Analysis of The Four Steps of Cellular Respiration

As you sit here reading this your body is going through a series of steps to give you the energy you need to live. As you blink, think, breath, walk, and talk your body uses this energy which it gets from the food you eat. Let’s take Gerald as an example. Gerald has just eaten an apple. His body is going to take the glucose from this apple and extracts the energy that is in glucose through a series of reactions called cellular respiration. Without cellular respiration Gerald would be dead. The four steps of cellular respiration are Glycolysis, Acetyl CoA Formation, The Krebs Cycle, and The Electron Transport Chain. By following the electrons in cellular respiration we can understand how Gerald’s apple, in part, gives him the energy to survive.

Cellular Respiration begins in glycolysis. Glycolysis breaks down a glucose molecule in order to take the energy stored in its bonds. This Glucose molecule comes from the food that we eat. For example, when Gerald ate his apple, the glucose from that apple will begin to be broken down in glycolysis. Now this glucose molecule will travel to the outside of a mitochondria, or in the cytoplasm of the cell, to begin the process. First the glucose molecule will be rearranged and using two ATPs it will add a phosphate one either side. This molecule is called fructose-1-6-bisphosphate. Due to the fact that this molecule is unstable it splits into two three carbon compounds with one phosphate on each. Next these two three carbon compounds turn into pyruvate. In this process, for each pyruvate, two molecules of ATPs and an NADH are produced. The ATP is produced when a phosphate is added to an ADP, and the NADH is produced when an NAD+ gets a h+ added to it. This means that an electron is added to the NAD+. This NADH will be later used in the electron transport chain which makes more ATPs. Since two ATPs were used in the first steps of glycolysis the end product of glycolysis makes 2 ATPs, 2 NADHs and 2 pyruvates. The breaking of the bonds in glucose and from the splitting of fructose-1-6-bisphosphatase release the electrons that the ATP and NADH capture. These electrons then move to the next step of cellular respiration which is the Acetyl CoA formation. 

The next step of cellular respiration is the Acetyl CoA formation. In this step one pyruvate produced in glycolysis moves into the matrix of the mitochondria. To begin this step the pyruvate loses a carbon which makes a two carbon compound that is called acetyl. When the pyruvate loses the one carbon it releases CO2. Due to the fact that a bond was broken to take away this CO2 electrons are released. These electrons get picked up by and NAD+ making and NADH+H+. According to the article What’s the Difference Between NAD+ and NADH,

“First, a charged hydrogen molecule (H+) and next, two electrons. As electrons are negatively charged, the combination of the positively charged NAD+ and H+, coupled with two electrons, effectively cancel each other out and neutralize the resulting NADH molecule.” Once the 2 carbon compound has been made, coenzyme A is added to the acetyl making it Acetyl CoA. the coenzyme is added to the acetyl in order to allow it to move through the two membranes of the mitochondria and into its matrix where the rest of the steps of cellular respiration will occur. Once the Acetyl CoA enters the mitochondria the coenzyme leaves and you are now left with the two carbon compound. The purpose of this coenzyme was to not allow an H+ to attach to the Acetyl. Because it was only a holding spot the bond in between the acetyl and coenzyme is very weak meaning not a lot of energy is stored there. In summary the purpose of this step of cellular respiration was to get the two move the two carbon compounds made from pyruvate into the mitochondria allowing it to be used for the next step of cellular respiration, The Krebs Cycle.

