Imagine yourself in the French countryside, in an Italian garden, or on the banks of the Mosel river in Germany. What do all these places have in common, besides a very pleasant afternoon? They are often coupled with a glass of wine and a plate of fine cheeses. These fermented food items are uniquely produced and available to you thanks to a special type of cellular respiration called fermentation!
- First, we will talk about fermentation.
- Then, we will look at the definition of fermentation in biology.
- After, we will discuss the processes of fermentation.
- Lastly, we will go over the differences between cellular respiration and fermentation, also look talk about the products of fermentation.
Fermentation is cellular respiration
To survive, cells need to perform actions, such as move, transport nutrients and synthesize molecules. To do so, they need energy, which is most frequently derived from sugar. Traditionally, cells convert sugar into energy through a process called cellular respiration.
Aerobic respiration is the most conventional method of producing energy, but it requires oxygen. Some cells, however, have developed the ability to perform respiration without oxygen, which is called anaerobic respiration.
Anaerobic respiration is respiration that occurs in the presence of oxygen. Anaerobic respiration is respiration that occurs in the absence of oxygen.
Fermentation is a special type of anaerobic respiration that does not use the electron transport chain.
Electron transport chain is a process that involves passing electrons from complex to complex, and finally to oxygen, creating a proton gradient that will be used to make ATP.
Interesting in learning more about this process? Check out "Electron Transport Chain"!
Respiration, whether aerobic or anaerobic, is a catabolic process.
Catabolism is the breakdown of large molecules into smaller molecules, which releases energy.
For anaerobic respiration to occur, a molecule must be able to substitute for oxygen, such as sulfate SO4-2. When no such molecule can functionally replace oxygen, the traditional metabolic pathways cannot be used. Lucky for cells (and for us!), there is another pathway to generating energy: fermentation.
Fermentation is the most common type of anaerobic fermentation. However, there are many other forms of anaerobic respiration, such as methanogenesis. Methanogenesis is a type of anaerobic fermentation characteristic of some archaea organisms (Fig. 1), which releases methane.
Figure 1. Halobacterium, by NASA. Licensed under licence by NASA.
Definition of Fermentation in Biology
Now that we know that aerobic and anaerobic respiration are, let's look at the definition of fermentation.
Fermentation is a type of metabolic pathway that converts sugar into energy in the form of adenosine triphosphate (ATP), without the presence of oxygen.
Fermentation occurs in two phases: glycolysis and NADH oxidation. The first phase, glycolysis, breaks down glucose with the help of NAD+, which generates ATP. The second phase, NADH oxidation, regenerates NAD+.
Well, that’s a lot of new terminology. So, like catabolism, let’s break all these terms down!
Essential Molecules
Glucose
Let's start with glucose. Glucose (Fig. 2) is a sugar whose chemical formula is C6H12O6. It is a chemical compound in the carbohydrate family because it contains carbon (C), hydrogen (H) and oxygen (O), with a 2:1 ratio of two hydrogens for every oxygen. Glucose is considered the main source of energy for most living organisms.
Fig. 2. Simplified diagram of the glucose molecule. Licensed under the GNU Free Documentation License.
ATP
Next, we have ATP. ATP (Fig. 3) is short for Adenosine triphosphate.
The structure of ATP consists of one adenosine bonded to three phosphates. When a phosphate bond is broken, energy is released. The resulting molecule, when missing a phosphate, is called adenosine diphosphate, or ADP. ADP can even lose a second phosphate, thus becoming adenosine monophosphate, or AMP.
To remember the difference between ATP and ADP, it may help to notice the difference in prefixes, di- and tri-. Di- means two, as in two phosphates, whereas tri- means three.
Fig. 3. Molecular structure of Adenosine Triphosphate (ATP), and its Functional Groups. Licensed under CC BY 3.0.
NADH
The final significant molecule is nicotinamide adenine dinucleotide, or NADH (Fig. 4). In its reduced form, it will lack a hydrogen (H+) and be identified as NAD+.
