MCAT Biology : Plasma Membrane and Transport

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #1241 : Mcat Biological Sciences

The __________ side of a plasma membrane receptor will bind to the ligand and the __________ side of the plasma membrane receptor will initiate a cell response.

Possible Answers:

intracellular . . . extracellular

intracellular . . . intracellular

extracellular . . . extracellular

extracellular . . . intracellular

Correct answer:

extracellular . . . intracellular

Explanation:

In signal transduction, a ligand binds to the extracellular side of the plasma membrane receptor. This initiates a cellular response that is facilitated by the intracellular side. The intracellular region can activate a G protein, bind to an effector, or initiate other cellular responses. These responses often result in a signal cascade that affects transcription factors and alters gene expression.

Example Question #22 : Cellular Structures And Organelles

Scientists use a process called Flourescent In-Situ Hybridization, or FISH, to study genetic disorders in humans. FISH is a technique that uses spectrographic analysis to determine the presence or absence, as well as the relative abundance, of genetic material in human cells. 

To use FISH, scientists apply fluorescently-labeled bits of DNA of a known color, called probes, to samples of test DNA. These probes anneal to the sample DNA, and scientists can read the colors that result using laboratory equipment. One common use of FISH is to determine the presence of extra DNA in conditions of aneuploidy, a state in which a human cell has an abnormal number of chromosomes. Chromosomes are collections of DNA, the totality of which makes up a cell’s genome. Another typical use is in the study of cancer cells, where scientists use FISH labels to ascertain if genes have moved inappropriately in a cell’s genome.

Using red fluorescent tags, scientists label probe DNA for a gene known to be expressed more heavily in cancer cells than normal cells. They then label a probe for an immediately adjacent DNA sequence with a green fluorescent tag. Both probes are then added to three dishes, shown below.  In dish 1 human bladder cells are incubated with the probes, in dish 2 human epithelial cells are incubated, and in dish 3 known non-cancerous cells are used. The relative luminescence observed in regions of interest in all dishes is shown below.

 

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The bladder cells in dish 1 begin to undergo programmed cell death, or apoptosis, when they initially become cancerous. If the cells form sodium-selective pores in their membranes to begin the process of cell death, sodium ions can begin to enter the cells without regulation. What will likely happen to a resting cell membrane potential when sodium enters?

Possible Answers:

It will become more positive, because less sodium will enter than potassium will leave

It will become more positive, because potassium will also enter

It will become more negative, because potassium will also enter

It will become more positive, because more sodium will enter than potassium will leave

It will become more negative, because more sodium will enter than potassium will leave

Correct answer:

It will become more positive, because more sodium will enter than potassium will leave

Explanation:

The pores formed are, according to the question, sodium selective. So it is unlikely that potassium concentration changes will be a major contributor to membrane potential changes. Since sodium is postively charged, and the ions entering are sodium, the inside of the cell will become more positively charged as sodium permeability goes up. We know that sodium will enter and potassium will leave due to the established gradients determined by sodium-potassium ATPase.

Example Question #1241 : Mcat Biological Sciences

What is the average resting potential of a nerve cell membrane?

Possible Answers:

-70mV

-130mV

7mV

0mV

Correct answer:

-70mV

Explanation:

Membrane potential is the difference between the electric potential inside the cell and the electric potential outside the cell. At rest, the membrane potential of most cells (including nerve cells) is between -70mV and -80mV due to the concentration of intracellular and extracellular potassium and sodium ions. The expulsion of sodium ions, in particular, contributes to positive charges outside the cell and lowers the charge inside.

Example Question #21 : Plasma Membrane And Transport

What is the conventional way of measuring membrane potential?

Possible Answers:

The potential outside the cell plus the potential inside the cell

The potential inside the cell multiplied by the potential outside the cell

The potential inside the cell minus the potential outside the cell

The potential outside the cell minus the potential inside the cell

Correct answer:

The potential inside the cell minus the potential outside the cell

Explanation:

Membrane potential is calculated by subtracting the potential outside the cell from the potential inside the cell.

A neuron usually has a negative resting membrane potential because the inside of the cell is more negative than the outside of the cell. This difference in polarity results from the uneven movement of sodium and potassium ions by the sodium-potassium pump. The amount of sodium ions pumped out of the cell (three) is higher than the amount of potassium ions pumped into the cell (two). The net export of positive ions contributes to the negative resting membrane potential.

Example Question #25 : Cellular Structures And Organelles

Which of the following is false regarding membrane potential?

Possible Answers:

During depolarization, the membrane potential increases because the outside of the cell becomes more negative

At resting membrane potential, a sodium ion inside the cell will have a higher electrical potential energy than a sodium ion outside the cell

Excitation of neurons alters membrane potential

The sodium-potassium pump helps maintain the membrane potential

Correct answer:

At resting membrane potential, a sodium ion inside the cell will have a higher electrical potential energy than a sodium ion outside the cell

Explanation:

Remember that depolarization is the first step of an action potential, during which the membrane potential increases rapidly. This rapid increase is attributed to the movement of sodium ions into the cell. Since sodium ions are positively charged, the inside of the cell becomes more positive and the outside of the cell becomes more negative. This change in polarity causes an increase in membrane potential. When a neuron is excited past the threshold stimulus, an action potential ensues. An action potential causes huge alterations in membrane potential due to the movement of ions during depolarization, repolarization, and hyperpolarization. The sodium-potassium pump constantly moves sodium and potassium ions against their concentration gradients, which helps maintain the negative resting membrane potential. 

