Membrane Transport Mechanisms Practice Questions

Correct answer: Passive diffusion of small nonpolar molecules directly through the lipid bilayer

Simple diffusion allows small nonpolar (hydrophobic) molecules to dissolve in the lipid bilayer and cross the membrane down their concentration gradient without the aid of proteins or energy. Polar or charged molecules cannot use this route effectively.

Correct answer: Facilitated diffusion

Facilitated diffusion uses specific membrane proteins (channels or carriers) to help polar or charged solutes cross the membrane down their concentration gradient without using ATP, classifying it as a passive transport mechanism.

Correct answer: O₂ (oxygen) gas

Oxygen is a small nonpolar gas that can dissolve in the hydrophobic core of the membrane and cross by simple diffusion; charged ions and large polar molecules generally require transport proteins.

Correct answer: Primary active transport uses ATP directly; secondary uses an ion gradient

Primary active transport directly uses ATP hydrolysis to move solutes against their gradient. Secondary active transport (cotransport) harnesses the energy stored in an ion gradient (established by primary active transport) to move another solute against its gradient.

Correct answer: Passive movement of water through membrane from low to high solute concentration

Osmosis refers to the passive movement of water across a semipermeable membrane from a region of lower solute concentration (higher water concentration) to higher solute concentration, aiming to equilibrate solute concentrations.

Correct answer: They are repelled by the bilayer's hydrophobic interior

The hydrophobic interior of the phospholipid bilayer repels charged or polar molecules; thus ions cannot cross by simple diffusion and require protein channels or pumps to traverse the membrane.

Correct answer: Secondary active transport via Na⁺ symport

Glucose reabsorption in the kidney often uses a Na⁺–glucose symporter: Na⁺ moves down its gradient, providing the energy to carry glucose against its gradient, a classic example of secondary active transport.

Correct answer: Primary active transport (e.g., Na⁺/K⁺-ATPase)

Primary active transport uses energy from ATP hydrolysis to move ions (e.g., Na⁺ out, K⁺ in) against their electrochemical gradients — a central mechanism for maintaining cellular ion balance.

Correct answer: Membrane proteins bind and carry specific solutes across the membrane

Carrier-mediated transport involves specific transmembrane proteins (carriers) that bind particular solutes and undergo conformational changes to move them across the membrane, either passively (facilitated diffusion) or actively.

Correct answer: Transport rate plateaus once all carriers are occupied

Carriers are a finite resource. As solute concentration rises, more carriers are occupied until every one of them is working continuously; beyond that point the transport rate reaches a maximum and further solute makes no difference. Simple diffusion has no such ceiling, which is why its rate does keep rising linearly with the concentration gradient.

Correct answer: Endocytosis

Endocytosis is the process by which cells engulf extracellular material (large molecules, particles, fluid) by forming a vesicle from the membrane — an energy-dependent form of transport distinct from diffusion or carrier-mediated processes.

Correct answer: It reduces membrane fluidity and passive leakiness

Cholesterol stabilizes and stiffens the phospholipid bilayer, reducing its passive permeability and making the membrane less leaky, thus influencing the selectivity and control of transport processes.

Correct answer: They are selective for specific ions and are usually gated

Ion channels are selective for particular ions (e.g., Na⁺, K⁺, Ca²⁺) and may be gated (e.g., by voltage or ligands), enabling controlled passive movement down the electrochemical gradient without energy input.

Correct answer: Exocytosis

Exocytosis is the vesicular transport mechanism by which cells export large proteins or other bulky molecules: vesicles fuse with the plasma membrane and release their contents extracellularly, a process that requires energy.

Correct answer: It transports two different molecules in opposite directions simultaneously

An antiporter exchanges two different solutes in opposite directions; one moves down its gradient (favorable), and the other moves against its gradient — a common form of secondary active transport.

Correct answer: Because carrier proteins ferry large polar glucose across the hydrophobic bilayer

Glucose is large and polar, unable to dissolve in the hydrophobic bilayer; facilitated diffusion uses specific transporters to move glucose across the membrane down its concentration gradient — something not possible via simple diffusion.

Correct answer: Keeping intracellular K⁺ high and Na⁺ low against their gradients

Maintaining high intracellular K⁺ and low Na⁺ concentrations requires the Na⁺/K⁺-ATPase pump, which uses ATP to transport ions against their gradients — a primary active transport process essential for cell homeostasis.

Correct answer: Facilitated diffusion uses a finite number of carriers that saturate when fully occupied

Facilitated diffusion depends on carrier proteins; because there are a limited number of such transporters, once they are all occupied the transport rate cannot increase further, resulting in saturation (Vmax). Simple diffusion through the lipid bilayer does not have this limitation.

Correct answer: High lipid solubility and no charge on the drug molecule

Hydrophobic, uncharged, lipid-soluble molecules diffuse more readily through the lipid bilayer by simple diffusion; this property enhances passive membrane permeation. Other features (size, polarity, charge) would hinder membrane passage.

