Cellular Respiration

Cellular respiration is the process that almost all living things use to make the energy they need to well, stay alive. This includes humans, plants, other animals, fungi, and bacteria.

There are two things we need to do cellular respiration, which are glucose, which is a type of sugar, and oxygen. Ok, so what cellular respiration does, is it takes this glucose and oxygen, and it creates three things: carbon dioxide, water, and, as we were talking about before, energy.

Cellular respiration takes place in cells, which are the basic unit of life. We have millions and millions of cells.

Ok, now the last thing you need to know before we get into the steps of this process is that there are two main types of cellular respiration: aerobic and anaerobic. The key difference between these two is that anaerobic respiration doesn’t need oxygen, whilst aerobic does. We’ll talk more about anaerobic respiration later.

Ok, now let’s dive a little deeper into how aerobic respiration works. There are 4 main steps of aerobic respiration: glycolysis, pyruvate oxidation, the Krebs Cycle (some people also call it the Citric Acid Cycle), and the Electron Transport Chain.

Let’s talk about glycolysis first. So glycolysis takes place in the cytoplasm of the cell, which is this thick, jelly-like fluid. There are two parts to glycolysis: the energy-investment, and the energy-payoff phase. In the energy-investment phase, energy is used, and in the payoff phase, its released. Now, the only thing needed for glycolysis is glucose. Glucose gets broken down a bunch of times until it finally forms two molecules of Pyruvate, or Pyruvic Acid. In addition, it also forms four molecules of ATP (but keep in mind it used 2 in the energy-investment phase), and two molecules of NADH.

The next step that happens is pyruvate oxidation. In pyruvate oxidation, one of the three carbons in pyruvate breaks off, releasing electrons, which forms more NADH. Also, carbon dioxide is formed. This new two carbon molecule is called Acetyl. This Acetyl gets fixed to Coenzyme A, forming Acetyl Coenzyme A. Also, an important thing to keep in mind is that this whole process happens twice for every glucose.

Ok, so now we have reached the Krebs Cycle, which is arguably the most complicated of all the steps. But don’t worry! My job is to keep things simple and understandable, so that’s what I’m going to do. The Krebs Cycle takes place in a structure in the cell called the mitochondrion. More specifically, in the mitochondrial matrix, which is a hollow space within the inner membrane of the mitochondrion. Now, remember that Acetyl Coenzyme A we made earlier. Well, we’re going to combine this with a compound called Oxaloacetate to create Citric Acid. Then, this Citric Acid goes through a bunch of different modifications to become Oxaloacetate again, which is why it’s called a cycle. Throughout this process, we also make 2 more ATPs, 6 NADHs, 4 Carbon Dioxides, two FADH2s. Also, just like Pyruvate Oxidation, this process happens twice, and the numbers I just mentioned are after 2 cycles.

Ok, we are done with the Krebs Cycle. Now let’s talk about the final step of cellular respiration, and the part that makes the most ATP energy: The Electron Transport Chain, or ETC for short. First of all, that NADH and FADH2 we made back in the Krebs Cycle will come in handy for this step, because they’ll supply it with electrons, and this super important for making ATP. Now, the ETC happens in the inner membrane of mitochondrion, which is this layer of phospholipids and proteins which separates the mitochondrial matrix from the intermembrane space. These NADHs and FADH2s release electrons into the ETC, in which they travel along. When they do this, they regenerate NAD+ and FAD respectively, which were important in creating these compounds in the first place. Now, in addition to the ETC, oxygen also accepts some electrons, creating water. Now, the movement of these electrons creates a proton gradient, which is basically the protons moving from a high to a low concentration. The high concentration is inside the mitochondrial matrix, so they move through the inner membrane into the intermembrane space, where there’s a lower concentration. Then, go back into the mitochondrial matrix through an enzyme called ATP synthase. This ATP synthase uses the energy of these flowing protons to make ATP, concluding aerobic respiration.

Ok, cool! Now let’s talk about anaerobic cellular respiration. To recap, anaerobic respiration does not need oxygen to produce energy. Anaerobic respiration also does glycolysis in the cytosol, however, it doesn’t do the Krebs Cycle or have an ETC. Instead, it does a process called fermentation, either lactic acid fermentation or ethanol fermentation. Fermentation also takes place in the cytosol.

Let’s talk about lactic acid fermentation first. Lactic acid uses the Pyruvate made in glycolysis as a starting material. Then, an NADH molecule comes in, donates electrons to this pyruvate, creating 2 lactate molecules.

In ethanol fermentation, we take the 3-carbon pyruvate, but this time, we break off one carbon, releasing carbon dioxide. NADH helps out here too, giving its electrons which ultimately forms the two carbon ethanol.