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ACT Membranes study materials
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ACT Membranes practice questions

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Question 1Membrane Structure and Composition

Which property is most characteristic of the phospholipid bilayer that forms the core of a cell membrane?

  1. It is a fluid structure with hydrophobic tails facing inward Correct answer.
  2. It is a rigid, fully hydrophobic solid layer
  3. It is a hydrophilic sheet with charged tails facing inward
  4. It is a static lattice of covalently cross-linked lipids

Explanation

Phospholipids have hydrophilic heads and hydrophobic tails; in water the tails cluster away from water, forming a bilayer interior. The noncovalent packing lets lipids move laterally, so the bilayer behaves as a fluid.

Question 2Membrane Structure and Composition

A transmembrane protein is most likely to have which feature in the region that spans the lipid core?

  1. A region of tightly bound water molecules
  2. A stretch of hydrophilic amino acids
  3. A segment enriched in hydrophobic amino acids Correct answer.
  4. A covalently attached lipid tail

Explanation

The bilayer interior is hydrophobic, so the portion of a protein embedded there is typically rich in nonpolar residues. Hydrophilic residues are usually positioned where the protein contacts the aqueous surfaces or a pore.

Question 3Membrane Structure and Composition

Which molecule would diffuse most readily through the lipid bilayer without a transport protein?

  1. A charged amino acid
  2. A large polar molecule such as glucose
  3. A small nonpolar gas such as oxygen Correct answer.
  4. A sodium ion

Explanation

Small nonpolar molecules dissolve in and cross the hydrophobic core without help. Ions and large polar molecules cannot pass the hydrophobic barrier readily and need proteins.

Question 4Membrane Structure and Composition

The fluid mosaic model describes the membrane primarily as which of the following?

  1. A double layer of proteins with lipid pores
  2. A rigid bilayer with proteins fixed in place
  3. A lipid bilayer with embedded proteins that can move laterally Correct answer.
  4. A protein monolayer coated with lipids

Explanation

The model pictures a fluid lipid bilayer in which proteins are embedded or attached and can move within the plane. This accounts for membrane fluidity and protein mobility.

Question 5Membrane Structure and Composition

Which component of the plasma membrane is primarily responsible for its selective permeability?

  1. Cholesterol esters
  2. Extracellular matrix fibers
  3. The hydrophobic lipid core and transport proteins Correct answer.
  4. Peripheral carbohydrates

Explanation

The hydrophobic core blocks most polar and charged solutes, while specific membrane proteins allow selected substances through. Together they produce selective permeability.

Question 6Membrane Structure and Composition

What is the effect of cholesterol on a membrane at moderate temperatures?

  1. It converts the bilayer into a monolayer
  2. It buffers fluidity, reducing it at high temperature and limiting packing at low temperature Correct answer.
  3. It increases fluidity without limit as temperature rises
  4. It removes all fluidity and makes the bilayer rigid

Explanation

Cholesterol inserts between phospholipids, restricting their motion at high temperature while preventing tight packing at low temperature. This broadens the range over which the membrane stays appropriately fluid.

Question 7Membrane Structure and Composition

Which structural feature distinguishes a phospholipid from a simple triglyceride?

  1. A steroid ring system
  2. A phosphate-containing head group Correct answer.
  3. Three fatty acid tails
  4. A single long hydrocarbon chain

Explanation

A phospholipid has a glycerol backbone with two fatty acid tails and a phosphate-containing head group. Triglycerides have three fatty acids and no phosphate group.

Question 8Membrane Structure and Composition

Which type of membrane protein is attached to the membrane surface without crossing the hydrophobic core?

  1. A peripheral membrane protein Correct answer.
  2. An integral pore-forming protein
  3. A multipass channel protein
  4. A single-pass transmembrane protein

Explanation

Peripheral proteins associate with the membrane surface, often through ionic interactions with head groups or with integral proteins. They do not span the hydrophobic core.

Question 9Membrane Structure and Composition

A researcher cools a membrane below its transition temperature. What change in the lipid bilayer is expected?

  1. Proteins are exported from the cell
  2. Lipids gain lateral mobility and the bilayer becomes more fluid
  3. The bilayer separates into two aqueous layers
  4. Lipids become more ordered and the bilayer becomes less fluid Correct answer.

Explanation

Lowering temperature reduces thermal motion, so lipid tails pack more tightly and the bilayer becomes less fluid. Below the transition temperature, ordered gel-like packing predominates.

