AP BIOLOGY

Detailed explanation-1: -Pyruvate dehydrogenase and carboxylase enzymes are localized to the mitochondrial matrix and, therefore, pyruvate must be transported from the cytosol through both the outer and the inner mitochondrial membranes.

Detailed explanation-2: -Phosphoenolpyruvate Carboxykinase (PEPCK) It is found in both the mitochondria (PEPCK-M) and the cytosol (PEPCK-C). Two different nuclear genes encode these two isoforms of PEPCK.

Detailed explanation-3: -Pyruvate is produced in the cytoplasm and is then transported into the mitochondria, where it is converted to acetyl-CoA; acetyl-CoA subsequently reacts with oxaloacetate to form citrate, thus entering the tricarboxylic acid (TCA) cycle.

Detailed explanation-4: -cerevisiae, pyruvate carboxylase is localized exclusively in the cytosol and lacks the peptide for mitochondrial targeting [3]. The cytosolic localization seems to afford these microorganisms the ability to produce large amounts of fumaric acid [3]. Biotin is an important regulator of pyruvate carboxylase activity.