18. In the citric acid cycle, acetyl-CoA reacts with ________ to produce _________.
(A) fumarate； ᵦ-ketoglutarate
(B) oxaloacetate； citrate
(C) citrate； isocitrate
(D) succinyl-CoA； oxaloacetate
(E) malate； cis-aconitate

14. Which of the following polysaccharides contains exclusively glucose in ᵦ(14) glycosidic bonds? (A) amylose (B) amylopectin (C) glycogen (D) cellulose (E) chitin

15. What is a polyribosome? (A) ribosomes that synthesize different subunits of the same protein (B) several ribosomes all attached to the same mRNA (C) ribosomes that are covalently bonded to each other in a polymer fashion (D) a ribosome that only produces proteins that will be excreted by the cell (E) none of the above

16. Which of the following DNA polymerases is correctly defined? (A) polymerase I: DNA repair (B) polymerase I: RNA primer excision (C) polymerase II: DNA repair (D) polymerase III: nucleotide incorporation during replication (E) all of the above

17. Which of the following is the regulatory step in cholesterol synthesis? (A) HMG-CoA synthase (B) HMG-CoA lyase (C) HMG-CoA reductase (D) squalene synthase (E) squalene epoxidase

19. Which of the following correctly identifies key elements present on the exterior of Gram-negative (GN) and Gram-positive (GP) bacteria? (A) GN: single lipid bilayer； GP: two lipid bilayers (B) GN: single peptidoglycan layer； GP: multiple peptidoglycan layers (C) GN: extensively crosslinked peptidoglycans； GP: uncrosslinked peptidoglycan (D) GN: contains both N-acetylmuramic acid and N-acetylglucosamine； GP: contains only N-acetylmuramic acid (E) none of the above

20. How many stereoisomers would be expected from a ketohexose? (A) 4 (B) 8 (C) 16 (D) 32 (E) 64

21. Which of the following does not specifically cause a membrane to be more rigid or fluid, but rather causes the Tm to occur over a greater range? (A) increasing the content of unsaturated fatty acids (B) raising the temperature (C) increasing the cholesterol content (D) decreasing the phosphatidylcholine content while increasing the phosphatidylethanolamine content (E) all of the above

22. Movement of ions across a cell membrane by the Na/K ATPase is best described as ________. (A) sodium moved outside to inside, potassium inside to outside (B) sodium moved inside to outside, potassium outside to inside (C) sodium moved inside to outside, potassium inside to outside (D) sodium moved outside to inside, potassium outside to inside (E) none of the above

23. Which of the following formulas would be a carbohydrate (A) C5H12O5 (B) C4H8O6 (C) C6H10O6 (D) C3H8O3 (E) C6H12O6

24. Which enzyme catalyzes the primary regulation site of glycolysis? (A) hexokinase (B) phosphofructokinase-1 (C) glyceraldehyde-3-phosphate dehydrogenase (D) phosphoglycerate kinase (E) pyruvate kinase