MCAT Biology : Plasma Membrane and Transport

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

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Example Question #1241 : Biology

Engulfment of a foreign pathogen is an example of ___________ and engulfment of extracellular fluid is an example of ___________; both are forms of ___________.

Possible Answers:

phagocytosis . . . pinocytosis . . . endocytosis

pinocytosis . . . pinocytosis . . . endocytosis

phagocytosis . . . phagocytosis . . . exocytosis

pinocytosis . . . phagocytosis . . . exocytosis

Correct answer:

phagocytosis . . . pinocytosis . . . endocytosis

Explanation:

Recall that endocytosis is the process by which solid particles and fluid and transported from the outside to the inside of the cell by the use of vesicles.

When a cell engulfs a foreign pathogen, it creates a vesicle known as a phagosome. The phagosome carries the pathogen within the cell and fuses with a lysosome, which allows hydrolytic enzymes to digest the pathogen. The process of engulfing the pathogen in a vesicle is known as phagocytosis.

A cell can also form a vesicle around fluids in the extracellular space, via the process of pinocytosis. This vesicle can also fuse with lysosomes, allowing the breakdown of small particulates in the vesicle, which can then be used for energy.

Both phagocytosis and pinocytosis are forms of endocytosis.

Example Question #41 : Cellular Structures And Organelles

ATP synthase is an enzyme that synthesizes ATP in the mitochondrion. Based on this information, which of the following is true regarding cellular transport and ATP synthase?

Possible Answers:

ATP synthase is important for both endocytosis and exocytosis

ATP synthase is irrelevant for both endocytosis and exocytosis

ATP synthase is important for exocytosis only

ATP synthase is important for endocytosis only

Correct answer:

ATP synthase is important for both endocytosis and exocytosis

Explanation:

ATP synthase is an important enzyme that is required for oxidative phosphorylation, a process that produces majority of the ATP in a cell. Both exocytosis and endocytosis are active processes. This means that they require energy (ATP) input; therefore, ATP synthase is essential for both endocytosis and exocytosis.

Example Question #1243 : Biology

A protein synthesized inside the cell does not undergo exocytosis. Which of the following is most likely true of this protein?

Possible Answers:

The protein was synthesized by a free-floating ribosome

The protein was synthesized by the ribosomes on rough endoplasmic reticulum

The protein was synthesized by the ribosomes on the nucleus

The protein was synthesized by the ribosomes on smooth endoplasmic reticulum

Correct answer:

The protein was synthesized by a free-floating ribosome

Explanation:

All proteins are synthesized by ribosomes, but the location of the ribosome can vary depending on the function and destination of the protein.

Ribosomes can be found as either as free-floating organelles in the cytosol or as part of the rough endoplasmic reticulum. The proteins synthesized on the rough endoplasmic reticulum will undergo modification and vesicular packaging, and are often destined for regions outside of the cell. These proteins can be expelled from the cell via exocytosis.

On the other hand, free-floating ribosomes in the cytosol produce proteins that are destined to remain in the cytosol. These proteins, once synthesized, are transported within the cytosol and never make contact with the cell exterior. The proteins from free-floating ribosomes do not undergo exocytosis.

Example Question #1241 : Biology

Proteins synthesized by ribosomes are processed and packaged in endoplasmic reticulum and Golgi apparatus, respectively. Packaged vesicles are then transported to their target location. Which of the following processes will these proteins most likely experience?

Possible Answers:

Exocytosis, because these proteins are destined for the cytosol

Exocytosis, because these proteins are destined for the extracellular space

Endocytosis, because these proteins are destined for the cytosol

Endocytosis, because these proteins are destined for the extracellular space

Correct answer:

Exocytosis, because these proteins are destined for the extracellular space

Explanation:

Exocytosis is the process by which materials and fluid are transported from the inside of the cell to the outside of the cell by the use of vesicles. Endocytosis is the process by which materials and fluid are transported from the outside to the inside of the cell by the use of vesicles. The question suggests that the proteins are synthesized inside the cell and then packaged into vesicles.

