AP BIOLOGY

Detailed explanation-1: -First, acetyl CoA combines with oxaloacetate, a four-carbon molecule, losing the CoA group and forming the six-carbon molecule citrate. After citrate undergoes a rearrangement step, it undergoes an oxidation reaction, transferring electrons to NAD+ to form NADH and releasing a molecule of carbon dioxide.

Detailed explanation-2: -So, the correct answer is ‘Oxaloacetic acid’.

Detailed explanation-3: -Acetyl CoA combines with the 4C-oxaloacetate to form a 6C-compound that is citric acid. Citric acid is then oxidized breaking one of the C-C bonds to form carbon dioxide and reduce NAD to NADH. Therefore citric acid loses a carbon to form the only 5C-compound in Kreb’s cycle, -ketoglutaric acid.

Detailed explanation-4: -To start the cycle, an enzyme fuses acetyl CoA and oxaloacetate together so that citric acid is formed (a 2-carbon molecule + a 4-carbon molecule = a 6-carbon molecule!). This is the first molecule that is made in the cycle and is where the cycle gets its name.

Detailed explanation-5: -In the presence of oxygen, a molecule of CO2 and two hydrogen atoms are split from pyruvic acid, and acetyl-coenzyme A (acetyl-CoA) is formed. Acetyl-CoA combines with oxaloacetic acid in the Krebs cycle to form the six-carbon atom citric acid.