Energy for survival
The main role of yeast is not to produce alcohol, carbon dioxide, and byproducts
By Michael Trommer
Energy cannot be created or destroyed. However, it can be transformed from one form into another. The preferred fuel for cells is normally carbohydrates, but fats and proteins can also be used.
Living cells are constantly doing work, and the energy needed for such work comes from the transformation of fuel, such as glucose, into residues with lower energetic content. The main glucose transformation processes are fermentation and aerobic respiration.
Universal pathway for carbohydrate metabolism in the absence of oxygen
General aspects of respiration and fermentation
Cells recharge ATP via two types of process: aerobic respiration and fermentation.
There are various types of fermentation, which differ in the kind of residue produced.
1 – Aerobic respiration
6C 6C (ATP)
C6H12O6 + 6 O2 g 6 CO2 + 6 H2O + energy
2 – Alcoholic fermentation
6C 2 2C 2C ATP ATP
C6H12O6 g 2 C2H2OH + 2 CO2 + energy
In fermentation, a good part of the glucose’s chemical energy remains stored in alcohol (this is why alcohol is a good fuel). Since good part of the glucose’s energy is not released during fermentation, it can be noticed that the respiration process releases more energy than fermentation.
One molecule of glucose in respiration releases enough energy to recharge 38 ATP. In fermentation, it releases only 2 ATP.
The fermentation process is divided into 10 different chemical stages, while respiration involves over 20 chemical reactions. Each stage in both processes is catalyzed by a specific enzyme. Both fermentation and respiration occur in many stages, and the energy is gradually transferred to the cell.
ATP: the “energy currency” of living beings
The energy released by fuel reaction (carbohydrate, fats, etc.) is never directly used for work in a substance (cell). Initially, it goes to the ATP in the form of a chemical bond very rich in energy.
In case there is energy available for fermentation or respiration, the ADP substance bonds to a phosphate group lost by the cell, turning into ATP. The ATP, in turn, can give up energy to the cellular work, forming ADP again. ATP is the direct energy source for cellular work in all living beings.
Less efficient process for energy production
At ambient temperature, glucose hardly reacts. Little reactive molecules can be stimulated if activation energy is provided.
Each one of the stages of fermentation is enabled by one specific enzyme.
1 – Molecule 6C (glucose) reacts with 2 molecules of ATP borrowed from the cell and gains 2 high-energy phosphate bonds (P ∼ 6C ∼ P).
2 – The molecule P ∼ 6C ∼ P breaks down into two molecules with 3 carbons and 1 energetic phosphate bond (3C ∼ P).
3 – To every molecule, 1 inorganic phosphate is given by the cell. Now there are 2 P ∼ 3C ∼ P molecules.
4 – Each molecule P ∼ 3C ∼ P reacts with 1 ADP and gives 1 phosphate and energy to it, turning it into ATP and recharging it to the ATP.
5 – Each molecule 3C ∼ P reacts with 1 ADP, gives 1 phosphate and energy to it, and turns into ATP. The two resulting 3C molecules are denominated pyruvic acid.
6 – Each pyruvic acid decarboxylates, loses 1 CO2, and turns into ethyl alcohol (2C), the final residue of the alcoholic fermentation.
The processes can be summarized as follows: in fermentation, 2 pyruvic acid molecules are obtained from 1 glucose molecule; the former lose CO2 and turn into ethyl alcohol, gaining 2 ATP. There is little biomass formation. In fermentation, ferment multiplication is minimal.
As an informative note
Glycogen is a storage of glucose in muscles and liver. It enables production of ATP energy in muscles and liver when there is insufficiency of oxygen.
The role of the substances NAD and FAD is to transport hydrogen. In respiration, they remove hydrogen atoms from the fuel and transfer them to oxygen, making water (H2O).
More efficient processes for energy production
The enzymes in all of the glycolysis stages can be found in the hyaloplasm. That is where that process occurs. The steps are identical to those of fermentation up to the stage where pyruvic acid is formed.
2-ATP profit for the cell (4 ATP produced - 2 ATP spent in the activation)
In addition to that, removal of hydrogen by the NAD occurs twice. However, pyruvic acid continues to degrade inside the mitochondria. On the other hand, the NADH2 of the glycolysis can enter the mitochondria and give their hydrogens to the oxygens in the respiratory chain.
Krebs cycle – grinding of acetic acid
Pyruvic acid originated from glycolysis loses hydrogen (CO2) and combines with a substance called coenzyme A. The result is called “active” acetic acid.
The acetic acid penetrates the Krebs cycle (2C) and takes part in the cycle, since the coenzyme A is deactivated in this stage.
A combination of acetic acid (2C) with a substance already present in the mitochondria and oxaloacetic acid (4C) results in the formation of citric acid. At the end of the cycle, the oxaloacetic regenerates. Therefore, it is not spent in the process. The cycle continues.
Balance: 3 NAD g 3 NADH2
1 FAD g 1 FADH2
1 ADP g 1 ATP
In every turn of the cycle, the elements above are produced, as well as 38 ATP, normally. As mentioned before, the respiration process uses carbohydrates, meats, and fats. (See figure below.) Cells prefer carbohydrates and glucose.
In this process, the hydrogens removed from the substrate by the NAD or the FAD combine with oxygen. However, in the mitochondria, the NADH2 never combines directly with oxygen. If the reaction were direct, the energy spent would be massive and possibly detrimental to the cell. The hydrogen passes stage after stage through a chain of intermediate electron acceptors in the mitochondria. When it reaches the final acceptor, oxygen, they make water. Each one of the interim reactions is slightly exergonic, and they release part of the total energy. This energy released in a control manner is then used by the cell for the synthesis of ATP.
NADH2 + ½ O2 g NAD + H2O + energy
In respiration, the ferment multiplication is very high. There is also high biomass formation.
Main compounds of the glycolytic pathway
Main compounds of the Krebs cycle