This process is common for a few different types of proteins. Membrane proteins will be integrated into these vesicles and introduced to the cell membrane by fusion of the vesicle. Some proteins destined for the lumens of cell organelles will also be packaged into vesicles, which will fuse with organelle membranes to deposit their contents. Most commonly, however, vesicles will fuse with the cell membrane and release protein contents into the extracellular space via exocytosis.

Golgi apparatus vesicles will never be involved in endocytosis. Of the two options involving exocytosis, only one offers the correct explanation (deposition of proteins into the extracellular space).

Example Question #1245 : Biology

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.

Another experiment shows that PrPC reacts with hormones that circulate among nervous tissue. As a transmembrane protein, what kinds of hormones are most likely to interact with PrPC?

I. Peptide hormones

II. Catecholamines

III. Steroid Hormones

Possible Answers:

II, only

I, II, and III

I and III

I and II, only

III, only

Correct answer:

I and II, only

Explanation:

Students should know that peptide hormones (and catecholamines, but this is not required to answer the question correctly as written here) interact with surface receptors and do not freely go through a membrane. They must interact with the transmembrane surface receptors to initiate a signal transduction cascade. In contrast, steroid hormones can bypass the transmembrance protein receptors by freely diffusing across the memberane, due to their small, nonpolar nature. In this case, only peptide hormones and catecholamines will require the facilitated diffusion mechanism provided by a transmembrane protein.

Example Question #201 : Cell Biology, Molecular Biology, And Genetics

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 is involved in the packaging of many neuronal transmembrane proteins, including membrane ion channels. Which of the following is true of a normal resting membrane potential?

(Note: membrane potential is typically measured relative to the cytosolic face.)

Possible Answers:

It is somewhat negative, due to a relatively high permeability via potassium carriers

It is somewhat negative, due to a relatively high permeability via potassium channels

It is somewhat positive, due to a relatively high permeability via sodium channels

It is somewhat positive, due to a relatively high permeability via potassium channels

It is somewhat negative, due to a relatively high permeability via sodium channels

Correct answer:

It is somewhat negative, due to a relatively high permeability via potassium channels

Explanation:

Compared to other compounds, the resting permeability of the cell membrane to potassium ions is quite high in most cells. During normal cell metabolism, the sodium-potassium pump moves three sodium ions out of the cytosol, and two potassium ions into the cytosol. The result is a high cytosolic potassium concentration, and an efflux of potassium through the permeable membrane.

It is important to note that the resting membrane potential is set by ion channels, and not ion carriers. The latter will change conformation during translocation of ions, while channels are simply conduits.

Example Question #41 : Plasma Membrane And Transport

Carbonic anhydrase is a very important enzyme that is utilized by the body. The enzyme catalyzes the following reaction:

A class of drugs that inhibits this enzyme is carbonic anhydrase inhibitors (eg. acetazolamide, brinzolamide, dorzolamide). These drugs are commonly prescribed in patients with glaucoma, hypertension, heart failure, high altitude sickness and for the treatment of basic drugs overdose.   

In patients with hypertension, carbonic anhydrase inhibitors will prevent the reabsorption of sodium chloride  in the proximal tubule of the kidney. When sodium is reabsorbed back into the blood, the molecule creates an electrical force. This electrical force then pulls water along with it into the blood. As more water enters the blood, the blood volume increase. By preventing the reabsorption of sodium, water reabsorption is reduced and the blood pressure decreases. 

When mountain climbing, the atmospheric pressure is lowered as the altitude increases. As a result of less oxygen into the lungs, ventilation increases. From the equation above, hyperventilation will result in more  being expired. Based on Le Chatelier’s principle, the reaction will shift to the left. Since there is more bicarbonate than protons in the body, the blood will become more basic (respiratory alkalosis). To prevent such life threatening result, one would take a carbonic anhydrase inhibitor to prevent the reaction from shifting to the left.  