Question 10Membrane Structure and Composition

Two phospholipids differ only in that one has saturated tails and the other has unsaturated tails. Which bilayer is expected to be more fluid at a given temperature?

  1. The bilayer with saturated tails
  2. The bilayer with unsaturated tails Correct answer.
  3. Both bilayers are equally fluid
  4. Neither bilayer is fluid at that temperature

Explanation

Double bonds in unsaturated tails create kinks that prevent tight packing, increasing fluidity. Saturated tails pack more closely and produce a less fluid bilayer.

Question 11Membrane Structure and Composition

A cell is placed in a solution that strips peripheral proteins from the membrane surface. Which interaction is most likely being disrupted?

  1. Hydrophobic anchoring within the bilayer core
  2. Ionic and hydrogen bonds with lipid head groups Correct answer.
  3. Covalent bonds within the lipid tails
  4. The glycerol backbone of phospholipids

Explanation

Peripheral proteins bind the membrane surface mainly through electrostatic and hydrogen-bond interactions with head groups or integral proteins. Treatments that disrupt these interactions release them without dissolving the bilayer.

Question 12Membrane Structure and Composition

Why does a small uncharged polar molecule such as water cross the bilayer slowly on its own but much faster through aquaporins?

  1. Aquaporins convert water into a nonpolar molecule
  2. Water cannot cross any lipid bilayer without energy
  3. Aquaporins add a phosphate to water
  4. Aquaporins provide a hydrophilic channel through the hydrophobic core Correct answer.

Explanation

The hydrophobic core impedes polar molecules, so water crosses only slowly by simple diffusion. Aquaporins form a hydrophilic pore that lowers the barrier and speeds water movement.

Question 13Membrane Structure and Composition

Which observation would best support the conclusion that membrane proteins can move within the bilayer?

  1. The bilayer thickness increases with temperature
  2. Lipids are found only in the outer leaflet
  3. Proteins remain clustered at one pole of the cell indefinitely
  4. Fluorescent labels on proteins become mixed across the membrane over time Correct answer.

Explanation

Lateral mixing of labeled proteins demonstrates that proteins diffuse within the plane of the membrane. Static clustering would instead suggest fixed positions.

Question 14Membrane Structure and Composition

A transmembrane protein has a positively charged residue located in the middle of its bilayer-spanning segment. What is the most likely consequence?

  1. The lipid tails would become hydrophilic
  2. The residue would be energetically unfavorable and destabilize the segment Correct answer.
  3. The protein would require a hydrophilic environment at that position
  4. The protein would be more stably embedded

Explanation

A charged residue in the hydrophobic core is energetically costly because it cannot be solvated by water. Such placement typically destabilizes the transmembrane segment unless a polar environment is provided.

Question 15Membrane Structure and Composition

Which change would tend to increase membrane fluidity at a fixed temperature?

  1. Decreasing the proportion of cholesterol in a rigid membrane Correct answer.
  2. Adding more saturated tails to the bilayer
  3. Increasing the length of the fatty acid tails
  4. Increasing the proportion of long saturated fatty acids

Explanation

Cholesterol can restrain lipid motion, so reducing it in an already rigid membrane can increase fluidity. Shorter tails and more unsaturation also raise fluidity.

Question 16Membrane Structure and Composition

The asymmetric distribution of lipids between the two leaflets of the plasma membrane is maintained mainly by which mechanism?

  1. Enzymes that move specific lipids between leaflets Correct answer.
  2. Covalent bonding of lipids to proteins
  3. Continuous fusion of vesicles with both leaflets equally
  4. Random thermal flipping of every lipid

Explanation

Flipping of polar head groups across the bilayer is energetically slow, so specific enzymes catalyze and maintain lipid asymmetry. This keeps different lipid types enriched in each leaflet.

Question 17Membrane Structure and Composition

Which statement best explains why integral membrane proteins can be removed only with detergents or organic solvents?

  1. They are held by covalent bonds to cholesterol
  2. They are attached only by weak ionic bonds to head groups
  3. They are located entirely outside the bilayer
  4. They are embedded in the hydrophobic core and require disruption of lipid packing Correct answer.

Explanation

Integral proteins have hydrophobic regions embedded in the bilayer core, so they must be freed by disrupting the lipids. Detergents and organic solvents solubilize the membrane and release them.

Question 18Membrane Structure and Composition

A membrane is enriched in phospholipids with very long, saturated tails. Compared with a membrane of shorter, unsaturated tails, what is the expected effect?

