MCAT (Biological and Biochemical)

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Living cells must connect with the outside world regularly to obtain the elements they require for living and to eliminate waste. Which of the following processes relies solely on the material's gradient to regulate the direction in which it flows across the cell membrane?

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Answer Explanation: (C)
Osmosis, also known as simple diffusion when referring to anything other than water, does not require energy and exploits the chemical component of potential energy to move material from a region of higher concentration to that of lower concentration. Passive transport moves material from one side of the membrane to the other according to gradient, but requires a special channel protein and deals with substances that cannot get through the membrane normally. Active transport always requires energy and moves substances against their concentration. Endocytosis is moving material from the outside of the cell into the inside, and involves invagination of the membrane around a material that is then brought into the cell as a vesicle. This usually is accompanied by a receptor that can start the process.

In order to divide and replicate themselves, somatic eukaryotic cells in the human body go through the cell cycle in a predictable pattern. The limitation point is the moment at which a cell decides to divide (R). How many chromosomes and chromatids does the cell have before this?

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Answer Explanation: (A)
The restriction point is the point of no return in the cell cycle process (the phases of the cell cycle in order, are G1/G0, R, S, G2, M) and also the last point before the genetic material starts to double. Although the cell is currently making enough protein to prepare for division, genetic replication has not started yet. Chromatids are identical copies of chromosomes. In the S, G2, and M phases, there are 92 chromatids in a somatic cell.

For a cell to divide effectively, all five steps of mitosis must be completed correctly. Match the mitotic stage to the appropriate component:

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Answer Explanation: (B) This answer would best be explained by describing each of the steps of mitosis with its components:
Prophase: dissolution of nucleoli, chromosomes become visible and nuclear envelope disappears, spindle fibers appear
Metaphase: spindle fibers attach to the chromosomes and the metaphase (equatorial) plate appears
Anaphase: chromosomes start to be pulled by their respective poles to opposite ends of the cell
Telophase: known as the early stage of cytokinesis, this is where the spindles will disappear and a nuclear membrane will be apparent around each set of chromosomes, chromosomes will start to disappear as they become uncoiled
Cytokinesis: formation of two distinct cells

Interactions between myosin and actin allow each sarcomere to shorten during muscle contraction. What other muscle contraction process, except the power stroke, necessitates the use of ATP?

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Answer Explanation: (A)
This is a unique and interesting concept specifically in one occurrence in the medical field: Rigor mortis. The stiffening of muscles after death is caused by an exhaustion of the body’s ATP supply. ATP in the relaxation stage binds to myosin, which is what releases the myosin head from the sarcomere and allows muscle to relax. When this does not occur, the muscle remains stiff. Hydrolyzation of the ATP molecule occurs during resting phase and the muscle is once again ready to contract.

During their adolescent years, Cystic Fibrosis (CF) patients frequently experience a loss of pancreatic activity. Which enzyme's absence, if isolated, would have the greatest impact on nutrient absorption?

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Answer Explanation: (B)
This question tests the knowledge of zymogens (enzymes that are secreted in inactive form) and the method in which pancreatic enzymes are turned on. Lipase would have a significant effect on absorption if it was lost, as it is secreted in active form. However, the product of trypsinogen, which is trypsin, activates four of the major enzymes of the pancreas (trypsinogen, chymotrypsinogen, elastinogen, carboxypeptidase) and allow them to break up their respective organic materials into absorbable components. Thus, loss of this enzyme would have the most extensive effects.

Neonatal Respiratory Distress Syndrome (NRDS) is a significant issue that occurs when a baby is born prematurely and their ability to assist oxygen passage through their alveolar membranes is disrupted. This is due to a lack of surfactant production. What role does surfactant play in the diffusion of oxygen?

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Answer Explanation: (C)
The wording of this answer will be somewhat different in presentation of information normally, so it will test the understanding of the concept. The main function that surfactant performs is that is decreases surface tension on the inner lining of the lung. The alveolar surfaces of the lung naturally have an elastic coil pressure that aid in expulsion of gases during the exhalation phase. However, without surfactant, there would be too much surface tension for them to remain open at the neutral state, and therefore would collapse and be unavailable for the next breath. Surfactant allows these alveoli to remain open and available for the next delivery of oxygen.

