Question 1

If all other conditions remain the same and the concentration of a nonpenetrating solute increases inside a cell, which is most likely to occur?
A) Water will tend to enter the cell because the interior has an increased osmolarity.
B) Water will tend to leave the cell because the interior has an increased osmolarity.
C) Water will tend to enter the cell because the interior has a decreased osmolarity.
D) Water will tend to leave the cell because the interior has a decreased osmolarity.
E) The solute will diffuse across the membrane until its concentration is equal on both sides of the membrane.

Accepted Answer

Water will tend to enter the cell because the interior has an increased osmolarity, leading to a higher concentration of solute inside the cell compared to outside. This difference in osmolarity causes water to move into the cell to dilute the internal solute concentration, aiming to achieve osmotic balance.

Question 2

The permeability of the plasma membrane to mineral ions:
A) is not influenced by channels formed by proteins.
B) is the same in all cell types.
C) is affected by differences in electrical charge on the two sides of the membrane.
D) is zero in all living cells.
E) is only possible because of the hydrophobic interior of the lipid bilayer.

Accepted Answer

The permeability of the plasma membrane to mineral ions is affected by differences in electrical charge on the two sides of the membrane, particularly due to the presence of ion channels that allow specific ions to pass through. This permeability is not the same in all cell types, and it is not zero in all living cells, as some cells rely on ion transport for various physiological processes. The hydrophobic interior of the lipid bilayer also contributes to the selectivity and regulation of ion transport.

Question 3

"Osmosis" refers to the movement of what substance across semipermeable membranes?
A) glucose
B) charged particles
C) lipid molecules
D) water
E) solutes

Accepted Answer

Osmosis refers specifically to the movement of water across semipermeable membranes.

Question 4

Ion channels in cell membranes:
A) are nonspecific.
B) are not affected by differences in electrical potential across the membrane.
C) may open in response to binding a ligand.
D) only allow ions to move from the extracellular fluid into the cell.
E) only allow ions to move from the intracellular fluid out of the cell.

Accepted Answer

Ion channels in cell membranes may open in response to binding a ligand, such as a neurotransmitter or hormone, which causes a conformational change in the channel protein and allows ions to pass through. The other options are incorrect; ion channels are usually specific for certain ion types, are affected by electrical potential differences, and can allow ions to move in either direction across the membrane.

Question 5

Which of the following is a feature that distinguishes active transport from facilitated diffusion?
A) saturation of transport rate
B) requirement for a carrier molecule
C) carrier molecules have specificity
D) presence of a transport maximum
E) requirement for metabolic energy

Accepted Answer

Active transport requires metabolic energy to move molecules against their concentration gradient, while facilitated diffusion relies on the concentration gradient to move molecules with the help of carrier molecules. The other features listed (saturation of transport rate, requirement for a carrier molecule, specificity of carrier molecules, and presence of a transport maximum) are shared by both active transport and facilitated diffusion.

Question 6

Which of the following statements about the Na+-K+ ATPase pump is false? A) It transports Na⁺ out of cells and K⁺ into cells. B) It binds to, and hydrolyzes, ATP. C) It is constantly active in all cells. D) Its activity requires the expenditure of metabolic energy. E) It transports Na⁺ and K⁺ in a 1:1 ratio.

Accepted Answer

The Na+-K+ ATPase pump transports Na+ and K+ in a 3:2 ratio, moving three Na+ ions out of the cell and two K+ ions into the cell per ATP molecule hydrolyzed.

Question 7

Which of the following statements regarding the diffusion of a nonpolar solute X across a cell membrane is correct?
A) At equilibrium the net flux of X across the cell membrane is zero.
B) At equilibrium, X will cease moving across the cell membrane.
C) Solute X will be unable to reach equilibrium because it is nonpolar.
D) Solute X will reach equilibrium, but the concentration will be much higher on the inside of the cell because of the smaller volume.
E) At equilibrium, movement of X across the membrane will be much faster in one direction than in the other.

Accepted Answer

Nonpolar solutes, like all solutes, will diffuse from an area of high concentration to an area of lower concentration until equilibrium is reached. At equilibrium, there will be no net movement of the solute across the membrane, but the solute will continue to move back and forth across the membrane at equal rates.

Question 8

If pure water and a solution containing a nonpenetrating solute are separated by a membrane that is permeable only to water, what would occur?
A) Water will diffuse by osmosis until the water concentrations in the two compartments become equal.
B) Water will diffuse by osmosis until all of the water is on the same side as the solute.
C) Water will diffuse by osmosis toward the side with the solute, until stopped by opposing hydrostatic pressure.
D) No movement will occur between the compartments.
E) Water will diffuse by osmosis away from the side with the solute, until stopped by hydrostatic pressure.

Accepted Answer

The answer of If pure water and a solution containing...

Question 9

Which is true regarding the mediated transport of a substance across a plasma membrane?
A) It depends upon the binding of that substance to a specific site on the membrane protein.
B) It depends upon movement of proteins from one side of the membrane to the other.
C) It always increases in direct proportion to the increasing concentration of the substance on one side of the membrane.
D) Both "depends upon the binding of that substance to a specific site on the membrane protein" and "depends upon movement of proteins from one side of the membrane to the other" are correct.
E) All of the choices are correct.

