MCAT Biology : MCAT Biological Sciences

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #44 : Cellular Processes And Functions

Both the sympathetic and the parasympathetic nervous systems are essential for homeostasis and for survival. For example, when we are trying to run away from a threat, the sympathetic nervous system is in full effect to allow us to escape from danger. However, when there is no obvious threat, the parasympathetic nervous system tends to be more in control. 

There are similarities and differences between the sympathetic and the parasympathetic nervous systems. In preganglionic nerve fibers, both the sympathetic and the parasympathetic nervous system utilize the neurotransmitter acetylcholine. Closer to the target organ, the parasympathetic nervous system remains dependent on acetylcholine whereas norepinephrine and epinephrine are the predominant neurotransmitters utilized by the sympathetic nervous system. 

When norepinephrine and epinephrine bind to their receptors, different effects are carried out based on the type of receptor, affinity, and location of the receptor. For example, epinephrine has a higher affinity for the beta-2 receptor. When epinephrine binds to the beta-2 receptor, common effects include vasodilation and bronchodilation. Norepinephrine has a stronger affinity for the alpha-1, alpha-2 and beta-1 receptors. When norepinephrine binds to its receptor, common effects on the body include vasoconstriction (alpha-1), increased heart rate (beta-1) and uterine contraction (alpha-1).

Patient A accidentally overdosed on a drug that activates the alpha-1, beta-1 and beta-2 receptors and is now experiencing a severe asthma attack. Which of the following second messengers should be regulated to treat the asthma attack? 

I. Decrease CAMP level

II. Decrease adenylate cyclase activity

III. increase phospholipase C activity

Possible Answers:

II only

None of these

III only

I only

I and II

Correct answer:

None of these

Explanation:

During a severe asthma attack, one should administer cpinephrine. Epinephrine binds to the beta-2 receptor. As mentioned in the passage, the activation of the beta-2 receptor will activate intercellular level of cyclic AMP. Cyclic AMP will then activate protein kinase A, which will then phosphorylate various proteins. Therefore, we want to increase cAMP levels, adenylate cyclase activity and decrease phospholipase C activity.

Example Question #11 : Cell Signaling

Sildenafil (commonly called Viagra) is a common drug used to treat erectile dysfunction and pulmonary arterial hypertension. Sildenafil's effect comes from its ability to cause vasodilation in smooth muscle cells. For this problem, we're only going to consider its effects on erections in males. 

Erectile dysfunction is a common medical problem in older men. Its most significant effect is the prevention of erections. Erections occur when there is an increase in blood flow via enlargement of an artery (vasodilation). Understanding the mechanism by which vasodilations occur is important in order to treat erectile dysfunction.  

Erections occur when nitric oxide  is released from an area in the penis and binds to guanylate cyclase in other cells of the penis, which creates cyclic guanosine monophosphate (cGMP) from GTP. cGMP causes a relaxation of the arterial wall in order to increase blood flow to the region, thereby causing an erection. cGMP is broken down over time by cGMP-specific phosphodiesterase type 5 (PDE5) into GTP, which reverses the effect and causes vasoconstriction on the arterial wall. Combatting this effect is the major method by which Viagra functions. 

Which of the following is not a possible mechanism by which Sildenafil treats erectile dysfunction?  

Possible Answers:

Increased breakdown of nitric oxide

Inhibition of PDE5 activity

Increase in cGMP production

Increase in nitric oxide release

Decrease in cGMP breakdown 

Correct answer:

Increased breakdown of nitric oxide

Explanation:

For this question we have to select an answer choice that would decrease the prolonging of vasodilation.

The only answer choice that decreases vasodilation is by increasing the breakdown of nitric oxide, which is the first messenger in this signal transduction cascade to cause vasodilation. If nitric oxide breakdown is increased, vasodilation would decrease.

Increase in cGMP production, decrease in cGMP breakdown, inhibition of PDE5 activity, and increase in nitric oxide release would all prolong vasodilation. 

 

Example Question #12 : Cell Signaling

Sildenafil (commonly called Viagra) is a common drug used to treat erectile dysfunction and pulmonary arterial hypertension. Sildenafil's effect comes from its ability to cause vasodilation in smooth muscle cells. For this problem, we're only going to consider its effects on erections in males. 

Erectile dysfunction is a common medical problem in older men. Its most significant effect is the prevention of erections. Erections occur when there is an increase in blood flow via enlargement of an artery (vasodilation). Understanding the mechanism by which vasodilations occur is important in order to treat erectile dysfunction.  

Erections occur when nitric oxide  is released from an area in the penis and binds to guanylate cyclase in other cells of the penis, which creates cyclic guanosine monophosphate (cGMP) from GTP. cGMP causes a relaxation of the arterial wall in order to increase blood flow to the region, thereby causing an erection. cGMP is broken down over time by cGMP-specific phosphodiesterase type 5 (PDE5) into GTP, which reverses the effect and causes vasoconstriction on the arterial wall. Combatting this effect is the major method by which Viagra functions. 

