Anaerobic Respiration  
Anaerobic Pathway

The end product of glycolysis is pyruvic acid. The fate of pyruvic acid depends on the presence or absence of oxygen. If oxygen is present, pyruvic acid can move into the Kreb Cycle (discussed later) where it is oxidized into CO2 and H2O. If oxygen is absent (anaerobic respiration), pyruvic is reduced and the end products are CO2 and ethanol (alcohol, wine) during fermentation in plants or lactic acid (sore muscles) in muscle cells. In your notes draw a road map listing the organic molecules in a step wise fashion used during fermentation. Upon completion of this "road map",  draw a input/output box in your notes indicating  what enters and leaves this pathway.  Include the organic molecules and the biological energy. 
 Study behavioral objectives 18-19 and read the pages indicated by E-7. 

In glycolysis, you studied the seven steps in the process by which cells extract chemical energy from glucose. By means of the initial stages in carbohydrate metabolism, the cell extracts enough energy to produce two ATP energy-storage molecules and two NADH molecules (represented by the green H above ) per glucose sugar molecule. Study the summary diagram above  of those seven steps. 

In the next web page during the  citric acid cycle, you will see  how the process of oxidative respiration enables the cell to extract essentially all of the chemical energy which remains in the end products of those seven steps-two pyruvic acid molecules and four hydrogen atoms. As long as oxygen is available, the process of aerobic carbohydrate metabolism continues via the pathway of oxidative respiration. This path makes possible the complete breakdown of sugar to carbon dioxide and water. 

But not all organisms have a ready and constant supply of oxygen. Many yeast and bacterial cells live in environments in which oxygen is in short supply or completely lacking. In such cases, the cell must turn to fermentation as its principle means of obtaining needed chemical energy. 

In fermentation (anaerobic respiration) , the seven steps illustrated above also occur. After the pyruvic acid, ATP and NADH  are produced, the cell rearranges the products of the initial seven stages to produce a set of stable molecules. Look at the illustration of the entire fermentation process above. 

 Take a close look at the reactions occurring in the two-step process which transforms the pyruvic acid and hydrogen to alcohol and carbon dioxide. Study the figures indicated by E-8. 
Step 1: 

pyruvic acid ---> acetaldehyde + carbon dioxide (word form) 
C3H4O3 ---> C2H4O + CO2 (symbol form) 
Step 2: 
acetaldehyde + NADH  ---> alcohol + NAD  (word form) 
C2H4 + NADH  ---> C2H6OH + NAD (symbol form)
Note: one NADH is used up per pyruvic acid molecule
First, the three-carbon pyruvic acid molecule is fragmented into a two-carbon molecule and a one-carbon molecule (carbon dioxide). Then the hydrogen atoms which were liberated in the earlier stages are combined with each new two-carbon molecule to complete the process with the formation of an alcohol molecule. As you can see by examining the two reactant/product expressions, the rearrangement process (click one): 
a. liberated a lot of energy for the cell 
b. absorbed a lot of energy 
c. has no energy consequences 
 The fermentation process is completed with the liberation the waste products of alcohol and carbon dioxide. At that point all of the chemical energy in glucose has been extracted that can extracted without using oxygen. In a stepwise series of chemical reactions, fermentation converts the single glucose fuel molecule to one ethyl alcohol molecule, one carbon dioxide molecule, and energy. If we look at the overall reactants and products of fermentation, we see that the energy produced is that which was produced early in the process and was sufficient for storage in two ATP molecules. Important: The two NADH molecules which were formed during glycolysis are both used up during the last stage. Therefore, during fermentation you can  not count the NADH as energy production. 
sugar ---> alcohol + carbon dioxide + energy 
C6H12O6 ---> 2C2H5OH + 2CO2 + E (stored in 2 ATP molecules) 
The ethyl alcohol produced in fermentation is the alcoholic component of beverages such as beer, wine, and whiskey. Brewer's yeast is the organism used to produce alcoholic beverages. If yeast is placed into a solution containing sugar (such as natural grape juice) and air is excluded from the container by an air lock, the yeast will metabolize the organic fuel of sugar to extract energy to sustain and perpetuate its life processes. 

Since yeast does not have any oxygen available during brewing, it must use the process of fermentation to obtain the energy it needs. 

 The fermentation process used by the yeast yields the alcohol the brewer is interested in. The carbon dioxide produced by the reaction is allowed to escape from the fermentation vat. If a brewer wants to control the amount of alcohol in a product (beer, 4 to 6%; wine, 10 to 12%), it is clear that he/she must control the amount of sugar in the original solution. 

