MCAT Chemistry Practice Questions & Answers

Why Is Chemistry/Biochemistry Important?

Why it is so important to have conceptual understanding of MCAT chemistry and biochemistry topics for your MCAT prep? As a healthcare professional, depending on your medical specialty, you will be working with patients, trying to understand the root cause of their health issues, administering drugs, anesthesia doses, and so on. All this has an underlying explanation. How does the body work? How do proteins fold? How do enzymes carry out basic reactions like digestion, breathing, and so on? Why does lung capacity depend on partial pressure of oxygen and carbon dioxide? How many milligrams of dose should be given to a patient based on his/her weight? As you see, to treat a patient, you need to have some basic knowledge about chemical reactions and biochemical processes, as well as MCAT chemistry equations and other high-yield MCAT topics. For these reasons, this section on MCAT tests your basic knowledge about necessary relevant topics, so that you not only look at the disease but can also understand the disturbance in the biochemical make-up leading to that.

With that being said, let’s dive into sample MCAT chemistry/biochemistry practice questions and answers! Remember to try the passages and questions by yourself first, without looking at the expert responses we provided.

MCAT Chemistry/Biochemistry Practice Passage #1

Alzheimer’s Disease(AD) is a neurodegenerative disease. One of the pathological events in the progression of AD is the conformational changes and misfolding of the amyloid beta protein(Aβ), concentrated in the synapses of neurons. Aβ has physiological importance and is mainly produced by amyloidogenic metabolism of amyloid precursor protein, by sequential action of β- and γ-secretases, leading to the liberation of peptide between 39 and 42 amino-acid residues. However, the production of amyloid beta exceeds the clearance from the body in a diseased condition, leading to target proteins attain toxicity following their transition from a α-helix to a β-sheet form. Such abnormal conformational transition exposes hydrophobic amino acid residues and promotes protein aggregation. Aβ senile plaques are extracellular depositions of Aβ that are largely 40 or 42 amino acid in length (Aβ40 and Aβ420). The Aβ40 form is the more common of the two, whereas Aβ42, consequent to the additional two hydrophobic C-terminus amino acids (isoleucine and alanine), is the more fibrillogenic and hence associated with AD. In    a study on the disease-associated mutant, (D23N)Aβ40, the researchers reported stabilized parallel and antiparallel β-sheets within the amyloid fibrils. A number of small molecule probes able to track and/or inhibit protein aggregation have been developed recently. Synthetic peptide derivatives, termed ‘β-sheet breakers’, have been developed that are able to inhibit amyloid formation by binding to monomeric precursors or by preventing fibril elongation by blocking fibril ends. Substitution of key residues in synthetic peptides corresponding to the amyloid core regions with prolines or incorporating N-methyl modified amino acids, prevents hydrogen bond formation crucial to the cross-β structure. Small molecules have several advantages over peptide-based owing to there small size and lack of peptide bonds.

Q1. In a diseased state, what causes the aggregation of amyloid beta peptide in the extracellular space

A.   Cleavage of the amyloid peptide at the C-terminus of the amyloid precursor protein

B.    Slow clearance of amyloid peptide from the body

C.    Conformational changes of the amyloid peptide

D.   Over activation of amyloid beta cleaving enzymes

Q2. According to the passage which of the following strategy can lighten the amyloid beta peptide load in the neurons

A.   Inhibition of γ-secretases

B.    Binding of beta sheet breakers peptides to the aggregates

C.    Prevention of hydrogen bond formation between conformationally misfolded amyloid peptides

D.   Targeting the already formed fibrils and their subsequent dissolution using beta sheet breakers.

Q3. Why small molecule inhibitors of protein aggregation have advantages over peptide-based beta sheet breakers?

A.   They can easily cross blood brain barrier.

B.    It is easy to carry out the structure activity relationship studies to find out the best inhibitor of aggregation

C.    They can surpass hydrolysis unlike peptide-based drugs in the body

D.   Both A and C

Q4. Why beta sheet conformation of any protein is more prone to aggregation?

A.   It has an exposed hydrophilic region which interacts with the aqueous environment

B.    It is rich in hydrophobic amino acids

C.    The hydrophobic amino acids are exposed to aqueous environment

D.   Beta sheet conformations are overall more stable than alpha helix conformations

Ref: CNS Neurol Disord Drug Targets. 2014 ; 13(7): 1280–1293

Ref: Agnès Rioux Bilan* et. al. Neural Regen Res 13(6):955-961.

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MCAT Chemistry/Biochemistry Practice Passage #2

