GRE Subject Test: Biochemistry, Cell, and Molecular Biology : Help with Membrane Transport

Study concepts, example questions & explanations for GRE Subject Test: Biochemistry, Cell, and Molecular Biology

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Example Questions

Example Question #1 : Help With Membrane Transport

Which of the following can freely diffuse through the cell membrane?

I. Oxygen

II. Potassium ions

III. Glucose

IV. Hydrogen ions

Possible Answers:

I, II, and IV

I and III

I only

I, II, III, and IV

Correct answer:

I only

Explanation:

Only small, nonpolar molecules can freely diffuse through the plasma membrane. From this group, only oxygen fits this description. Hydrogen and potassium ions are charged and, therefore, carry a polar element that cannot diffuse through the membrane. Glucose is too large, and also contains a polar aldehyde group. Oxygen atoms generally contribute to molecular polarity due to their abundance of valence electrons. When arranged symmetrically, however, such as in the oxygen molecule, the oxygen atoms result in zero net polarity.

Transmembrane proteins and channels are used for facilitated diffusion of glucose and small ions.

Example Question #2 : Help With Membrane Transport

Which of the following is false with regard to the LDL-receptor?

Possible Answers:

It releases its cargo in the low pH environment of the endosome

It is shuttled back to the cell surface after it delivers its cargo

Once bound to cargo, it enters the cell via COPII-mediated endocytosis

Its cargo is low density lipoprotein

Correct answer:

Once bound to cargo, it enters the cell via COPII-mediated endocytosis

Explanation:

The LDL-receptor enters the cell via clathrin-mediated endocytosis, not COPII. COPII is involved in budding off of the endoplasmic reticulum.

All of the other given answer options are true.

Example Question #3 : Help With Membrane Transport

Which of the following is false concerning facilitated diffusion?

Possible Answers:

It moves substances down their electrochemical gradients

It is typically used to move charged or polar substances

It requires energy in order to move substances

It requires the use of proteins in the membrane

Correct answer:

It requires energy in order to move substances

Explanation:

Facilitated diffusion is used to move polar or charged substances down their electrochemical gradients. Although this does require the use of integral membrane proteins, no energy is used during this process, as it is still a form of diffusion.

Example Question #4 : Help With Membrane Transport

When will water have a net flow into a cell?

Possible Answers:

When the cell is hypotonic to its surrounding environment

When the cell is in a hypotonic solution

When there is more solute outside of the cell compared to inside the cell

When the cell is in an isotonic solution

Correct answer:

When the cell is in a hypotonic solution

Explanation:

The direction for the diffusion of water, or osmosis, can be predicted by comparing the tonicities of the cell and its surroundings. When the cell has more solute in it than the external environment, water will flow into the cell in order to help even out the concentrations.

As a result, water will flow into the cell when it is found in a hypotonic solution.

Example Question #5 : Help With Membrane Transport

A person is found to have a defect in his ATP synthase gene. Which of the following will be halted in his cells?

Possible Answers:

More than one of these are true

Sodium-potassium pump because it involves active transport

Sodium-potassium pump because it involves passive transport

Potassium channels because it involves active transport

Correct answer:

Sodium-potassium pump because it involves active transport

Explanation:

ATP synthase is a molecule used to generate energy for the cell (in the form of ATP). Lack of ATP synthase will halt many active processes that require energy. Recall that sodium-potassium pump is found on multitude of cells in the body and functions to maintain the intracellular concentration of ions. This pump transports sodium and potassium ions against its concentration gradient (sodium out of and potassium into the cell). Since it is transporting against a concentration gradient, the sodium-potassium pump carries out active transport and, therefore, requires energy. Decreased ATP synthase activity will lead to decreased ATP levels and, subsequently, affect the sodium-potassium pump.

Potassium channels are selective molecules (selective for potassium) that transport potassium ions along its concentration gradient (from inside of cell to outside); therefore, this is a type of passive transport and does not require energy.

Example Question #6 : Help With Membrane Transport

A researcher observes that a molecule is brought inside the cell via membrane transporters. What can the researcher conclude about this type of transport?

Possible Answers:

The molecule is found in higher concentration outside the cell

Cell does not expend energy to transport this molecule

Cell expends energy to transport this molecule

None of these can be concluded

Correct answer:

None of these can be concluded

Explanation:

Membrane transporters are molecules found on plasma membranes that act as gateway for several molecules that enter and exit cells. These transporters are usually specific for one or two molecules and transport them across the cell membranes. The membrane transporters can transport ions/molecules along or against the concentration gradients; therefore, membrane transporters perform both passive and active transport. Transporters that move molecules along concentration gradient carry out a special type of passive transport called facilitated diffusion whereas transporters that move against concentration gradient carry out active transport (requires energy).

The question doesn’t state whether the molecule is brought along or against its concentration gradient; therefore, the membrane transporter could be a facilitated transporter or an active transporter.

Example Question #7 : Help With Membrane Transport

In a normal individual, which of the following molecule(s) always enter(s) the cell via passive transport?

I. Calcium

II. Potassium

III. Sodium

Possible Answers:

I only

I and II

II only

I and III

Correct answer:

I and III

Explanation:

There are two types of membrane transport: passive and active transport. Passive transport involves movement of molecules along its concentration gradient. This is a spontaneous process and does not require energy. Examples of passive transport include simple diffusion of lipophilic molecules and facilitated transport of molecules involving a membrane transporter (such as an ion channel). Active transport, on the other hand, moves molecules against their concentration gradient. This is not a spontaneous process and, therefore, requires energy.

To answer this question we need to recall the relative concentrations of the three ions presented in the question. Potassium ions are found in higher concentrations inside the cell whereas calcium and sodium are found in higher concentrations outside the cell. This means that moving calcium and sodium ions inside the cell will involve moving along their respective concentration gradients (from high to low concentration) whereas moving potassium ions inside the cell will involve moving against the concentration gradient; therefore, sodium and calcium come into the cell via passive transport whereas potassium enters via active transport.

Example Question #8 : Help With Membrane Transport

Which of the following transport mechanisms requires ATP hydrolysis to move a molecule against its concentration gradient?

Possible Answers:

Cotransport

Secondary active transport

Facilitated diffusion

Passive diffusion

Active transport

Correct answer:

Active transport

Explanation:

Secondary active transport and cotransport are the same thing, and while solutes can be moved against their concentration gradient, ATP is not used. Passive and facilitated diffusion do not require ATP either. Active transport is the only transport mechanism that uses ATP to move molecules against their concentration gradient. 

All GRE Subject Test: Biochemistry, Cell, and Molecular Biology Resources

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