21. Which of the following statements correctly describes the roles of insulin and glucagon in glucose homeostasis?
(A) Insulin promotes glycogenolysis and gluconeogenesis in the liver, while glucagon promotes glycolysis and glycogenesis.
(B) Insulin stimulates glucose uptake in muscle and adipose tissues by upregulating GLUT4 transporters, whereas glucagon primarily acts on the liver to increase glucose production.
(C) Insulin inhibits lipogenesis and enhances β-oxidation of fatty acids, whereas glucagon stimulates lipogenesis and inhibits β-oxidation.
(D) Both insulin and glucagon act through receptor tyrosine kinases to mediate their effects on target tissues.
(E) Insulin and glucagon are secreted by pancreatic β-cells in response to high glucose levels.

17. How is the electron transport chain related to oxidative phosphorylation? (A) The electron transport chain serves as the electron donor for oxidative phosphorylation. (B) Oxidative phosphorylation provides protons as the electron acceptor for the electron transport chain. (C) These two processes are entirely separate. (D) Both processes occur simultaneously in the cytoplasm. (E) None of the above.

18. Which of the following inhibitors can have its inhibitory effect overcome by increasing substrate concentration? (A) Competitive inhibitor. (B) Non-competitive inhibitor. (C) Uncompetitive inhibitor. (D) Irreversible inhibitor. (E) None of the above.

19. Which of the following statements about CRISPR-Cas9 is FALSE? (A) CRISPR-Cas9 can be used to introduce specific mutations in a precise manner. (B) Off-target effects, where unintended DNA sequences are edited, are a potential concern with CRISPR-Cas9. (C) CRISPR-Cas9 is primarily limited to editing the genomes of simpler organisms such as bacteria and yeast. (D) gRNA directs the Cas9 protein to a complementary target DNA sequence for cleavage. (E) CRISPR-Cas9 has the potential to treat genetic diseases by repairing or correcting disease-causing mutations.

20. Which of the following statements about nitrogen metabolism in biological systems is INCORRECT? (A) The enzyme glutamine synthetase catalyzes the ATP-dependent conversion of glutamate and ammonia to glutamine, serving as a key regulatory point in nitrogen assimilation. (B) The urea cycle is the primary pathway for nitrogen excretion in most terrestrial vertebrates, converting ammonia into urea for safe excretion. (C) Nitrogen fixation by nitrogenase requires ATP and is inhibited by molecular oxygen due to the enzyme's oxygen sensitivity. (D) Nitrate reductase and nitrite reductase are exclusively found in prokaryotes and play a critical role in reducing nitrate to ammonia for assimilation. (E) Aminotransferases (transaminases) use pyridoxal phosphate (PLP) as a cofactor to transfer amino groups between amino acids and a-keto acids, linking amino acid metabolism with the citric acid cycle.

22. Which of the following statements about oncogenes and tumor suppressor genes is INCORRECT? (A) Oncogenes are typically derived from proto-oncogenes through activating mutations, which lead to a gain of function and promote uncontrolled cell proliferation. (B) Tumor suppressor genes usually require both alleles to be inactivated by mutations or deletions to lose their function, a concept described by the "two-hit hypothesis." (C) Mutations in oncogenes are often dominant, while mutations in tumor suppressor genes are typically recessive. (D) Oncogenes are involved in processes such as apoptosis and DNA repair, whereas tumor suppressor genes primarily regulate growth and cell division. (E) The p53 protein, a tumor suppressor, plays a key role in the DNA damage response by inducing cell cycle arrest or apoptosis.

23. Which of the following hormones or neurotransmitters is NOT derived from an amino acid? (A) Serotonin. (B) Thyroxine (T4). (C) Dopamine. (D) Testosterone. (E) Epinephrine.

24. Which of the following organelles plays a crucial role in the modification and sorting of secretory proteins? (A) Endoplasmic reticulum. (B) Lysosome. (C) Autophagosome. (D) Golgi apparatus. (E) Peroxisome.

25. Which enzyme is deficient in phenylketonuria? (A) Phenylalanine hydroxylase. (B) Tyrosine hydroxylase. (C) Phenylalanine transaminase. (D) Phenylalanine deaminase. (E) Tyrosine kinase. 

1. Which of the following statements is correct regarding acetyl-CoA carboxylase? (A) Acetyl-CoA carboxylase is an allosteric enzyme. (B) Acetyl-CoA carboxylase is regulated by hormones via changes in its phosphorylation state. (C) Acetyl-CoA carboxylase is the most important enzyme in the regulation of lipogenesis. (D) All of the above are correct

2. Which of the following statements is wrong regarding the regulation of acetyl-CoA carboxylase? (A) Citrate promotes the conversion of the enzyme from an inactive dimer to an active polymeric form (B) Long-chain acyl-CoA promotes inactivation of the enzyme by phosphorylation of the enzyme. (C) Long-chain acyl-CoA promotes the tricarboxylate transporter, which transports citrate out of mitochondria into the cytosol. (D) All of the above are correct