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Basic Energy Concepts
Types of Catabolism
Catabolism of Fats
Catabolism of Proteins
Summary of Catabolism
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Summary of Catabolism
©2000 Timothy Paustian, University of Wisconsin-Madison
Before we go on and take a look at what cells do with all the energy they have so diligently extracted, it is useful to tie together the most important concepts of catabolism.
Microbes can be classified by the type of catabolism they carry out. A useful way to look at these catabolic classifications is to follow the path of the electrons from catabolic substrate to terminal end product.
In fermentation electrons are extracted from a relatively reduced organic compound and eventually end up on a more oxidized organic molecule. An example is the alcoholic fermentation of glucose to ethanol carried out by yeast. Energy yields are typically low and products are not fully oxidized; there is a large amount of energy left in the final product.
In aerobic respiration electrons are extracted from organic compounds and oxygen is the terminal electron accepter. The organic substrate is often completely oxidized to H2O and CO2 and a large amount of energy is extracted. Much more than with fermentation.
In anaerobic respiration electrons are extracted from organic (and sometimes inorganic sources) and donated to an inorganic molecule that is not oxygen. There are several different types of anaerobic respiration, but the most common terminal electron accepters are nitrate, sulfate and carbonate. Anaerobic respiration typically extracts more energy than fermentation, but less than aerobic respiration.
Some of the more interesting ways of doing business involve organisms that use an inorganic molecule as both their electron donor and their electron accepter. Often these organisms will use CO2 as their sole carbon and energy source.
As an example, some microbes are capable of using H2 as there electron donor and donating these electrons to Fe+3 to form Fe+2. Ferrous iron (Fe+2) is much more soluble than ferric iron (Fe+3) and will leach from anaerobic environments. Eventually the ferrous iron in solution will encounter oxygen and a non-biological oxidation will take place.
4Fe+2 + O2 + 8OH- + 2H2O 4Fe(OH)3
Iron hydroxide is very insoluble and will precipitate forming brown deposits. Aerobic areas that drain bogs (anaerobic aquatic environments) will often have iron deposits accumulate where oxygen comes in contact with the ferrous iron being produced by microbial activity in the bog. These iron bogs are common in cooler parts of the world..
Finally there is photosynthesis.
It is also possible to categorize microbes according to their metabolic processes.
Obligate Aerobes - Only grow in the presence of oxygen and they are normally dependent upon aerobic respiration for their energy
Obligate respirers - Only respire either aerobically or anaerobically.
Obligate anaerobe - Can only survive in the absence of oxygen.
Obligate fermenter - Can only ferment their energy source and are incapable of respiration. Many of these microbes produce lactic acid as a end product and are important in food industry. They also are common inhabitants of our body.
Obligate anaerobic respirer - These microbes must exist in anaerobic environments and they can only respire. This group includes sulfate reducers and methanogens. Some are extremely sensitive to oxygen, becoming nonviable if exposed to air for just a few minutes.
Facultative anaerobe - Capable of growth in the absence or presence of oxygen. If oxygen is present, they will used oxidative phosphorylation, if not, they will ferment the carbon source.
Some organisms are capable of only one type of catabolism, but most are more versatile, being able to carry out several metabolic methods depending upon what the environment dictates. Microbes as a group display tremendous catabolic diversity. It is not too far fetched to state that if a chemical reaction exists that can generate energy, it is possible to find a microbe that takes advantage of it for growth.
Microbes that are capable of carrying out several different metabolic pathways on a particular substrate have a problem - they must choose which method to use. Microbes regulate their metabolism based upon what the outside world is telling them. Normally a cell will use the pathway that is most advantageous; for catabolism that means the pathway that results in the most energy per unit of substrate metabolized. For example, a facultative anaerobe will have a means to sense the amount of oxygen available. If oxygen concentrations are high, the respiratory enzymes of oxidative phosphorylation will be synthesized. If conditions are anaerobic, fermentative enzymes will be made.
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