Overview of Cellular Respiration & Its Steps

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  • 0:01 What Is Cellular Respiration?
  • 0:57 ATP and Activation Energy
  • 1:49 Glycolysis
  • 2:58 Aerobic Respiration
  • 4:49 Anaerobic Fermentation
  • 6:22 Lesson Summary
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Lesson Transcript
Instructor: Kevin Newton

Kevin has edited encyclopedias, taught middle and high school history, and has a master's degree in Islamic law.

Ever wonder how your body turns nutrients and oxygen into energy? Or maybe how beer or champagne get both bubbles and alcohol? It turns out that both processes are a direct result of cellular respiration.

What Is Cellular Respiration?

So by now you've probably figured out that the food you eat doesn't just go into the bottomless pit. Instead, it provides energy necessary for life. However, how does the energy present in an apple get broken down to be the fuel for life? Not surprisingly, such action takes place on an extremely small level. Once the process of digestion is complete, that apple exists as a bunch of carbohydrate strings. The path from carbohydrate strings to energy is known as cellular respiration and is one of the most essential functions of any animal. In this lesson, we'll look at the process of cellular respiration in a variety of organisms. In the case where the body has no carbohydrates to metabolize, the body can also turn fats and proteins into energy. However, for this lesson, we'll focus on carbohydrates. Spoiler alert, oxygen is not always necessary!

ATP and Activation Energy

In many ways, cellular respiration is like a party. Until someone really gets it started, there's just a lot of standing around. For cellular respiration, the process of getting the party started is called achieving activation energy. For glucose molecules, which are the carbohydrates that are the most common source of energy in the body, the extrovert that provides the activation energy is called ATP. ATP is an acronym for adenosine triphosphate, but everyone just refers to it as ATP. In fact, it's so efficient at holding energy that it is often used as an energy reservoir itself. ATP is one of the most important molecules in the body, as the process of cellular respiration would grind to a halt without it. And if cellular respiration grinds to a halt, the body just dies.


So if ATP is the extrovert and activation energy is the act of that first cannonball into the swimming pool, then glycolysis is the reason for having the party in the first place. It is the first reaction in the process of cellular respiration and turns glucose into pyruvate. This pyruvate, also called pyruvic acid, is an intermediate step on the road to energy for most organisms, but is an important step nonetheless. Glycolysis can happen with or without oxygen, as it only requires ATP and glucose. It helps to kickstart the whole process, as it turns glucose into pyruvate. As we will see, pyruvate is much easier to use as fuel. Think about it like this…when you are sleepy, you really can't do much in the way of having a party. However, after a couple of cups of coffee, then you're ready to mingle. Glycolysis is that process of getting a couple of cups of coffee in you so you can really start creating some energy. However, what gets you through the day boils down to either aerobic respiration or anaerobic fermentation.

Aerobic Respiration

However, glycolysis is not enough. Instead, we, like most other animal life, gain energy through a process known as aerobic respiration. It is at this stage that oxygen becomes necessary, which is why we have to breathe. With more oxygen around, the pyruvate molecules can become further oxidized into ATP, and more ATP means more energy. This process happens in the mitochondria of the cells, which is why they are often referred to as the power plants of the cell.

This occurs in two steps. First, the Krebs cycle makes the process of gaining ATP, and therefore more energy, so efficient that it has been called one of the evolutionary wonders that allowed more complex life to exist.

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