Question

What is fermentation? Under what conditions does it occur? What is its purpose? Describe the major differences between anaerobic and aerobic respiration in terms of reactions, products, and energy yield. Yeast is capable of both aerobic and anaerobic respiration, but frequently performs anaerobic respiration regardless of the oxygen content of its environment. What advantage might this provide the yeast?

Answer 1

Fermentation is a type of anaerobic process wherein glucose is broken down to acid leading to generation of ATP. When glucose is metabolized to pyruvate, if oxygen is not present, it undergoes fermentation. Pyruvate is broken down to acetaldehyde and CO2, by pyruvate carboxylase. Acetaldehyde is broken down to ethanol and carbon dioxide by alcohol dehydrogenase. Net gain of ATP in fermentation is 2, which is obtained via glycolysis. In lactic acid fermentation, pyruvate is converted to lactate, with reduction of NADH to NAD+ by lactate dehydrogenase. Its purpose is to degrade the sugars for energy generation in oxygen limiting conditions. Fermentation allows the organism to grow in oxygen deprived conditions and thus, can generate limited amount of energy. It regenerates the NAD+ from NADH, that is used during glycolysis.

Fermentation is carried out by yeast and some bacteria. It is carried out where is lack of oxygen, under anaerobic conditions. However, the organism need to possess the enzymes required for converting pyruvate to ethanol. Fermentation is carried out to regenerate the remove electrons and H+ ions from NADH, which is formed during glycolysis. NAD+ is produced from NADH.

Differences between Anaerobic and Aerobic respiration:

Anaerobic respiration	Aerobic respiration
This takes place in absence of oxygen	This requires oxygen as terminal electron acceptor in electron transport chain.
It involves glycolysis, but no Krebs cycle or electron transport chain.	There are three processes in aerobic respiration- glycolysis , TCA cycle and electron transport chain.
Glucose is first broken down to pyruvate. There are two types of fermentations for fate of pyruvate: 1) Lactic acid fermentation: Pyruvate is converted to lactic acid by lactate dehydrogenase. NADH is converted to NAD+. 2) Alcohol fermentation: Pyruvate is converted by pyruvate carboxylase to acetaldehyde and CO2. Acetaldehyde is converted by alcohol dehydrogenase to ethanol. NADH is converted to NAD+.	In glycolysis, glucose is broken down to pyruvate. Pyruvate is converted to acetyl CoA by pyruvate dehydrogenase. Acetyl CoA produces CO2 when it passes through the Krebs cycle. NADH and FADH2 generated donate electrons to the ETC to produce ATP. ATP synthase produces ATP from ADP and Pi.
Products of Anaerobic respiration are either lactic acid or ethanol and CO2 along with ATP.	Products are CO2, water and ATP.
Yield of ATP= 2 (from glycolysis).	Yield of ATP= 32 ATP (when NADH= 2.5 ATP, FADH2=1.5 ATP). Earlier books mention it as between 36-38 ATP (when NADH=3ATP, FADH2= 2 ATP).

Yeast requires oxygen for growth. Hence, they will utilize aerobic respiration. Fermentation is used by the yeast to break down sugars to produce ethanol and CO2. Fermentation is carried out by yeast till the sugar levels decrease. This is known as Crabtree effect. When yeasts are grown on high sugar substrates, they will activate anaerobic respiration, in order to degrade the sugars fast. This leads to formation of alcohol. Most yeast will only tolerate alcohol levels of 15%. When the sugar levels decline, the yeast will inactivate the fermentation enzymes and will shift to aerobic respiration. The ethanol produced will also inhibit growth of other bacteria or microbes. Once the preferred carbohydrate is depleted, the yeast will then use the ethanol. Thus, yeasts will have a selective advantage over other organisms in a high sugar environment.