MCAT Biology : Plasma Membrane and Transport

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #182 : Cell Biology, Molecular Biology, And Genetics

Which of the following are true about a cell's phospholipid bilayer?

Possible Answers:

Consists of both polar heads and polar tails

Consists of outward facing non-polar tails and polar heads packed between

Consists of both non-polar heads and non-polar tails

Consists of outward facing polar heads and non-polar tails packed between

Correct answer:

Consists of outward facing polar heads and non-polar tails packed between

Explanation:

Each phospholipid consists of a polar phosphate head and two non-polar lipid tails. The phospholipid bilayer consists of polar heads facing the inside and outside of the cell, which interact with polar, aqueous environments. The non-polar hydrocarbon tails are packed on the inside of the bilayer, as far away from the polar environments as possible.

Example Question #32 : Cellular Structures And Organelles

Which is not a difference between prokaryotic and eukaryotic cells?

Possible Answers:

Prokaryotic and eukaryotic cells have different sizes of ribosomes

Eukaryotic cells contain membrane-bound organelles that prokaryotes do not

Eukaryotic cells have a plasma membrane, while prokaryotic cells do not

Prokaryotic cells lack a nucleus

Eukaryotes can be multicellular, while prokaryotes are always unicellular

Correct answer:

Eukaryotic cells have a plasma membrane, while prokaryotic cells do not

Explanation:

Both prokaryotic and eukaryotic cells have plasma membranes that separates the cell's contents from the external environment. Prokaryotic cells frequently have more external layers (cell wall and cell capsule), since they are generally more exposed and require additional protection from the environment, but these layers will always lie one top of the fundamental cell membrane.

Prokaryotes have 70S ribosomes, while eukaryotes have 80S ribosomes. Eukaryotic cells contain membrane-bound organelles, such as the nucleus, while prokaryotes lack such organelles. Prokaryotic organisms are always unicellular, while eukaryotic organisms can be either unicellular (protists) or multicellular.

Example Question #31 : Plasma Membrane And Transport

Which of the following structures plays the biggest role in cell containment?

Possible Answers:

Mitochondria

Phospholipid bilayer

Lysosomes

Peroxisomes

Correct answer:

Phospholipid bilayer

Explanation:

Cell containment, as the name suggests, is the process by which the contents of a cell (organelles, cytoskeleton, etc.) are contained within a region of space inside the cell. This means that there must be a barrier that prevents the contents from leaking out of the cell. This barrier is the phospholipid bilayer, or cell membrane. Phospholipid bilayers can be found in plasma membranes or in the membranes that cover the organelles. Recall that most organelles in an eukaryotic cell contain a phospholipid bilayer that separates the contents of the organelle from the cytosol.

Example Question #34 : Cellular Structures And Organelles

Which of the following is important for proper cell containment?

I. Semipermeability of the cell membrane

II. Acidity of lysosomes

III. Mitochondrial DNA

Possible Answers:

I only

I and II

II and III

II only

Correct answer:

I only

Explanation:

Proper cell containment depends heavily on the properties of the phospholipid bilayer. The cell membrane is semipermeable; this means that the cell membrane only permits certain molecules to pass through. If the cell membrane were permeable to everything, then the contents inside the cell can easily traverse the cell membrane and exit the cell. The semipermeability of the cell membrane is very important to maintain cell containment and regulate the homeostatic environment of the cell interior.

The acidity of lysosomes and presence of mitochondrial DNA are irrelevant to cell containment. The acidity of lysosomes is important for eliminating biological waste products inside the cell, and the mitochondrial DNA is important to produce unique mitochondrial proteins; however, they do not play a role in maintaining the contents of the cell or an organelle.

Example Question #31 : Plasma Membrane And Transport

Cryptosporidium is a genus of gastrointestinal parasite that infects the intestinal epithelium of mammals. Cryptosporidium is water-borne, and is an apicomplexan parasite. This phylum also includes Plasmodium, Babesia, and Toxoplasma. 

Apicomplexans are unique due to their apicoplast, an apical organelle that helps penetrate mammalian epithelium. In the case of cryptosporidium, there is an interaction between the surface proteins of mammalian epithelial tissue and those of the apical portion of the cryptosporidium infective stage, or oocyst. A scientist is conducting an experiment to test the hypothesis that the oocyst secretes a peptide compound that neutralizes intestinal defense cells. These defense cells are resident in the intestinal epithelium, and defend the tissue by phagocytizing the oocysts. 

