Introduction : Glucose happens to be important and common fuel for many organisms.
The process of breaking down of glucose in multistep manner to extract energy present in its C-C bonds (chemical bonds) is called “Cellular Respiration“.
Only green plants, Algae and Cyanobacteria have the ability to prepare their own food by the process of “Photosynthesis“. During light reaction phase of photosynthesis, chloroplast present in leaves, convert trapped sunlight into energy in the form of ATP and NADPH.
ATP and NADPH will be used in next phase of photosynthesis called as Calvin cycle or Dark Reaction to fix CO2 to form carbohydrates such as glucose, sucrose or starch. ATP and NADPH acts a connecting link between light phase and dark phase (Calvin cycle) of photosynthesis.
The non-green part of plants – trunk, roots etc. rely on oxidation of food or cellular respiration in cells for release of energy and eventually trapping this released energy to synthesize ATP.
It is very important to note that during the oxidation of food, the energy present in them is released at once or single step. It takes multiple steps and requires many enzymes to trap chemical energy from food in the form of ATP.
Energy produced from food cannot be directly used by our cell. It is necessary to direct the energy extracted from food to synthesize ATP. This ATP will be broken whenever the cell is in need to energy for its functions.
Hence ATP is called energy currency of cell.
Cellular respiration can takes place in presence (Aerobic) and also in absence of oxygen (Anaerobic).
Glycolysis is the first and common step towards oxidation of glucose in both types of respiration.
The end product of glycolysis i.e. Pyruvic acid can enter either Krebs Cycle and Oxidative phosphorylation, if oxygen is present for the cells. In absence of oxygen, prokaryotic and some unicellular eukaryotic cells undergoes Fermentation or anaerobic respiration.
Fermentation is nothing but glycolysis plus one or two additional reactions after that.
Pyruvic acid formed as end product of glycolysis can further be oxidized in the absence of oxygen by the process of fermentation.
Two main types of fermentation are: Lactic acid fermentation and Ethanol fermentation.
Lactic acid fermentation is seen in animals and some type of bacteria (lactobacillus – bacteria in yogurt).
Lactic acid fermentation can be seen in mammalian red blood cells, which lack mitochondria and also in skeletal muscles (where supply of oxygen is very less).
Pyruvic acid is converted to lactic acid in absence of oxygen to yield lactic acid. The enzyme which catalyzes this reaction is called lactate dehydrogenase.
NADH transfers its electrons directly to pyruvate to generate NAD+ in this reaction.
Ethanol fermentation is a process in which pyruvic acid from glycolysis is converted into ethanol in two step process.
In the first step, Pyruvic acid is converted to acetaldehyde by losing carboxyl group and in second step acetaldehyde gets reduced to ethanol as NADH passes its electrons (0xidized) to acetaldehyde to regenerate NAD+, which can now take part in glycolysis as an electron carrier.
The enzymes pyruvic acid dehydrogenase and ethanol dehydrogenase catalyzes the first and second step respectively.
No ATP is synthesized during conversion of pyruvate to ethanol and lactic acid. So the net yield of ATP from fermentation is only 2 ATPs (from glycolysis) per glucose molecule.
Apart from these two, many other fermentation methods take place in bacteria.
Fermentation was discovered by Louis Pasteur, who described fermentation as “la vie sans l’air” (“life without air”).
Fermentation is not the only way of anaerobic respiration. In some bacteria and archaea, an inorganic molecule such as sulfate, nitrate or sulfur will acts as final electron acceptor for an electron transport chain instead of oxygen as observed in aerobic respiration.
Importance of fermentation :
As we have seen fermentations yield very less energy and less than 7% of the energy of glucose is released after complete combustion of glucose. The final product of the fermentations ( lactic acid and ethanol) still possess energy which anaerobic respiration fail to process further.
So why do organisms or cells depend on such an inefficient pathway like anaerobic respiration for energy instead of high ATP yielding aerobic respiration?
The answer lies in the fact that for this type of respiration, oxygen is not at all required. This allows organisms to survive and flourish in difficult habitats such as soils, deep water, and skin pores, where availability or penetration of oxygen is highly difficult or impossible.
The fermentation of pyruvic acid by yeast produces the ethanol found in alcoholic beverages.
Another advantage of fermentations is that many of our food products such as sour cream, yogurt, various cheeses, beer, wine, and sauerkraut are the result of fermentations.
Introduction Cellular Respiration : Part I
Glycolysis : Part II Cellular respiration.
Fermentation : Part III Cellular Respiration
Krebs Cycle : Part IV Cellular Respiration.
Electron Transport Chain and Oxidative Phosphorylation : Part V Cellular Respiration
Image Credit : Openstax Biology