The electron transport chain, which is embedded in the mitochondrial membrane, is responsible for producing new ATP molecules for the cell to use. This is performed by a positive gradient of H+ ions forming outside the membrane and then passing back through ATP synthase, a specialized channel. The energy generated converts an ATP to an ATP, a process known as oxidative phosphorylation. What is the name of the mechanism that allows H+ to return to the mitochondrial interior?

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Answer Explanation: (B)
This question attempts to lead the reader into retrieving memory for the electron transport chain which is really not necessary. Understanding the definitions of these terms is sufficient, and knowing that the gradient of H+ goes from high to low from outside to inside through a membrane protein fits the definition of passive transport.

A ventricular septal defect is a hole in the septum between the right and left ventricles that is a common human heart abnormality. Which of the following statements is correct if a patient has this defect?

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Answer Explanation: (C)
This question aims to assess the knowledge of cardiovascular anatomy. Deoxygenated blood runs from the inferior and superior vena cavae and into the right atrium. It flows from there to the right ventricle and then up through the pulmonary vascular system. A hole in the ventricular septum would allow for this blood to be mixed between ventricles. Since the left ventricle is at a higher pressure, oxygenated blood will flow from the left ventricle to the right.

Acute Myeloid Leukemia (AML) is a type of cancer in which myeloid progenitor cells divide rapidly while maintaining their semi-stem cell status. Which of the following cell types would develop from this stem cell under normal circumstances?

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Answer Explanation: (A)
This question assesses knowledge of the pathway that gives rise to cell types in blood-forming cells. Platelets, which arise from megakaryocytes and megakaryocytic precursors, arise from the myeloid population. The other answers that would have been correct are neutrophils, basophils, eosinophils, and macrophages. Lymphoid progenitor cells are the lymphoid tissue equivalent to myeloid progenitor cells, and give rise to plasma cells, T cells, and NK cells.

A patient arrives at the hospital after being bitten by a dog that was "behaving erratically," according to him. The incision is scabbed up and bleeding profusely. The dog was apprehended by animal control and described as foaming at the mouth and extremely hostile. When a rabies infection is suspected, the patient is given a serum containing rabies antibodies generated inside a horse. What kind of immunity is this an example of?

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Answer Explanation: (B)
Passive immunity is defined as transfer of antibodies from an individual to a patient. This immunity lasts only for as long as the lifespan of the antibodies (3-6 months) and the patient does not continue to produce them. Conversely, active immunity is stimulation of the immune system to produce its own antibodies, such as a vaccine. Natural and artificial are terms to describe how the patient was infected.

Which of the following compounds can pass through the descending loop of Henle of the nephron of the kidney?

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Answer Explanation: (A)
This question is important to understand the function of the kidney, and relies on the reader’s knowledge of what permeability is. Since the descending loop of Henle is only freely permeable to water and no other substances, the concentration of solutes inside the nephron becomes very high and the water surrounding it very dilute. This is what allows the ascending loop to reabsorb so much water using the gradient and necessary ions for the body flow with it (eg. Magnesium, chloride, sodium).

The hypothalamus regulates the body's stress response by generating CRF, which leads to the systemic release of cortisol, the body's stress hormone. Which mechanism eventually suppresses this trigger?

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Explanation:
Cortisol acts as an inhibitor after its release on the anterior pituitary and the hypothalamus, indicating a sufficient stress response to the body and blocking release of more CRF. This is very important, as prolonged cortisol can start to harm the body. Although cortisol does cause vasoconstriction, it is not the feedback mechanism. The body also does not run out of CRF. CRF-inhibitory factor does not exist.

Pregnancy tests are highly sensitive and work by detecting B-hCG, or human chorionic gonadotropin, levels in urine. What tissue secretes this hormone, and what is its function?

