What do alcohol fermentation, acetyl CoA formation, and the krebs cycle have in common?

A. All produce water.

B. All produce carbon dioxide.

C. All produce ATP.

D. All produce alcohol.

The correct answer is B: All produce carbon dioxide.

What alcohol fermentation, acetyl CoA formation, and the Kreb’s cycle have in common is that they are all processes that occur at various times during cellular respiration. Alcohol fermentation, however, only occurs in anaerobic respiration.

Anaerobic respiration is cellular respiration in the absence of oxygen. Aerobic cellular respiration is respiration that occurs when oxygen is present. Acetyl CoA formation and the Kreb’s cycle only occur during aerobic cellular respiration. These two processes occur in the mitochondrion of the cell.

Alcohol (ethanol) fermentation
Alcohol (ethanol) fermentation

These three processes all involve enzymes and the coenzyme, nicotinamide adenine dinucleotide (NAD). They also involve energy either in the form of adenine trinucleotide (ATP) or NADH2. All three processes also occur after the first stage of respiration has ended.

This first stage is glycolysis which is the splitting of the glucose (sugar) molecule to form pyruvate. When oxygen is present the pyruvate enters the aerobic respiration processes, while when oxygen is absent, the pyruvate enters the fermentation reactions.

The formation of acetyl CoA and the Kreb’s cycle both involve decarboxylation reactions. Decarboxylation is the removal of carbons in a reaction.

Carbons are removed from molecules and released as carbon dioxide in both cases. The coenzyme NAD is reduced to form NADH2 in both acetyl CoA formation and the Kreb’s cycle.

Alcohol fermentation

In the absence of oxygen, the products of glycolysis may undergo alcohol fermentation. This usually happens in plants and fungi when oxygen is absent. Alcohol fermentation is also known as ethanol fermentation since ethanol is the final product of the fermentation process.

Glycolysis occurs first to form two pyruvate molecules from a single glucose molecule. During glycolysis ATP is formed from ADP and NADH2 is formed from NAD.

At the end of alcohol fermentation, two ethanols are formed, each having two carbons. Two molecules of carbon dioxide are released during the process.

Each of the pyruvate molecules is acted on by pyruvate decarboxylase to produce acetaldehyde, which has two carbons. Carbon dioxide is also released during this reaction. The acetaldehyde is then broken down into ethanol.

An alcohol dehydrogenase enzyme catalyzes this reaction in which acetaldehyde is broken down. NADH2 that is present is then oxidized during this final step to regenerate NAD.

Acetyl CoA

Pyruvate is the product formed at the end of glycolysis. Glycolysis is the first stage of cellular respiration when sugar is split.

The pyruvate molecule formed during glycolysis is actively transported into the mitochondrion in aerobic respiration. Pyruvate that is produced then undergoes further modification.

A carboxyl group is lost from the pyruvate in a decarboxylation reaction. This then is released as carbon dioxide. The pyruvate is also oxidized by what is known as a pyruvate dehydrogenase complex. In the process, NADH2 is formed from NAD.

The end product that is formed from the oxidation of pyruvate, is an acetyl group, which consists of two carbons. This acetyl group then joins with coenzyme A to form what is known as acetyl CoA.

This is the molecule that will then enter the Kreb’s cycle. Lipids (fats) and proteins can all be converted to acetyl CoA for entry into the Kreb’s cycle. Fats are first broken down to their component glycerol and three fatty acids.

These then are converted to palmitic acid which eventually produces acetyl CoA. The palmitic acid produces eight acetyl CoA molecules.

Acetyl CoA is an important molecule in cell metabolism. It is not only a required molecule for the Kreb’s cycle but it is also the main molecule needed to synthesize fatty acids.

Kreb’s cycle

The Kreb’s cycle is also known as the tricarboxylic acid cycle or citric acid cycle. This process involves several steps during which molecules are acted on by enzymes. During some of the steps, carbon dioxide is released and molecules such as NAD, FAD, and ADP are reduced.

The acetyl groups in acetyl CoA have only two carbons each. What happens next is that an acetyl group, with its two carbons, combines with oxaloacetate.

The oxaloacetate has four carbons, which means that the end product of this reaction is a molecule that has six carbons. This six carbon molecule is citrate.

This reaction is catalyzed by the enzyme citrate synthase. During the Kreb’s cycle, some carbon is released in the form of carbon dioxide. The carbons are successively removed from the citrate in a series of steps.

In the process of the decarboxylation of citrate, different intermediates are formed and carbon dioxide is released when some of these are formed. The citrate changes form to produce isocitrate.

The first intermediate formed from isocitrate, where there is a loss of carbon, is the five carbon α-ketoglutarate. Carbon dioxide is released and NAD is reduced during this stage.

This then breaks down to form a four carbon succinyl CoA. A molecule of NAD is also reduced and carbon dioxide is released during this step. Succinate is then formed, during which time more energy is produced.

The succinate is then converted into fumarate, during which time FADH2 is produced. The fumarate then becomes a four carbon malate that then becomes a four-carbon oxaloacetate. NAD is reduced to form NADH2.

Recycling

The oxaloacetate is then ready to combine again with acetyl CoA at the beginning of the cycle again. This is why the process is a cycle; molecules are being constantly recycled during the process for reuse when the next acetyl group enters the cycle.

Energy is also produced during various stages of the Kreb’s cycle. This energy is important for the final stage of respiration.

Energy production

Energy is supplied when the bonds are broken during the processing of the sugar molecule. This energy reduces ADP to ATP.

Similar reactions occur in which FAD forms FADH2 and NAD forms NADH2. The FAD is a coenzyme like NAD. The molecule FAD is flavin adenine dinucleotide which is reduced to FADH2.

Both the NADH2 and FADH2 are coenzymes that store energy and carry electrons into the electron transport chain, which is the final stage of aerobic cellular respiration.

The electron transport chain is the final stage of aerobic respiration in which the most ATP is generated.

References

  1. Editors of Encyclopedia Britannica (2018). Fermentaion. Retrieved from Britannica.com.
  2. PH Raven, RF Evert, SE Eichhorn (1987). Biology of plants. Worth Publishers.
  3. Editors of Encyclopedia Britannica (2018). Cellular respiration. Retrieved from Britannica.com.
  4. Editors of Encyclopedia Britannica (2018). Tricarboxylic acid cycle. Retrieved from Britannica.com.
  5. C Rye, R Wise, V Jurukovski, J DeSaix, J Choi, and Y Avissar. (2017). Biology. Rice University.

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