Cell Physiology Exam 4 Study Guide: Membrane Structure, Transport, and Protein Trafficking

Vocabulary practice flashcards covering membrane structure, fluidity, lipid and protein mobility, transport mechanisms across membranes, action potentials, and intracellular protein trafficking from Cell Physiology Exam 4.

Gorter and Grendel Experiment

An experiment demonstrating that biological membranes consist of a lipid bilayer by extracting lipids from a known count of human red blood cells, spreading them as a monolayer on a water trough using a movable barrier, and showing that the monolayer area was approximately twice that of the intact cells.

Transmembrane Domain

A segment of an integral membrane protein, commonly an $\alpha$-helix consisting of $20\text{ to }26$ predominantly hydrophobic amino acids, that spans the phospholipid bilayer.

Monotopic Integral Membrane Protein

An integral membrane protein that spans the membrane bilayer exactly once.

Bitopic Integral Membrane Protein

An integral membrane protein that spans the membrane bilayer twice.

Polytopic Integral Membrane Protein

An integral membrane protein that spans the membrane bilayer more than twice.

Hydropathy Plot

A plot measuring the free energy change of transferring successive segments of a protein from a non-polar to a polar solvent to identify transmembrane segments; a positive free energy change indicates non-polar regions that form transmembrane domains, whereas a negative value indicates polar regions.

Sodium Dodecyl Sulfate (SDS)

A detergent that denatures proteins and coats them with a uniform negative charge, eliminating intrinsic charge differences so that proteins migrate toward the positive pole during SDS-PAGE strictly according to size.

O-linked Glycosylation

The enzymatic attachment of oligosaccharides to the hydroxyl groups of serine or threonine amino acid residues within the lumen of the RER and Golgi apparatus.

N-linked Glycosylation

The enzymatic attachment of oligosaccharides to the amide nitrogen of asparagine residues occurring in the lumen of the rough endoplasmic reticulum and Golgi.

Extravasation

The multi-step process by which white blood cells (leukocytes) migrate out of the blood circulation, across the blood vessel endothelial wall, and into surrounding tissue spaces at sites of infection or tissue damage.

Selectins

Carbohydrate-binding lectins expressed on endothelial cells of blood vessel walls that bind specific oligosaccharides on leukocyte surfaces, causing leukocytes to stick loosely and roll along the vessel wall during extravasation.

Fluid-Mosaic Model

A structural model of biological membranes describing a bilayer wherein lipids and proteins diffuse laterally within the plane of the membrane, embedded with a mosaic of integral, lipid-linked, and peripheral proteins.

Cholesterol (Membrane Function)

A sterol lipid in animal cell membranes that increases membrane rigidity near the outer portion of a leaflet, yet maintains distal fluidity when saturated fatty acid tails are abundant; highly concentrated within lipid rafts.

Lipid Raft

A specialized membrane microdomain enriched in cholesterol, sphingomyelin, and specific sets of functionally associated proteins that moves and functions together as a coordinated unit.

Fluorescence Recovery After Photobleaching (FRAP)

An experimental technique used to measure the rate and mobility of membrane protein lateral diffusion by photobleaching a defined fluorescent area with a laser and measuring the rate of fluorescence recovery over time.

Transverse Diffusion (Flip-Flop)

The movement of a phospholipid from one leaflet of the bilayer to the opposing leaflet, an extremely rare spontaneous event because it requires the polar head group to traverse the central hydrophobic core.

Scramblase

An enzyme in the endoplasmic reticulum membrane that randomly moves phospholipids between the cytosolic and luminal leaflets to ensure symmetric membrane growth.

Flippase

A Golgi membrane enzyme that catalytically transfers specific phospholipids from the luminal leaflet to the cytosolic leaflet, establishing the asymmetric lipid composition of the Golgi and plasma membranes.

Erythrocyte Membrane Skeleton (Cortex)

A supportive protein meshwork underlying the red blood cell plasma membrane composed of hexagonal spectrin filaments, ankyrin, and the transmembrane protein Band 3, providing mechanical strength, resilience, and deformability.

Glycophorin

A heavily glycosylated red blood cell integral membrane protein that imparts a strong negative charge to the cell surface, preventing red blood cells from adhering to each other or to endothelial walls.

Passive Transport (Facilitated Diffusion)

The movement of a solute down its concentration or electrochemical gradient across a membrane through channels or carriers without the expenditure of metabolic energy.

Active Transport

The energy-requiring transport of a substance across a biological membrane against (or up) its concentration or electrochemical gradient.

Glucose Transporter (GLUT)

A passive carrier protein that binds two glucose molecules on one side of the membrane, undergoes a conformational change, and releases them on the opposite side down their concentration gradient.

