Multiple Choice
The Krebs cycle occurs in which portion of the cell?
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8. Respiration
Krebs Cycle
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In the Krebs cycle, what happens to pyruvic acid after it enters the mitochondrion?
A
It is broken down into lactic acid via fermentation.
B
It is converted into acetyl-CoA, which then enters the Krebs cycle.
C
It is directly converted into glucose through gluconeogenesis.
D
It is used to synthesize fatty acids.
Verified step by step guidance1
Step 1: Understand the context of the problem. The Krebs cycle (also known as the citric acid cycle) is a key metabolic pathway that occurs in the mitochondria and is involved in energy production. Pyruvic acid is a product of glycolysis, which occurs in the cytoplasm.
Step 2: Recognize that pyruvic acid must be transported into the mitochondrion to participate in the Krebs cycle. Once inside, it undergoes a preparatory step before entering the cycle.
Step 3: Learn the chemical transformation that occurs. Pyruvic acid is converted into acetyl-CoA through a process called oxidative decarboxylation. This reaction is catalyzed by the pyruvate dehydrogenase complex and involves the removal of a carbon atom as CO₂ and the addition of coenzyme A.
Step 4: Note that acetyl-CoA is the molecule that directly enters the Krebs cycle. It combines with oxaloacetate to form citrate, which is the first step of the cycle.
Step 5: Clarify misconceptions. Pyruvic acid is not broken down into lactic acid in the mitochondrion (this occurs during fermentation in anaerobic conditions), nor is it directly converted into glucose or used to synthesize fatty acids in this context.
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