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Answer Explanation: (B)
This question is helpful to understand both menstruation and fertilization. The corpus luteum, after follicular rupture, lasts for two weeks, and secretes progesterone, which is responsible for preparing the lining of the uterus for implantation. It is the loss of this hormone that leads the the menstrual phase of the cycle. If a fertilized egg, or blastocyst, is implanted into the endometrium, however, it secretes B-hCG, which maintains the corpus luteum and allows for survival and proliferation of the endometrium to begin the pregnancy. This hormone is detectable 1-2 weeks after conception.

A scientist uses mouse models to study the response of blood hormones to high levels of glucose in the body and discovers a previously unknown hormone. This hormone is located both inside and outside of cells when performing its function, increases the amount of nucleic acid in the cell when operating, and is formed from a structure having four hydroxycarbon rings, according to his data. Which hormone will this most likely be classified as?

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Answer explanation: (D)
Steroid hormones, derived from cholesterol, act on intracellular receptors and produce mRNA specific to the outcome they produce. Thus, they are found both inside and out of the cell. Peptide hormones, such as insulin, act on outer membrane receptors, but are polarized, so they cannot enter the cell. Amino acid hormones can have the ability to enter the cell, but are usually made up of very small combinations of amino acids, and never of cholesterol, a four hydroxycarbon ring.

A scientist stimulates a group of neurons in the hypothalamus using electrodes and records their membrane potential changes. When she activates them for the first time, she notices a 100mV difference in membrane potential. There is no response when she tries another stimulus right after the first. When she increases the voltage applied to the second shock, though, she sees a similar response to the first. In terms of membrane potential, what time period of stimulation would the second shock be characterized?

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Answer Explanation: (D)
This question requires secondary knowledge to determine not only which refractory period the neurons are at during the second shock, but to also extrapolate that to a depolarization curve. During depolarization and repolarization, the neuron is in what is called the absolute refractory period, meaning no amount of stimulus will lead to a firing. During hyperpolarization, if a stronger than normal stimulus is applied, there will be a response. This is known as the relative refractory period. Resting potential will show a response to the same amount of stimulus as any other time. This is depicted in the graph above:

A rare genetic disease with X-linked recessive transmission is discovered in a newborn's genetic test. Which of the following claims about this disorder's pedigree is most likely correct?

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Answer Explanation: (A)
Although a pedigree will almost always be a visual question, it is very important to be able to understand the logic of heritability to increase accuracy on these questions. Because a male can only pass an X chromosome to a daughter, if he is affected, he will produce all affected daughters. Mitochondrial DNA is passed exclusively by the mother and therefore will affect all her descendants. Females are twice as likely to be affected as males in X-linked dominant disorders, since they have two X chromosomes. Finally, X-linked recessive disorders tend more towards males, since they only need one inherited copy to be affected.

The most accurate definition of how enzymes facilitate processes, based on the diagram and knowledge about enzymes, would be:
Enzymes play a critical role in the biological world's energy production process. Below is an example of a typical energy reaction:

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Answer Explanation: (C)
The most important concept that needs to be known for enzymes is that they do not change the overall free energy of a reaction. Rather, they are facilitators of reactions by making the transitional state (The peak of the red curve) easier to attain, which reduces the activation energy needed. They do not increase either the reactant or the product, and the overall result of the reaction is the same.

Which of the following responses, based on this graph, would increase the reaction's velocity the fastest?
The Michaelis-Menten Equation defines the speed of a reaction in terms of substrate amount and enzyme affinity for that substrate. It's frequently depicted in the same way as the graph below:

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Answer Explanation: (D)
The key of understanding the Michaelis-Menten equation is to view it in terms of the slope of the velocity increase in relation to the amount of increase of substrate. The slope is the highest below the Km point, which is when half of the sites of the enzyme are full. After this point, you begin to get diminishing returns on the increase in velocity as substrate is added. After Vmax, no amount of substrate will increase the rate of reaction.

What products may C. sporogenes make with 6 molecules of glucose, based on this graph?
Clostridium sporogenes is an obligate anaerobe that relies on the glycolytic route for energy. The following figure depicts the path:

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Answer Explanation: (B)
A vital component to understanding the glycolysis pathway, and one that is not well elicited on the graph but must be known, is that when fructose 1,6-DP splits in step 4, it produces 2 molecules of glyceraldehyde-3-phosphate. Hence, every step after this occurs in double. Therefore, although it looks as though 1 molecule of glucose yields 1 molecule of pyruvate, it is actually the case that all yields after step 4 are doubled. This is an easy way to be tripped up on the exam.

