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Test Prep MCAT Test Exam
Medical College Admission Test: Verbal Reasoning, Biological Sciences, Physical Sciences, Writing Sample (Page 29 )

Updated On: 30-Jan-2026
Page 29 of 164
PDF with 811 Questions

The process of depolarization triggers the cardiac cycle. The electronics of the cycle can be monitored by an electrocardiogram (EKG). The cycle is divided into two major phases, both named for events in the ventricle:
the period of ventricular contraction and blood ejection, systole, followed by the period of ventricular relaxation and blood filling, diastole.
During the very first part of systole, the ventricles are contracting but all valves in the heart are closed thus no blood can be ejected. Once the rising pressure in the ventricles becomes great enough to open the aortic and pulmonary valves, the ventricular ejection or systole occurs. Blood is forced into the aorta and pulmonary trunk as the contracting ventricular muscle fibers shorten. The volume of blood ejected from a ventricle during systole is termed stroke volume.
During the very first part of diastole, the ventricles begin to relax, and the aortic and pulmonary valves close. No blood is entering or leaving the ventricles since once again all the valves are closed. Once ventricular pressure falls below atrial pressure, the atrioventricular (AV) valves open. Atrial contraction occurs towards the end of diastole, after most of the ventricular filling has taken place. The ventricle receives blood throughout most of diastole, not just when the atrium contracts.
: Electronic and pressure changes in the heart and aorta during the cardiac cycle.
Figure 1


According to Fig. 1, the opening of the aortic and pulmonary valves is NOT associated with:

  1. ventricular systole.
  2. a rise and fall in aortic pressure.
  3. a drop and rise in left ventricular volume.
  4. the third phase of the cardiac cycle.

Answer(s): C

Explanation:

Close analysis of Figure 1 (compare to previous questions especially question 2 reveals that during the period that the aortic and pulmonary valves are OPEN, the curve for left ventricular volume drops (= decreases) but does not rise (= increase).



Several models have been developed for relating changes in dissociation constants to changes in the tertiary and quaternary structures of oligomeric proteins. One model suggests that the protein's subunits can exist in either of two distinct conformations, R and T. At equilibrium, there are few R conformation molecules: 10 000 T to 1 R and it is an important feature of the enzyme that this ratio does not change. The substrate is assumed to bind more tightly to the R form than to the T form, which means that binding of the substrate favors the transition from the T conformation to R.
The conformational transitions of the individual subunits are assumed to be tightly linked, so that if one subunit flips from T to R the others must do the same. The binding of the first molecule of substrate thus promotes the binding of the second and if substrate is added continuously, all of the enzyme will be in the R form and act on the substrate. Because the concerted transition of all of the subunits from T to R or back, preserves the overall symmetry of the protein, this model is called the symmetry model. The model further predicts that allosteric activating enzymes make the R conformation even more reactive with the substrate while allosteric inhibitors react with the T conformation so that most of the enzyme is held back in the T shape.
Experiment Evaluating Non-Symmetry Model Enzymes
Experiments were performed with enzyme conformers that did not obey the symmetry model. The data is summarized in Figure 1.

Equilibrium distribution of two conformers at different temperatures given the free energy of their Figure 1:
interconversion. (modified from Mr.Holmium).
What assumption is made about the T and R conformations and the substrate?

  1. In the absence of any substrate, the T conformation predominates.
  2. In the absence of any substrate, the R conformation predominates.
  3. In the absence of any substrate, the T and R conformations are in equilibrium.
  4. In the absence of any substrate, the enzyme exists in another conformation, S.

Answer(s): A

Explanation:

Paragraph 1. Information concerning the relative amounts of T and R conformations present before substrate is
added is given in the passage.



Many nutrients required by plants exist in soil as basic cations: Mg2+, Mn2, and Ca2. A soil's cation-exchange capacity is a measure of its ability to adsorb these basic cations as well as exchangeable hydrogen and aluminum ions. The cation-exchange capacity of soil is derived from two sources: small clay particles called micelles consisting of alternating layers of alumina and silica crystals, and organic colloids.
Replacement of A13+ and Si4+ by other cations of lower valence creates a net negative charge within the inner layers of the micelles. This is called the soil's permanent charge. For example, replacement of an atom of aluminum by calcium within a section where the net charge was previously zero, as shown below, produces a net charge of -1, to which other cations can become adsorbed.

