19. Your liver produces 90% of the cholesterol found in your body. When cholesterol levels get too high, the first enzyme in the pathway of cholesterol synthesis is inhibited. This is an example of__?
(A) positive feedback；
(B) denaturation；
(C) negative feedback；
(D) equilibrium.

15. Competitive inhibition of enzymes occurs__? (A) when a substance binds to an enzyme at a site away from the active site；(B) when the product, instead of the reactant of a reaction binds to the active site；(C) when a substance other than the substrate binds at the active site of an enzyme；(D) by blocking the production of an enzyme.

16. Which statement about simple diffusion is correct? (A) does not require energy；(B) utilizes proteins to move molecules across a membrane；(C) cannot occur without a membrane present；(D) moves molecules against a concentration gradient.

17. If an animal cell has a greater concentration of solute than its environment, the cell__? (A) will not experience a net gain or loss of water；(B) is hypertonic to the environment；(C) is isotonic to the environment；(D) is hypotonic to the environment.

18. The molecular reactants for photosynthesis are__? (A) glucose and carbon dioxide；(B) glucose and sunlight；(C) glucose and water；(D) water and carbon dioxide.

20. Which of the following is NOT a net product or reactant of photosynthesis? (A) oxygen；(B) glucose；(C) carbon dioxide；(D) ATP

21. The main reason that cellular aerobic respiration needs to occur step by step instead of a single, big reaction is__? (A) cells produce the enzymes needed for cellular respiration very slowly； (B) too much energy would be released as heat, and destroy the cell；(C) cells don't store enough oxygen to absorb all the energy in one release；(D) cells don't have enough mitochondria to catalyze the larger, single reaction.

22. During glycolysis molecules of glucose are__? (A) broken down in oxidation, liberating the carbon atoms as CO2；(B) bonded in a reduction to form three molecules of pyruvate；(C) bonded covalently to form two molecules of pyruvate；(D) broken down by enzymes to form two molecules of pyruvate.

23. The Krebs cycle runs__? (A) six times for each glucose, because each carbon pulled from the original molecule will power the Krebs cycle；(B) 12 times for each glucose, because all of these processes are associated with the Krebs cycle；(C) three times for each glucose, to power the electron transport proteins in hydrogen transport；(D) twice for each glucose, acting on the two-carbon molecule fragments from glycolysis, carried as acetyl CoA.

24. Fermentation is most common in__? (A) fungi, decomposing dead plants；(B) human muscle cells；(C) both human muscle cells, and intestinal bacteria；(D) bacteria, in mammal intestines.

25. Glycolysis does not require__? (A) oxygen；(B) ATP；(C) NAD+；(D) glucose.