Outline the importance of the citric acid cycle in metabolism
Only an outline of the importance of the citric acid cycle (CAC) was expected, and not a
detailed understanding. Candidates struggled to appreciate its importance. For a good
answer candidates were expected to mention that the CAC has two main functions: energy
production and biosynthesis. Some understanding of the overall reaction
( AcCoA + 3NAD+ + FAD + GDP + Pi + 2H2O 2CO2 + 3NADH + FADH2 + GTP +
CoA ), the link between oxidation of metabolic fuel (carbohydrates, lipids and protein) and
aerobic energy production (oxidative phosphorylation), that Acetyl CoA is final common
product of these oxidations (Glucose via glycolysis (cytoplasm), Free fatty acids via β
oxidation (mitochondrial matrix) and Amino acids – glucogenic and ketogenic reactions),
that CAC is first stage of aerobic respiration, requires oxygen to proceed, that the CAC
together with oxidative phosphorylation accounts for 2/3 to 3/4 of ATP generated from fuel
oxidation and anabolic involvement, whereby several biosynthetic pathways utilize CAC
intermediates, e.g. Glucose biosynthesis (gluconeogenesis) via malate then oxaloacetate, fatty acid and cholesterol biosynthesis via citrate + CoA AcCoA, Amino acid biosynthesis
via reductive amination and transamination reactions, Porphyrin (organic component of
haeme) via succinyl CoA and Purine and pyrimidine biosynthesis – precursors of nucleotide
bases for DNA and RNA