In the conversion of pyruvate to acetyl CoA, each pyruvate molecule loses one carbon atom with the release of carbon dioxide.
During the breakdown of pyruvate, electrons are transferred to NAD+ to produce NADH, which will be used by the cell to produce ATP.
In the final step of the breakdown of pyruvate, an acetyl group is transferred to Coenzyme A to produce acetyl CoA.
Key Terms
acetyl CoA: a molecule that conveys the carbon atoms from glycolysis (pyruvate) to the citric acid cycle to be oxidized for energy production

When the carboxyl group of pyruvic acid is removed, the resulting compound is acetyl CoA. Let's break down the process and understand why this is the correct answer:

1. Pyruvic acid structure:
Pyruvic acid is a three-carbon molecule with a carboxyl group (-COOH) attached to one end. The other end contains a carbonyl group (C=O).

2. Removal of carboxyl group:
To convert pyruvic acid into acetyl CoA, the carboxyl group is removed as carbon dioxide (CO2). This process is called decarboxylation.

3. Formation of acetyl group:
After the carboxyl group is removed, the remaining two-carbon fragment is called an acetyl group (-COCH3). This acetyl group is then combined with coenzyme A (CoA) to form acetyl CoA.

4. Role of coenzyme A:
Coenzyme A is a molecule derived from vitamin B5 (pantothenic acid). It plays a crucial role in various metabolic reactions, including the conversion of pyruvic acid to acetyl CoA. CoA acts as a carrier molecule, binding to the acetyl group and facilitating its transfer to other metabolic pathways.

5. Significance of acetyl CoA:
Acetyl CoA is an important molecule in cellular respiration as it serves as the entry point for the citric acid cycle (also known as the Krebs cycle or TCA cycle). In this cycle, acetyl CoA is further oxidized to produce energy in the form of ATP.

6. Other options:
- Alcohol: The removal of the carboxyl group does not result in the formation of an alcohol.
- Aldehyde: Although pyruvic acid contains a carbonyl group, the removal of the carboxyl group does not result in the formation of an aldehyde.
- Ketone: The removal of the carboxyl group does not result in the formation of a ketone.

Therefore, the correct answer is option A, acetyl CoA.