MCAT Prep Questions

MCAT Chemistry Sample Passage and Questions

Pseudoionone (PSI) is a terpene ketone derived from essential oils with vast applications in pharmaceutical, fragrance and cosmetic industries. PSI is typically produced from the aldol condensation of citral with ketone catalyzed by sodium hydroxide and lithium hydroxide with yields of 60–80%. Various undesired side reactions exist with these homogeneous catalysts such as the self-condensation of acetone and the secondary reactions involving the PSI produced. These homogeneous catalysts are also highly corrosive in nature and require neutralization steps upon reaction completion. Thus, the re-use of the catalyst is not practical.

Selection of a selective catalyst can increase the total yield and also reduce the formation of byproducts. Heterogeneous catalysts are easily separated from the reaction mixture and therefore, can be regenerated and reutilized. These advantages give rise to a more environmentally friendly process.

The importance of PSI encourages researchers to constantly improve the synthetic process. Table 1 shows the recent modifications and improvements that have been accomplished in the last decade to increase the efficiency of PSI production.

Recently, a new PSI derivative, 11,15-dimethylhexadeca-8,10,14-trien-7-one, was reportedly synthesized in the aldol condensation of citral and 2-octanone with sodium ethoxide catalyst. Regrettably, the new PSI derivative is not characterized in the paper mentioned. One of the weaknesses of this experiment is the use of sodium ethoxide where it is typically made by treating sodium metal with ethanol . In addition to being a costly material, sodium also reacts violently in the presence of water or oxidizing agent. Sodium ethoxide, which is usually used in the form of liquid with ethanol, is highly corrosive due to its strong base nature. Apart from that, inorganic salts formed in the neutralization steps upon reaction completion are undesirable for industry.

Amberlyst A-26 OH is a strong base, anionic, macroreticular polymeric resin based on the crosslinked styrene divinylbenzene copolymer containing quaternary ammonium groups. It is highly suitable to serve as a green catalyst. In its solid form, it is nonhazardous and can be used in a heterogeneous catalysis. After a reaction reaches completion, it can be recovered by simple filtration. Furthermore, many researchers have shown that this catalyst can be regenerated with ease using a 1 m NaOH solution.

Reference: An improved method for aldol condensation catalyses by Amberlyst A-26 OH: application in the synthesis of pseudoionone derivative, 11,15-dimethylhexadeca-8,10,14-trien-7-one; Daniel J.W. Chong, Florence H.L. Chong and Jalifah Latip. Green Processing and Synthesis

https://doi.org/10.1515/gps-2015-0126

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MCAT Biology Sample Passage and Questions

Since the discovery of sickle cell disease (SCD) in 1910, enormous strides have been made in the elucidation of the pathogenesis of its protean complications, which has inspired recent advances in targeted molecular therapies. In SCD, a single amino acid substitution in the β-globin chain leads to polymerization of mutant hemoglobin S, impairing erythrocyte rheology and survival. Sickle cell anemia is caused by homozygosity of the beta-S (β^S) allele, which differs from the wild-type β-allele by a single nucleotide polymorphism in which GTG is substituted for GAG in the sixth codon of the β-globin gene. This leads to replacement of a hydrophilic glutamic acid residue (Glu) with a hydrophobic valine residue (Val) at the sixth position in the β-globin chain, resulting in a mutated hemoglobin tetramer HbS (α2βs2) in the erythrocytes of individuals with sickle cell anemia.

Over the past 7 decades, scientists have characterized three major pathobiological processes (HbS polymerization, vaso-occlusion, and hemolysis-mediated endothelial dysfunction) that drive clinical disease. A single-nucleotide polymorphism in the β-globin gene leads to substitution of valine for glutamic acid at the sixth position in the β-globin chain. Following deoxygenation, the mutated hemoglobin (HbS) molecules polymerize to form bundles. The polymer bundles result in erythrocyte sickling by increasing cellular rigidity and distort the erythrocyte membrane, which in turn results in impaired rheology of the blood and aggregation of sickle erythrocytes with neutrophils, platelets, and endothelial cells to promote stasis of blood flow, referred to as vaso-occlusion.

Hemoglobin (Hb) polymer bundles also promote hemolysis or lysis of erythrocytes, which releases cell-free Hb into the blood circulation. Oxygenated Hb (Fe²⁺) promotes endothelial dysfunction by depleting endothelial nitric oxide (NO) reserves to form nitrate (NO₃⁻) and methemoglobin (Fe³⁺).

Some of the approved or potential therapies prevent HbS polymerization and rescue erythrocyte deformability. Other strategies include allosterically modifying HbS oxygen affinity. In addition to anti-polymerization or anti-sickling therapies, several antiadhesion therapies are approved or being tested.

Reference: Annu Rev Pathol. 2019 Jan 24; 14: 263–292. Pathophysiology of Sickle Cell Disease

Prithu Sundd, Mark T. Gladwin and Enrico M. Novelli

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MCAT Physics Sample Passage and Questions

There are four kind of vision problems. Myopia (Nearsightedness), Hyperopia (Farsightedness), Astigmatism and Presbyopia. In Myopia an elongated eyeball, a thick lens, or rigid or highly curved cornea may cause light that enters your eyes to deposit in front of the retina instead of directly on it. The nearsighted (myopic) eye converges rays from a distant object in front of the retina; thus, they are diverging when they strike the retina, producing a blurry image. This can be caused by the lens of the eye being too powerful or the length of the eye being too great. Difficulties seeing distant objects is the hallmark sign of this problem. Prescription eyewear and refractive surgeries may help correct this error. Concave lenses change the angle of refraction for light rays, so that they converge farther back on the retina.

In Hypermetropia (Farsightedness), as the opposite of myopia, there is an unusually short eyeball and a less curved cornea or thinner lens which directs the received light to the back portion of your retina. The farsighted (hyperopic) eye is unable to converge the rays from a close object by the time they strike the retina, producing blurry close vision. You may have a blurred close-range eyesight yet intact distant vision when this happens.

Correction of nearsightedness requires a diverging lens that compensates for the overconvergence by the eye. The diverging lens produces an image closer to the eye than the object, so that the nearsighted person can see it clearly. This reduces the power of an eye that is too powerful. To determine the spectacle power needed for correction, you must know the person’s far point—that is, you must know the greatest distance at which the person can see clearly. Then the image produced by a spectacle lens must be at this distance or closer for the nearsighted person to be able to see it clearly. It is worth noting that wearing glasses does not change the eye in any way. The eyeglass lens is simply used to create an image of the object at a distance where the nearsighted person can see it clearly.

Reference:

https://opentextbc.ca/openstaxcollegephysics/chapter/vision-correction/

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