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 #1 : Plasma Membrane And Transport

Each of the following membrane transport processes requires the use of specific proteins that allow for movement across the plasma membrane EXCEPT __________.

Possible Answers:

primary active transport

simple diffusion

facilitated transport via permeases

facilitated transport via channels

secondary active transport

Correct answer:

simple diffusion

Explanation:

Plasma membranes of the cell are permeable to molecules that pass through the phospholipid bilayer easily, namely small nonpolar molecules. Due to this specificity in permeability, membrane proteins are often required to transport molecules across the bilayer. Simple diffusion occurs when a substance passes through a membrane without the aid of an intermediary. All forms of facilitated transport, along with active transport, require the aid of specific membrane proteins. Thus, simple diffusion is the correct answer.

Example Question #2 : Plasma Membrane And Transport

The cell is the most basic functional unit of life.  Everything that we consider to be living is made up of cells, and while there are different kinds of cells, they all have some essential features that link them all together under the category of "life."  One of the most important parts of a cell is the membrane that surrounds it, seperating it from the rest of the environment.

While organisms from the three main domains live in incredibly different environments, they all possess similar cell membranes.  This phospholipid bilayer protects the cell, giving it a way to allow certain things in while keeping other things out.  Though organisms from different domains have different kinds of fatty linkages in their membranes, they all serve this essential purpose.

Membranes contain all kinds of essential proteins and signal molecules that allow the inside of the cell to respond to the outside of the cell.  In a multicellular eukaryote, this ability can be used to allow cells to communicate.  In a bacterial colony, an extracellular signal could be used to signal other bacteria.  Signals cascade through a series of molecular pathways that go from the outside of the cell all the way to the nucleus and back out again, giving the cell control on a genetic level.  This allows cellular responses to be quick and effective, and it also allows the cell to control how long it stays in that state.

One of the most important membrane proteins is the sodium-potassium pump. What would happen to a cell if this pump suddenly stopped working?

Possible Answers:

The environment would become hypotonic and the cell would lyse.

The environment would become hypotonic and the cell would shrivel.

The environment would become hypertonic and the cell would shrivel.

Nothing, the cell would be fine.

The environment would become hypertonic and the cell would lyse.

Correct answer:

The environment would become hypotonic and the cell would lyse.

Explanation:

The sodium-potassium pump serves to move three sodium ions out of the cell and two potassium ion into the cell.  These ions both have a plus one charge, so when the pump functions, it creates an environment where there are more solutes on the outside of the cell.  if it stopped working, the cell would stop moving sodium out, and since it is a polar molecule, it can't cross the cell membrane on its own.  There would be more solutes inside the cell than on the outside, and water would flow into the cell towards the higher solute concentration, causing the cell to swell and lyse.

Example Question #1 : Cellular Structures And Organelles

The sodium-potassium pump helps to maintain electrolyte gradients through use of ATP. Which of the following best describes this type of transport?

Possible Answers:

Diffusion

Facilitated diffusion

Osmosis

Filtration

Active transport

Correct answer:

Active transport

Explanation:

Active transport most correctly describes this type of movement, as it uses ATP as an energy source. In contrast, the other four choices are all different types of passive transport, constituting types of movement where no energy source is needed. Diffusion is simply the net movement of particles down their concentration gradient. Facilitated diffusion is a similar concept, but uses specialized transport proteins. Osmosis describes the movement of water, and lastly, filtration is the movement of both solute and water molecules.

Example Question #2 : Cellular Structures And Organelles

Assume that there are thirty sodium ions outside the cell and twenty potassium ions inside the cell. What will happen after one cycle of the sodium-potassium pump?

