How do we get energy? Where does it come from? Spoiler alert: your body is constantly making energy for you to use, and it comes from all the chemical reactions happening within your body. This is called your Metabolism. The process by which those chemical reactions take place is called Cellular Respiration.

This video will help introduce you to the process of cellular respiration

We are going to start outside the cell with a molecule of glucose that we got by eating a delicious piece of fruit. Before you proceed, try to predict what will first happen with the glucose (using your knowledge gained by the video above).

We start out in the cytoplasm, which is the fluid of the cell, outside the mitochondria. A molecule of glucose first needs to be processed before it can become ATP. The first step in cellular respiration is called Glycolysis. If we break up the word, we have glyco (meaning glucose) and lysis (which means to break down). Already we can infer that glycolysis involves the breaking down of glucose. To start the process, we must first input 2 ATP molecules (remember, to make an omelette you have to break a few eggs!). A compound called NADH helps transport electrons, and 2 NADH molecules are produced. The end product of glycolysis is pyruvate, which is needed to start the next phase of cellular respiration.

This video will help you remember the important steps in glycolysis! (You don't need to know the complicated chemical names, just pay attention to the mechanisms, reactants, and end products!)

We are now inside the mitochondria. The second phase of cellular respiration is the Citric Acid Cycle. This is also sometimes called the Krebs cycle, after the scientist who did a lot of work to help discover the mechanisms behind this part of our metabolism. In the citric acid cycle, or CAC for short, our pyruvate molecule from glycolysis undergoes several processing steps. The first step is the conversion of pyruvate to a molecule called Acetyl-CoA, which is a coenzyme. Coenzymes accept electrons from other molecules that are trying to get rid of them (getting rid of electrons is called oxidation, and reduction is when compounds accept those electrons). Here, carbon dioxide is released as a byproduct. Have you ever heard that when you exhale, it's carbon dioxide? That's true! And this is where it comes from! You may be wondering why it's called the citric acid cycle. In the beginning, but after glycolysis, Acetyl CoA combines with another molecule to form the compound citrate. During the CAC, 2 more molecules of ATP are made, along with 2 FADH2, and 6 NADH molecules. Don't worry too much about NADH and FADH2, just know they are coenzymes and they are electron carriers! As you can see below, the CAC is called a cycle because it continues on and on! The main products we want to focus on are the electron carriers, which are very important for our next step!

This video will help you remember some of the more important aspects of the Citric Acid Cycle, a.k.a The Krebs Cycle (same guy from the glycolysis video!)

The final step in cellular respiration is called the Electron Transport Chain, also known as ETC or ETS (electron transport system). This, to me, is the most important step! This is where the majority of our ATP is made. Up until this point, we have 2 ATPs from glycolysis and 2 ATPs from the CAC. 4 ATPs is not a lot, so the rest of our ATP is generated in the ETC! The main products of this step are ATP molecules!

The electron carriers like NADH and FADH2 that we made in the CAC are very important to this step. The energy that came from the electrons is used pump protons across the inner membrane of the mitochondria. This is called an electrochemical proton gradient. Protons are pumped from the inside of the mitochondria to the intermembrane space (not quite inside or outside the mitochondria, but in the middle!). The protons need to get out, so they have to use a channel. But they need help, and ATP synthase is the helper enzyme (all enzymes end in -ase, and usually are named after what they do. This enzyme helps synthesizes ATP). This actually makes the proton pump "spin," creating kinetic energy! The energy here comes from actual movement, and ADP (adenosine diphosphate) is combined with another phosphate group to form ATP! This happens over and over, electrons are accepted and let go, hence why it is called a chain. The final electron acceptor is oxygen, which brings us back to why this is called aerobic respiration. If oxygen is not present, the ETC cannot function. Approximately 32-24 ATPs are made from the ETC. With the 4 from the first two steps, we now have a theoretical yield of 36-38 ATPs per 1 molecule of glucose.

Your body has billions of mitochondria! Most of them are concentrated in your muscles where you need the most energy. These processes are happening all the time and insanely fast! So the next time you go for a walk, eat an apple, or even relax on the couch, remember how hard your body is working to provide energy for you!

Check out this fun game/tutorial to help you understand the complex process of cellular respiration (You will need Java!):

Still don't get it? Check out these supplementary resources that may turn on some lights! They include quizzes and animations that will help you visualize some of this material, I know it is difficult since it is a very abstract concept!