Cellula(r)espiration

Anaerobic cellular respiration vs. aerobic cellular respiration:

To my understanding, there are 3 main types of cellular respiration: Anaerobic, Aerobic, and fermentation.

Aerobic respiration takes place in the presence of oxygen, and produces the most ATP in the cycle from one molecule of glucose compared to the others. The chemical formula for glucose going through this aerobic cycle is as follows:

C6H12O6 + 6 O2  –> 6 CO2  + 6 H2O  + Energy (ATP + heat)

Sidenote: although this is the formula for glucose, carbohydrates, fats, and proteins can also start the cycle, but it does get more complicated.

Anyways, in this cycle, the glucose molecule is being oxidized while the oxygen is being reduced. But what does this mean? Oxidation and reduction processes are called redox reactions. Redox reactions are ones that transfer electrons between reactants. When something is oxidized, it loses electrons, and when something is reduced it gains electrons. (remember oil rig; oil is harnessed, while the rig gains the oil). Following this are the terms oxidizing agent and reducing agent, which what I thought at first was straight forward, but it’s actually the opposite of what I thought. The oxidizing agent is one that accepts the electrons, and the reducing agent is the electron donor. Tricky!

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In aerobic cellular respiration, the molecule undergoes 3 main stages: glycolysis, the citric acid cycle, and oxidative phosphorylation.

In glycolysis, which takes place in the cytoplasm of the cell, an organic molecule is broken down into two pyruvate. From this breaking down of glucose, 2 ATP are produced through substrate level phosphorylation (to be further explained). Additionally, 2 NADH grab electrons from this step and head toward the oxidative phosphorylation step. I thought it was interesting that to start this step, 2 ATP are put into the cycle, and in turn, it creates 4 ATP, making a net total of 2 ATP in the glycolysis step. The great thing about glycolysis is that oxygen is not needed, so it can occur in either aerobic or anaerobic respiration.

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Sidenote: Substrate level phosphorylation:

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Although this process creates ATP, it is much less efficient because it takes a long time to make a lot of ATP. Hence, why the glycolysis and krebs cycle step create 2 ATP each. 

The next part of glycolysis is when the pyruvate enters the mitochondria, where the oxidation of pyruvate is completed. Before the pyruvate enters the citric acid cycle, it converts into acetyl coenzyme A, which is the link step between glycolysis and the citric acid cycle. In this link step, 2 CO2 molecules are released and 2 more NADH molecules carry electrons.

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The citric acid cycle: In this step, 4 CO2 molecules are released, 6 NADH and 2 FADH2 pick up electrons, and 2 ATP are created through substrate level phosphorylation. It takes place in the inner mitochondrial matrix. It completes the breakdown of pyruvate into CO2 and ATP. I have a question though; where does this CO2 go? Out of the mitochondria, and if so, where?

The last step in aerobic respiration is oxidative phosphorylation. The two electron carriers, NADH and FADH, donate electrons to this last step to power the ATP synthase. This step takes place in the inner membrane (or cristae) of the mitochondria. As the carriers are dropping off electrons, electrons drop in free energy as they go down the chain and are finally passed to O2 , forming H2O. The interesting thing about this step is that it doesn’t directly create ATP, but instead breaks the large free-energy drop from food to O2 into smaller steps that release energy in manageable amounts. Now this is where chemiosmosis comes in. When the electrons enter the electron chain, proteins pump H+ ions that are present in the mitochondrial matrix into the intermembrane space. Following this, the H+ is pumped back out of the membrane by the ATP synthase. The ATP synthase uses the exergonic flow of the H+ ions to fuel the phosphorylation of ATP. This H+ gradient is referred to as a proton-motive force, emphasizing its capacity to do work.

Sidenote: Exergonic -energy is released to the surroundings. The bonds being formed are stronger than the bonds being broken. Endergonic- energy is absorbed from the surroundings. The bonds being formed are weaker than the bonds being broken.

This whole pumping-the-ions-back-and-forth is an example of chemiosmosis, which is the use of energy in a H+ gradient to drive cellular work. This step is so important! Without the ATP synthase, the chemiosmotic gradient is disturbed, and is not regulated, which the cell needs! The cell creates a steady amount of ATP with the ATP synthase, instead of random spurts of energy. For example, say that you’re sitting in a classroom, and everyone immediately gets up and starts sprinting out of the hall. You get up and join them, and at first, you make energy for this through anaerobic respiration, but after a little, you breath in O2 to create more energy to continue sprinting. But what if you didn’t have that steady, reliable ATP synthase to finish the most important step in the cell? You wouldn’t be able to sprint for very long, do to the unregulated synthesis of ATP.

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Oxidative Phosphorylation

So that is basically the whole (aerobic) cellular respiration process! It’s important to remember that the electron carriers are recycled in the process- there is a finite amount of them in each cell.

But what if O2 is not present in the cell?

This is where fermentation and anaerobic respiration come in. Without O2, the electron transport chain step fails, so in such a case, glycolysis couples with fermentation or anaerobic respiration to produce ATP. The difference between fermentation and anaerobic is this: Anaerobic respiration uses an electron transport chain with a final electron acceptor other than O2, and fermentation uses substrate-level phosphorylation instead of an electron transport chain to generate ATP.

Fermentation consists of two main types: alcoholic and lactic acid. Alcoholic fermentation is where pyruvate is broken into ethanol. First, CO2 is released from the pyruvate. Second, the acetaldehyde is reduced to ethanol.

In lactic acid fermentation, pyruvate is produced by NADH and has no CO2 release in the cycle, creating lactate as the end product.

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For a summary of all these cycles:

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The last thing we talked about this week is obligate and facultative anaerobes. Obligate anaerobes are cells that carry out only fermentation and anaerobic respiration; they cannot survive in the presence of O2. Facultative anaerobes, which could be yeast or many other bacteria, can survive using fermentation or cellular respiration. In facultative, pyruvate is a fork in the metabolic road that leads to two alternative catabolic routes.

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Facultative

I think the one thing I would like to go over a little more is reduction and oxidation reactions (redox reactions) and what part they play in cellular respiration. I’m still a little confused on that.

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One thought on “Cellula(r)espiration

  1. Good work. I like the way you framed your learning and reflected on the processes as you progressed.

    Once CO2 is created as a waste product it diffuses out of the mitochondria into the cell and then into the bloodstream where it is carried away to the lungs. At this point we breath it out!

    Redox reactions are all about the movement of electrons (as biologists we can follow the hydrogens). We are following the electrons because this is the same thing as following the energy. When something becomes oxidized, its free energy goes down (its ability to do work). But, the molecule that is becoming reduced, gets a boost in energy (upto a point). When NAD+ strips away electrons and becomes NADH, it now has electrons (read: more available free energy than before). We can then use that NADH to transport that energy to another location and then put that energy to work by dropping off the electrons. NAD+ becomes reduced to NADH. REmember RIG (reduction is gaining). Not only is it gaining electrons and hydrogens but also energy in this case. Full reduction is what we see at the end of the cellular respiration equation in the form of H2O but there is very little to no free energy left available.

    Hope that helps.

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