Nitric oxide is which of these types of signals? 

Possible Answers:

Endocrine signal

Autocrine signal

Paracrine signal

Growth hormone

Neurotransmitter

Correct answer:

Paracrine signal

Explanation:

Nitric oxide, as stated in the passage, is a signal that is sent from an area in the penis to another area within the penis. Since this is signaling to nearby cells, it is an example of paracrine signaling. 

Example Question #1381 : Biology

The cellular membrane is a very important structure. The lipid bilayer is both hydrophilic and hydrophobic. The hydrophilic layer faces the extracellular fluid and the cytosol of the cell. The hydrophobic portion of the lipid bilayer stays in between the hydrophobic regions like a sandwich. This bilayer separation allows for communication, protection, and homeostasis. 

One of the most utilized signaling transduction pathways is the G protein-coupled receptor pathway. The hydrophobic and hydrophilic properties of the cellular membrane allows for the peptide and other hydrophilic hormones to bind to the receptor on the cellular surface but to not enter the cell. This regulation allows for activation despite the hormone’s short half-life. On the other hand, hydrophobic hormones must have longer half-lives to allow for these ligands to cross the lipid bilayer, travel through the cell’s cytosol and eventually reach the nucleus. 

Cholesterol allows the lipid bilayer to maintain its fluidity despite the fluctuation in the body’s temperature due to events such as increasing metabolism. Cholesterol binds to the hydrophobic tails of the lipid bilayer. When the temperature is low, the cholesterol molecules prevent the hydrophobic tails from compacting and solidifying. When the temperature is high, the hydrophobic tails will be excited and will move excessively. This excess movement will bring instability to the bilayer. Cholesterol will prevent excessive movement.

Which of the following are associated with the G protein-coupled receptor? 

I. Adenylate cyclase

II. Phospholipase C 

III. Diacylglycerol 

Possible Answers:

I, II and III 

III only

II and III only

II only

I only

Correct answer:

I, II and III 

Explanation:

Adenylate cyclase is utilized in the G protein-coupled receptor pathway to convert ATP to cAMP. Phospholipase C and diacylglycerol are also part of the G protein-coupled receptor pathway. 

Example Question #331 : Cell Biology, Molecular Biology, And Genetics

The cellular membrane is a very important structure. The lipid bilayer is both hydrophilic and hydrophobic. The hydrophilic layer faces the extracellular fluid and the cytosol of the cell. The hydrophobic portion of the lipid bilayer stays in between the hydrophobic regions like a sandwich. This bilayer separation allows for communication, protection, and homeostasis. 

One of the most utilized signaling transduction pathways is the G protein-coupled receptor pathway. The hydrophobic and hydrophilic properties of the cellular membrane allows for the peptide and other hydrophilic hormones to bind to the receptor on the cellular surface but to not enter the cell. This regulation allows for activation despite the hormone’s short half-life. On the other hand, hydrophobic hormones must have longer half-lives to allow for these ligands to cross the lipid bilayer, travel through the cell’s cytosol and eventually reach the nucleus. 

Cholesterol allows the lipid bilayer to maintain its fluidity despite the fluctuation in the body’s temperature due to events such as increasing metabolism. Cholesterol binds to the hydrophobic tails of the lipid bilayer. When the temperature is low, the cholesterol molecules prevent the hydrophobic tails from compacting and solidifying. When the temperature is high, the hydrophobic tails will be excited and will move excessively. This excess movement will bring instability to the bilayer. Cholesterol will prevent excessive movement.

Which of the following molecules can be found inside of a cell? 

I. Cyclic adenosine monophosphate (cAMP) 

II. Protein kinase A

III. Protein kinase C

Possible Answers:

II and III 

II only

III only 

I, II, and III

I only

Correct answer:

I, II, and III

Explanation:

Cyclic adenosine monophosphate, protein kinase A and protein kinase C are all second messengers in the G protein-coupled receptor pathway. Since they are second messengers, they amplify and transmit the signal throughout different compartments of the cell.  

Example Question #1 : Other Cell Processes

The mechanism by which a macrophage engulfs cellular debris and/or pathogens is best described as __________.

Possible Answers:

Exocytosis

Phagocytosis

Pinocytosis

Receptor-mediated endocytosis

Correct answer:

Phagocytosis

Explanation:

Macrophages are immune cells that phagocytize cell debris and pathogens, then fuse with lysosomes to digest the material. Receptor-mediated endocytosis is a form of endocytosis primarily involved in viral infection or protein uptake. Pinocytosis refers to the uptake of liquid, specifically, from the extracellular medium. Exocytosis is the opposite of what is being described in this question.

Example Question #2 : Other Cell Processes

Which cells would be expected to have the highest rate of endocytosis?

Possible Answers:

B-cells

Osteoblasts

Neurons

Erythrocytes

Correct answer:

Neurons

Explanation:

Endocytosis is the uptake of extracellular materials in vesicles. After release of neurotransmitter into the synaptic cleft, neurons are able to re-uptake the neurotransmitter via endocytosis. This allows them to recycle the cell membrane and molecular components that are released during an action potential.