Possible Answers:

There will be 32 sodium ions outside the cell and 23 potassium ions inside the cell

There will be 27 sodium ions outside the cell and 18 potassium ions inside the cell

There will be 28 sodium ions outside the cell and 17 potassium ions inside the cell

There will be 33 sodium ions outside the cell and 22 potassium ions inside the cell

Correct answer:

There will be 33 sodium ions outside the cell and 22 potassium ions inside the cell

Explanation:

To answer this question you need to know the directionality of the sodium-potassium pump and the number of ions pumped each cycle. Remember that each cycle of the sodium-potassium pump moves three sodium ions to the outside of the cell and two potassium ions to the inside of the cell. The amount of sodium ions outside the cell will increase by three and the amount of potassium ions inside the cell will increase by two.

The final result after one cycle of the sodium-potassium pump will be 33 sodium ions outside the cell and 22 potassium ions inside the cell.

Example Question #3 : Cellular Structures And Organelles

Why does the sodium-potassium pump require ATP to function properly?

Possible Answers:

The concentration of sodium and potassium ions is greater inside the cell

The concentration of sodium and potassium ions is greater outside the cell

The concentration of sodium ions is greater inside the cell and the concentration of potassium ions is greater outside the cell

The concentration of sodium ions is greater outside the cell and the concentration of potassium ions is greater inside the cell

Correct answer:

The concentration of sodium ions is greater outside the cell and the concentration of potassium ions is greater inside the cell

Explanation:

When a membrane channel, such as the sodium-potassium pump, requires energy (ATP) to transport molecules it means that the channel is moving molecules against their concentration gradient. This mode of transport is called active transport.

Recall that the sodium-potassium pump moves three sodium ions out of the cell and two potassium ions into the cell per cycle. Since it uses active transport, the sodium-potassium pump must move both sodium and potassium ions against their respective concentration gradients. This means that the concentration of sodium ions is greater outside the cell and the concentration of potassium ions is greater inside the cell.

Note that symporters exist in which facilitated diffusion of one ion is used to pull a second ion against its concentration gradient without the use of ATP. In this manner, ATP is not always necessary to transport an ion against its concentration gradient. When both ions are moving against their gradients, however, or when only one ion is being transported, ATP will be needed.

Example Question #6 : Plasma Membrane And Transport

Which of the following is true about the sodium-potassium pump?

Possible Answers:

It is an antiporter because it transports sodium ions to the outside of the cell and potassium ions to the inside of the cell

It is an antiporter because it transports sodium ions to the inside of the cell and potassium ions to the outside of the cell

It is a symporter because it transports sodium ions to the outside of the cell and potassium ions to the inside of the cell

It is a symporter because it transports sodium ions to the inside of the cell and potassium ions to the outside of the cell

Correct answer:

It is an antiporter because it transports sodium ions to the outside of the cell and potassium ions to the inside of the cell

Explanation:

Antiporters are proteins that carry molecules in opposite directions, whereas symporters are proteins that carry molecules in the same direction. The sodium-potassium pump transports sodium ions out of the cell and potassium ions into the cell. The movement of ions occurs in opposite directions; therefore, the sodium-potassium pump is classified as an antiporter.

Example Question #4 : Cellular Structures And Organelles

Which of the following is true of an electrochemical gradient?

Possible Answers:

Electrochemical gradients are only created by neutral molecules

Proton motive force in the mitochondria is generated from a type of electrochemical gradient

Electrochemical gradients drive the movement of molecules in active transport

To maintain the gradient, it is essential to have equal concentrations of a molecule on the intracellular and extracellular sides of a membrane

Correct answer:

Proton motive force in the mitochondria is generated from a type of electrochemical gradient

Explanation:

An electrochemical gradient is a gradient that is created by concentration differences of ions between the inside and the outside of the cell. If the concentration of molecules is equal on both sides of the cell, then the electrochemical gradient is depleted.

An electrochemical gradient will also involve electric potential, since the concentration discrepancy involves ion gradients. Recall that electric potential is dependent on the charges of molecules; therefore, an electrochemical gradient is only created when there is an unequal amount of ions present on both sides of the cell, not neutral molecules.