$\text{Na}^+/\text{Glucose}$ Symport

A secondary active transport carrier on the apical/luminal surface of intestinal epithelial cells that couples the downhill influx of $\text{Na}^+$ to the uphill movement of glucose into the cytoplasm.

$\text{Na}^+/\text{K}^+$ ATPase Pump

An active transport pump that alternates between an unphosphorylated conformation (E1) binding three cytosolic $\text{Na}^+$ ions and a phosphorylated conformation (E2) binding two extracellular $\text{K}^+$ ions, using ATP hydrolysis to maintain ion gradients.

Digitalis

A drug that produces stronger cardiac muscle contractions and a slower heart rate by inhibiting the $\text{Na}^+/\text{K}^+$ ATPase pump, dissipating the $\text{Na}^+$ gradient and consequently disabling the $\text{Na}^+-\text{Ca}^{2+}$ exchanger so that $\text{Ca}^{2+}$ is retained inside cardiac cells.

Ligand-Gated Ion Channel

An ion channel whose opening and closing is regulated by the binding of a specific chemical molecule (ligand).

Voltage-Gated Ion Channel

An ion channel that opens or closes in response to changes in the electrical potential across the plasma membrane.

$\text{K}^+$ Leak Channel

A non-gated ion channel whose opening and closing is not controlled by voltage, ligands, or mechanical stress, allowing passive efflux of $\text{K}^+$ to generate the negative resting membrane potential.

Resting Membrane Potential

The steady-state electrical potential difference across the plasma membrane of a non-excited cell, typically about $-70\,mV$, indicating that the cytosolic side is $70\,mV$ more negative than the extracellular environment.

Threshold Potential

The critical membrane depolarization level (typically $-55\,mV$ to $-40\,mV$) required to open voltage-gated $\text{Na}^+$ channels and initiate an action potential.

Refractory State of $\text{Na}^+$ Channels

An inactivated, closed configuration that voltage-gated $\text{Na}^+$ channels rapidly enter following depolarization, rendering them temporarily unresponsive to voltage changes and ensuring unidirectional nerve impulse propagation.

Saltatory Conduction

The rapid propagation of action potentials along a myelinated axon, where the impulse jumps between unmyelinated nodes of Ranvier enriched in voltage-gated $\text{Na}^+$ channels, increasing conduction velocity approximately $20\text{-fold}$.

Signal Sequence (Signal Peptide)

A short sequence of amino acids within a newly translated protein that serves as a molecular address tag targeting the protein to a specific cellular compartment or organelle.

Nuclear Localization Signal (NLS)

An internal amino acid sequence tag on a fully folded cytosolic protein that directs its binding to nuclear import receptors for transport through the nuclear pore complex.

Signal Recognition Particle (SRP)

A cytosolic GTP-binding ribonucleoprotein complex that binds the exposed hydrophobic signal sequence of a nascent polypeptide on a ribosome, halting elongation until docked to the SRP receptor at the RER membrane.

Translocon

A protein-conducting aqueous channel embedded in the RER membrane that opens upon SRP displacement, allowing the nascent polypeptide to enter the ER lumen.

Signal Peptidase

An ER luminal enzyme that removes the N-terminal signal sequence from a nascent polypeptide during co-translational translocation, producing a new N-terminal end.

KDEL Sequence

A four-amino-acid retrieval signal (Lys-Asp-Glu-Leu) present at the C-terminus of resident soluble ER proteins that binds KDEL receptors in the Golgi to package the proteins into COPI retrograde transport vesicles.

Unfolded Protein Response (UPR)

A cellular protective stress pathway triggered by accumulated unfolded proteins in the ER lumen, leading to transcriptional upregulation of chaperone genes and global translational attenuation.

Mannose-6-Phosphate (M6P)

A carbohydrate modification covalently added to lysosomal enzymes in the cis-Golgi that is recognized by M6P receptors in the trans-Golgi network to sort and package enzymes into lysosome-bound vesicles.

I-Cell Disease (Mucolipidosis II)

A genetic disorder resulting from a deficiency in the cis-Golgi enzyme that attaches phosphate to mannose on lysosomal enzymes, causing the enzymes to fail sorting to lysosomes and be secreted outside the cell instead.

Start-Transfer Sequence

An internal hydrophobic signal peptide that is not cleaved by signal peptidase, remaining embedded within the lipid bilayer to initiate translocation of downstream polypeptide regions.

Stop-Transfer Sequence

A hydrophobic amino acid sequence within a translocating polypeptide that halts transfer through the translocon and directs lateral integration of the segment into the membrane as an anchor.