She sets up the following experiment:

As the neutralizing compound was believed to be secreted by the oocyst, the scientist collected oocysts onto growth media. The oocysts were grown among intestinal epithelial cells, and then the media was collected. The media was then added to another plate where Toxoplasma gondii was growing with intestinal epithelial cells. A second plate of Toxoplasma gondii was grown with the same type of intestinal epithelium, but no oocyst-sourced media was added.

Upon studying cryptosporidium under a microscope, the scientist in the passages notices that each oocyst has a thick cell wall. When the scientist puts the oocysts into pure water, the oocysts remain intact while other cells swell. Against what kind of stress is the oocyst cell wall protecting?

Possible Answers:

Tensile

Dilutional

Hydropic

Halo-tonic

Osmotic

Correct answer:

Osmotic

Explanation:

As described in the question, the shell is protecting against osmotic stress. We would expect osmotic stress to be present when we put the oocysts in pure water, which is hypotonic relative to the oocyst itself. The tendency of non-protected cells to burst in such a condition is the result of osmotic stress caused by the influx of water diffusing across the membrane.

Example Question #36 : Cellular Structures And Organelles

An erythrocyte is placed into a solution with a large concentration of sodium. The erythrocyte is in a __________ solution and it will __________.

Possible Answers:

hypertonic . . . burst 

hypertonic . . . crenate 

hypotonic . . . burst 

hypotonic . . . crenate 

Correct answer:

hypertonic . . . crenate 

Explanation:

Because there is a smaller concentration of solute inside the cell than in the solution, it is said to be in a hypertonic solution. This will cause water from inside the cell to move outside the plasma membrane to account for this difference; therefore, the cell will crenate (shrink).

Example Question #37 : Cellular Structures And Organelles

An erythrocyte is placed in a solution of 0.9% sodium ions. Erythrocytes contain a concentration of approximately 0.9% sodium chloride. This is a __________ solution, relative to the cell, and the cell will __________.

Possible Answers:

isotonic . . . swell

hypotonic . . . lyse

hypertonic . . . crenate

isotonic . . . remain the same

Correct answer:

hypotonic . . . lyse

Explanation:

The trick to understanding this question is to remember that the sodium chloride actually consists of equal parts sodium ions and chlorine ions; therefore, the internal concentration of solute inside the red blood cell is actually twice the concentration of solute outside the cell. Since the cell contains a higher solute concentration compared to the extracellular environment, the solution is considered hypotonic. Water will flow toward the more concentrated region via osmosis, entering the cell and causing it to swell. Eventually, the swelling can cause the cell to lyse.

Example Question #38 : Cellular Structures And Organelles

What is the function of cholestrol in the cell plasma membrane?

Possible Answers:

Promote membrane fluidity

Promote cell-to-cell recognition

Provide ATP 

Transport molecules across the membrane

Allow movement of charged particles through the membrane

Correct answer:

Promote membrane fluidity

Explanation:

The major purpose of cholestrol in the plasma membrane is to maintain membrane fluidity. Carbohydrates and glycoproteins function in cell-to-cell recognition, and proteins function in the transport of particles through the membrane. Charged particles can't freely pass through the membrane unless it is through a carrier protein.

Example Question #39 : Cellular Structures And Organelles

Cells can acquire substances from the extracellular environment through a process called endocytosis, of which there are multiple subtypes.

Which of the following describes the process by which extracellular fluid is taken up by small invaginations of the cell membrane?

Possible Answers:

Phagocytosis

Receptor-mediated endocytosis

Pinocytosis

Clathrin-mediated endocytosis

Correct answer:

Pinocytosis

Explanation:

Pinocytosis, or "cell drinking", refers to the process by which extracellular fluids are engulfed by invaginations of the cellular membrane to create vesicles of the fluid. Phagocytosis is a similar mechanism that acquires extracellular solids into the cell. Receptor-mediated endocytosis occurs when hormones and nutrients bind to a ligand-specific receptor on the cellular membrane and clathrin-mediated endocytosis involves entrance to the cell via clathrin-coated pits.

Example Question #1241 : Biology

What is the name for cellular intake of fluid and small solutes?

Possible Answers:

Endocytosis

Phagocytosis

Receptor-mediated endocytosis

Pinocytosis

Correct answer:

Pinocytosis

Explanation:

The best answer would be pinocytosis, or "cell-drinking". Pinocytosis is a specific type of endocytosis; while pinocytosis refers to the uptake of fluids, endocytosis refers to any process involving cellular uptake via vesicles.

Phagocytosis is cellular uptake of large molecules or pathogens, and receptor-mediated endocytosis requires a particular ligand to initiate endocytosis.

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