Kaplan MCAT General Chemistry Review

The blood-brain barrier is a unique part of the human nervous system. Endothelial cells lining blood vessels in the central nervous system (CNS) are more tightly attached to one another than in other parts of the human body. As a result, there is limited permeability of both small and large molecules from the circulation into the cerebrospinal fluid (CSF).

These tightly sealed endothelial cells have both advantages and disadvantages in the human system. The CNS is a fragile, essential part of the human body, and the endothelial cells serve as a barrier. Multiple characteristics of any given molecule affect its permeability: its polarity, size, weight, charge, and degree of protein binding in the blood. Nonpolar molecules pass more effectively from the bloodstream into the CSF. Smaller particles, such as water, and small, charged particles will also move with varying ease across this barrier. Water moves freely, but charged ions can take hours to equilibrate between the systemic circulation and the CSF.

When disease afflicts the CNS, it is necessary to deliver drugs to the cerebrospinal fluid for delivery into the tissues of the brain and spinal cord. On the other hand, some extremely effective chemotherapeutic agents, such as cisplatin, are beneficial when they do not cross the blood-brain barrier because they are neurotoxic when they penetrate the CNS. Alternatively, when beginning general anesthesia for a surgical procedure, it is essential that anesthetic agents penetrate from the systemic circulation into the CSF to alter consciousness and systemic muscle tone during the procedure.

1. Based on the passage, which of the following characteristics would be essential for any pharmaceutical intended for use as a general anesthetic?

A. The molecule should be nonpolar.

B. The molecule should be directly delivered to the CNS without going through the systemic circulation first.

C. The molecule should be polar.

D. The molecule should be slow-acting.

2. What can be logically inferred from the passage about charge and its effect on a molecule’s permeability of the blood-brain barrier?

A. Charged molecules are more likely to associate with one another tightly in the blood stream, inhibiting diffusion into the CSF.

B. Uncharged molecules are more likely to be able to diffuse between endothelial cells.

C. Uncharged molecules are less likely to be transported through endothelial cells.

D. Charged molecules are more soluble in the bloodstream than in the CSF.

3. What can be logically inferred from the passage about the role of the blood-brain barrier in supporting human life?

A. A permeable CNS is essential in allowing diffusion of nutrients from the peripheral circulation into the CNS.

B. The micro-environment of the CNS is similar to that of the systemic circulation.

C. The blood-brain barrier limits the flow of damaged or infected cells from the CSF into the systemic circulation.

D. The blood-brain barrier adaptively protects the CNS from toxins or other possible insults originating in the systemic circulation.

4. Based on the information in the passage, what type of intermolecular force has the MOST influence on molecules that pass easily through the blood-brain barrier?

A. Ion-dipole interactions

B. Dipole-dipole interactions

C. Hydrogen bonding

D. Dispersion forces

5. Which of the following statements is NOT true when relating formal charge with permeation across the blood-brain barrier?

I. A formal charge of zero guarantees permeability through the blood-brain barrier.

II. A negative formal change on one or more atoms in a molecule will improve its permeability of the blood-brain barrier.

III. Two molecules, both with formal charges of zero, will be equally permeable through the blood-brain barrier.

A. I only

B. III only

C. II and III only

D. I, II, and III

6. Cisplatin, a commonly used chemotherapeutic agent, is PtCl₂(NH₃)₂. What type of bond forms between each of the NH₃ groups and the central platinum?

A. Coordinate covalent bond

B. Polar covalent bond

C. Nonpolar covalent bond

D. Ionic bond

7. Phenytoin, shown below, is an anti-seizure medicine. It is one of many drugs that is actively transported out of the CNS by cellular transporters. What would be the best estimate of the geometry around the central carbon to which the arrow points?

A. Square planar

B. Tetrahedral

C. Trigonal pyramidal

D. Octahedral

8. Which of the following BEST describes the relationship between resonance structures and molecular polarity?

A. The most stable resonance structures maximize polarity.

B. If a molecule has more than one important resonance structure, it is more likely to be a polar molecule than another molecule without such resonance structures.

C. The most important resonance structures spread out and minimize formal charge.

D. Resonance structures will counterbalance the natural polarity of a bond.

PASSAGE II (QUESTIONS 9–16)