Which of the following is the semi-fluid transition zone of the mantle between the lithosphere and the mesosphere?Asthenosphere
Atmosphere
Biosphere
Geosphere

Answer:

Simplified diagram of pyruvate oxidation. Pyruvate—three carbons—is converted to acetyl CoA, a two-carbon molecule attached to coenzyme A. A molecule of coenzyme A is a necessary reactant for this reaction, which releases a molecule of carbon dioxide and reduces a NAD+ to NADH.

Simplified diagram of pyruvate oxidation. Pyruvate—three carbons—is converted to acetyl CoA, a two-carbon molecule attached to coenzyme A. A molecule of coenzyme A is a necessary reactant for this reaction, which releases a molecule of carbon dioxide and reduces a NAD+ to NADH.

In eukaryotes, this step takes place in the matrix, the innermost compartment of mitochondria. In prokaryotes, it happens in the cytoplasm. Overall, pyruvate oxidation converts pyruvate—a three-carbon molecule—into acetyl two-carbon molecule attached to Coenzyme A—producing an t, N, A, D, H, end text and releasing one carbon dioxide molecule in the process. Acetyl  C, o, A, end text acts as fuel for the citric acid cycle in the next stage of cellular respiration.

Pyruvate oxidation steps

Pyruvate is produced by glycolysis in the cytoplasm, but pyruvate oxidation takes place in the mitochondrial matrix (in eukaryotes). So, before the chemical reactions can begin, pyruvate must enter the mitochondrion, crossing its inner membrane and arriving at the matrix.

In the matrix, pyruvate is modified in a series of steps:

More detailed diagram of the mechanism of pyruvate oxidation.

1. A carboxyl group is removed from pyruvate and released as carbon dioxide.

2. The two-carbon molecule from the first step is oxidized, and NAD+ accepts the electrons to form NADH.

3. The oxidized two-carbon molecule, an acetyl group, is attached to Coenzyme A to form acetyl CoA.

More detailed diagram of the mechanism of pyruvate oxidation.

A carboxyl group is removed from pyruvate and released as carbon dioxide.

The two-carbon molecule from the first step is oxidized, and NAD+ accepts the electrons to form NADH.

The oxidized two-carbon molecule, an acetyl group, is attached to Coenzyme A to form acetyl CoA.

Image credit: "Oxidation of pyruvate and the citric acid cycle: Figure 1" by OpenStax College, Biology, CC BY 3.0

Step 1. A carboxyl group is snipped off of pyruvate and released as a molecule of carbon dioxide, leaving behind a two-carbon molecule.

Step 2. The two-carbon molecule from step 1 is oxidized, and the electrons lost in the oxidation are picked up 2 \text{NADH}NADHstart text, N, A, D, H, end text are generated from \text{NAD}^+NAD

Step 3. The oxidized two-carbon molecule—an acetyl group, highlighted in green—is attached to Coenzyme A (\text{CoA}CoAstart text, C, o, A, end text), an organic molecule derived from vitamin B5, to form acetyl \text{CoA}CoAstart text, C, o, A, end text. Acetyl \text{CoA}CoAstart text, C, o, A, end text is sometimes called a carrier molecule, and its job here is to carry the acetyl group to the citric acid cycle.

The steps above are carried out by a large enzyme complex called the pyruvate dehydrogenase complex, which consists of three interconnected enzymes and includes over 60 subunits. At a couple of stages, the reaction intermediates actually form covalent bonds to the enzyme complex—or, more specifically, to its cofactors. The pyruvate dehydrogenase complex is an important target for regulation, as it controls the amount of acetyl \text{CoA}CoAstart text, C, o, A, end text fed into the citric acid cycle^{1,2,3}

1,2,3

start superscript, 1, comma, 2, comma, 3, end superscript.

If we consider the two pyruvates that enter from glycolysis (for each glucose molecule), we can summarize pyruvate oxidation as follows:

Two molecules of pyruvate are converted into two molecules of acetyl \text{CoA}CoAstart text, C, o, A, end text.

Two carbons are released as carbon dioxide—out of the six originally present in glucose.

2 \text{NADH}NADHstart text, N, A, D, H, end text are generated from \text{NAD}^+NAD

+

start text, N, A, D, end text, start superscript, plus, end superscript.

Why make acetyl \text{CoA}CoAstart text, C, o, A, end text? Acetyl \text{CoA}CoAstart text, C, o, A, end text serves as fuel for the citric acid cycle in the next stage of cellular respiration. The addition of \text{CoA}CoAstart text, C, o, A, end text helps activate the acetyl group, preparing it to undergo the necessary reactions to enter the citric acid cycle.

Explanation: