MCAT Biology : Cellular Processes and Functions

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

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:

Inhibition of PDE5 activity

Decrease in cGMP breakdown 

Increase in nitric oxide release

Increase in cGMP production

Increased breakdown of nitric oxide

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:

Paracrine signal

Neurotransmitter

Autocrine signal

Growth hormone

Endocrine signal

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 #13 : Cell Signaling

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:

III only

I, II and III 

I only

II and III only

II 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 #14 : Cell Signaling

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:

I only

III only 

II and III 

II only

I, II, and III

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:

Phagocytosis

Receptor-mediated endocytosis

Pinocytosis

Exocytosis

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:

Osteoblasts

B-cells

Erythrocytes

Neurons

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

Necrosis

Apoptosis

Cellular senescence

Cellular differentiation

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 #4 : Other Cell Processes

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 a normal process, and is usually preceeded by necrosis

It is an abnormal process, and usually leads to necrosis

It is a normal process, and usually leads to necrosis

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

It is a normal process, and does not lead 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 #5 : 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, II, and III

II and III

I and III

I only

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.

Example Question #6 : Other Cell Processes

There are two models for the operation of the Golgi apparatus in eukaryotic cells. As it is difficult to visualize the operation of cells at the molecular level in real time, scientists typically rely on static electron micrographs to see the morphology of organelles. As a result, the dynamic operation of these organelles can sometimes be unclear.

Cisternal Maturation Hypothesis

In the cisternal maturation hypothesis, the cisternae of the Golgi apparatus evolve. Proteins leave the endoplasmic reticulum, and enter the cis-Golgi. The cisterna of the cis-Golgi then matures, with its enzymatic contents and internal environment changing as it becomes the medial-Golgi, and, eventually, the trans-Golgi.

In this model, the proteins never physically leave their membrane-bound cisternae during their transit across the Golgi. Instead, the entire unit of contents remains within the evolving cisternae.

Vesicular Transport Hypothesis

In contrast to the cisternal maturation hypothesis, the vesicular transport hypothesis posits that the cis-, medial-, and trans-Golgi cisternae are more static structures. Instead of evolving around their contents, the contents are physically shuttled via vesicular intermediates from each cisterna to the next.

In the case of vesicular transport, vesicles are shuttled along microtubules. Motor proteins facilitate this movement, with unique proteins being used for each direction of movement along a microtubule.

The Golgi apparatus has been implicated in cell death pathways. A scientist finds that a cell death pathway involving the Golgi is used in the breakdown of extra skin between digits during embryonic development. This pathway __________.

Possible Answers:

is called necrosis, and mainly involves the release of hydrolytic enzymes from within the cell

is called apoptosis, and mainly involves the absorption of hydrolytic enzymes from outside the cell

is called necrosis, and is a normal physiologic process also involved in the elimination of cancer cells

is called necrosis, and mainly involves the absorption of hydrolytic enzymes from outside the cell

is called apoptosis, and mainly involves the release of hydrolytic enzymes from within the cell

Correct answer:

is called apoptosis, and mainly involves the release of hydrolytic enzymes from within the cell

Explanation:

Apoptosis is the normal, physiologic elimination of cells. These include cancer cells, skin cells as described in the question, and others. It mainly involves the release of sequestered enzymes and mediators from within an apoptotic cell, such as from the Golgi apparatus or lysosomes.

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