  1. Greater thickness and reduced fluidity Correct answer.
  2. No change in thickness or fluidity
  3. Reduced thickness and greater fluidity
  4. Loss of the bilayer structure entirely

Explanation

Long saturated tails pack tightly and extend farther, increasing bilayer thickness and reducing fluidity. Shorter, unsaturated tails pack less well and yield a thinner, more fluid bilayer.

Question 19Membrane Structure and Composition

Which feature of the plasma membrane allows a cell to keep its internal ion composition different from the surrounding fluid?

  1. A completely impermeable lipid bilayer
  2. Selective transport proteins plus a hydrophobic barrier Correct answer.
  3. Absence of cholesterol
  4. Uniform lipid composition in both leaflets

Explanation

The hydrophobic core blocks free ion movement, and specific pumps and channels control which ions cross. Together these produce a composition that differs from the outside fluid.

Question 20Membrane Structure and Composition

A protein is anchored to the outer leaflet of the plasma membrane by a short covalently attached fatty acid chain. How is this protein best classified?

  1. A soluble cytosolic protein
  2. An integral protein with a transmembrane segment
  3. A lipid-anchored membrane protein Correct answer.
  4. A peripheral protein bound only by ionic interactions

Explanation

Covalent attachment of a lipid to a protein tethers it to the membrane without a transmembrane segment. Such proteins are classified as lipid-anchored.

Question 21Membrane Structure and Composition

Why is the hydrophobic effect central to bilayer self-assembly in water?

  1. Nonpolar tails are excluded from water and cluster together Correct answer.
  2. Lipids are pushed together by active transport
  3. Hydrophilic heads repel each other in water
  4. Water forms covalent bonds with lipid tails

Explanation

Water molecules hydrogen-bond to each other and exclude nonpolar tails, which minimizes unfavorable contact between water and hydrocarbon. This drives tails to cluster inward and heads to face water, forming a bilayer.

Question 22Membrane Structure and Composition

A cell membrane is treated so that its cholesterol is removed. Which outcome is most consistent with the role of cholesterol?

  1. All transport proteins stop working
  2. Fluidity becomes more sensitive to temperature changes Correct answer.
  3. The bilayer immediately dissolves
  4. The membrane becomes completely impermeable to water

Explanation

Cholesterol broadens the temperature range over which the membrane remains appropriately fluid. Without it, fluidity varies more sharply with temperature.

Question 23Membrane Structure and Composition

A transmembrane channel lining is lined with polar residues while its exterior facing the lipid core is hydrophobic. Why is this arrangement functionally important?

  1. It lets ions pass through a hydrophilic pore while stabilizing the protein in the bilayer Correct answer.
  2. It allows the channel to covalently bond to lipid tails
  3. It prevents the protein from being embedded in the membrane
  4. It makes the lipid core hydrophilic

Explanation

A polar interior provides a favorable pathway for charged solutes, while a hydrophobic exterior matches the lipid core and stabilizes the protein's position. This dual arrangement is typical of channel proteins.

Question 24Membrane Structure and Composition

Which combination of lipid changes would most strongly decrease membrane fluidity?

  1. Longer tails and fewer double bonds Correct answer.
  2. Shorter tails and more cholesterol
  3. More cholesterol in an already fluid membrane
  4. Shorter tails and more double bonds

Explanation

Longer tails increase hydrophobic packing and fewer double bonds allow tighter alignment, both reducing fluidity. The combination therefore most strongly lowers fluidity.

Question 25Membrane Structure and Composition

Why does the plasma membrane's lipid asymmetry matter for cell function?

  1. It removes the need for a hydrophobic core
  2. It makes both leaflets chemically identical
  3. It prevents any protein from binding the membrane
  4. It allows different recognition and signaling events on each face of the membrane Correct answer.

Explanation

Different lipids on each leaflet create distinct surface properties, supporting processes such as recognition and signaling at the outer face and cytosolic interactions at the inner face. Asymmetry is therefore functionally meaningful.

Question 26Membrane Structure and Composition

Which of the following best describes the arrangement of phospholipid molecules in a bilayer?

  1. Hydrophobic tails face inward, hydrophilic heads face outward. Correct answer.
  2. Both heads and tails are uniformly distributed throughout the bilayer.
  3. Hydrophilic tails face inward, hydrophobic heads face outward.
  4. Phospholipids are arranged in a single layer with heads facing the interior.

