39. Which of the following statements about human respiration is true?
(A) Most of the carbon dioxide in the blood is bound to hemoglobin.
(B) Increased pH level decreases affinity of hemoglobin for O2 and improves the unloading reaction.
(C) At normal systemic venous PO2, about 25% of the hemoglobin is in the form of oxyhemoglobin.
(D) Surfactants can increase the surface tension of the lungs.
(E) During an unforced exhalation, alveolar pressure is greater than atmospheric pressure.

35. Which of the following does not tend to promote speciation? (A) The founder effect (B) Gene flow (C) Natural selection (D) Polyploidy (E) Disruptive selection

36. What is the response a plant would react to an attack from herbivores? (A) Leaf abscission to prevent further loss of tissue. (B) Early flowering to reproduce before being eaten. (C) Production of chemical compounds for defense or to attract predators of the herbivores. (D) Production of physical defenses, such as thorns. (E) Production of thicker bark and cuticle to make it more difficult to eat.

37. A healthy young man ingests a large volume of distilled water and 50 min later a study is undertaken. In which of the following parts in his kidneys would the liquid with the greatest osmolarity be found? (A) Proximal tubule (B) The bottom of the loop of Henle of the cortical nephron (C) The bottom of the loop of Henle of the juxtamedullary nephron (D) Distal tubule of the juxtamedullary nephron (E) The innermost medullary collecting duct

38. Which of following statements about human digestive function is incorrect? (A) When food bolus enters the stomach, gastrin is secreted into the gastric lumen. (B) Secretin released from duodenum inhibits the secretion of gastric juice. (C) CCK released from duodenum stimulates the secretion of pancreatic enzymes. (D) The vagus nerve activates the parietal cells of stomach when a bite of food enters the mouth. (E) Increased sympathetic activity would slow down the motility of digestive tract.

40. Which event occurs after the first heart sound and before the second heart sound in human cardiac cycle? (A) Ventricular relaxation (B) Aortic systolic pressure (C) AV valves open (D) P wave of the ECG (E) Blood volume in ventricle increases

41. In a neuroscience lab, a student increased the concentration of K+ outside a neuron from 5 mM to 10 mM, while maintaining other ion concentrations as they were. What would happen to the neuron’s membrane potential? (A) The membrane potential would become more negative. (B) The membrane potential would become less negative. (C) The membrane potential would remain the same. (D) The membrane potential would be closer to the K+ equilibrium potential. (E) There would be no potential difference across the membrane.

42. Which of the following is not true about helper T cells? (A) They function in both cell-mediated and humoral immune responses. (B) They bear surface CD4 molecules. (C) They recognize polysaccharide fragments presented by infected cells. (D) They are subject to infection by HIV. (E) When activated, they secrete cytokines that stimulate other lymphocytes.

43. Which of the following statements about vision is not true? (A) Rods are more light-sensitive than cones and are responsible for night vision. (B) Color vision results from the presence of three subclasses of cones in the retina. (C) Perception of visual information takes place in the cerebral cortex. (D) All information from the left eye goes to the right visual cortex and all information from the right eye goes to the left visual brain. (E) The ciliary muscles relax if you look away from this page and focus your eyes on a distant object and the lens is flattened.

44. Which of the following sensory receptors is incorrectly paired with its category? (A) Photoreceptor — rod cell in retina (B) Nociceptor — muscle spindle (C) Chemoreceptor — taste receptor in tongue (D) Mechanoreceptor — hair cell in cochlea (E) Electroreceptor — hair cell in ampulla of Lorenzini

45. Within a few weeks of treatment with the drug 3TC, a patient’s HIV population consists entirely of 3TC-resistant viruses. What is the best explanation? (A) HIV has the ability to change its surface proteins and resist vaccines. (B) The patient must have become reinfected with 3TC-resistant viruses. (C) HIV began making drug-resistant versions of reverse transcriptase in response to the drug. (D) A few drug-resistant viruses were present at the start of treatment, and natural selection increased their frequency. (E) Some viruses developed drug resistance and then passed their function.