MCAT TPR Exam 1

Two forces of equal strength F pulling (or pushing) in opposite directions along an axis of symmetry of a solid cause the solid to be in tension (or compression), and the ratio ofF to the solid's cross-sectional area A (perpendicular to the direction of the forces) is called stress. A tensile stress causes the solid to lengthen, while a compressive stress causes it to shorten. In either case, the ratio of the magnitude of the change in length, t.L, to the original length, 1.{), of the solid is called the strain. Simply put, stress causes strain. Human bone Is subject to a variety of stresses during a typlcalllfetlme. Under normal stress conditions, the strain in bone is very small, and the bone will return to its original shape after the stress is removed. However, if the strain exceeds the yield point as shown In Figure 1, the bone behaves plastically, and the strain no longer returns to zero when the stress is removed. Nevertheless. a bone can undergo considerable strain beyond the yield point before fracturing. Figure 1 compares stress to strain In typical human bone under tensile stress. Within the elastic region, the ratio of stress to strain is a constant known as Young's modulus: E = FIA flLIL0 The area under the curve in Figure 1 represents the elastic energy density, that is, the energy per unit volume of bone necessary to create the corresponding strain. The energy required per volume of bone to create a fracture would be equal to the area under the entire curve. elastic energy density at the yield point plastic region ~ fracture yield point __ ~_: ___ _.. ' • ! i Figure 1 Stress vs. strain in typical human bOne Cartilage is nearly as strong as bone in resisting compression, but it does not resist tension or shear as well. Its flexibility makes it ideal connective tissue for joints and bone endings. ===-----= = Passage 1 of,7i ... -. • Hide Time:f'''01:09:01 sabl -n;;;•f ~I A cyclist collides with a tree, and the collision creates an Intense compressive stress on the cyclist's left humerus (upper arm bOne). causing a fracture. Which of the fOllowing changes in the initial conditions most likely could have prevented the fracture? 0 A. A decrease in his velocity by a factor of two 0 B. A decrease in his mass by a factor of two o c. An Increase in his mass by a factor of two o D. A decrease in the cross-sectional area of his humerus by a factor of two If the graph in Figure 1 were redrawn for cartilage undergoing tensile stress, then the angle 8 would be: 0 A. greater, because cartilage has a smaller Young's modulus than does bOne. o B. greater, because cartilage has a larger Young's modulus than does bOne. o C. smaller, because cartilage has a smaller Young's modulus than does bone. 0 D. smaller, because cartilage has a larger Young's modulus than does bOne. Item 3 An increase In the cross-sectional area of a typical human bOne would most likely: o A. decrease the Young's modulus for that bOne, because the stress would be decreased. OB. oc. OD. uem• increase the Young's modulus for that bone, because the strain would be decreased. not affect the Young's modulus for that bone, but the strain for any given stress would be decreased. not affect the Young's modulus for that bone, but the strain for any given stress would be increased. Pagers disease Is characterized by massive new bone formation resulting in softemng of the bone. A patient with this disease may exhibit bOne deformities such as bowing of the tibia or humerus. Compared to normal bOne, a sample of bone from an Individual with Paget's disease would most likely have: o A. a smaller Young's modulus and a lower yield point. 0 B. a smaller Young's modulus and a higher yield point. 0 C. a larger Young's modulus and a lower yield point. o D. the same Young's modulus and a lower yield point. A human tibia with a cross-sectional area of 2 cm2 undergoes a 1% change in length When compressed by a force of 20,000 N. What is the approximate elastic energy density within the bone while compressed If the bone has not reached its yield point? o A. 5 x 1 03 J/m3 EXHIBIT NEXT MARK REVIEW Chemical indicators, used in the determination of pH, are either synthetic or naturally-occurring organic compounds of intermediate molecular weight Indicators operate by changing color depending upon the solution tested. Chemically, indicators act as weak acids or weak bases in aqueous solution, thus reacting in acid-base equilibria. For example, weak acid indicator HX reacts with water as shOwn below: HX(aq) ~ H·{aq) + x-{aq) Reaction 1 For an indicator to be suitable. It must have different colors for its undissociated and its dissociated state in order to distinguish between acidic and basic media. For example, In a medium with a high (H"], the equilibrium will shift to the left, favoring the reactants. In this case, the color of the solution will match that of the undissociated acid form of the indicator. Similarly, in a strongly basic solution, the color of the solution will match that of the dissociated form of the indicator. However, the color change associated with acidic and basic solutions is relative; that is, the pH at which the color change takes place depends upon the acid/base strength of the Indicator Itself. It follows that indicators have an acid ionization constant However, the human eye requires an acidic concentration ten times greater than the basic concentration to perceive the pure color of the indicator's acidic reading. This is also true for basic concentrations. Therefore, for the indicators to show the undissociated (acidic) form, the hydrogen ion concentration must be (H•) = 10Ka. and for the Indicators to shOw the dissociated (basic) form, the hydrogen ion concentration must be [H•) = Ka/10. The associated pH functions would be: { pK. -1 pH= pK.