NADH and NAD+ are molecular compounds whose essential function is to act as an electron donor or receiver.
Fig. 4. Simplified diagram of Nicotinamide Adenine Dinucleotide (NADH). Note the two Hydrogen (H) at the top of the molecule, where NADH gets reduced and oxidized. Public domain.
Fermentation Process in Biology
In biology, fermentation is the metabolic process cells use to generate energy, or ATP, without the primary assistance of oxygen. It occurs in a simpler fashion than aerobic respiration, in that it takes only two steps: glycolysis and NADH oxidation.
Glycolysis
The first step in anaerobic respiration, glycolysis, occurs in the cytoplasm.
Glycolysis is the breakdown of a glucose molecule into two pyruvate, or pyruvic acid, molecules, which yields four ATP molecules.
Glycolysis occurs in a long sequence of steps, summarized below:
Phosphorylation, wherein two ATP molecules, one after the other, lose a phosphate group to a glucose molecule.
The modified glucose molecule splits into triose phosphate.
NAD+ take a hydrogen (H+) from the triose phosphate.
ADP takes a phosphate from the modified triose phosphate. The modified molecule is now a phosphoglycerate. This step generates ATP.
Phosphoglycerate is modified into phosphoenolpyruvate, losing H2O in the process.
ADP takes a phosphate from phosphoenolpyruvate, resulting into our end product, pyruvate, or pyruvic acid. This step generates ATP.
The previous list is an abridged summary. The full ten steps of glycolysis can be seen in this diagram (Fig. 5).
Fig. 5. The full ten steps of glycolysis. licensed by CC BY-SA 4.0
Glycolysis can be summarized and simplified into a single equation:
Glucose + 2 NAD+ + 2 ATP ⇾ 2 Pyruvate + 2 NADH + 4 ATP + 2 H+
Pyruvate, or pyruvic acid, is the resulting molecule from glycolysis. Its chemical formula is C3H3O3-. Pyruvate will undergo more catabolism in the next phase. Look! Four ATP molecules have been produced and can be used for energy. We have ATP to power our cells!
So, why can’t we just stop here? Unfortunately, the NAD+ required for glycolysis is in short supply. To produce more ATP, the cell needs a supply of both sugar and NAD+. The next phase, fermentation, will meet this demand.
NADH Oxidation
At this point, after glycolysis, we have an overabundance of pyruvic acid (pyruvate) and NADH. And, we also need to generate more NAD+ to continue the synthesis of ATP via glycolysis.
But, how can we regenerate the NADH to NAD+? Here, oxidation will solve our issue.
Oxidation is the loss of electrons or protons.
Oxygen is the most common oxidizing agent, but unfortunately under anaerobic conditions oxygen is in very low supply. Yet, by definition, oxidation can still occur! Oxygen is the most common oxidizing agent because of its high electronegativity, but in its absence, some cells have found a substitute.
During fermentation, NADH becomes oxidized to NAD+, which replenishes the demand for NAD+ during glycolysis. The oxidization is made possible by some catalyst, such as an enzyme, yeast, or bacteria. There are two major types of fermentation, named after their by-product:
Alcohol fermentation: Fermentation of pyruvate by yeast, which yields ethyl alcohol as a by-product. It is used to make wines, beers, and spirits.
Lactic acid fermentation: Fermentation of pyruvate by bacteria, which yields lactic acid as a by-product. It is used to make cheeses and yogurts.
To summarize, the combined metabolic phases of glycolysis and NADH Oxidation are displayed below (Fig. 6).
Difference between Cellular Respiration and Fermentation
You may be asking yourself, why isn't all respiration anaerobic? Anaerobic respiration has distinctly less metabolic pathways than aerobic respiration. To answer that question, we observe how many ATP can be produced from one single molecule of glucose.
Aerobic respiration can yield up to 36 ATP per molecule, whereas anaerobic respiration can only yield a paltry two ATP per molecule of glucose.
