MCAT (Biological and Biochemical) — Questions and Answers

Question 1: 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?

Active Transport
Endocytosis
Osmosis (Correct answer)
Passive Transport

Correct answer: Osmosis

Osmosis is a specific type of passive transport that involves the net movement of water molecules across a selectively permeable membrane. This movement occurs solely down the water potential gradient, from an area of higher water concentration (lower solute concentration) to an area of lower water concentration (higher solute concentration), without the input of cellular energy.

Question 2: 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?

46 chromosomes, 46 chromatids (Correct answer)
23 chromosomes, 46 chromatids
46 chromosomes, 92 chromatids
23 chromosomes, 23 chromatids

Correct answer: 46 chromosomes, 46 chromatids

Before the S phase (DNA replication) and the restriction point (R) in the cell cycle, a human somatic cell is in G1 phase. In this phase, the cell contains 46 chromosomes, and each chromosome consists of a single chromatid. DNA replication in the S phase would then duplicate each chromatid, resulting in 46 chromosomes, each with two sister chromatids (totaling 92 chromatids).

Question 3: For a cell to divide effectively, all five steps of mitosis must be completed correctly. Match the mitotic stage to the appropriate component:

Metaphase — Formation of two distinct nuclear membranes
Prophase — Dissolution of nucleoli (Correct answer)
Anaphase — Separation of the cytoplasm and organelles
Cytokinesis — Separation of sister chromatids to their respective poles
Telophase — Formation of the equatorial plate

Correct answer: Prophase — Dissolution of nucleoli

During prophase of mitosis, the chromatin condenses into visible chromosomes, the nuclear envelope begins to break down, and the nucleoli disappear. The dissolution of nucleoli is a characteristic event of prophase, preparing the cell for chromosome segregation. Other options describe events that occur in different stages or are incorrect.

Question 4: 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?

Myosin-actin detachment (Correct answer)
Tropomyosin-troponin interaction
Myosin-actin interaction
Calcium-troponin interaction

Correct answer: Myosin-actin detachment

ATP plays a crucial role in muscle contraction beyond the power stroke. Specifically, ATP binding to the myosin head causes it to detach from the actin filament, allowing the muscle to relax and prepare for another contraction cycle. Without ATP, myosin remains bound to actin, leading to rigor mortis.

Question 5: 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?

Elastinogen
Trypsinogen (Correct answer)
Carboxypeptidase
Lipase

Correct answer: Trypsinogen

Trypsinogen is the inactive precursor to trypsin, a crucial pancreatic enzyme responsible for digesting proteins in the small intestine. Its absence would severely impair the breakdown of proteins into smaller peptides and amino acids, leading to significant malabsorption of essential nutrients. While other enzymes are important, trypsin is a primary protease.

Question 6: 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?

Dilates blood vessels
Increases surface permeability
Maintains alveoli in an open state (Correct answer)
Depresses cilia of the lung

Correct answer: Maintains alveoli in an open state

Surfactant is a lipoprotein complex that reduces the surface tension within the alveoli of the lungs. By lowering surface tension, surfactant prevents the small alveolar sacs from collapsing during exhalation, thus maintaining them in an open state. This ensures a larger surface area for efficient gas exchange, including oxygen diffusion.

Question 7: 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?

Passive diffusion
Passive transport (Correct answer)
Endocytosis
Active transport

Correct answer: Passive transport

The movement of H+ ions back into the mitochondrial interior through ATP synthase occurs down their electrochemical gradient, from an area of higher concentration to lower concentration. This process does not require direct cellular energy input and is therefore a form of passive transport, specifically chemiosmosis, which drives ATP synthesis.

Question 8: 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?

The percentage of CO2 in the right ventricle would be higher.
The percentage of oxygen in the left atrium would be lower.
The percentage of oxygen in the right ventricle would be higher. (Correct answer)
The percentage of oxygen in the right atrium would be lower.

Correct answer: The percentage of oxygen in the right ventricle would be higher.

A ventricular septal defect (VSD) allows oxygenated blood from the left ventricle (which pumps to the body) to mix with deoxygenated blood in the right ventricle (which pumps to the lungs). This shunting of blood means that the right ventricle will receive blood with a higher oxygen concentration than it normally would, as it's mixed with blood from the systemic circulation.

Question 9: 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?

Platelet (Correct answer)
NK cell
Lymphoid progenitor
T cell

Correct answer: Platelet

Myeloid progenitor cells are multipotent stem cells that differentiate into various blood cell types, including granulocytes (neutrophils, eosinophils, basophils), monocytes, erythrocytes (red blood cells), and megakaryocytes, which then produce platelets. NK cells and T cells, on the other hand, develop from lymphoid progenitor cells.

Question 10: 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?