Correct answer: Passive diffusion cannot move ions or polar molecules efficiently

Because the lipid bilayer is hydrophobic, passive diffusion cannot efficiently move ions or polar molecules across the membrane; these require specialized transport proteins or active processes to maintain cellular homeostasis.

Correct answer: Uptake of a large protein such as an antibody

Large molecules or particles (such as proteins) cannot pass through protein channels or diffuse across the membrane; vesicular processes (endocytosis) are required for their internalization.

Correct answer: Carries a single solute down its concentration gradient

A uniporter is a type of carrier protein that facilitates passive transport of one kind of solute down its concentration gradient — without coupling to another solute or using ATP.

Correct answer: It pumps more Na⁺ out than K⁺ in, making the interior more negative

The Na⁺/K⁺-ATPase exchanges 3 Na⁺ out for 2 K⁺ in per ATP hydrolyzed, creating a net loss of positive charges from the cell, which helps establish the negative resting membrane potential crucial for many cell functions.

Correct answer: Glucose entry into gut cells driven by the Na⁺ gradient

Glucose uptake that relies on Na⁺ moving down its gradient to drive glucose uptake against its gradient (via Na⁺–glucose symporter) is a classic example of secondary active transport, coupling one solute’s favorable movement to another’s unfavorable movement.

Correct answer: The lipid solubility of the molecule

According to Fick's Law, the rate of simple diffusion is directly proportional to the lipid solubility (partition coefficient) and the concentration gradient of the substance.

Correct answer: Chemically related molecules binding to the same transporter site

Competitive inhibition occurs when two or more similar substances compete for the same binding site on a carrier protein, reducing the transport rate of each.

Correct answer: Transports two different solutes in the same direction

A symporter (or cotransporter) is a secondary active transporter that moves two different species in the same direction, typically using the gradient of one to pull the other.

Correct answer: 3 Na+ out, 2 K+ in

The pump moves 3 sodium ions out of the cell and 2 potassium ions into it for every ATP molecule hydrolysed. Because more positive charge leaves than enters, the pump is electrogenic and helps maintain both the ionic gradients and the resting membrane potential.

Correct answer: An ion concentration gradient established by a primary pump

Secondary active transport does not use ATP directly; instead, it uses the potential energy stored in an electrochemical gradient (usually Na+) created by primary active transport.

Correct answer: They both require a specific membrane protein and exhibit saturation

Both are 'protein-mediated,' meaning they use specific transporters that can become saturated when all binding sites are occupied (Vmax).

Correct answer: Phagocytosis

Phagocytosis ('cell eating') is a form of vesicular transport used to internalize large particulate matter like bacteria or cell debris.

Correct answer: The rate of transport levels off despite increases in concentration

The transport maximum (or Vmax) is reached when all available carrier proteins are saturated with solute, meaning the transport rate cannot increase further.

Correct answer: Water molecules

Aquaporins are water channels that allow for the rapid, passive movement of water across cell membranes in response to osmotic gradients.

Correct answer: They can open or close in response to chemical or electrical stimuli

Gated channels respond to specific triggers, such as voltage changes (voltage-gated) or ligand binding (ligand-gated), to regulate ion flow.

Correct answer: Facilitated diffusion

GLUT transporters facilitate the passive movement of glucose down its concentration gradient into the cell; this is a form of facilitated diffusion.

Correct answer: Exocytosis

Exocytosis is the process where intracellular vesicles fuse with the plasma membrane to release their contents (like neurotransmitters) into the ECF.

Correct answer: Creates a net charge imbalance across the membrane

A pump is electrogenic if it moves a different number of charges in each direction (e.g., 3 Na+ out vs 2 K+ in), contributing to the membrane potential.

Correct answer: Potassium ion (K+)

Charged ions like K+ have extremely low permeability in the hydrophobic lipid bilayer and cannot cross by simple diffusion; they require channels or pumps.

Correct answer: Secondary active symport

SGLT uses the downward gradient of Sodium to pull Glucose 'uphill' into the cell in the same direction, making it a secondary active symporter.

Correct answer: Osmosis refers specifically to the movement of solvent (water)

While both are passive, osmosis is the diffusion of water across a semi-permeable membrane from an area of low solute concentration to high solute concentration.

Correct answer: The non-specific uptake of extracellular fluid and small solutes

Pinocytosis ('cell drinking') is a form of endocytosis where the cell gulps droplets of extracellular fluid into tiny vesicles.

Correct answer: An antiporter

An antiporter (or exchanger) moves two substances in opposite directions; the NCX uses the sodium gradient to eject calcium from the cell.

Correct answer: A flexible bilayer with proteins that can move laterally

The fluid mosaic model describes the membrane as a dynamic, fluid phospholipid bilayer with embedded 'mosaic' proteins that facilitate transport.

Correct answer: Hypotonic

If the ECF has a lower solute concentration than the ICF, it is hypotonic, which would cause water to enter the cell by osmosis.