There lies the true power of oxygen! Look on the bright side, with fermentation, we get to enjoy the flavorful fermented food by-products.
Products of fermentation
Chemically, fermentation produces by-products such as lactic acid, ethanol, hydrogen gas, carbon dioxide, ATP, acetone and some proteins or enzymes. Yet, most people are familiar with the fermentation process because it gives rises to a unique food category: fermented foods.
Fermentation has yielded many fermented foods that have enriched our lives. Wine, beer, and liquors are fermented food beverages that humankind has enjoyed since the dawn of civilization.
Another staple of civilization is bread. Alcohol fermentation give bread its bubbles inside the crumb. Other than fermented foods, ethanol fermentation produces an energy resource, ethanol fuel, which can power some motorized vehicles equipped with internal combustion engines.
Another early fermentation process is the further fermentation of ethanol beverages into vinegar, using lactic acid fermentation. Vinegar has provided humans with its preservative properties during the pickling process, in times when mechanical refrigeration was unavailable.
Cheese is another well-loved fermented food, where milk sugars are converted into lactic acid. Similarly, yogurt and sour cream are fermented foods derived from milk sugars. Lactic acid gives these fermented foods their sour taste.
Fermented sausages, such as salami or pepperoni, are a type of meat preservation process used globally. In Asian countries, fermented beans, such as miso, are popular fermented foods derived by lactic acid fermentation.
Even chocolate is a product of fermentation! Lactic acid producing bacteria ferment cocoa pulp, and modifies its flavor profile into the familiar chocolate taste.
Lactic acid fermentation of cabbage yields sauerkraut and kimchi. Soy sauce and Worcestershire sauce can be added to the list of fermented foods derived from lactic acid fermentation.
Fermentation - Key takeaways
- Fermentation is a type of anaerobic metabolic pathway that converts sugar into energy, or ATP, in the absence of oxygen.
- Fermentation consists of the glycolysis phase and the NADH oxidation phase.
- The glycolysis phase uses reduced nicotinamide adenine dinucleotide, or NAD+ to break down glucose into pyruvate.
- The NADH oxidation phase uses pyruvate to regenerate NAD+ into NADH.
- Two of the most common type of fermentation are alcohol fermentation (ethanol fuel, wine, and spirits) and lactic acid fermentation (cheese and yogurt).
References
- Fig. 1. Cluster of cells of Halobacterium sp. strain NRC-1 (https://upload.wikimedia.org/wikipedia/commons/a/a1/Halobacteria.jpg) by NASA is under license by NASA (https://www.jpl.nasa.gov/jpl-image-use-policy)
- Fig. 2. Représentation de Haworth de la molécule de glucose. (https://upload.wikimedia.org/wikipedia/commons/thumb/6/64/Glucose_Haworth.png/752px-Glucose_Haworth.png?20111018174511) by Steinsky (https://en.wikipedia.org/wiki/User:Steinsky) under the GNU Free Documentation License (https://commons.wikimedia.org/wiki/Commons:GNU_Free_Documentation_License,_version_1.2)
- Fig. 3. 230 Structure of Adenosine Triphosphate (ATP)-01 (https://upload.wikimedia.org/wikipedia/commons/7/7e/230_Structure_of_Adenosine_Triphosphate_%28ATP%29-01.jpg) by OpenStax College is licensed by CC BY 3.0 (https://creativecommons.org/licenses/by/3.0)
- Fig. 4. NADH phys alt (https://upload.wikimedia.org/wikipedia/commons/e/e6/NADH_phys_alt.svg) by NEUROtiker (https://commons.wikimedia.org/wiki/User:NEUROtiker) is public domain under Wikimedia Commons (https://commons.wikimedia.org/wiki/Main_Page)
- Fig. 5. Glycolysis pathway 2 (https://upload.wikimedia.org/wikipedia/commons/4/47/Glycolysis_pathway_2.png) by Eunice Laurent (https://commons.wikimedia.org/w/index.php?title=User:EunLau&action=edit&redlink=1) is licensed by CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)
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