BMEG 245: Membrane Structure & Transport (Ch. 11 + 12)

What is the structure of a cell membrane?

A cell membrane is a lipid bilayer in which proteins are embedded; it forms a two-ply sheet that separates the cell interior from the environment and controls molecular traffic.

Why are phospholipids called amphipathic?

Because each molecule has a hydrophilic (polar) head that interacts with water and hydrophobic (non-polar) tails that avoid water, allowing bilayer formation.

Why do phospholipids form bilayers spontaneously in water?

Hydrophobic tails cluster to avoid water while hydrophilic heads face outward; this minimizes free energy and makes the bilayer energetically favorable.

What feature makes the bilayer self sealing

Any tear exposes hydrophobic tails to water, which is energetically unfavorable, so the membrane rapidly reseals to restore a closed bilayer.

What is the most common phospholipid in cell membranes?

Phosphatidylcholine, which has a choline-phosphate head group.

What is the function of cholesterol in membranes?

Cholesterol fills gaps between phospholipids, reducing permeability and stiffening the membrane while maintaining fluidity.

What does "amphipathic" mean?

It means having both hydrophilic and hydrophobic parts within the same molecule.

How do phospholipids move within the bilayer?

They can rotate, flex, and laterally diffuse freely; transverse “flip-flop” movements require enzymes like scramblase or flippase.

What is the role of scramblase?

Scramblase transfers random phospholipids between leaflets in the ER to equilibrate growth of both sides of the bilayer.

What is the role of flippase?

Flippase selectively transfers specific phospholipids to the cytosolic side of the plasma membrane, maintaining asymmetric lipid distribution.

How are new membranes made in the cell?

Membrane synthesis begins in the ER; new phospholipids are added by enzymes and distributed by scramblase and vesicle transport.

What is a liposome?

A closed spherical vesicle formed from pure phospholipids in water, used experimentally to model membranes.

What does "membrane orientation is preserved" mean?

Once a protein or lipid is positioned facing a particular side of the membrane, that orientation is maintained through vesicular transport.

What are integral membrane proteins?

Proteins embedded within the lipid bilayer, often spanning it as α-helices or β-barrels.

How does an α helix allow a protein to cross the membrane?

The hydrophobic side chains face outward toward lipid tails while the polar backbone is hydrogen-bonded internally, forming a stable transmembrane helix.

What is a β barrel membrane protein?

A cylinder of antiparallel β-sheets forming a hydrophilic pore across the membrane, common in porins.

How can membrane proteins be extracted?

By using detergents: mild non-ionic detergents (like Triton X-100) preserve structure, while strong ionic detergents (like SDS) denature proteins.

What reinforces the plasma membrane of red blood cells?

The spectrin cytoskeletal cortex, a lattice that maintains the cell’s biconcave shape and flexibility.

How can the lateral movement of membrane proteins be restricted?

Proteins can be tethered to the cell cortex, extracellular matrix, other cells, or separated by diffusion barriers.

What is the glycocalyx?

A carbohydrate layer on the extracellular surface composed of glycolipids, glycoproteins, and proteoglycans that protects cells and aids in recognition.

Why is the plasma membrane called "fluid"?

Lipids and many proteins move laterally, giving the membrane a dynamic, fluid-mosaic character.

What creates a membrane potential?

Differences in the concentration and electrical charge of ions across the plasma membrane, typically −20 to −200 mV (inside negative).

How do ions cross lipid bilayers without proteins?

Only small non-polar molecules diffuse freely; ions and polar molecules require transport proteins.

What are the two main types of transport proteins?

Channels (form pores for specific ions) and transporters (bind solutes and undergo conformational changes to move them).

What determines passive diffusion rate through a pure lipid bilayer?

Smaller, more hydrophobic molecules diffuse faster; large or charged molecules cross very slowly.

What is the difference between channel and transporter?

Channels provide a continuous pore for ions when open, while transporters alternate between conformations to move bound molecules.

What is a concentration gradient?

The difference in solute concentration across a membrane that drives passive movement from high to low concentration.

What is a membrane potential?

The voltage difference across a membrane due to unequal ion distribution, influencing ion movement.

What is an electrochemical gradient?

The combined effect of concentration gradient and membrane potential that determines the net driving force for an ion’s movement.

What is osmosis?

The passive movement of water across a semipermeable membrane from low solute concentration to high solute concentration.

How do animal cells prevent osmotic swelling?

By pumping out ions (e.g., Na⁺) to balance osmotic pressure and maintaining rigid cytoskeletal support.

How do plant cells resist osmotic swelling?

Their cell wall exerts turgor pressure that counteracts osmotic influx of water.

What is passive transport?

Movement of molecules down their electrochemical gradient without energy input.

What is active transport?

Movement of molecules against their electrochemical gradient using energy (ATP, ion gradient, or light).

Describe the Na⁺/K⁺ ATPase pump.

