Cellular Respiration: Electron Transport Chain and Fermentation
The Respiratory Chain: Concept and Objectives
Definition
The respiratory chain consists of the transport of electrons from reduced coenzymes, NADH + H+ and FADH2, to oxygen. This transport takes place in the membrane of mitochondrial cristae.
Objectives
- In this process, most of the energy contained in glucose and other organic compounds is obtained and stored as ATP.
- Simultaneously, the electron carrier coenzymes are recovered in their oxidized form, which allows for further chemical oxidation of glucose and other organic substances.
- Water is obtained as a waste product.
Respiratory Chain: Mechanism
In the membrane of mitochondrial cristae, electron transport occurs from NADH or FADH2 to oxygen, as shown in the figure (not provided in this text). This electron transport generates a proton transport by complexes I, II, and III from the matrix to the intermembrane space. Each complex can pump two protons. The output of these protons through ATPases serves to synthesize ATP, with 1 ATP produced for every two protons, similar to what occurs in chloroplasts.
NADH can reduce complex I, resulting in 3 ATP being obtained for each molecule of NADH. FADH2 cannot reduce complex I and donates its two electrons to Co-Q (coenzyme Q). This is why FADH2 generates only 2 ATP.
The electrons are finally transferred to oxygen, which, together with two protons from the medium, forms a molecule of H2O:
2H+ + 2e– → 1/2O2 + H2O
Each NADH originating in the mitochondria yields 3 ATP. However, in eukaryotes, NADH originating in the hyaloplasm during glycolysis can produce only 2 ATP. This is because this NADH cannot traverse the mitochondrial membrane and must give up its electrons to a chemical intermediary, which in turn donates them to FAD in the mitochondria. This is not the case in prokaryotes.
Anaerobic Fermentations
Oxidation of NADH + H+ and FADH2 in the respiratory chain requires oxygen as the terminal electron acceptor. This allows for the recovery of NAD+, enabling glycolysis and the Krebs cycle to continue.
If there is no oxygen, NADH + H+ and FADH2 accumulate, and the energy-obtaining process is interrupted. Under these anaerobic conditions, certain microorganisms and cells, such as our muscle cells, regain oxidized coenzymes through various metabolic pathways known as anaerobic fermentation.
For some microorganisms (anaerobes), fermentations are their only source of energy. They are called anaerobes because they cannot survive in an oxygen-containing environment, as it is lethal to them. Others, known as facultative anaerobes, use these pathways as an emergency mechanism during periods when oxygen is unavailable.
In fermentation, glucose is not completely degraded to CO2 and H2O, but rather undergoes incomplete degradation of the carbon chain. Depending on the product obtained, we have the following types of fermentation:
A) Lactic Acid Fermentation
This type of fermentation is carried out by bacteria used to make yogurt and by muscle cells when they do not receive a sufficient supply of oxygen, such as during strenuous exercise. In lactic acid fermentation, pyruvic acid is reduced to lactic acid by means of NADH + H+. This allows for the recovery of NAD+, enabling further degradation of glucose molecules.
B) Alcoholic Fermentation
In alcoholic fermentation, pyruvic acid is transformed into ethyl alcohol (ethanol). This fermentation is carried out by yeasts such as Saccharomyces. It is a process of great industrial importance, as different types of yeast lead to a variety of alcoholic beverages, including beer, wine, and cider.
In bread making, yeast is added to the dough. The fermentation of starch from the flour produces CO2 bubbles, making the bread spongy. In this case, the alcohol produced is lost during the cooking process.
Alcoholic fermentation has the same objective as lactic acid fermentation: the recovery of NAD+ under anaerobic conditions. In alcoholic fermentation, pyruvate is decarboxylated to acetaldehyde, which is then reduced by NADH to ethanol.