+1 for undissociated for dissociated Thus, the range of pH for Indicators can be determined. Table 1 pH Ranges lor various Indicators Indicae or Methyl violet Methyl red Bromthymol blue Phenol red Phenophthalein Alizarin yellow Passage 2 of 7io-, •. Hide Time-.~·. 01:08:49 • • Disable nffiei. • pH range 0.2 to 1.6 4.8 to 6.0 6.0 to 7.6 6.4 to 8.0 8.2to 10.0 IO.lto 12.0 BACK Methyl orange Is an Indicator that Is red in its undissociated form and yellow in its dissociated form. II the Ka for methyl orange is 1.6 x 10-4, which of the following gives the best pH range lor its undissociated and dissociated forms? o A. 1.4 to 3.4 0 B. 2.8 to 4.8 0 c. 4.0 to 6.0 0 D. 6.2 to 8.2 Which one of the following graphs best depicts the titration curve for the addition of a strong base to a strong acid? OA. t pH OB. t pH oc. t pH 00. t pH Volumeof _ strong base added Volume of strong base - added Volumeof _ strong base added Volumcof _ strong base added What is the pOH of an aqueous solution whose hydrogen ion concentration is measured to be 1 0 -s M? OA. 3 OB. 5 oc. 7 00. 9 Formic acid, HGOOH (Ka = 1.8 x 10""). dissociates in water according to Reaction 1. What Is the hydrogen ion concentration in a 0 .05 M aqueous solution of HCOOH? OA. 9.0 X 10-6 M OB. 3.6 X 10"" M oc. 3.0 X 10-3 M 00. 6.0 X 10-2 M EXHIBIT MARK REVIEW ton flow in neurons can be characterized as an electrical circuit for both the resting neuron (Figure 1) and the active neuron (Figure 2). In Figure 1, the axon membrane can be treated as a capacitor, slow leakage Channels as a 25 MO resistor, and the Na'IK' pump as a voltage generator. The voltage across the membrane Is - 70 mv from the eX1erior to the interior of the cell. and in a resting axon, there Is no net transfer of charge across the axon membrane. Rgure 2 includes the additional Na • innux (a 4 k o resistor) of an action potential. Other ion nuxes are Ignored. axon membrane axon membrane Passage 3 of 7io-,.. _ Hide Time,..·· ot:o8:38 • Disable -n.n.;;_ • extracellular fluid lealcage channels intrncellulnr fluid Figure 1 Resting neuron extracellular fluid intracellular fluid Figure 2 Active neuron Na•fK• pump According to the figures, decreasing which of the following would create the greatest increase In charge stored per unit voltage on an axon membrane in its rest state? 0 A. Leakage channel resistance 0 B. Na' channel resistance 0 c. Area of the membrane surfaces 0 D. Thidness or the membrane During an axon's rest state, a net positive charge travels across the leakage channels and across the Na ' tK' pump: 0 A. at equal rates and In the same direction. 0 B. at equal rates but In opposite directions. 0 c. at different rates but In the same direction. 0 D. at different rates and In opposite directions. Ouabain Irreversibly inhibits the Na'/K' pump. The effect of this on ihe equivalent circuit model ror the resting axon would be to cause: 0 A. increased potential difference across the resistors and the capacitor. 0 B. decreased potential difference across the resistors and the capacitor. 0 c. decreased potential difference across the capac~or and increased potential difference across the resistors. 0 D. decreased potential difference across the resistors and increased potential difference across the capacitor. If the active neuron in Figure 2 is clamped at a constant voltage and the total current through the Na • channels plus the leakage channels is 2.5 ~JA, what is the voltage drop across the membrane? OA. 1 mv OB. 10mv oc. 62.5mV OD. 62.5V Across the membrane of an axon in its rest state: 0 A. the potential is higher on the outside of the cell than the inside, and electric field lines run from the outside to the inside. 0 B. the potential is higher on the outside of the cell than the inside, and electric field lines run from the inside to the outside. 0 c. the potential is lower on the outside of the cell than the inside, and electric field lines run from the outside to the inside. 0 D. the potential Is lower on the outside of the cell than the inside. and electric field lines run from the inside to the outside. nem 1s BACK t Q time, tThe change in Q, the magnitude of charge on each surface of the axon membrane thai is depicted in the graph above, could be produced by Which of the following? 0 A. A decrease in the resistance or the Na • channels 0 B. A steady decrease In the voltage across the axon membrane 0 C. Inactivity of the Na'IK' pump 0 D. An increase in the resistance of the leakage channels EXHIBIT MARK REVIEW ~am Name MCAT fp~ Pract!~ Test 1 Without Essay t:Mit a Score I I End Section I I Suspend Questions on the right are not based on any passage and are not related to each ~ .... -------- other. Which one of the following molecules will NOT have a net dipole moment?