12.3. Fermentation

In fermentation, say by yeast, the incomplete oxidation of glucose is achieved under anaerobic conditions by sets of reactions where pyruvic acid is converted to CO₂ and ethanol. The enzymes, pyruvic acid decarboxylase and alcohol dehydrogenase catalyse these reactions. Other organisms like some bacteria produce lactic acid from pyruvic acid.

Fermentation is a process that takes place without oxygen (anaerobic) where glucose is not fully oxidized. In yeast, a type of fungus, the conversion of glucose to ethanol and carbon dioxide occurs when pyruvic acid breaks down during the fermentation process. Key enzymes like pyruvic acid decarboxylase and alcohol dehydrogenase facilitate these transformations. This same pyruvic acid can lead to lactic acid when processed by certain bacteria or in the muscles during intense exercise when oxygen is low.

Think of fermentation like making bread. When you mix flour (glucose) with yeast (which ferments the glucose), the yeast produces carbon dioxide (the gas that makes the bread rise) and ethanol (which evaporates during baking). This is similar to how the yeast creates alcohol in beverages or even how our muscles create lactic acid when we exercise hard.

In both lactic acid and alcohol fermentation not much energy is released; less than seven percent of the energy in glucose is released and not all of it is trapped as high energy bonds of ATP. Also, the processes are hazardous – either acid or alcohol is produced.

Fermentation is not an efficient process for energy production compared to aerobic respiration. It only captures about 7% of the energy that was stored in the glucose molecule, resulting in the creation of products like lactic acid or ethanol, which can be harmful in high concentrations. This inefficiency is one reason why organisms prefer aerobic processes when oxygen is available, as they can extract far more energy.

Imagine trying to power a car with just a few drops of fuel, resulting in a very short trip. That’s similar to how fermentation works—it can “run” but not very effectively. On the other hand, using gasoline (aerobic respiration) allows for a long, fruitful drive.

What is the net ATPs that is synthesised (calculate how many ATP are synthesised and deduct the number of ATP utilised during glycolysis) when one molecule of glucose is fermented to alcohol or lactic acid?

When glucose undergoes fermentation, it first goes through glycolysis where 2 ATP molecules are consumed and 4 ATP are generated, resulting in a net yield of 2 ATP molecules. However, fermentation itself does not produce additional ATP. Thus, regardless of whether it results in alcohol or lactic acid, the total ATP production from one glucose molecule during fermentation remains at 2, compared to the much higher yields in aerobic conditions.

Think of a student preparing for exams by studying late at night. They put in lots of effort (energy spent on studying, like ATP consumed) but only gain a limited understanding (only 2 ATP gained), much like glucose turning into lactic acid or ethanol through fermentation.

Yeasts poison themselves to death when the concentration of alcohol reaches about 13 percent. What would be the maximum concentration of alcohol in beverages that are naturally fermented?

In yeast fermentation, alcohol is the byproduct of glucose breakdown. However, as the alcohol concentration increases, it eventually becomes toxic to the yeast itself; typically, this becomes lethal around a concentration of 13%. Therefore, naturally fermented beverages cannot exceed this concentration without killing the yeast involved in their production.

This is similar to how some plants can only absorb a certain amount of water before the roots start to rot. In both cases, there's a limit to what can be tolerated; for yeast, it's the alcohol concentration, while for the plant, it's the water levels.

What then is the process by which organisms can carry out complete oxidation of glucose and extract the energy stored to synthesise a larger number of ATP molecules needed for cellular metabolism?

The process that allows for complete oxidation of glucose is aerobic respiration, which involves multiple steps including glycolysis, the Krebs cycle, and the electron transport chain. This sequence effectively breaks down glucose all the way to carbon dioxide and water, yielding up to 38 ATP molecules from one glucose molecule, which is significantly more than what fermentation can offer.

Think of aerobic respiration as a well-planned road trip with multiple fuel stops allowing you to travel far and efficiently, versus fermentation, which is like a quick errand that drains your resources fast, giving you only a little return before you run out of gas.

--