Explanation

Phospholipids are amphipathic: their hydrophilic heads interact with water, while their hydrophobic tails avoid water. In a bilayer, the tails cluster in the interior and the heads face the aqueous environments on both sides.

Question 27Membrane Structure and Composition

Which type of lipid is most commonly found in the inner leaflet of the plasma membrane, contributing to membrane asymmetry?

  1. Phosphatidylserine Correct answer.
  2. Cholesterol
  3. Sphingomyelin
  4. Phosphatidylcholine

Explanation

Phosphatidylserine is an aminophospholipid that is actively kept on the cytoplasmic (inner) leaflet by flippases. Its exposure on the outer leaflet is a signal for apoptosis. Phosphatidylcholine and sphingomyelin are more abundant on the outer leaflet.

Question 28Membrane Structure and Composition

What is the primary role of the hydrophobic core of the lipid bilayer?

  1. To facilitate the rapid diffusion of ions across the membrane.
  2. To serve as a barrier to the free movement of most polar and charged molecules. Correct answer.
  3. To anchor peripheral membrane proteins.
  4. To provide a site for covalent modification of lipids.

Explanation

The hydrophobic core excludes water and charged or polar substances, creating a permeability barrier. Only small nonpolar molecules and gases can diffuse through it relatively easily. Ions and most polar molecules require transport proteins.

Question 29Membrane Structure and Composition

Which of the following is a function of membrane proteins?

  1. Acting as transporters for ions and polar molecules. Correct answer.
  2. Synthesizing phospholipids in the bilayer.
  3. Maintaining the fluidity of the membrane by filling spaces between lipids.
  4. Providing a hydrophobic barrier to water.

Explanation

Membrane proteins serve diverse functions including transport, signaling, and cell recognition. Transporters allow ions and polar molecules to cross the hydrophobic barrier. Cholesterol, not proteins, modulates fluidity, and the lipid bilayer itself forms the hydrophobic barrier.

Question 30Membrane Structure and Composition

A transmembrane protein has a single hydrophobic alpha helix that spans the bilayer. Which characteristic must the amino acid side chains in this segment have?

  1. They must be small enough to allow ions to pass through the helix.
  2. They must be predominantly charged to interact with the polar heads.
  3. They must be predominantly nonpolar to interact favorably with the lipid tails. Correct answer.
  4. They must be able to form covalent bonds with phospholipids.

Explanation

The interior of the bilayer is hydrophobic, so the transmembrane segment of a protein must be composed largely of nonpolar amino acids to be stable within the lipid core. Charged or polar residues are typically found at the membrane-water interfaces or in aqueous channels.

Question 31Membrane Structure and Composition

In a membrane, cholesterol molecules are interspersed among phospholipids. How does cholesterol affect the packing of lipid tails at temperatures above the transition temperature?

  1. It decreases packing by disrupting tail interactions, making the membrane more fluid.
  2. It increases packing by filling gaps between tails, making the membrane less fluid. Correct answer.
  3. It has no effect on packing because it is too small to interact with tails.
  4. It causes the tails to become fully extended and rigid.

Explanation

At temperatures above the transition temperature, cholesterol's rigid ring structure fills spaces between phospholipid tails, reducing their movement and thereby decreasing fluidity. This is one reason cholesterol is described as a fluidity buffer.

Question 32Membrane Structure and Composition

Which of the following lipids is a glycolipid commonly found in the outer leaflet of the plasma membrane?

  1. Galactocerebroside Correct answer.
  2. Cardiolipin
  3. Phosphatidylinositol
  4. Phosphatidylethanolamine

Explanation

Galactocerebroside is a glycosphingolipid with a sugar (galactose) head group, making it a glycolipid. Glycolipids are typically found on the outer leaflet of the plasma membrane. The other options are phospholipids.

Question 33Membrane Structure and Composition

A membrane protein is found to be associated with the inner leaflet of the plasma membrane and can be released by treatment with a high-salt solution. What type of protein is this most likely to be?

  1. A peripheral membrane protein held by electrostatic interactions. Correct answer.
  2. An integral membrane protein with multiple transmembrane domains.
  3. A lipid-anchored protein covalently attached to a fatty acid.
  4. A transmembrane protein with a covalently attached sugar chain.

Explanation

Peripheral membrane proteins associate with the membrane surface through noncovalent interactions, such as electrostatic bonds with lipid head groups or with other proteins. High salt disrupts these interactions, releasing the protein without disrupting the bilayer.

