Fermentation breaks down glucose in the absence of oxygen.
Fermentation is the process of reducing pyruvate to either ethanol or lactate to regenerate NAD+ from NADH. Fermentation uses pyruvate as the acceptor of high energy electrons from NADH. The NAD+ produced by reducing pyruvate anaerobically is available for re-use in the glycolytic pathway so that more ATP can be created. Under anaerobic conditions, each glucose molecule can generate one ATP molecule.
There are two examples of the fermentation process; the reduction of pyruvate to ethanol and the reduction of pyruvate to lactate in human muscle cells.
Alcohol Fermentation
There are two steps in the alcohol fermentation process to produce ethanol. The first step removes a carboxyl group from pyruvate that produces acetaldehyde. The second step involves NADH passing its electrons to acetaldehyde which regenerates NAD+ and forms ethanol. Alcohol dehydrogenase is the enzyme that converts acetaldehyde to ethanol.
Lactic Acid Fermentation
When oxygen supplies are running low, many prokaryotes and eukaryotes use lactic acid fermentation to regenerate NAD+. Lactate dehydrogenase is the enzyme that catalyzes this process converting pyruvate to lactate.
Practice Questions
Khan Academy
MCAT Official Prep (AAMC)
Official Guide B/B Section Passage 1 Question 4
Practice Exam 2 B/B Section Question 45
Practice Exam 3 B/B Section Passage 6 Question 33
Sample Test B/B Section Question 45
Key Points
• Fermentation is the breakdown of glucose in the absence of oxygen.
• There are two common types of fermentation; alcohol fermentation and lactic acid fermentation
• The net equation of alcohol fermentation is: Glucose + 2ADP + 2Pi + 2H+ → 2 ethanol + 2ATP + 2H2O + 2CO2
• The net equation of lactic acid fermentation is: Glucose + 2ADP + 2Pi → 2 lactate + 2ATP + 2H2O
Key Terms
Fermentation: the metabolic process by which organic molecules are converted into acids, gases, or alcohol
Glycolytic pathway: the cellular metabolic pathway of the simple sugar glucose to yield pyruvic acid and ATP as an energy source
Glucose: a simple monosaccharide (sugar) with a molecular formula of C6H12O6; it is a principal source of energy for cellular metabolism
Adenosine triphosphate (ATP): a multifunctional nucleoside triphosphate used in cells as a coenzyme, often called the “molecular unit of energy currency” in intracellular energy transfer
NADH: nicotinamide adenine dinucleotide (NAD) carrying two electrons and bonded with a hydrogen (H) ion; the reduced form of NAD
Pyruvate: any salt or ester of pyruvic acid; the end product of glycolysis
Alcohol dehydrogenase: an enzyme that converts acetaldehyde to ethanol
Lactate dehydrogenase: the enzyme that catalyzes this process converting pyruvate to lactate