During resting membrane potential, a sodium ion inside the cell will not have a higher electrical potential energy than a sodium ion outside the cell. Recall that the electrical potential energy is higher when the electrical potential is higher. The resting membrane potential is negative; therefore, the inside of the cell has a lower potential than the outside of the cell. Since the potential inside the cell is lower, a sodium ion inside the cell will have a lower potential than a sodium ion outside the cell.

Example Question #26 : Cellular Structures And Organelles

Which of the following is required to generate a membrane potential?

I. A concentration gradient of ions

II. Presence of neurotransmitters

III. Semipermeable membrane

Possible Answers:

II only

I only

I and II

I and III

Correct answer:

I and III

Explanation:

Two main factors are required for a membrane potential. First, there must be a concentration or electrochemical gradient of ions. A potential difference occurs between two regions in space that have an uneven distribution of charges; therefore, for a membrane potential there must be a concentration gradient of ions (a difference between the ion concentrations) between the outside and the inside of the cell. Second, there must be a semipermeable membrane that separates the interior of the cell from the exterior. If a semipermeable membrane isn’t present, then the ions can undergo simple diffusion, which will equilibrate the concentration of ions. This would diminish the concentration gradient and the potential difference would become zero. 

Neurotransmitters are not required for establishing a membrane potential. They are chemicals that bind to receptors on membranes and initiate a cellular response.

Example Question #21 : Cellular Structures And Organelles

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 scientist realizes that the PrPC protein functions in normal cells to help regulate the cell membrane potential. Her research shows that cells with PrPC have a normal resting membrane potential at around –70 mV. Activation of PrPC causes depolarization, with a peak depolarization at around +60 mV. What ion, also present in action potentials, is PrPC most likely allowing to flow freely?

Possible Answers:

Ca2+

K+

Cl–

Na+

Mg2+

Correct answer:

Na+

Explanation:

Students should know the main players in establishing action potentials are K+ and Na+.  Further, Na+ inward flow through open channels brings an action potential to a peak depolarization of about +60 mV, which is sodium's equilibrium potential

Example Question #181 : Cell Biology, Molecular Biology, And Genetics

Which of the following is generally permeable to the cell membrane?

Possible Answers:

Albumin

Glucose

Potassium ions

Testosterone

Correct answer:

Testosterone

Explanation:

Albumin, glucose, and potassium ions are all examples of NON-permeable solutes. To be permeable, a solute must be small and nonpolar. Albumin, the main osmoregulatory protein, is bulky and too large to cross the membrane freely. Glucose is also large, as well as polar, and cannot cross the membrane. Potassium cannot freely cross due to the positive charge on the ion. All of these will require facilitated means of entering the cell.

Testosterone is a steroid hormone, and as such is small and nonpolar. All steroid hormones have intracellular receptors, and are able to enter a cell freely. Of the four choices, it is the only solute that can permeate the cell membrane.

Example Question #21 : Plasma Membrane And Transport

Plasma membrane channels are classified as which of the following?

Possible Answers:

Integral membrane proteins because they contain hydrophobic regions that can span the phospholipid bilayer

Peripheral membrane proteins because they contain hydrophobic regions that can span the phospholipid bilayer

Peripheral membrane proteins because they are not amphipathic

Integral membrane proteins because they are not amphipathic

Correct answer:

Integral membrane proteins because they contain hydrophobic regions that can span the phospholipid bilayer

Explanation:

Integral membrane proteins are proteins that span the entire membrane, whereas peripheral membrane proteins are proteins that associate only with only one side of the membrane (the "periphery").

Plasma membrane channels are proteins that facilitate the exchange of ions and other molecules between the extracellular and intracellular sides of a cell. To accomplish this task, a channel must span through the membrane (phospholipid bilayer); therefore, membrane channels are classified as integral membrane proteins. Recall that the inside of a phospholipid bilayer is extremely hydrophobic. Since like dissolves like, a membrane channel must contain hydrophobic regions that can interact with the interior of the phospholipid bilayer.

Amphipathic molecules contain both polar and nonpolar regions. Integral proteins (and most peripheral proteins) are amphipathic molecules. Phospholipids are also amphipathic molecules because they contain a polar head and a nonpolar tail.

Example Question #30 : Cellular Structures And Organelles

Which of the following is true regarding plasma membrane channels?

Possible Answers:

The hydrophobic regions on plasma membrane channels are made up of lipids

Facilitated diffusion of molecules occurs at a slower rate than simple diffusion

Water can traverse the phospholipid bilayer via simple or facilitated diffusion

Some plasma membrane channels use facilitated diffusion to move molecules against their electrochemical gradients

Correct answer:

Water can traverse the phospholipid bilayer via simple or facilitated diffusion

Explanation:

Water molecules are small and can travel through the membrane via simple diffusion. The rate of simple diffusion, however, is extremely slow due to the polarity of water molecules and their reluctance to enter the hydrophobic core of the membrane. Faster transportation of water molecules occurs via facilitated diffusion by specialized membrane channels called aquaporins; therefore, water can be transported via simple diffusion and facilitated diffusion.

Hydrophobic regions on membrane channels are essential to span the hydrophobic portions of the phospholipid bilayer. Although they are hydrophobic, lipids are not the main components of the hydrophobic regions. The hydrophobic regions in a membrane channel consist of hydrophobic amino acids that contain nonpolar side chains. 

Remember that facilitated diffusion and simple diffusion are both forms of passive transport; therefore, these processes do not require energy and transport molecules from a region of high concentration to low concentration. They don’t move molecules against their electrochemical gradient. 

Facilitated diffusion occurs at a much higher rate than simple diffusion. Facilitated diffusion uses membrane channels to transport molecules; therefore, it is much easier for molecules to traverse through a channel (facilitated diffusion) than the phospholipid bilayer (simple diffusion).

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