Looking at the chemistry detailed in the overview of the citric acid cycle at the top of the first page of Panel 13–2, why do you suppose it is useful to link the two-carbon acetyl group to another, larger carbon skeleton, oxaloacetate, before completely oxidizing both of the acetyl-group carbons to CO₂?

Panel 13–2

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pyruvate COO NAD+ COO™ H-C-OH NAD* NADH + H STEP 7 NADH + H STEP 8 CH₂ malate 4C coo H₂O HS-COA coenzyme A fumarate 4C COO™ CH T CH coo COO™ C=O CH₂ coo oxaloacetate 4C next cycle FADH₂ STEP 6 FAD acetyl CoA 2C CH₂-C-S-COA COO™ c=o CH₂ coo oxaloacetate 4C succinate 4C COO™ CH₂ CH₂ Coo CITRIC ACID CYCLE HS-COA STEP 1 H₂O STEP 5 GTP HS-COA GDP COO™ CH₂ HO-Ċ-COO™ H₂O T CH₂ Coo citrate 6C succinyl CoA 4C COO™ CH₂ Overview of the complete citric acid cycle. The two carbons from acetyl CoA that enter this turn of the cycle (shadowed in red) will be converted to CO₂ in subsequent turns of the cycle: the two carbons in the starting oxaloacetate (shadowed in blue) will be converted to CO₂ in this cycle. CH₂ c=o I S-COA a-ketoglutarate 5C STEP 2 isocitrate 6C HO-CH STEP 4 NAD COO™ CH₂ HC-COO™ too™ STEP 3 NADH + H СОО CH₂ T CH₂ c=0 coo- HS-CoA NAD NADH + H CO₂