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What Is Cellular Respiration (Simplified)

Cellular Respiration is simply respiration(using oxygen) at a cellular level. When you learn the 3 steps of cellular respiration you will be able to connect why animals must breath oxygen and eat food to live. Sugar from food and oxygen are used in cellular respiration to create energy for the billions of cells in your body to live. Although there are other ways to make energy from the other nutrients in food, the focus of this article is to describe how cellular respiration uses sugar.


Cellular Respiration

A simplified approach to remembering cellular respiration is to break it down into 3 major steps and learn them separately. Each step further break down sugar and make molecules that are needed in the next step. Some simple rules you will need to accept right now are(this will make sense later):

  • Cellular respiration makes energy from sugar.
  • Cellular Respiration needs Oxygen to occur.
  • Cellular Respiration takes place in the mitochondria; the powerhouse of the cell.



Glycolysis is the first step of cellular respiration and commonly begins with the simple sugar glucose. Through a series of steps a single molecule of glucose is broken down into 2 molecules of Pyruvic Acid to be used in Step 2 of Cellular Respiration.

The simplified steps of Glycolysis are show below but there is a more detailed description in my article about memorizing Glycolysis. The importance of glycolysis is the breakdown of a molecule of sugar. Not much energy is produced in this step.

[table caption=”Steps of Glycolysis”] Stage,Molecules
Splitting,Fructose-1 6-Bispohsphate
Splitting,G3P and DAP
Splitting,(2 molecules)G3P
Energy Generation,(2x) 13-Bisphosphoglycerate
Energy Generation,(2x) 3-Phosphoglyercerate
Energy Generation,(2x) 2-Phosphoglycerate
Energy Generation,(2x) Phosphoenolpyruvate
Energy Generation,(2x) Pyruvic Acid
Glycolysis starts with glucose and ends with Pyruvic Acid
[table caption=”Products of Glycolysis”]



Pyruvic Acid(used in the next step of cellular respiration)



Citric Acid Cycle

The second step of Cellular Respiration has many names; Citric Acid Cycle, Krebs Cycle, or TCA Cycle. Pyruvic Acid from Glycolysis is modified and brought inside of the Mitochondria where the rest of cellular respiration occurs. The Citric Acid Cycle is a repeating process that will make a majority of the molecules that are needed to make cellular energy. The Citric Acid Cycle makes the necessary precursors for the final step of cellular respiration.

Citric Acid Cycle Molecules
The numbers represent the amount of Carbons in a molecule. Essentially the number of carbons tells you how big the molecule is as it is being broken down.

If you remember that 2 molecules of Pyruvic Acid where made in Glycolysis, this should let you know that the Citric Acid Cycle does 2 cycles for every molecule of Glucose. ATP is energy while FADH2 and NADH are precursors that make multiple molecules of ATP in the final step of cellular respiration.

Here is a summary of the end products, but a more detailed article with more pictures and details can be found here: Memorize Krebs Cycle.


1 cycle, 2 CO2; 1 ATP; 1 FADH2; 3 NADH

2 cycles, 4 CO2; 2 ATP; 2 FADH2; 6 NADH




Electron Transport Chain

The electron transport chain is the final step of cellular respiration that uses the oxygen that we inhale to produce energy in the form of ATP. The electron transport chain occurs on the membrane of the mitochondria, and uses the molecules made in Glycolysis and the Citric Acid Cycle to power the creation of ATP.

The NADH2 makes 3 ATP while FADH2 will produce 2 ATP. NADH2 makes more ATP because it enters the electron transport chain at an earlier stage and provides more momentum for ATP production. Think about this like a staircase in the image below. Whoever starts higher up will be moving faster and have more energy when it reaches the bottom.

Original image of stairs by Hansueli Krapf

As you can see from the image the NADH2 and FADH2 bring electrons with them. The electrons powers the hydrogen atoms to build up in one location. The hydrogen atoms then use there large numbers to create ATP. The oxygen we breath comes in to play by accepting the electrons after they have done their job. This is important because free electron floating around the body can cause harm to DNA and other molecules. For a detailed description of this see my article about the Electron Transport Chain.


The over goal of cellular respiration is to make energy in the form of ATP. Below I have an image that summarizes how much ATP is produced in each stage with 1 molecule of ATP.
ATP Cellular Respiration

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One comment

  1. Thank you so much! Now I can see the connections with the three steps.