Accepted Answer

Mediated transport depends upon the binding of the substance to a specific site on the membrane protein, as this allows for selective transport and regulation of substances crossing the membrane. The other options are not necessarily true for all types of mediated transport.

Question 10

Which is TRUE regarding diffusion?
A) It depends upon the random motion of molecules.
B) It results in net movement of molecules from regions of low concentration to regions of high concentration.
C) It is the principal mechanism by which molecules are moved over large distances in the body.
D) It requires energy in the form of ATP.
E) It requires integral membrane proteins to occur.

Accepted Answer

Diffusion is the movement of molecules from an area of high concentration to an area of low concentration as a result of the random motion of molecules.

Question 11

Which is true about mediated transport across cell membranes?
A) It refers to the movement of ions through protein channels.
B) It refers both to simple diffusion and to the active transport of molecules.
C) It is characterized by saturable carriers and a maximum velocity of transport.
D) As the concentration gradient across a membrane increases, the transport rate always increases.
E) It is nonspecific; any transporter can transport any molecule across the cell membrane.

Accepted Answer

Mediated transport across cell membranes involves specific carriers that can become saturated, resulting in a maximum velocity of transport. This is known as facilitated diffusion or active transport, and it differs from simple diffusion, which does not involve carriers and is not saturable. The transport rate may not always increase as the concentration gradient increases due to saturation of the carriers. Additionally, specific transporters are needed for specific molecules, making this process generally specific rather than nonspecific.

Question 12

Which is a form of cellular transport that involves the movement of molecules into cells without passing through the plasma membrane's structural matrix?
A) diffusion
B) osmosis
C) facilitated diffusion
D) endocytosis
E) apoptosis

Accepted Answer

Endocytosis is the cellular process of internalization of substances by the formation of vesicles derived from the plasma membrane. It involves the engulfment of molecules or particles and their transportation into the cell without passing through the plasma membrane's structural matrix. This makes it the only option among the given choices that matches the given description.

Question 13

Which is true about mediated transport of substances across cell membranes?
A) It involves a specific membrane protein that functions as a carrier molecule.
B) It always involves the movement of substances against a concentration gradient.
C) It is always directly coupled to the splitting of ATP molecules.
D) There is no limit to how fast it can occur as the concentration gradient becomes larger.
E) It is the main mechanism for transporting hydrophobic molecules across membranes.

Accepted Answer

Mediated transport involves the use of specific membrane proteins, such as carrier molecules, to transport substances across the membrane. This process is different from simple diffusion, which does not require the use of membrane proteins.

Question 14

If the ATP-generating mechanisms in a cell are poisoned and the cell depletes its ATP reserves, which would occur first?
A) Primary active transport of molecules would cease.
B) Secondary active transport of molecules would cease.
C) Facilitated diffusion of molecules would cease.
D) Ion concentration gradients would reach equilibrium across the cell membrane.
E) All transport processes would cease immediately when the ATP was depleted.

Accepted Answer

Primary active transport requires the direct input of ATP to move molecules against their concentration gradients. Therefore, if ATP levels are depleted, primary active transport will cease first. Secondary active transport and facilitated diffusion rely on concentration gradients established by primary active transport, so they will also eventually cease if primary active transport stops. However, ion concentration gradients may remain for some time after ATP is depleted, so answer D is not the best choice. Answer E is incorrect because transport processes would not immediately cease but would gradually slow down as ATP levels decreased.

Question 15

Which of the following would decrease the net flux of a penetrating solute into a cell?
A) increasing the permeability constant for that solute
B) decreasing the temperature
C) increasing the concentration of the solute in the extracellular fluid
D) increasing the area of the cell membrane
E) decreasing the thickness of the membrane

Accepted Answer

Decreasing the temperature would decrease the kinetic energy of the solute molecules, thus reducing their movement and the net flux into the cell.

Question 16

What is the best description of a secondary active transport process that cotransports an amino acid with Na+?
A) Both Na+ and the amino acid bind on the side of the membrane where the Na+ concentration is lowest, and undergo net movement to the other side of the membrane.
B) Na+ and the amino acid bind to opposite sides of the membrane, and Na+ undergoes net transport from the side of the membrane with higher Na+ concentration toward the side with lower concentration, while the amino acid moves in the opposite direction.
C) Both Na+ and the amino acid bind on the side of the membrane where the Na+ concentration is higher, and both are transported to the side of the membrane where the Na+ concentration is lower.
D) Both Na+ and the amino acid bind to the side of the membrane where the amino acid concentration is highest and are transported toward the side of the membrane where the amino acid concentration is lower.
E) Na+ and the amino acid bind to opposite sides of the membrane, and the amino acid undergoes net transport from the side of the membrane with lower Na+ concentration to the side of the membrane where the Na+ concentration is higher.