Question 34Membrane Structure and Composition

Which of the following would most likely increase the fluidity of a lipid bilayer at a given temperature?

  1. Introducing cis double bonds into the fatty acid tails. Correct answer.
  2. Increasing the proportion of saturated fatty acids.
  3. Increasing the length of the phospholipid tails.
  4. Adding cholesterol to a membrane already above its transition temperature.

Explanation

Cis double bonds create kinks in the fatty acid tails, preventing tight packing and increasing membrane fluidity. Longer tails and saturated tails pack more tightly and decrease fluidity. Cholesterol generally decreases fluidity at high temperatures.

Question 35Membrane Structure and Composition

Which of the following best explains why phospholipids spontaneously form bilayers in aqueous environments?

  1. Hydrogen bonds between tails stabilize the bilayer structure.
  2. Covalent bonds form between adjacent phospholipid molecules.
  3. The hydrophobic effect drives tails to aggregate away from water, while heads interact with water. Correct answer.
  4. The bilayer is stabilized by the presence of integral membrane proteins.

Explanation

The hydrophobic effect is the primary driving force: water molecules form ordered cages around exposed hydrophobic tails, which is entropically unfavorable. Aggregating tails away from water minimizes this, while hydrophilic heads remain in contact with water. This self-assembly does not require covalent bonds or proteins.

Question 36Membrane Structure and Composition

A cell is treated with a drug that inhibits flippase enzymes. What is the most likely immediate consequence for the plasma membrane?

  1. Loss of lipid asymmetry between the two leaflets. Correct answer.
  2. Failure of integral proteins to insert into the membrane.
  3. Increased fluidity of the membrane.
  4. Complete dissolution of the lipid bilayer.

Explanation

Flippases are ATP-dependent enzymes that move specific lipids (such as phosphatidylserine) from the outer to the inner leaflet, maintaining lipid asymmetry. Inhibiting them would allow lipids to equilibrate between leaflets, reducing asymmetry.

Question 37Membrane Structure and Composition

Which of the following is a characteristic of lipid rafts?

  1. They are found only in the inner leaflet of the plasma membrane.
  2. They are enriched in polyunsaturated phospholipids.
  3. They are sites of high fluidity and rapid lipid diffusion.
  4. They are enriched in cholesterol and sphingolipids. Correct answer.

Explanation

Lipid rafts are microdomains of the plasma membrane that are enriched in cholesterol and sphingolipids, which pack tightly together. They are more ordered and less fluid than surrounding membrane regions and are involved in signaling and membrane trafficking.

Question 38Membrane Structure and Composition

A membrane has a high proportion of phosphatidylethanolamine (PE), which has a relatively small head group. What effect does this lipid shape have on the membrane?

  1. It increases the thickness of the hydrophobic core.
  2. It causes the membrane to become more permeable to ions.
  3. It promotes curvature, such as in the inner leaflet of a curved membrane. Correct answer.
  4. It favors the formation of a flat bilayer.

Explanation

Phosphatidylethanolamine is a cone-shaped lipid because its head group is small relative to its tail volume. Cone-shaped lipids tend to induce negative curvature, which is important in processes like membrane budding and fusion.

Question 39Membrane Structure and Composition

Which of the following best describes the role of the glycocalyx in the plasma membrane?

  1. It serves as a site for ATP synthesis.
  2. It is involved in cell recognition and adhesion. Correct answer.
  3. It maintains the fluidity of the membrane.
  4. It provides a hydrophobic barrier to water-soluble molecules.

Explanation

The glycocalyx is the carbohydrate-rich layer on the outer surface of the plasma membrane, formed by glycolipids and glycoproteins. It functions in cell-cell recognition, adhesion, and protection. It does not form a hydrophobic barrier or synthesize ATP.

Question 40Membrane Structure and Composition

In an experiment, a fluorescent lipid probe is inserted into the outer leaflet of a cell membrane. Over time, the probe is found in the inner leaflet. Which process is most likely responsible?

  1. Simple diffusion through the hydrophobic core.
  2. Flippase-mediated translocation.
  3. Transbilayer movement catalyzed by a scramblase. Correct answer.
  4. Endocytosis followed by vesicle fusion.

Explanation

Scramblases are enzymes that facilitate the bidirectional movement of lipids between leaflets, leading to equilibration. Flippases are typically unidirectional and specific for certain lipids. Simple diffusion of lipids across the bilayer is very slow, and endocytosis would not directly move a lipid probe from one leaflet to the other without vesicle trafficking.