Carbonic anhydrase inhibitors are useful in patients with a drug overdose that is acidic. The lumen of the collecting tubule is nonpolar. Due to the lumen's characteristic, molecules that are also nonpolar and uncharged are able to cross the membrane and re-enter the circulatory system. Since carbonic anhydrase inhibitors alkalize the urine, acidic molecules stay in a charged state.

How will excess intake of a carbonic anhydratase inhibitor affect the blood's osmolarity if not properly regulated by the body?

Possible Answers:

Decrease the blood's osmolarity

Make the blood hypoosmotic

Increase the blood's osmolarity

No change to the blood

Make the blood less hyperosmotic

Correct answer:

Increase the blood's osmolarity

Explanation:

As mentioned from the passage, carbonic anhydrase inhibitors will prevent water reabsorption at the proximal tubule. As a result, there will be less water in the blood. Osmolarity is a measurement of the amount of solutes divided by the fluid volume. Since the fluid volume will decrease, the osmolarity will increase.  

Example Question #41 : Plasma Membrane And Transport

Nuclear transport is a very important concept of study in modern cellular biology. Transport of proteins into the nucleus of an organism requires energy in the form of GTP, which is attached to a protein called Ras-related Nuclear protein (RAN).

RAN is a monomeric G protein found in both the cytosol as well as the nucleus and its phosphorylation state plays an important role in the movement of proteins into and out of the nucleus. Specifically, RAN-GTP and RAN-GDP binds to nuclear import and export receptors and carries them into or out of the nucleus. They also play a role in dropping off cargo that import and export receptors hold onto. RAN's functions are controlled by two other proteins: RAN guanine exchange factor (RAN-GEF) and RAN GTPase activating protein (GAP). RAN-GEF binds a GTP onto RAN, while RAN-GAP hydrolyzes GTP into GDP. As a result, there is a RAN-GTP and RAN-GDP concentration gradient that forms between the cytosol and nucleus.

Further research indicates that RAN-GTP and RAN-GDP form a concentration gradient across the nuclear membrane. The nuclear pore is only permeable to RAN-GTP. 

Given that our cells at any given time can only have  total concentration of RAN-GDP, with the cytosol having 3 times the concentration of RAN-GDP than in the nucleus, what can be said about the motion of RAN-GDP across the nuclear membrane? 

Possible Answers:

RAN-GDP does not move across the membrane 

RAN-GDP will move into the nucleus when RAN-GTP concentrations increase

RAN-GDP will move into the nucleus when RAN-GTP concentrations diminish

RAN-GDP will move from the nucleus to the cytosol

RAN-GDP will flow from the cytosol into the nucleus

Correct answer:

RAN-GDP does not move across the membrane 

Explanation:

For this question, the concentration values are not important, but simply the fact that the nuclear membrane is not permeable to RAN-GDP, as it is only permeable to RAN-GTP. Because of this impermeability, RAN-GDP will not be able to move into or out of the nucleus, and will therefore stay in whichever side it is on. 

Example Question #201 : Cell Biology, Molecular Biology, And Genetics

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. 

cGMP is also found in cells of the eye, and is known to open sodium channels. Given that Sildenafil functions by inhibiting phosphodiesterases, which of the following is also inhibited by Sildenafil in the eye? 

Possible Answers:

Depolarization

Active transport

Hyperpolarization

Photoreception

Facilitated diffusion

Correct answer:

Hyperpolarization

Explanation:

For this question we have to consider the effect of Sildenafil inhibiting phosphodiesterases (such as PDE5) on the eye. 

Inhibition of PDE5 in the eye would cause an increase in cGMP, which is given as something that opens ion channels. Since ion channels don't get to close, this means that the cell can never hyperpolarize, since hyperpolarization requires a voltage drop across a membrane. Since ion channels are not ever being closed, an electrochemical gradient cannot be formed and therefore the cell can never hyperpolarize. 

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