60. Tetanus refers to
(A) the partial sustained contraction of major supporting muscles.
(B) the all-or-none contraction of a single muscle fiber.
(C) a stronger contraction resulting from the activation of multiple motor units.
(D) a smooth and sustained contraction, resulting from wave summation.
(E) a state of muscle fatigue caused by the depletion of ATP and the accumulation of lactate.

56. In what order would you pass through tissues when moving from the pith to the epidermis in a plant possessing secondary vascular tissue? (A) primary phloem  →  primary xylem  →  secondary phloem  →  secondary xylem. (B) primary xylem → secondary xylem  → . vascular cambium  →  secondary phloem  →  primary phloem.(C) primary phloem  →  secondary phloem  →  secondary xylem →  primary xylem. (D) secondaryxylem  →  primary xylem  →  primary phloem  →  secondary phloem. (E) secondary phloem  →  primary phloem →  primary xylem  → secondary xylem.

57. Following clear-cutting of a deciduous forest several hundred years ago, the land was colonized by herbaceous species which, over time, were replaced largely by shrubs, then by forest trees. Assuming the growth of the shrubs and trees was enhanced by the soil-holding properties of the herbaceous plants, which of the following processes best describes the progression from herbaceous plants to forest trees? (A) secondary succession; inhibition. (B) primary succession; toleration. (C) secondary succession; facilitation. (D) primary succession; inhibition. (E) primary succession; facilitation.

58. Which of the following statements about fire is INCORRECT? (A) Fire is one kind of disturbance. (B) Communities such as grasslands and· chaparrals depend on periodic fire to .maintain. (C) Fire suppression by humans may increase the severity of the fires. (D) Fire suppression is necessary for the protection of threatened and endangered forest species. (E) Secondary succession follows a fire.

59. Which of the following is the CORRECT sequence that occurs during the excitation and contraction of a muscle cell?  1. Tropomyosin shifts and unblocks the cross-bridge binding sites.  2. Calcium is released and binds to the troponin complex.  3. Transverse tubules depolarize the sarcoplasmic reticulum.  4. The thin filaments are ratcheted across the thick filaments by the heads of the myosin molecules using energy from ATP.  5. An action potential in a motor neuron causes the axon to release acetylcholine, which depolarizes the muscle cell membrane.  (A) 5, 2, 3, 1, 4 (B) 2, 1, 3, 5, 4 (C) 2, 3, 4, 1, 5 (D) 5, 3, 1, 2, 4 (E) 5, 3, 2, 1, 4

61. Which of the following best describes how an axon grows toward its target cell? (A) The axon grows in a direct path, attracted by signal molecules released by target cells. (B) Cells along the growth path release signal molecules that either attract or repel the axon, and the interaction of CAMs on the growth cone and neighboring cells may provide tracks that guide axon growth. (C) Nerve growth factor released by astrocytes stimulates a neural progenitor cell to differentiate into a neuron, whose axon then grows toward an increasing concentration of signal molecules. (D) The axon produces growth-promoting proteins only in its growth cone, causing the axon to grow in an outward direction toward its target cells. (E) Glia first migrate to the target cells, leaving a trail of CAMs along the path that the growth cone of the axon follows.

62. If an egg cell contained EDTA~ a chemical that binds calcium and magnesium, what effects would this have on reproduction? (A) The acrosomal reaction would be blocked. (B) The fusion of sperm and egg nuclei would be blocked. (C) The fast block to polyspermy would not occur. (D) The fertilization envelope would not be formed. (E) The zygote would not contain maternal and paternal chromosomes.

63. Which of the following would probably have the greatest effect on convergent extension? (A) Stopping DNA synthesis. (B) Stopping mRNA synthesis. (C) Releasing an enzyme that digests glycoproteins. (D) A and B only. (E) A, B and C.

64. Which of the following is common to both avian and mammalian development? (A) holoblastic cleavage. (B) epiblast and hypoblast. (C) trophoblast. (D) yolk plug. (E) gray crescent.

65. How do the estrous and menstrual cycles compare? (A) Endometrial bleeding occurs during the menstrual cycle; the endometrium is reabsorbed by the uterus during the estrous cycle. (B) There are more pronounced behavioral changes during menstrual cycles than during estrous cycles. (C) There are stronger effects of season and climate on menstrual cycles. (D) Copulation can only occur during the period surrounding ovulation in both the estrous and menstrual cycles. (E) The length of both cycles averages 28 days.

66. A toxin that specifically blocks active transport in the loop of Henle might result in which of the following? (A) A decrease in the interstitial concentration ofNaCl. (B) A decrease in the filtrate concentration ability of the kidney. (C) An increase in the amount of interstitial urea to maintain interstitial osmolarity. (D) A and C only. (E) A, B and C.