Question 41Membrane Transport

Which property of the plasma membrane allows small nonpolar molecules such as oxygen to cross it most readily?

  1. A charged carbohydrate coat
  2. A hydrophobic lipid interior Correct answer.
  3. A hydrophilic lipid core
  4. A rigid protein scaffold

Explanation

The membrane's interior is formed by the fatty-acid tails of phospholipids, creating a hydrophobic barrier. Nonpolar gases dissolve in this region and diffuse through easily, while polar and charged substances are impeded.

Question 42Membrane Transport

A cell is placed in a hypertonic solution. What is the immediate net movement of water?

  1. Water leaves the cell, causing it to shrink Correct answer.
  2. Water enters the cell until it bursts
  3. Water is actively pumped out by carrier proteins
  4. Water moves equally in both directions with no net change

Explanation

In a hypertonic solution the solute concentration outside is higher, so the external water potential is lower. Water moves by osmosis from the higher water potential inside the cell to the outside, causing shrinkage.

Question 43Membrane Transport

Which process moves substances against their concentration gradient using energy from ATP?

  1. Facilitated diffusion
  2. Active transport Correct answer.
  3. Osmosis
  4. Simple diffusion

Explanation

Active transport uses energy, typically from ATP hydrolysis, to move solutes against their gradient via pumps. The other listed processes are passive and depend on the gradient rather than opposing it.

Question 44Membrane Transport

Which structural feature of the fluid mosaic model describes proteins embedded within the phospholipid bilayer?

  1. Cholesterol molecules
  2. Peripheral membrane proteins
  3. Integral membrane proteins Correct answer.
  4. Glycolipids

Explanation

Integral proteins are embedded within or span the hydrophobic core of the bilayer. Peripheral proteins instead associate with the membrane surface.

Question 45Membrane Transport

Which molecule most readily diffuses directly through the phospholipid bilayer without a transport protein?

  1. Sodium ion
  2. Glucose
  3. Amino acid
  4. Carbon dioxide Correct answer.

Explanation

Small nonpolar molecules such as carbon dioxide dissolve in the hydrophobic core and cross by simple diffusion. Ions, sugars, and amino acids are polar or charged and require proteins.

Question 46Membrane Transport

What role does cholesterol play in the plasma membrane of animal cells?

  1. It forms the hydrophilic heads of phospholipids
  2. It transports ions across the bilayer
  3. It moderates membrane fluidity across temperature changes Correct answer.
  4. It serves as a primary energy source for the cell

Explanation

Cholesterol sits between phospholipid tails, reducing fluidity at high temperatures and preventing tight packing at low temperatures. This buffers the membrane against temperature-induced changes.

Question 47Membrane Transport

Facilitated diffusion differs from simple diffusion in that facilitated diffusion

  1. requires a membrane protein Correct answer.
  2. requires ATP hydrolysis
  3. moves solutes against the gradient
  4. only transports nonpolar gases

Explanation

Facilitated diffusion uses specific channel or carrier proteins to move polar solutes down their gradient. Like simple diffusion, it is passive and requires no energy.

Question 48Membrane Transport

A red blood cell placed in distilled water will most likely

  1. remain unchanged in volume
  2. shrink due to water loss
  3. swell and lyse due to water entry Correct answer.
  4. actively pump water outward

Explanation

Distilled water is hypotonic relative to the cytoplasm, so water enters the cell by osmosis. Without a cell wall, the cell swells and eventually bursts.

Question 49Membrane Transport

The sodium-potassium pump moves ions in which pattern?

  1. Three Na+ in and two K+ out per ATP
  2. Three Na+ out and two K+ in per ATP Correct answer.
  3. One Na+ out and one K+ in per ATP
  4. Two Na+ out and three K+ in per ATP

Explanation

The pump exports three sodium ions and imports two potassium ions for each ATP hydrolyzed. This unequal exchange contributes to the negative resting membrane potential.

Question 50Membrane Transport

Which statement best explains why the sodium-potassium pump is described as electrogenic?

  1. It creates a physical electrical wire across the membrane
  2. It generates heat during ATP hydrolysis
  3. It moves unequal numbers of positive charges, contributing to membrane potential Correct answer.
  4. It neutralizes all charges on both sides of the membrane

Explanation

Because the pump exports three positive charges while importing two, the net transfer of charge alters the membrane potential. This unequal charge movement is what makes the pump electrogenic.