Cellular Respiration        

Acknowledgement: this page is adapted from a page found at http://www.biologycorner.com/bio3/notes-respiration.html

The process by which your cells transfer the energy in organic compounds (glucose, starch, carbohydrates) to ATP, the byproduct of this reaction is water and carbon dioxide. It occurs in the cell's cytosol (cytoplasm) and the cell's mitochondria,  the energy producer or "powerhouse" of the cell.

Cellular Respiration occurs in three stages

1. Glycolysis
2. Krebs Cycle
3. Electron Transport Chain
(oxidative phosphorylation)

1. Glycolysis (anaerobic: does not require oxygen)

  1. Occurs in the cytosol

  2. Glycolysis has 10 steps, each step is catalyzed by a different enzyme

  3. glucose (6 carbons) is broken down into two pyruvate molecules (3 carbons each)

  4. ATP consuming phase - 2 ATP are consumed

  5. ATP producing phase - 4 ATP are produced



    • If O2  is available, each NADH can be used to make 3 ATP in oxidative phosphorylation (Electron Transport)

Animation of Glycolysis

  • If you remember one thing about glycolysis, remember that starting with one molecule of glucose, C6H12O6, glycolysis yields 2 molecules of the 3-carbon acid, pyruvate (i.e. pyruvic acid) , 2 ATP, 2 NADH and does not require O2



2. Krebs Cycle (also known as the CITRIC ACID CYCLE) (aerobic--requires oxygen)

  1. Occurs in the matrix of the mitochondria

  2. The breakdown of glucose is completed and CO2 is produced

  3. In the presence of O2, the two pyruvate molecules produced by glycolysis travel to mitochondria

  4. Krebs Cycle has 8 steps, each step is catalyzed by a different enzyme

  5. Each turn of the Krebs cycle requires the input of one 2-carbon acetyl-CoA and two carbons are released as C02

    • One glucose molecule entering glycolysis results in the production of 2 pyruvate molecules which are converted to 2 molecules of acetyl-CoA,

    • Each turn of the Krebs cycle uses only ONE acetyl-coA, so it takes two complete turns of the Krebs cycle per glucose

  6. The NADH and FADH2 created in the Krebs cycle are used by electron transport system to create large amounts of ATP

    • 3 ATP per NADH and 2 ATP per FADH2 that enter electron transport

Animation of Krebs Cycle

  • If you remember one thing about the Krebs cycle, remember each pyruvate molecule yields 4 NADH, 1 FADH2, 1 ATP and 3 CO2 molecules
  • Double this number, for each glucose molecule that enters glycolysis because the Krebs cycle makes 2 turns, one for each pyruvate


3. The Electron Transport Chain (also known as oxidative phosphorylation)

  1. Produces the energy that drives the synthesis of ATP in oxidative phosphorylation

  2. Consists of molecules (mostly proteins) that are embedded in the inner mitochondria

  3. Cofactors are attached to these proteins.  The cofactors are alternately reduced and oxidized as they accept and donate electrons

  4. The initial electron acceptor in the chain is a flavoprotein (FMN) and it accepts an electron from NADH. The electron is then passed down a series of molecules to oxygen, which is the final electron acceptor. It is then combined with a couple of hydrogen ions, H+, to form water. The process will not function in the absence of oxygen.

  5. FADH2 and NADH both donate electrons to the chain

  6. The electron transport chain doesn't make any ATP itself, instead these reactions are coupled to others to produce ATP, a process called CHEMIOSMOSIS

Inner membrane

of the mitochondrion













View both of these animations!  They will totally help you understand!

Animation of Electron Transport Chain         Another Animation of Electron Transport Chain 

The NET RESULTS of the Aerobic Respiration

  1. Each NADH produces 3 ATP

  2. Each FADH2 produces 2 ATP (since FADH2 enters the ETC at a lower step)  

  3. Do the math: 34 ATP produced in the ETC + 2 from glycolysis + 2 from Krebs = 38 total ATP from each glucose

  4. However....In some cells 2 ATP are used to move NADH into the mitochondria from the cytoplasm. Therefore, a grand total of 36 ATP produced from each glucose molecule

Anaerobic Respiration

In the absence of oxygen (remember that oxygen is the final electron acceptor) the cell goes through a process called FERMENTATION

Fermentation takes place under anaerobic conditions (no oxygen present). There are two types of fermentation, alcoholic fermentation and lactic acid fermentation.

Alcoholic Fermentation: occurs in yeasts, fungi and some bacteria. 

                           Pyruvate (C3)      Acetaldehyde (C2) + CO2  

Acetaldehyde (C2)  +  NADH      2 Ethanol (C2)   +  NAD+

Lactic Acid Fermentation:  occurs in some bacteria and oxygen deprived human muscle cells where lactate is produced as a waste product. Muscle cramps are caused by increased acidity caused by the lactate build up.

NADH  +  Pyruvate (C3)      Lactic Acid (C3) +  NAD+

Anaerobic Respiration Steps
  1. Glycolysis occurs, and pyruvate is produced, the pyruvate enters the Krebs cycle and produces NADH2 and FADH2, and some ATP

  2. The problem occurs in the ETC, because there is no oxygen to be the final electron acceptor

  3. The ETC carrier proteins become saturated with electrons which prevents NADH from unloading the 2 electrons it carries--hence, NAD+ cannot be regenerated normally and NADH levels increase, while NAD+ levels increase.

  4. The primary goal of fermentation is to regenerate NAD+.  NAD+ is required for Glycolysis to function. Fermentation generates  very little ATP: Only 2 ATP per glucose