An electrochemical gradient acts as a driving force to move molecules from a region of high concentration (high potential) to a region of low concentration (low potential). This movement is observed in simple diffusion and facilitated diffusion. In active transport, however, molecules move from an area of low concentration to an area high concentration. This means that active transport is not driven by the electrochemical gradient and that molecules move against the electrochemical gradient in active transport.

Proton motive force is the main driving force that pumps protons from the intermembrane space to the matrix of a mitochondrion. This pumping is coupled with ATPase (the enzyme that synthesizes ATP); therefore, the proton motive force drives the synthesis of ATP in mitochondria. The proton motive force arises from an electrochemical gradient of hydrogen ions (protons). During the electron transport chain, protons are pumped into the intermembrane space. This leads to an increase in the concentration of protons and, subsequently, the electric potential in the intermembrane space. The electrochemical gradient created from this phenomenon drives the protons from a region of high electric potential (intermembrane space) to a region of low electric potential (matrix); therefore, proton motive force comes from an electrochemical gradient.

Example Question #5 : Cellular Structures And Organelles

One component of the immune system is the neutrophil, a professional phagocyte that consumes invading cells. The neutrophil is ferried to the site of infection via the blood as pre-neutrophils, or monocytes, ready to differentiate as needed to defend their host.

In order to leave the blood and migrate to the tissues, where infection is active, the monocyte undergoes a process called diapedesis. Diapedesis is a process of extravasation, where the monocyte leaves the circulation by moving in between endothelial cells, enters the tissue, and matures into a neutrophil.

Diapedesis is mediated by a class of proteins called selectins, present on the monocyte membrane and the endothelium. These selectins interact, attract the monocyte to the endothelium, and allow the monocytes to roll along the endothelium until they are able to complete diapedesis by leaving the vasculature and entering the tissues.

The image below shows monocytes moving in the blood vessel, "rolling" along the vessel wall, and eventually leaving the vessel to migrate to the site of infection.

 

Untitled

The movement of monocytes between endothelial cells can best be characterized as __________.

Possible Answers:

passive transport

facilitated diffusion

paracellular transport

transcellular transport

pinocytic transport

Correct answer:

paracellular transport

Explanation:

Paracellular transport moves material between cells, while transcellular transport moves things through cells; thus, this is an example of paracellular transport.

Facilitated diffusion, pinocytosis, and passive transport all involve the entrance of a substance into a cell. Monocytes are transferring location, but are not entering another cell in the process.

Example Question #2 : 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.

Since PrPC is a transmembrane protein, what are we most likely to find in the part of the protein that spans the membrane?

Possible Answers:

Amphipathic amino acid residues

Hydrophobic amino acid residues

Neither hydrophilic nor hydrophobic amino acid residues

Hydrophilic amino acid residues

Both hydrophilic and hydrophobic amino acid residues

Correct answer:

Hydrophobic amino acid residues

Explanation:

The core of the lipid bilayer of all eukaryotic cells contains lipid; therefore, transmembrane proteins have a hydrophobic-rich series of residues in the area that spans the membrane.

Example Question #10 : Plasma Membrane And Transport

Which of the following best describes the composition of the plasma membrane of an animal cell?

Possible Answers:

Phosphoprotein and cholesterol

Phospholipids, sphingolipids, cholesterol, and protein, with some carbohydrate

Cholesteryl esters, proteins, and a small but significant amount of triglycerides

A lipid bilayer formed mainly from cholesterol with protein attached to both sides

Lipoproteins in which triglycerides are a major component

Correct answer:

Phospholipids, sphingolipids, cholesterol, and protein, with some carbohydrate

Explanation:

The major components of the plasma membrane of an animal cell are lipids and proteins, with a small amount of carbohydrate components. The major lipid components are glycerophospholipids, sphingolipids, and some cholesterol. The amount of cholesterol varies depending upon certain factors, such as temperature, and helps maintain the fluidity of the membrane. Thus, the correct answer is phospholipids, sphingolipids, cholesterol, and protein, with some carbohydrate.

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