What additional types of settings, besides HbF, would cause the dissociation curve to shift to the left?
Fetal hemoglobin (Hb-F) has a greater affinity for oxygen than adult hemoglobin (Hb-A), allowing a fetus to get enough oxygen from the mother's blood. The following is a representation of a normal hemoglobin-dissociation curve that shows this increased affinity:

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Answer Explanation: (D)
The shifts in the hemoglobin-dissociation curve are important to know to understand how oxygen gets offloaded into tissues. The essential idea is that areas that need oxygen more than the blood should create environments that allow oxygen to come off of hemoglobin and into these tissues. The Bohr Effect explains why hemoglobin loses affinity for oxygen in areas of increased CO2 and decreased pH (these are usually seen together, such as in metabolically active tissue during exercise). It is important to remember that increased H+ ions and decreased pH are interchangeable. In areas where CO2 decreases (such as the lungs), hemoglobin in the blood binds to oxygen more readily. BPG stabilizes the low oxygen affinity state of hemoglobin.

An asthma attack brings a patient to the emergency room. For the past hour, the patient has been hyperventilating and has a blood pH of 7.52. Although the patient does not appear to respond to treatment, a subsequent blood pH result of 7.41 is obtained. Why is it that a normal blood pH isn't reassuring?

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The correct answer is: D
When a patient with an asthma attack does not respond to treatment and has been hyperventilating for over an hour, he or she may become fatigued and may not be able to maintain hyperventilation. In this case, the patient begins to decrease his or her breathing rate and is not receiving adequate oxygen. By extension, carbon dioxide is trapped in the blood, and the pH begins to drop. Despite the fact that this pH is normal at the moment, this patient is crashing and may start demonstrating acidemia in the near future. While the kidneys should compensate for alkalemia, this is a slow process and would not normalize the blood pH within an hour; further, adequate compensation by the kidneys would actually be a reassuring sign, eliminating choice (B). There is no evidence to believe the measurement was inaccurate, eliminating choice (A). Finally, after treatment, the patient should return to a normal blood pH with adequate ventilation and would not be expected to overcompensate by becoming acidemic, eliminating choice (C).

Which of the following enzyme kinetics statements is FALSE?

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B: Most enzymes in the human body operate at maximal activity around a temperature of 37°C and a pH of 7.4, which is the pH of most body fluids. In addition, as characterized by the Michaelis–Menten equation, enzymes form an enzyme–substrate complex, which can either dissociate back into the enzyme and substrate or proceed to form a product. So far, we can eliminate choices (A), (C), and (D), so let’s check choice (B). An increase in the substrate concentration, while maintaining a constant enzyme concentration, leads to a proportional increase in the rate of the reaction only initially. However, once most of the active sites are occupied, the reaction rate levels off, regardless of further increases in substrate concentration. At high concentrations of substrate, the reaction rate approaches its maximal velocity and is no longer changed by further increases in substrate concentration.

Which statement concerning fat-soluble vitamins is correct?
I. Vitamin D protects against cancer because it is a biological antioxidant.
II. Vitamin K is necessary for the posttranslational introduction of calcium-binding sites.
III. Vitamin A is metabolized to retinal, which is important for sight.
IV. Vitamin E is important for calcium regulation.

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D: Vitamin A is metabolized to retinal, which is important for sight. Vitamin D is metabolized to calcitriol, which is important for calcium regulation. Vitamin E is made up of tocopherols, which are biological antioxidants. Vitamin K is necessary for the introduction of calcium binding sites, such as during the posttranslational modification of prothrombin.

Which of the following statements about diffusion and osmosis is correct?

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B: The movement of any solute or water by diffusion or osmosis is dependent only on the concentration gradient of that molecule and on membrane permeability.

All of the following statements about the DNA molecule are correct, with the exception of

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CORRECT ANSWER D Explanation:
All of the other statements are true, but the DNA and RNA molecules do not have the same structure.

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