Figure 1
A pH-dependent charge develops when hydrogen dissociates from hydroxyl moieties on the outer surfaces of the clay micelles. This leaves negatively-charged oxygen atoms to which basic cations may adsorb. Likewise, a large pH-dependent charge develops when hydrogen dissociates from carboxylic acids and phenols in organic matter.
In most clays, permanent charges brought about by substitution account for anywhere from half to nearly all of the total cation-exchange capacity. Soils very high in organic matter contain primarily pH-dependent charges.
In a research study, three samples of soil were leached with a 1 N solution of neutral KCl, and the displaced A13+ and basic cations measured. The sample was then leached again with a buffered solution of BaCl2 and triethanolamine at pH 8.2, and the displaced H+ measured. Table 1 gives results for three soils tested by this method.
Table 1


Due to the buffering effect of the soil's cation exchange capacity, just measuring the soil solution's pH will not indicate how much base is needed to change the soil pH. In another experiment, measured amounts of acid and base were added to 10-gram samples of well-mixed soil that had been collected from various locations in a field. The volumes of the samples were equalized by adding water. The results were recorded in Figure 2.

Figure 2
Which column(s) in Table 1 represent(s) the permanent charge of the soil micelles?

  1. A13+
  2. H+
  3. A13+ and Basic Cations
  4. A13+ and H+

Answer(s): C

Explanation:

The permanent charge would be represented by the ions that are removed at a neutral pH. The passage tells us that the permanent charge is the net negative charge held in the inner layer of the micelles that is generated when lower valence cations displace other cations like A13+ and Si4+. This part of the soil's charge is not dependent on the pH and will be displaced by the low valence ion K+ in the KC1 solution. Therefore, since A13+ and the basic cations were displaced by the KC1 solution, together they represent the permanent charge.



Several models have been developed for relating changes in dissociation constants to changes in the tertiary and quaternary structures of oligomeric proteins. One model suggests that the protein's subunits can exist in either of two distinct conformations, R and T. At equilibrium, there are few R conformation molecules: 10 000 T to 1 R and it is an important feature of the enzyme that this ratio does not change. The substrate is assumed to bind more tightly to the R form than to the T form, which means that binding of the substrate favors the transition from the T conformation to R.
The conformational transitions of the individual subunits are assumed to be tightly linked, so that if one subunit flips from T to R the others must do the same. The binding of the first molecule of substrate thus promotes the binding of the second and if substrate is added continuously, all of the enzyme will be in the R form and act on the substrate. Because the concerted transition of all of the subunits from T to R or back, preserves the overall symmetry of the protein, this model is called the symmetry model. The model further predicts that allosteric activating enzymes make the R conformation even more reactive with the substrate while allosteric inhibitors react with the T conformation so that most of the enzyme is held back in the T shape.
Experiment Evaluating Non-Symmetry Model Enzymes
Experiments were performed with enzyme conformers that did not obey the symmetry model. The data is summarized in Figure 1.

Equilibrium distribution of two conformers at different temperatures given the free energy of their Figure 1:
interconversion. (modified from Mr.Holmium).
The substrate binds more tightly to R because:

  1. T has a higher affinity for the substrate than R.
  2. R has a higher affinity for the substrate than T.
  3. there are 10 000 times more T conformation molecules than R conformation molecules.
  4. the value of the equilibrium constant does not change.

Answer(s): B

Explanation:

If a molecule has a high affinity for something, it is likely to be associated with it maximally. The substrate binds more tightly to the R conformation even though the R conformation is present in small amounts because R has
a higher affinity for the substrate than T.



Several models have been developed for relating changes in dissociation constants to changes in the tertiary and quaternary structures of oligomeric proteins. One model suggests that the protein's subunits can exist in either of two distinct conformations, R and T. At equilibrium, there are few R conformation molecules: 10 000 T to 1 R and it is an important feature of the enzyme that this ratio does not change. The substrate is assumed to bind more tightly to the R form than to the T form, which means that binding of the substrate favors the transition from the T conformation to R.
The conformational transitions of the individual subunits are assumed to be tightly linked, so that if one subunit flips from T to R the others must do the same. The binding of the first molecule of substrate thus promotes the binding of the second and if substrate is added continuously, all of the enzyme will be in the R form and act on the substrate. Because the concerted transition of all of the subunits from T to R or back, preserves the overall symmetry of the protein, this model is called the symmetry model. The model further predicts that allosteric activating enzymes make the R conformation even more reactive with the substrate while allosteric inhibitors react with the T conformation so that most of the enzyme is held back in the T shape.
Experiment Evaluating Non-Symmetry Model Enzymes
Experiments were performed with enzyme conformers that did not obey the symmetry model. The data is summarized in Figure 1.

Equilibrium distribution of two conformers at different temperatures given the free energy of their Figure 1:
interconversion. (modified from Mr.Holmium).
The symmetry model would NOT account for an enzyme:

  1. with many different biologically active conformations.
  2. which engages in positive cooperativity.
  3. with a complex metal cofactor.
  4. which is a catalyst for anabolic reactions.

Answer(s): A

Explanation:

The symmetry model describes an instance of something which may be described as positive cooperativity (paragraph 2). The model does not exclude the enzyme from having cofactors, and places no restriction on what the enzyme's function will be. However, the symmetry model does not account for the existence of any other conformations than the two described (paragraph 2).



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