Glycolysis & the Citric Acid Cycle

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  • 0:02 Cellular Respiration
  • 1:25 Glycolysis
  • 3:10 The Citric Acid Cycle
  • 5:12 Lesson Summary
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Lesson Transcript
Instructor: Christopher Muscato

Chris has a master's degree in history and teaches at the University of Northern Colorado.

How does food actually become energy? Through cellular respiration! In this lesson we will examine two main components of respiration and discover exactly how they break down food into useable energy.

Cellular Respiration

This is a hamburger. And this is Butch the bodybuilder. Butch is very strong and has lots of muscles, but to create those muscles, his body has to be able to take this hamburger, extract the energy contained inside the proteins and carbohydrates, and use that energy to build up new muscle tissues. This process is just as complex as it sounds, and there are several things at work here. Foremost is cellular respiration, the process by which cells release the energy in food molecules and make it available for use.

The main goal of cellular respiration is the creation of ATP, a molecule that packages and transports energy in a way that the rest of the cell can use. Creating ATP requires a series of metabolic pathways, chemical reaction chains in which the product of one reaction powers the next reaction. The metabolic pathways include glycolysis and the citric acid cycle. These pathways are manufactured by enzymes, molecules that speed up chemical reactions. So, one enzyme is in charge of the first part of the pathway, and it creates a product used to start the second reaction, so on and so forth. That's the basics, but what do you say we take a closer look? Let's see what real bodybuilding is all about.

Glycolysis

So, cells turn organic molecules into useable energy through cellular respiration. That's the big picture. But, we can break this down further into a few metabolic pathways that make respiration possible. The first is glycolysis, the pathway responsible for breaking down glucose into an even simpler molecule called pyruvate. Glucose is a basic form of carbohydrate called a simple sugar and one of the most common sources of energy. So, the body ingests food, from that food carbohydrates are extracted, and from those we get glucose. Now glycolysis occurs. Through glycolysis, one glucose molecule will turn into two pyruvate molecules. This process happens in the cytoplasm of the cell.

Glycolysis really has two distinct phases. In phase one, two molecules of ATP are consumed by the cycle to generate the energy needed to break down the glucose into a molecule called glyceraldehyde 3-phosphate. Then in phase two, this molecule is converted into pyruvate. Breaking apart the molecules releases the energy within the molecular bonds, and that's what is packaged as ATP. Now, phase two yields four ATP, but remember that we had to use two ATP to get this process started, so the net gain is really two ATP. Glycolysis also yields two molecules of NADH, a molecule important to many pathways as an electron donor, but the main function of this cycle is to create a net gain of two ATP and those two molecules of pyruvate.

The Citric Acid Cycle

Once we've finished with glycolysis, we can move onto stage two of respiration, the citric acid cycle, also called the Krebs cycle after the scientist who discovered it. The goal of this pathway is to release the energy within the pyruvate molecules created by glycolysis. Remember how we defined these pathways as reactions that power other reactions? Glycolysis produces the molecules that are processed by the citric acid cycle. The citric acid cycle occurs in the mitochondria of the cell and will eventually break pyruvate all the way down to inorganic substances like carbon dioxide and water, thus releasing all of the energy inside the molecule.

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