Artificial
Passive (Correct answer)
Active
Natural

Correct answer: Passive

This is an example of passive immunity because the patient is receiving pre-formed antibodies (from a horse) rather than producing their own immune response. This provides immediate, but temporary, protection against the rabies virus. Active immunity would involve the patient's own immune system generating antibodies, typically through vaccination or natural infection.

Question 11: Which of the following compounds can pass through the descending loop of Henle of the nephron of the kidney?

H2O (Correct answer)
Cl-
Na+
K+

Correct answer: H2O

The descending loop of Henle is highly permeable to water but relatively impermeable to solutes like Na+, Cl-, and K+. As the filtrate moves down this segment, water passively exits the tubule into the hypertonic medullary interstitium, concentrating the filtrate. This selective permeability is crucial for the kidney's ability to concentrate urine.

Question 12: 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?

Release of CRF-inhibitory factor.
Constriction of blood flow to the adrenal gland.
Exhaustion of the body’s store of CRF.
Negative feedback loop of cortisol. (Correct answer)

Correct answer: Negative feedback loop of cortisol.

The body's stress response, involving CRF and cortisol, is primarily regulated by a negative feedback loop. Elevated levels of cortisol in the bloodstream act on the hypothalamus and pituitary gland to inhibit further release of CRF and ACTH, respectively. This mechanism helps to prevent excessive and prolonged stress responses, maintaining homeostasis.

Question 13: 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?

Endometrium, cell division
Blastocyst, corpus luteum maintenance (Correct answer)
Corpus luteum, self-maintenance
Blastocyst, increase in blood flow

Correct answer: Blastocyst, corpus luteum maintenance

Beta-hCG (human chorionic gonadotropin) is secreted by the trophoblast cells of the developing blastocyst shortly after implantation. Its primary function is to maintain the corpus luteum in the ovary, preventing its degeneration. The corpus luteum then continues to produce progesterone, which is essential for maintaining the uterine lining and supporting the early pregnancy.

Question 14: 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?

Amino acid
Peptide
Enzymatic
Steroid (Correct answer)

Correct answer: Steroid

Steroid hormones are lipid-soluble, allowing them to easily cross cell membranes and function both inside and outside cells. Their characteristic structure is based on a four-ring carbon skeleton. Steroids often act as transcription factors, binding to intracellular receptors and influencing gene expression, which leads to an increase in nucleic acid (RNA) synthesis.

Question 15: 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?

Repolarization
Depolarization
Resting potential
Hyperpolarization (Correct answer)

Correct answer: Hyperpolarization

The period immediately following an action potential where a neuron is unresponsive to a normal stimulus is the absolute refractory period, often coinciding with repolarization and hyperpolarization. However, the fact that an *increased voltage* can elicit a response indicates the relative refractory period. This period occurs during hyperpolarization, where the membrane potential is more negative than the resting potential, requiring a stronger stimulus to reach the threshold for a new action potential.

Question 16: 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?

All daughters of an affected male will be affected. (Correct answer)
All descendants on the maternal side will have the disorder.
There will be equal distribution of males and females affected.
Females will be approximately twice as affected as males in this family.

Correct answer: All daughters of an affected male will be affected.

For an X-linked recessive disorder, an affected male (X^rY) passes his X chromosome, which carries the recessive allele (X^r), to all of his daughters. Therefore, all daughters will inherit this recessive allele. While they may not express the phenotype if they are heterozygous carriers (X^rX^N), they are genetically 'affected' in the sense of carrying the disease allele.

Question 17: 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:

Stabilizing the product to make the energy needed to produce less demanding.
Increasing the amount of available reactant to induce more volume.
Reducing the activation energy needed to start the reaction. (Correct answer)
Increasing the energy change in the reaction so that more energy is available for subsequent reactions.

Correct answer: Reducing the activation energy needed to start the reaction.

Enzymes function as biological catalysts by lowering the activation energy required for a reaction to proceed. They do this by stabilizing the transition state, which makes it easier for reactants to convert into products. This reduction in activation energy increases the reaction rate without altering the overall energy change (ΔG) or the equilibrium of the reaction.

Question 18: 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:

Increasing the amount of substrate from a starting point of Km
Increasing the amount of substrate from a starting point above Km
No amount of substrate will increase the rate of reaction.
Increasing the amount of substrate from a starting point below Km (Correct answer)

Correct answer: Increasing the amount of substrate from a starting point below Km

The Michaelis-Menten graph shows that at very low substrate concentrations (well below Km), the reaction velocity is directly proportional to the substrate concentration. This is because there are many free enzyme active sites available to bind substrate. Therefore, increasing the substrate from a point below Km will lead to the fastest initial increase in reaction velocity as more enzyme active sites become occupied.