It hydrolyzes one ATP to pump 3 Na⁺ out and 2 K⁺ in, maintaining resting potential and ion gradients; consumes ~30 % of cell ATP.

What type of active transport uses ATP directly?

Primary active transport, as in ATP

What are the three main classes of active transporters?

ATP-driven pumps, gradient-driven (coupled) pumps, and light-driven pumps.

What is a coupled transporter?

A transporter that uses the downhill movement of one solute to drive the uphill transport of another.

Describe a glucose Na⁺ symport.

It simultaneously transports Na⁺ and glucose into cells; Na⁺ moving down its gradient drives glucose uptake against its gradient.

What is a uniport?

A transporter that moves a single type of molecule in one direction.

What is an antiport?

A coupled transporter that moves two solutes in opposite directions.

How is glucose transported across intestinal epithelial cells?

Na⁺-driven symport on the apical side imports glucose; passive glucose uniport on the basal side releases it to blood.

What maintains the Na⁺ gradient driving symports?

The Na⁺/K⁺ ATPase pump, which expels Na⁺ to keep cytosolic Na⁺ low.

What is a Ca²⁺ pump’s function?

It removes Ca²⁺ from the cytosol (into ER or extracellular space) to keep cytosolic Ca²⁺ very low, allowing rapid signaling.

What is a gradient driven pump?

A secondary active transporter that uses one ion’s gradient (often Na⁺ or H⁺) to move another molecule against its gradient.

How do plant and bacterial cells generate gradients for transport?

They use H⁺ pumps instead of Na⁺ pumps to drive secondary active transport.

What are ion channels?

Selective, gated pores allowing rapid passive ion movement across membranes.

What is meant by “selectivity filter” in ion channels?

A narrow region of the channel that fits specific ions by size and charge, ensuring selectivity.

Why can ions move through channels faster than through transporters?

Channels do not undergo major conformational changes for each ion; millions of ions pass per second when open.

What are the main types of gated ion channels?

Voltage-gated, ligand-gated, and mechanically-gated channels.

How do voltage gated Na⁺ and K⁺ channels coordinate an action potential?

Na⁺ channels open first to depolarize the membrane; K⁺ channels open later to repolarize and restore resting potential.

What is depolarization?

A rapid increase in membrane potential (less negative) due to Na⁺ influx during an action potential.

What is repolarization?

Restoration of negative membrane potential caused by K⁺ efflux after depolarization.

How does the delayed opening of K⁺ channels ensure signal directionality?

It prevents backward propagation by repolarizing regions behind the advancing action potential.

What is the resting membrane potential of most animal cells?

Approximately −70 mV, maintained by K⁺ leak channels and Na⁺/K⁺ pumps.

What is a nerve action potential?

A traveling wave of electrical excitation resulting from sequential opening of voltage

What is the approximate speed of an action potential?

Up to about 100 meters per second along myelinated axons.

What happens at a synapse?

The electrical signal triggers neurotransmitter release, which crosses the synaptic cleft and binds receptors on the next cell, converting back into an electrical signal.

What triggers neurotransmitter release?

Depolarization opens voltage-gated Ca²⁺ channels, and Ca²⁺ influx causes synaptic vesicles to fuse with the plasma membrane.

What converts electrical signals to chemical signals?

The release of neurotransmitters from presynaptic vesicles into the synaptic cleft.

What converts chemical signals back to electrical signals?

Neurotransmitter

Name the main excitatory and inhibitory neurotransmitters.

Excitatory: Glutamate; Inhibitory: GABA and Glycine.

What are monoamine neurotransmitters?

Dopamine, serotonin, norepinephrine, and epinephrine—regulate mood, reward, and alertness.

What neurotransmitters act at neuromuscular junctions?

Acetylcholine, which triggers muscle fiber depolarization.

How is an action potential propagated unidirectionally?

Refractory period: Na⁺ channels inactivated immediately after opening, preventing re-excitation behind the wave.

What is a patch-clamp recording used for?

Measuring the activity of individual ion channels by monitoring current flow through a small patch of membrane.

What are mechanically-gated ion channels?

Channels that open in response to physical deformation of the membrane; important in touch, hearing, and mechanosensation.

How does the plant Mimosa pudica use ion channels?

Mechanically-gated channels initiate voltage signals that propagate through voltage-gated channels, causing leaves to fold.

What defines the direction of glucose transport through its transporter?

It depends solely on glucose concentration gradient since glucose is uncharged.

What role does the Na⁺/K⁺ pump play in nerve cell signaling?

It maintains Na⁺ and K⁺ gradients essential for generating action potentials; without it, depolarization couldn’t occur.

Why is electrical signaling faster than chemical signaling?

Because changes in membrane potential propagate instantly along the membrane, while chemical transmission requires diffusion and receptor activation.

Why are chemical signals still necessary at synapses?

They allow signal modulation, integration, and specificity between different neurons.