The correct option is 1 and 4 only.
Explanation
Fermentation is the incomplete oxidation of glucose under anaerobic conditions (absence of oxygen). It occurs in many prokaryotes, unicellular eukaryotes like yeast, and sometimes in animal muscle cells. The process involves the breakdown of pyruvic acid formed during glycolysis into end products such as ethanol and carbon dioxide (alcoholic fermentation) or lactic acid (lactic acid fermentation).
Statement-wise Analysis
- Statement 1 is Correct: In fermentation, there is only a partial breakdown of glucose. Consequently, a very small amount of energy is released. It releases less than seven % of the energy stored in glucose, and not all of it is trapped as high-energy bonds of ATP.
- Statement 2 is Incorrect: For glycolysis to continue, the cell requires a steady supply of NAD+. During fermentation, the reducing agent NADH is oxidised to NAD+. This regeneration of NAD+ ensures that glycolysis can proceed to generate ATP, albeit in small amounts. The process is slow compared to aerobic respiration.
- Statement 3 is Incorrect: Yeasts have a limited tolerance for alcohol, which is a metabolic waste product for them. Yeasts poison themselves to death when the concentration of alcohol reaches approximately 13 %. They cannot tolerate concentrations well above 20 % naturally.
- Statement 4 is Correct: The accumulation of fermentation by-products can be hazardous to the cells. For example, the accumulation of alcohol is toxic to yeast cells at high concentrations, and the accumulation of lactic acid in animal muscle cells during strenuous exercise leads to fatigue and muscle cramps.
Key Takeaway
Fermentation is an anaerobic process yielding low energy (less than 7% of glucose potential) and involves the re-oxidation of NADH to NAD+. The end products, such as alcohol or acid, can be toxic to the organism if accumulated in high concentrations.