Accepted Answer

In secondary active transport, Na+ typically moves down its concentration gradient, providing the energy to transport the amino acid against its gradient. Both bind on the side with higher Na+ concentration and move to the side with lower Na+ concentration.

Question 17

In general, polar molecules diffuse more rapidly through the lipid bilayer part of cell membranes than do nonpolar molecules. This statement is:
A) true.
B) false, because polar molecules diffuse through less rapidly because the bilayer is polar throughout its width.
C) false, because polar molecules diffuse through less rapidly because the bilayer is nonpolar throughout its width.
D) false, because nonpolar molecules diffuse through more rapidly because much of the bilayer is nonpolar.
E) false, because the rate of diffusion of nonpolar and polar molecules through the bilayer is essentially the same as long as the molecules are the same size.

Accepted Answer

The lipid bilayer part of cell membranes is primarily nonpolar, which allows nonpolar molecules to diffuse through more rapidly than polar molecules. Polar molecules have difficulty crossing the nonpolar interior of the bilayer.

Question 18

A cell is placed into a 1 millimolar solution of substance X and over time you witness the concentration of X inside the cell increase to 5 millimolar. What is the best explanation for this observation?
A) X is moving into the cell by simple diffusion.
B) X is moving into the cell by diffusion through a protein channel.
C) X is moving into the cell by facilitated diffusion.
D) X is moving into the cell by primary active transport.
E) Water is leaving the cell by osmosis.

Accepted Answer

The concentration of substance X inside the cell has increased from 1 millimolar to 5 millimolar, which cannot be explained by simple diffusion or facilitated diffusion. This suggests an active transport mechanism, specifically primary active transport, which requires ATP to move molecules against their concentration gradient. Therefore, option D is the best explanation for this observation.

Question 19

Which of the following statements regarding the structure and function of cell membranes is correct?
A) The phospholipid bilayer is arranged so that the hydrophilic heads of the phospholipid molecule face the extra- and intracellular fluids.
B) Some proteins integrated into the membrane serve as channels for the passage of nonpolar molecules through the membrane.
C) Peripheral membrane proteins function as channels associated with transport of ions through the membrane.
D) Cholesterol molecules function to transport ions from one side of the plasma membrane to the other.
E) The plasma membrane is a layer of phospholipid molecules with their hyrophilic head groups in contact with the extracellular solution and the hydrophobic tail groups in contact with the intracellular solution.

Accepted Answer

The phospholipid bilayer is arranged so that the hydrophilic heads of the phospholipid molecule face the extra- and intracellular fluids, making it a selectively permeable barrier that allows some molecules to pass through while preventing others from doing so. B and C are incorrect because proteins that serve as channels for passage of nonpolar molecules are actually integral membrane proteins, not peripheral. Cholesterol molecules found in the plasma membrane play a role in regulating the fluidity of the membrane and are not involved in transport of ions. In contrast to choice E, the hydrophobic tail groups are in contact with other hydrophobic molecules within the membrane, not the intracellular fluid.

Question 20

By what transport mechanism does glucose enter most cells?
A) diffusion through the lipid bilayer
B) primary active transport
C) secondary active transport
D) diffusion through a protein channel
E) facilitated diffusion

Accepted Answer

Glucose enters most cells through facilitated diffusion, in which it moves down its concentration gradient through a protein channel. This type of transport does not require energy, and the protein channel helps to facilitate the movement of glucose through the hydrophobic interior of the lipid bilayer.

Quiz 4: Movement of Molecules Across Cell Membranes

If all other conditions remain the same and the concentration of a nonpenetrating solute increases inside a cell, which is most likely to occur?

The permeability of the plasma membrane to mineral ions:

"Osmosis" refers to the movement of what substance across semipermeable membranes?

Ion channels in cell membranes:

Which of the following is a feature that distinguishes active transport from facilitated diffusion?

Which of the following statements about the Na+-K+ ATPase pump is false?

Which of the following statements regarding the diffusion of a nonpolar solute X across a cell membrane is correct?

If pure water and a solution containing a nonpenetrating solute are separated by a membrane that is permeable only to water, what would occur?

Which is true regarding the mediated transport of a substance across a plasma membrane?

Which is TRUE regarding diffusion?

Which is true about mediated transport across cell membranes?

Which is a form of cellular transport that involves the movement of molecules into cells without passing through the plasma membrane's structural matrix?

Which is true about mediated transport of substances across cell membranes?

If the ATP-generating mechanisms in a cell are poisoned and the cell depletes its ATP reserves, which would occur first?

Which of the following would decrease the net flux of a penetrating solute into a cell?

What is the best description of a secondary active transport process that cotransports an amino acid with Na+?

In general, polar molecules diffuse more rapidly through the lipid bilayer part of cell membranes than do nonpolar molecules. This statement is:

A cell is placed into a 1 millimolar solution of substance X and over time you witness the concentration of X inside the cell increase to 5 millimolar. What is the best explanation for this observation?

Which of the following statements regarding the structure and function of cell membranes is correct?

By what transport mechanism does glucose enter most cells?