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Cellular Respiration

The cellular process which transfers chemical energy from glucose to ATP.

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Cellular Respiration: Regents Level

Why eat?

Because we're hungry. Not necessarily. But biologically speaking…we eat to get energy. The food we eat is broken down, the glucose extracted, and that energy is converted into an energy molecule called ATP.

Cellular Respiration

What happens to the energy stored in glucose during photosynthesis? How do living things make use of this stored energy? The answer is cellular respiration. This process releases the energy in glucose to make ATP (adenosine triphosphate), the molecule that powers all the work of cells.

There are two types of respiration reactions. We have:

Aerobic Respiration: A reaction that requires oxygen and produces 36 ATP molecules.

Anaerobic Respiration: A reaction the requires no oxygen and produces 2 ATP.

Aerobic Cellular Respiration

What is Aerobic Respiration?

Aerobic respiration is the process that requires the use of oxygen to release energy from the bonds in organic molecules like glucose. This process is efficient in that it produces 36 molecules of ATP energy. We carry out aerobic respiration while at rest or during any low intensity to moderate intensity activity. 

What Do We Need For Aerobic Respiration? 

An easy way to remember the raw materials for aerobic respiration is to remember two things that we need to do in order to live; eating and breathing. Eating and breathing provides us with the two raw materials that are needed for aerobic cell respiration. In order to carry out aerobic respiration we need glucose and oxygen.

What Does Aerobic Respiration Produce?

Aerobic cell respiration produces three products. First and foremost, aerobic cell respiration produces 36 adenosine triphosphate (ATP) molecules.It also produces two waste products that we breath out; water and carbon dioxide. Wait, you don't believe we produce water ? Check out the the image below. The fog you see is water vapor coming produced by cellular respiration. 


License: CC BY-NC 3.0

What Is The Reaction For Aerobic Cellular Respiration? 

                                     C6H12O6 + 6 O2 -----> 6 H2O + 6 CO2 + 36 ATP

Where Does Cellular Respiration Happen?

The first part of aerobic cell respiration takes place in the cytoplasm of the cell. However, ATP molecules are produced inside mitochondria  of the cell. The mitochondria is the "powerhouse' of the cell because it is the site where ATP molecules are formed. If you look below, there is a diagram of a mitochondria. 

 The mitochondria has an inner and outer membrane. Cellular respiration occurs within the following structures:

Intermembrane Space: The space between the cristae and the inner membrane.

Matrix:  The fluid that fill the inner membrane of the mitochondria

License: CC BY-NC 3.0

What Happens In Aerobic Cellular Respiration?

Cellular respiration occurs in three stages:

  • Glycolysis: Occurs in the cytoplasm of the cell outside of the mitochondria.
  • Kreb's Cycle: Occurs inside the matrix of the mitochondria
  • Electron Transport Chain: Occurs inside the mitochondria

Glycolysis ( Glyco- Glucose and Lysis- Seperate): Glycolysis occurs out in the cytoplasm. The main purpose of is to seperate glucose  (6 carbon atoms) into two pyruvic acid (3 carbon) molecules and make NADH energy for the Krebs Cycle. Pyruvic acid moves into the mitochondria for the Krebs Cycle and 2 ATP are created. A molecule of NADH carries electrons into the mitochondria.


Raw Materials Used: Glucose
Products Made: 2 ATP, 2 Pyruvic Acid Molecules and 1 NADH (electron carrier) molecule. 

Kreb's Cycle - Kreb's Cycle occurs in the matrix of the mitochondria. The main job of the Kreb Cycle is to produce energy for the final step, the electron transport chain. Kreb cycle breaks down pyruvic acid for energy. It increases the energy in NADH made from glycolysis and makes another energy rich molecule, FADH2, for the third and final phase of respiration.

Raw Materials Used: 2 Pyruvic Acid Molecules
Products Made:  carbon dioxide (waste), 2 ATP and high energy molecules of NADH and FADH2.

Electron Transport Chain - The main job of the electron transport chain is to create ATP. The third and final step in cellular respiration occurs between the matrix and the inner membrane space of the mitochondria. NADH and FADH2 force protons out of the matrix. This creates a concentration gradiant with a high concentration of protons (H+) outside the matrix and a low concentration inside the matrix. Protons re-enter the mitochondria and create ATP along the way.

Oxygen that is breathed in combines with protons (H+) to form water that we breath out. 

Raw Materials Used: NADH and FADH2
Products Made: 32 ATP (ENERGY!) and Water (waste).

Why Do We Need Cellular Respiration?

We need cellular respiration to replenish energy that we use. ATP has three phosphates (adenosine tri-phosphate). When energy is used, a bond is broken between two of the phosphate molecules creating ADP (adenosine di-phosphate). Cellular respiration creates a new bond between ADP and a phospate molecule, creating ATP. See the diagram below.

License: CC BY-NC 3.0

Anaerobic Cellular Respiration

Anaerobic fermentation is the process in which organisms produce energy without oxygen. There are two types of anaerobic cellular respiration:

1. Lactic Acid Fermentation - In the cytoplasm.
2. Alcohol Fermentation- In the cytoplasm.

 Lactic Acid Fermentation

Lactic fermentation occurs in animals and in bacteria. Have you ever felt like this after running the mile in gym?

License: CC BY-NC 3.0

Yes?  When you run out of oxygen, your body goes into oxygen debt and then begins anaerobic respiration in addition to aerobic respiration. Running becomes painful because of the lactic acid build up in your muscles. Lactic acid fermentation is the process where glucose is broken down into lactic acid in the absence of oxygent. Here's how it happens.

Step 1- Glycolysis occurs splitting glucose into pyruvate and creates 2 ATP

Step 2- Pyruvate is further broken down into lactic acid so electrons can be used to break down glucose for another set of energy. 

Step 3- Lactic acid builds up and you start to cry.

License: CC BY-NC 3.0

Raw Materials:
Products: Lactic Acid and 2 ATP.


Alcohol Fermentation 

Alcohol fermentation is the process where glucose is broken down into alcohol,carbon dioxide and 2 ATP.  First, just like aerobic respiration and lactic acid fermentation, glucose is brokend down into pyruvate and 2 ATP are formed. The pyruvates are further broken down into ethyl alcohol and carbon dioxide. This also allows for electrons to be transferred to break down more glucose.

Cellular Respiration Overview

Here's another review video from Paul Anderson of Bozeman High School in Montana.

 Here is a quick overview of cell respiration that goes over the basics of cellular respiration. 
*Note: The TCA Cycle = Kreb's Cycle

Lesson Check: Cellular Respiration 

Please complete the lesson check below on cellular respiration. Remember, you may use your notes or the reading above to help you answer the questions.


Image Attributions

  1. [1]^ License: CC BY-NC 3.0
  2. [2]^ License: CC BY-NC 3.0
  3. [3]^ License: CC BY-NC 3.0
  4. [4]^ License: CC BY-NC 3.0
  5. [5]^ License: CC BY-NC 3.0

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