One of the most beautiful, majestic, and fascinating metabolic pathways are known to man is cellular respiration. In addition, it is one of the most difficult. I felt like I had tripped and fallen into a can of organic-chemistry-flavored alphabet soup when I first heard about it! But now you don't have to worry about it. I am here to solve all your queries.
Fortunately, once you understand it, cellular respiration doesn't scare you as much. To begin, let's take a high-level look at cellular respiration by tracing the four major stages' connections to one another.
A glucose molecule gradually decomposes into carbon dioxide and water during cellular respiration. During the process, reactions that transform glucose directly generate some ATP. Oxidative phosphorylation, on the other hand, results in the production of significantly more ATP later on. The movement of electrons through the electron transport chain, a collection of proteins embedded in the inner membrane of the mitochondrion, powers oxidative phosphorylation.
The electron carriers NAD + and FAD transport these electrons to the electron transport chain. When they gain electrons, these carriers transform into NADH and FADH2. To make things clear, when it says +NADH, or +FADH 2, this is what is happening. The molecule is not forming from scratch; rather, it is merely being changed into the form that carries electrons.
NAD+ + 2e- + 2H+ → NADH + H+
FAD + 2e- + 2H+ → NADH2
To see how a glucose molecule is converted into carbon dioxide and how its energy is harvested as ATP NADH/FADH2 in one of your body's cells, let’s walk step by step through the four stages of cellular respiration.
1. Glycolysis. In glycolysis, glucose—a six-carbon sugar—undergoes a series of chemical transformations. In the end, it gets converted into two molecules of pyruvate, a three-carbon organic molecule. In these reactions, ATP is made, and NAD+ is converted to NADH.
2. Pyruvate oxidation. Each pyruvate from glycolysis goes into the mitochondrial matrix—the innermost compartment of mitochondria. There, it’s converted into a two-carbon molecule bound to Coenzyme A, known as acetyl CoA. Carbon dioxide is released and NADH is generated.
3. Citric acid cycle. The acetyl CoA made in the last step combines with a four-carbon molecule and goes through a cycle of reactions, ultimately regenerating the four-carbon starting molecule. ATP, NADH, and FADH2 are produced, and carbon dioxide is released.
4. Oxidative phosphorylation. The NADH and FADH2 made in other steps deposit their electrons in the electron transport chain, turning back into their "empty" forms (NAD+ and FAD). As electrons move down the chain, energy is released and used to pump protons out of the matrix, forming a gradient. Protons flow back into the matrix through an enzyme called ATP synthase, making ATP. At the end of the electron transport chain, oxygen accepts electrons and takes up protons to form water.
Glycolysis can take place without oxygen in a process called fermentation. The other three stages of cellular respiration—pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation—require oxygen to occur. Only oxidative phosphorylation uses oxygen directly, but the other two stages can't run without oxidative phosphorylation.
I hope, now, the idea doesn’t bother you anymore.
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