Question

When pyruvate is used to form acetyl CoA, the product has only two carbon atoms. What happened to the third carbon?

Short answer

The third carbon is released as carbon dioxide (CO2).

Step-by-step solution

- Understanding Pyruvate

Pyruvate is a three-carbon molecule that is produced during glycolysis, the first step of cellular respiration.

- Conversion to Acetyl CoA

Explain that pyruvate undergoes a process called oxidative decarboxylation to be converted into acetyl CoA. During this process, one molecule of pyruvate loses one carbon atom in the form of carbon dioxide (CO2).

- Role of Enzyme Complex

The enzyme complex responsible for this conversion is called pyruvate dehydrogenase. It facilitates the removal of the carbon as CO2 and attaches the remaining two-carbon molecule to coenzyme A, forming acetyl CoA.

- Fate of the Third Carbon

The third carbon atom from pyruvate is released as a molecule of carbon dioxide (CO2) during the decarboxylation process.

- Summary

Summarize that the conversion of pyruvate to acetyl CoA involves the loss of one carbon atom as CO2, leaving the two remaining carbon atoms to form acetyl CoA.

Key concepts

oxidative decarboxylation

Oxidative decarboxylation is a crucial step in cellular respiration. It involves the removal of a carbon atom from a molecule through an oxidation reaction.

In the case of pyruvate, a three-carbon molecule is transformed into a two-carbon molecule called acetyl CoA. This transformation helps in the release of a carbon atom in the form of carbon dioxide (CO2). The process is called 'oxidative' because it involves the oxidation of pyruvate by removing electrons, which are then transferred to NAD+, converting it to NADH. This reaction is vital as it bridges glycolysis and the Krebs cycle in cellular respiration.

To summarize:

• Oxidative decarboxylation removes a carbon as CO2.
• It converts NAD+ to NADH.
• This step is essential for linking glycolysis to the Krebs cycle.

pyruvate dehydrogenase complex

The pyruvate dehydrogenase complex (PDC) is a group of enzymes that work together to convert pyruvate into acetyl CoA.

The PDC performs a series of reactions to achieve this conversion:

• First, pyruvate enters the mitochondria and the PDC removes the third carbon as CO2 in an oxidative decarboxylation reaction.
• Next, the remaining two-carbon molecule is bonded to coenzyme A (CoA) to produce acetyl CoA.
• This complex also reduces NAD+ to NADH, storing energy in the form of high-energy electrons.

The pyruvate dehydrogenase complex is incredibly efficient and crucial for energy production during cellular respiration. By converting pyruvate into acetyl CoA, PDC enables the entry of carbon molecules into the Krebs cycle, where more energy will be harvested.

carbon dioxide release

During the conversion of pyruvate to acetyl CoA, one carbon atom is released as carbon dioxide (CO2).

This happens through the process of oxidative decarboxylation catalyzed by the pyruvate dehydrogenase complex. Let's break it down:

• The original pyruvate has three carbon atoms.
• One of those carbon atoms is removed in the form of CO2.
• This reaction helps to streamline the molecule for the next steps of cellular respiration.

Releasing carbon dioxide is a way for the cell to dispose of excess carbon atoms while capturing high-energy electrons in the form of NADH. Eventually, the release of CO2 is part of how cells maintain their energy balance and appropriately manage carbon usage.

cellular respiration

Cellular respiration is the process cells use to produce energy from nutrients.

It consists of several stages:

• First, glycolysis breaks down glucose into two molecules of pyruvate.
• Next, pyruvate is converted into acetyl CoA by the pyruvate dehydrogenase complex.
• Then, acetyl CoA enters the Krebs cycle, where it undergoes a series of reactions to produce ATP, NADH, and FADH2.
• Finally, the electron transport chain uses the high-energy electrons from NADH and FADH2 to produce even more ATP.

Each step is interconnected, and the conversion of pyruvate to acetyl CoA is a critical juncture, ensuring that the energy extracted from glucose can continue to be processed efficiently. Understanding these steps is essential to comprehending how energy flows through living organisms.