The Krebs Cycle begins when the acetyl is attached to a four carbon compound called oxaloacetate. The product of this happening is a molecule called citrate. Citrate is a six carbon compound and is the first product of the Krebs cycle (Previous two sentences were paraphrased from The Krebs Cycle worksheet). The article, Oxidation of Pyruvate and The Citric Acid Cycle posted on LumenLearning.com explains. “Unlike glycolysis, the citric acid cycle is a closed loop: the last part of the pathway regenerates the compound used in the first step. The eight steps of the cycle are a series of redox, dehydration, hydration, and decarboxylation reactions that produce two carbon dioxide molecules, one GTP/ATP, and reduced forms of NADH and FADH2.” The Kreb cycle is also considered an aerobic cycle because it requires oxygen. This cycle requires oxygen because the “NADH and FADH2 produced must transfer their electrons to the next pathway in the system, which will use oxygen.” as explained in the previously used article, Oxidation of Pyruvate and The Citric Acid Cycle. Once the citrate is made a bond breaks in between one carbon and electrons and CO2 are released. These electrons are picked up by an NADH. This NADH will be used in other steps in cellular respiration by providing its electrons as energy. Once this step is complete we are left with a five carbon compound. This five carbon compound then breaks another bond releasing more energy and let’s go of another CO2. When this occurs An NAD+ and an ADP pick up these electrons making one NADH molecule and one ATP molecule. They will later be used to provide energy for more steps of cellular respiration. After the second CO2 is separated we are left with succinate which is four carbon compound. The next step of the Krebs Cycle is to convert succinate to oxaloacetate. In order to do this the succinate must first change into fumarate which is an enzyme. In order to do this the succinate transfers two hydrogens to a carrier protein called FAD which in result creates FADH2. FADH is a carrier protein much like NADH however it has some characteristics that are unique. According to the article Oxidation of Pyruvate and The Citric Acid Cycle, “The energy contained in the electrons of these atoms is insufficient to reduce NAD+ but adequate to reduce FAD. Unlike NADH, this carrier remains attached to the enzyme and transfers the electrons to the electron transport chain directly” Once the fumarate is made, it is then changed into malate. This occurs when water is added to fumarate. In the very last step malate is turned back into oxaloacetate. This is done by oxidizing the malate. Now the cycle has been completed. However the Kreb Cycle must happen one more time, since for each pyruvate the cycle turns twice. Meaning that at the end of these two cycles you would have created 6 NADH, 2 FADH, 2ATP, and 4CO2. The NADHs FADHs and ATPs are all responsible for carrying the electrons from the Krebs cycle. These electrons which were obtained when high energy bonds were broken will be used throughout the steps of cellular respiration in order for the cell to have enough energy to complete these steps. The FADH and some of the NADH will now directly move to the next step of cellular respiration with is the Electron Transport Chain (ETC) providing energy for it to work.

The Electron Transport chain is the final step of cellular respiration. In this step the most amount of ATPs are created. Unlike in the previous steps like glycolysis and the Krebs cycle where only two are produced, the electron transport chain produces 34 ATPs. Not only this but it also creates water by the end of the process. Just as how glycolysis occurs in the cytoplasm of the cell and the krebs cycle occurs in the matrics of the mitochondria electron transport chain occurs in the cristae, or inner membrane of the cell. The Electron Transport Chain consists of three proton complexes which runs across the inner mitochondrial membrane. To begin, an NADH from the Krebs Cycle moves to the first protein complex. Here the electrons from the NADH move through the protein complex releasing a hydrogen ion into the inter membrane space. Energy is needed for this to happen because since there are already existing hydrogen ions in the membrane space the hydrogens do not want to move to that area. Once this has been completed the electrons move to the second protein complex there the same process occurs, releasing an electron into the membrane. Last, electron moves to the very last protein complex where the last of its energy is used to pump another hydrogen into the membrane space. This electron is now a low energy proton and needs something to attach to so that it has somewhere to go since it cannot just be let go. This electron attaches to an oxygen and two hydrogens in result making H2O. It is crucial for this step to occur since the electrons don’t attach to the O2 and hydrogen ions the electron transport chain can get blocked and as a result kill the cell. Not only are electrons used from the NADH that came from the Krebs Cycle but also from the FADH2 that was also created. The same process that the NADH went through the FADH2 does the same. However this time it does not begin in the first protein complex in the second one. This means that in produces one less hydrogen than the NADH does. The next step of the ETC is ATP synthase. In this process a hydrogen ion from the space between membranes moves through the ATP synthase making the ATP. The reason why energy is created when this occurs is because while the area where the hydrogens are is positively charged inner part of the membrane is negatively charged. “...it's basically the same as a battery you’ve got a compartment with a positive and one compartment with a negative, when you allow those charges to flow from the positive to a negative… if you can remember physics that is current, that's what current is and so what we’re generating right here is an electrical protons.” Stated in the video called ETC and Chemiosmosis made by Eric N Madrid. In short this quote explains that since the positive ions are moving to a negatively charged space they are creating energy. This energy is what turns the ADP into ATP. As previously said the ETC produces 34 ATPs meaning that this process of chemiosmosis will occur 34 times. The Electron Transport Chain is a very important part of cellular respiration due to the fact that it produces the most amount of ATP or energy. The water produced from this step will be released from your body through your sweat or as you breath and the ATP that you created will be used in your body as energy.

Gerald has finished his apple. He greatly enjoyed each bite of the juicy green fruit knowing that by eating it he is giving his body the sustenance to breath. He may not directly know all these specific steps of cellular respiration but his body knows what is needed to do to give him energy. His body moves the electrons from Glycolysis to Acetyl CoA formation to The Kreb cycle and finally to the Electron transport chain. At the end of this process his body will have made 38 ATPs that his body can use for energy. Now that we have followed these electrons throughout these processes we can now understand how these processes occur and Geralds afternoon snack is giving him the energy to live out his day. And now Gerald must go back to writing his essay for Mr. Childress’s class and as his fingers type away he will use the ATP his body has made and hopefully he’ll get an A on his essay.