next page, I will be discussing another similar, yet different process known as photosynthesis. Please do check it out! I hope to see you in the next one!

By the way, the pictures above are not mine. They are all cited at the bottom left corner of the pictures, or they are cited below the picture. If they are not cited, I got them from my biology textbook, which I cited on the welcome page. Please check it out - it is an amazing textbook, full of interesting information.