Erythrocytes are red blood cells, osteoblasts are bone-producing cells, and B-cells are responsible for producing antibodies. None of these cell types plays a significant role in particulate uptake. One should note, however, that while B-cells do not frequently use endocytosis, cytotoxic T-cells are able to phagocytose cellular debris through endocytosis.

Example Question #3 : Other Cell Processes

Prions are the suspected cause of a wide variety of neurodegenerative diseases in mammals. According to prevailing theory, prions are infectious particles made only of protein and found in high concentrations in the brains of infected animals. All mammals produce normal prion protein, PrPC, a transmembrane protein whose function remains unclear. 

Infectious prions, PrPRes, induce conformational changes in the existing PrPC proteins according to the following reaction:

PrPC  + PrPRes  → PrPRes + PrPRes

The PrPRes is then suspected to accumulate in the nervous tissue of infected patients and cause disease. This model of transmission generates replicated proteins, but does so bypassing the standard model of the central dogma of molecular biology. Transcription and translation apparently do not play a role in this replication process.

This theory is a major departure from previously established biological dogma. A scientist decides to test the protein-only theory of prion propagation. He establishes his experiment as follows:

Homogenized brain matter of infected rabbits is injected into the brains of healthy rabbits, as per the following table:

Rabbit 1 and 2: injected with normal saline on days 1 and 2

The above trials serve as controls.

Rabbit 3 and 4: injected with homogenized brain matter on days 1 and 2

The above trials use unmodified brain matter.

Rabbit 5 and 6: injected with irradiated homogenized brain matter on days 1 and 2

The above trials use brain matter that has been irradiated to destroy nucleic acids in the homogenate.

Rabbit 7 and 8: injected with protein-free centrifuged homogenized brain matter on days 1 and 2

The above trials use brain matter that has been centrifuged to generate a protein-free homogenate and a protein-rich homogenate based on molecular weight.

Rabbit 9 and 10: injected with boiled homogenized brain matter on days 1 and 2

The above trials use brain matter that have been boiled to destroy any bacterial contaminants in the homogenate.

A novel cell line is created that lacks PrPC. These cells are functionally normal except they grow uncontrollably and do not appear to have a finite number of divisions. PrPC is thus most likely involved in which process?

Possible Answers:

Inflammation

Cellular differentiation

Necrosis

Apoptosis

Cellular senescence

Correct answer:

Apoptosis

Explanation:

Apoptosis is programmed cell death. If cells are growing uncontrollably without a natural end to their growth patterns, they are likely not undergoing apoptosis.

Example Question #1401 : Mcat Biological Sciences

Most scientists subscribe to the theory of endosymbiosis to explain the presence of mitochondria in eukaryotic cells. According to the theory of endosymbiosis, early pre-eukaryotic cells phagocytosed free living prokaryotes, but failed to digest them. As a result, these prokaryotes remained in residence in the pre-eukaryotes, and continued to generate energy. The host cells were able to use this energy to gain a selective advantage over their competitors, and eventually the energy-producing prokaryotes became mitochondria.

In many ways, mitochondria are different from other cellular organelles, and these differences puzzled scientists for many years. The theory of endosymbiosis concisely explains a number of these observations about mitochondria. Perhaps most of all, the theory explains why aerobic metabolism is entirely limited to this one organelle, while other kinds of metabolism are more distributed in the cellular cytosol.

Cells with abnormalities in critical systems, such as mitochondria, will often undergo apoptosis. Which of the following is true of apoptosis?

Possible Answers:

It is an abnormal process, and usually leads to necrosis

It is a normal process, and does not lead to necrosis

It is a normal process, and is usually preceeded by necrosis

It is an abnormal process, and does not lead to necrosis

It is a normal process, and usually leads to necrosis

Correct answer:

It is a normal process, and does not lead to necrosis

Explanation:

Apoptosis and necrosis are distinct processes. Apoptosis is a biologically healthy death of cells, self-limiting and helpful in the elimination of abnormal cells. Necrosis is a pathological state of cell death, and drives inflammation.

Example Question #3 : Other Cell Processes

Which of the following are common occurances in apoptosis that are not seen during necrosis?

I. Membrane blebbing

II. DNA fragmentation

III. Cell shrinkage

 

Possible Answers:

I only

I and III

I, II, and III

II and III

Correct answer:

I and III

Explanation:

Apoptosis is a highly organized system of programmed cell death. Necrosis is premature cell death, typically caused by outside forces. In apoptosis membrane blebbing and bulging is observed, but the membrane itself maintains its integrity. In necrosis the membrane is usually disrupted. In both apoptosis and necrosis there is DNA fragmentation. Apoptosis is unique, however, in the fact that it has organized DNA fragmentation (during necrosis it is random). Apoptotic cells typically shrink, while necrotic cells swell.

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