Question 19: 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:

7 Pyruvate, 7 ATP, 7 NADH, 7H+, 7 H2O
12 Pyruvate, 12 ATP, 12 NADH, 12H+, 12 H2O (Correct answer)
2 Pyruvate, 2 ATP, 2 NADH, 2H+, 2 H2O
6 Pyruvate, 6 ATP, 6 NADH, 6H+, 6 H2O

Correct answer: 12 Pyruvate, 12 ATP, 12 NADH, 12H+, 12 H2O

Glycolysis is the metabolic pathway that breaks down one molecule of glucose into two molecules of pyruvate. For each molecule of glucose, glycolysis produces a net of 2 ATP, 2 NADH, 2 H+, and 2 H2O. Therefore, with 6 molecules of glucose, the process will yield 6 * 2 = 12 molecules of pyruvate, 12 ATP, 12 NADH, 12 H+, and 12 H2O.

Question 20: 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:

Increased temperature
Decreased pH
Increased BPG
Decreased CO2 (Correct answer)

Correct answer: Decreased CO2

A left shift in the oxygen-hemoglobin dissociation curve indicates an increased affinity of hemoglobin for oxygen. This shift is caused by conditions that favor oxygen binding, such as decreased temperature, increased pH (decreased H+), decreased 2,3-BPG, and decreased CO2. Decreased CO2, specifically, reduces the formation of carbaminohemoglobin and the Bohr effect, thereby increasing hemoglobin's oxygen affinity.

Question 21: 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?

The normal blood pH reading is likely inaccurate.
The patient’s kidneys may have compensated for the alkalemia.
The patient’s blood should ideally become acidemic for some time to compensate for the alkalemia.
The patient may be descending into respiratory failure. (Correct answer)

Correct answer: The patient may be descending into respiratory failure.

An asthma patient hyperventilating would initially blow off CO2, leading to respiratory alkalosis (pH 7.52). If the patient's pH then normalizes (7.41) despite not responding to treatment and still having an asthma attack, it suggests they are no longer able to hyperventilate effectively. This inability to blow off CO2 indicates worsening respiratory function and impending respiratory failure, as CO2 levels would be rising, bringing the pH down towards normal or even acidosis.

Question 22: Which of the following enzyme kinetics statements is FALSE?

Most enzymes operating in the human body work best at a temperature of 37°C.
An increase in the substrate concentration (at constant enzyme concentration) leads to proportional increases in the rate of the reaction. (Correct answer)
Maximal activity of many human enzymes occurs around pH 7.4.
An enzyme–substrate complex can either form a product or dissociate back into the enzyme and substrate.

Correct answer: An increase in the substrate concentration (at constant enzyme concentration) leads to proportional increases in the rate of the reaction.

At low substrate concentrations, increasing substrate does lead to a proportional increase in reaction rate. However, as substrate concentration continues to increase, the enzyme active sites become saturated. Once saturation is reached, further increases in substrate concentration will no longer proportionally increase the reaction rate, as the enzyme is working at its maximal velocity (Vmax). Therefore, the statement that increases are *proportional* at all substrate concentrations is false.

Question 23: 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.

I only
I and IV only
II and IV only
II and III only (Correct answer)

Correct answer: II and III only

Vitamin K is essential for the post-translational modification of clotting factors, specifically the carboxylation of glutamate residues, which creates calcium-binding sites necessary for coagulation. Vitamin A is metabolized to retinal, a component of rhodopsin, which is crucial for vision. Therefore, statements II and III are correct.

Question 24: Which of the following statements about diffusion and osmosis is correct?

Diffusion and osmosis rely on the electrochemical gradient of all compounds in a cell.
Diffusion and osmosis rely on the electrochemical gradient of only the compound of interest. (Correct answer)
Diffusion and osmosis cannot occur simultaneously.
Diffusion and osmosis will proceed in the same direction if there is only one solute.

Correct answer: Diffusion and osmosis rely on the electrochemical gradient of only the compound of interest.

Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration, driven by the concentration gradient of that specific substance. Osmosis is the diffusion of water across a selectively permeable membrane, driven by the water potential gradient, which is influenced by the solute concentration. Both processes are specific to the substance moving and are driven by its own electrochemical gradient, not the gradients of all other compounds.

Question 25: All of the following statements about the DNA molecule are correct, with the exception of

It is contained in human chromosomes.
It is a double helix.
It is made of nucleotides.
Its structure is identical to the RNA molecule. (Correct answer)

Correct answer: Its structure is identical to the RNA molecule.

DNA and RNA are both nucleic acids but have distinct structural differences. DNA is typically a double helix, contains deoxyribose sugar, and uses thymine as a base. RNA is typically single-stranded, contains ribose sugar, and uses uracil instead of thymine. Therefore, their structures are not identical.

MCAT (Biological and Biochemical)

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?