In order to function properly, the human body needs to maintain an optimal blood pH between 7.35 and 7.45. Large deviations from this range are extremely dangerous. Any condition in which the blood pH drops below 7.35 is known as acidosis. If the pH rises above 7.45, it is known as alkalosis. To prevent acidosis or alkalosis, the balanced functioning of body’s chemical buffer system in the blood with the body's respiratory and urinary systems is very important. The chemical buffer system functioning in blood plasma include plasma proteins, phosphate, bicarbonate and carbonic acid buffers. In the bicarbonate-carbonic acid buffer system, bicarbonate is regulated in the blood by sodium. When sodium bicarbonate (NaHCO₃), comes into contact with a strong acid, such as HCl, carbonic acid (H₂CO₃), which is a weak acid, and NaCl are formed. When carbonic acid comes into contact with a strong base, such as NaOH, bicarbonate and water are formed. Bicarbonate ions and carbonic acid are present in the blood in a 20:1 ratio at biological pH. With 20 times more bicarbonate than carbonic acid, this capture system is most efficient at buffering changes that would make the blood more acidic. This is useful because most of the body’s metabolic wastes, such as lactic acid and ketones, are acids. Carbonic acid levels in the blood are controlled by the expiration of CO₂ through the lungs. The level of bicarbonate in the blood is controlled by the renal system, where bicarbonate ions in the renal filtrate are conserved and passed back into the blood. The respiratory system contributes to the balance of acids and bases in the body by regulating the blood levels of carbonic acid . CO₂ in the blood readily reacts with water to form carbonic acid, and the levels of CO₂ and carbonic acid in the blood are in equilibrium. When the CO₂ level in the blood rises (as it does when you hold your breath), the excess CO₂ reacts with water to form additional carbonic acid, lowering blood pH. Increasing the rate and/or depth of respiration (which you might feel the “urge” to do after holding your breath) allows you to exhale more CO₂. The loss of CO₂ from the body reduces blood levels of carbonic acid and thereby adjusts the pH upward, toward normal levels. As you might have surmised, this process also works in the opposite direction. Excessive deep and rapid breathing (as in hyperventilation) rids the blood of CO2 and reduces the level of carbonic acid, making the blood too alkaline. This brief alkalosis can be remedied by rebreathing air that has been exhaled into a paper bag. Rebreathing exhaled air will rapidly bring blood pH down toward normal.

Q1. Which of the following would increase blood pH.

A.    an IV injection of lactated Ringers (which contains the base lactate)

B.    Holding your breath for long time

C.    Cardiac arrest

D.   Chronic Pulmonary disease

Q2. Which of the following pairs can act as a buffer system?

A.   NaOH/ NaCl

B.    CH₃COOH/NaOH

C.    NH₄OH/NH₄Cl

D.   HCl/NH₄OH

Q3. The ability of hemoglobin (Hb) to carry oxygen throughout the body as oxyhemoglobin (HbO₂) is dependent on the pH of the blood. What effect would acidosis have on the ability of a patient to transport oxygen?

HbH⁺ + O₂ ↔ HbO₂ + H⁺

A.   Remains unchanged

B.    Decreases the ability to carry oxygen

C.    Increases the ability to carry oxygen

D.   Increases first and then decreases.

MCAT Chemistry/Biochemistry Practice Passage #3

Polycyclic aromatic compound formed by fusing two or more aromatic rings together. They are known environmental contaminants. The inherent properties of PAHs such as heterocyclic aromatic ring structures, hydrophobicity, and thermostability have made them recalcitrant and highly persistent in the environment. The existence of dense π electrons on aromatic rings is responsible for the biochemical persistence of PAHs and their resistant to nucleophilic attack. Phenanthrene, Napthalene, Anthracene, Pyrenes are some of the most common PAHs found in environment. All these structures have significant resonance stabilization due to its three aromatic rings. All of the resonance structures are aromatic as described by Huckel Rule.

Bacteria play an important role in the removal of polycyclic aromatic hydrocarbons (PAHs) from polluted environments. In marine environments, Cycloclasticus is one of the most prevalent PAH-degrading bacterial genera. Cycloclasticus sp. is known to degrade naphthalene, phenanthrene, pyrene, and other aromatic hydrocarbons. To study the degradation capacity, Cells were grown supplemented with naphthalene, phenanthrene, or pyrene as the sole carbon source. Cultures of bacteria were incubated for between 0 and 10 days. Abiotic control assays without bacterial inoculation were conducted in parallel. After induction, cells were harvested by centrifugation and resuspended in 100 ml ASM. Bacterial growth was monitored by measuring culture ODs at 600 nm.

FIG 1 PAH degradation efficiency and growth of Cycloclasticus sp. P1 with several PAHs as the sole carbon source. The left axis shows PAH degradation efficiency as the residual PAH percentage after growth. The right axis shows cell density (OD600) under optimal growth conditions of 28°C, 3.0% salinity, and pH of 7.0. Vertical bars represent means ± standard deviations (SDs) of results from triplicate treatments.

The results clearly indicate at the ability of the Cycloclasticus in degrading the PAH and facilitating the cell proliferation. The study is important in understanding the biodegradation of ecotoxic PAHs and its future application

Wang et.al. ASM Journals, Applied and Environmental Microbiology, Vol. 84, No. 21.

Q1. Which of the following will have most resonance structures?

A. Benzene

B. Naphthalene

C. Anthracene

D. Phenanthrene

Q2. Cycloclasticus is able to degrade PAHs into nontoxic metabolites. What is the correct order for the degradation specificity for the bacteria?

A.   Phenanthrene > Napthalene> Pyrene

B.    Napthalene> Pyrene> Phenanthrene

C.    Napthalene> Phenanthrene> Napthalene

D.   Pyrene> phenanthrene> Napthalene

Q3. According to Huckle Rule, which of the following is not an aromatic compound?

A.   1,4-Cyclohexadiene

B.    Benzene

C.    Pyridine

D.   Napthalene

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