 The amount of sugar made available to the yeast will limit the amount of all products of fermentation-the alcohol (for the brewer), the carbon dioxide (as waste), and the energy (for the yeast). The yeast cells will die after a while if more sugar is not made available to them. Why? 

Answer: They need the energy to survive. 

Comparing Energy Yields: Fermentation Versus Respiration

 Yeast is a good example of a creature which uses the process of fermentation very effectively. The process certainly works well, for there are numerous creatures that use it to the exclusion of all other energy-liberating processes. But how does fermentation as an energy-liberating process compare with aerobic respiration ? Look at the illustration below, which shows both processes for metabolizing a glucose molecule.Respirtion which produces 38 ATP in the table below will be studied on the next web page. 

 
 

Which statement best describes the amount of energy derived by the two methods? (Click one.) 

a. Fermentation yields about the same energy as does oxidative respiration. 
b. Fermentation yields less energy than does oxidative respiration. 
c. Fermentation yields far less energy than dues oxidative respiration. 
 The only energy extracted from glucose by the fermentation process is that which is released up to the point when pyruvic acid is produced. What additional energy is made available to the cell by respiration that is not obtained through fermentation? 

Answer: The chemical energy stored in the pyruvic acid and the hydrogen atoms. 

As you will learn in the next section, oxidation of the two pyruvic acid molecules produces sufficient energy to convert thirty-six ADP molecules into ATP. Thus the oxidative respiration of one glucose molecule provides the cell with thirty-six more storage units of energy than does the fermentation of one glucose molecule. The illustration provides a simple summary of the two energy-extraction processes. 

As you can see, compared to respiration, fermentation is shown to be a very inefficient process of energy-extraction for a cell. 

 
Summary

Energy is a necessity of life; to the cell, the energy which can be obtained from each molecule of organic fuel is very important. The cell has evolved the mechanisms for extracting, through a complex series of chemical reactions, all the chemical energy stores in the organic fuel it produces or takes in as food. 

Respiration, photosynthesis (in which the sun's energy is transformed into chemical energy in complex organic fuels), and the energy-exchange process of the AMP/ADP/ATP molecular systems are all examples of very complex biological processes. These complex mechanisms, through which living matter obtains, stores, and uses energy, have all evolved because they are effective means through which life can be sustained and perpetuated. 

The story of life is the story of the use of energy. Deprived of the mechanisms for using readily available energy, all living matter would immediately succumb to the trends described by thermodynamics-without energy to build and maintain complex structures, living matter could not remain highly organized and dynamic. Life would quickly slip into static disorganization. 

Photosynthesis locks up the energy of the sun in highly organized organic molecules. This energy can be unlocked by both plants and animals via the processes of fermentation and respiration. All living matter engages in the continual quest for energy in usable forms. The struggle for life is best expressed as a struggle for energy. 

Review

Look at this illustration of the complete process of fermentation: 

Note: The NADH (green H) formed during glycolysis are used up during the last step of fertilization. Since glucose was broken into two pyruvic acids, two carbon dioxides and two alcohols will be formed during fermentation. 

1. The first seven steps of fermentation are exactly the same as the initial seven steps of oxidative respiration. (Click T or F.) 

2. In Step 8, energy is: 

a. liberated 
b. used 
c. neither used nor liberated 
3. In Step 9, energy is: 
a. used 
b. liberated 
c. neither used nor liberated 
4. All the energy from fermentation: 
a. was liberated during the first seven steps 
b. was used up by the cell 
c. is still to be liberated in further steps 
5. In Steps 8 and 9, the CO2 and alcohol are: 
a. further processed for energy 
b. excreted as waste products by the cell 
6. In the process of carbohydrate metabolism: 
a. fermentation yields about the same energy as oxidative respiration 
b. fermentation yields less energy than oxidative respiration 
c. fermentation yields far less energy than oxidative respiration 

                                             input/output box 
                                      (write in your notes and learn) 

   Organic Molecules -   Input: one glucose   
                                    Output: 2 ethanol and 2 carbon dioxide molecules  
    Energy Output - ATP ( 2 used during activation and 4 synthesized = net of 2) 
                              NADH (none, 2 formed during glycolysis but used during 
                              the  last step of fermentation) 
 

Fermentation  produces a net of 2 ATP. 
This is the end of  lesson five. Click here to go back to the home page and lesson six  Respiration - Krebs Cycle.    click 

Created by the Multimedia Development Lab, Academic Technology Services.  
Last modified October 29, 1997.