Cell Biology: Cell Membranes, Membrane Transport, Endocytosis, Junctions, and the Nucleus

Comprehensive practice flashcards covering cellular membrane ultrastructure, transport kinetics, ion pumps and channels, endocytosis mechanisms, intercellular junctions, and nuclear morphology.

In freeze-fracture electron microscopy of cell membranes, what do the terms "P-face" and "E-face" designate?

The P-face (protoplasmic face) corresponds to the half of the split lipid bilayer facing the cytoplasm (inner leaflet), while the E-face (exoplasmic face) corresponds to the half facing the extracellular space (outer leaflet).

Which landmark experiment verified the lateral mobility of proteins across the plasma membrane?

The fusion of a human cell and a mouse cell to create a heterokaryon, after which fluorescently labeled surface membrane proteins rapidly intermixed throughout the hybrid membrane via lateral diffusion.

What are four primary physiological functions of the glycocalyx?

1) Promotes cell adhesion; 2) Defines cellular identity through terminal glucides on glycoproteins and glycolipids that act as cell-type antigens; 3) Participates in immunity and serves as receptor carbohydrate moieties for chemical signals and hormones; 4) Stores extracellular cations, specifically binding $Ca^{2+}$ via sialic acid.

What are the four classifications of endogenous chemical mediators based on signaling distance and transport route?

1) Neurotransmitters (diffuse across synaptic clefts); 2) Endocrine mediators (hormones transported via the bloodstream over long distances); 3) Paracrine mediators (local regulators diffusing through interstitial fluid to target adjacent cells); 4) Autocrine mediators (act on receptors of the same cell that secreted them).

What structural motif characterizes calmodulin, and how many calcium ions are required for its full activation?

Calmodulin is an $\alpha$-helix-loop-helix protein containing four EF-hand motifs (EF1 through EF4); binding of $4$ $Ca^{2+}$ ions triggers a conformational change that enables it to bind and activate target enzymes.

Which scientists received the 2012 Nobel Prize in Chemistry for their structural and functional discoveries regarding G-protein-coupled receptors (GPCRs)?

Robert Lefkowitz and Brian Kobilka.

What is the pathogenic mechanism by which Vibrio cholerae enterotoxin induces profuse secretory diarrhea?

Cholera toxin binds to ganglioside GM1 receptors on small intestine enterocytes and enters the cytosol, where it constitutively activates adenylyl cyclase; the resulting rise in cyclic AMP ($\text{cAMP}$) hyper-activates protein kinase A, causing mass efflux of $Cl^-$ into the intestinal lumen, accompanied by $Na^+$ and $H_2O$, leading to rapid dehydration.

How do the ionophores gramicidin and valinomycin differ in their physical mechanism of transporting cations?

Gramicidin forms a fixed transmembrane channel that permits the rapid passage of $Na^+$, whereas valinomycin functions as a mobile carrier ("navette") that chelates a hydrated $K^+$ ion, diffuses across the hydrophobic core of the bilayer, and unloads it on the opposite side.

What transport role does the erythrocyte membrane Band 3 protein fulfill?

Band 3 functions as an electroneutral anion exchanger facilitating the facilitated diffusion (antiport) of bicarbonate ($HCO_3^-$) and chloride ($Cl^-$) across the red blood cell membrane.

What four distinct mechanisms trigger the gating of membrane ion channels?

1) Membrane potential shifts (voltage-gated); 2) Extracellular ligand binding (extracellular ligand-gated); 3) Intracellular messenger binding (intracellular ligand-gated); 4) Mechanical tension transmitted via cytoskeletal links (mechanically gated).

What are the four primary structural and functional classes of ATP-driven pumps?

1) P-type pumps (phosphorylate their own polypeptide chains to pump ions such as $H^+$, $K^+$, $Na^+$, or $Ca^{2+}$); 2) ABC transporters (hydrolyze ATP to export or import small molecules); 3) V-type proton pumps (hydrolyze ATP to acidify organellar lumens by pumping $H^+$); 4) F-type ATP synthases (utilize proton gradients in mitochondria and chloroplasts to run in reverse and synthesize ATP).

What is the transport stoichiometry of the $Na^+/K^+$ ATPase per ATP molecule hydrolyzed, and which specific glycoside inhibits it?

It exports $3$ $Na^+$ ions and imports $2$ $K^+$ ions per ATP hydrolyzed; its activity is selectively inhibited by ouabain, which binds to its extracellular $K^+$-binding site.

What is the domain organization of the CFTR ABC transporter, and what mutation most commonly causes cystic fibrosis?

CFTR consists of two transmembrane domains (TMD1 and TMD2), two nucleotide-binding domains (NBD1 and NBD2), and a regulatory domain; the most frequent disease-causing mutation is a deletion of a single phenylalanine residue ($\Delta F508$).

How does the secondary active $Na^+/\text{glucose}$ symport system transport glucose against its concentration gradient?

It couples the downhill entry of $Na^+$ along its steep electrochemical gradient (from $[Na^+]_{out} = 0.14\,\text{M}$ to $[Na^+]_{in} = 0.014\,\text{M}$ established by the $Na^+/K^+$ pump) to drive the uphill import of glucose molecules into the cytosol.

Which macromolecular gatekeepers manage protein import into the nucleus and the mitochondrial matrix?

Nuclear pore complexes mediate nuclear import when importins bind Nuclear Localization Signals (NLS) on cargo proteins; TOM (Translocase of the Outer Membrane) and TIM (Translocase of the Inner Membrane) complexes transport precursor proteins into the mitochondria.

Which four principal leukocyte types carry out phagocytosis in humans?

Dendritic cells, macrophages (activated monocytes), eosinophils, and neutrophils.

What causes senescent erythrocytes to undergo phagocytosis in the spleen after their approximately $90\,\text{day}$ lifespan?

Aging red blood cells develop chemical surface modifications that present neoantigens, which are recognized as non-self by splenic macrophages to trigger targeted phagocytosis.

What signaling cascade controls directional actin polymerization in neutrophils responding to a chemoattractant gradient?

Binding to chemoattractant GPCRs liberates $G\beta\gamma$ subunits to activate $PI(3)K\gamma$, converting $PtdIns(4,5)P_2$ into $PtdIns(3,4,5)P_3$; this recruits pleckstrin homology (PH) domain proteins that trigger small GTPases to execute localized cytoskeletal reorganization.

What are the four sequential steps through which neutrophils extravasate into inflamed tissue?

1) Activation (by mediators such as histamine and thrombin); 2) Rolling (tethering mediated by L-selectin and endothelial cell receptors); 3) Adhesion (firm attachment to the endothelium); 4) Transmigration (diapedesis driven by chemoattractants including leukotriene D4, C5a, and N-formyl methionine).

How does antibody opsonization accelerate pathogen engulfment by macrophages?

Opsonizing antibodies (such as $IgG$) bind surface antigens on the pathogen, leaving their constant Fc domains projecting outward; macrophage Fc receptors recognize and bind these Fc regions, prompting actin-mediated pseudopod extension and phagocytic cup formation.

What enzymatic defect underlies Chronic Granulomatous Disease (CGD), and what is its clinical consequence?

A genetic defect in one of the subunits of the NADPH oxidase complex ($p22$, $p47$, $p40$, $gp91$, $p67$, or Rac) eliminates the oxidative burst and production of reactive oxygen species (ROS), leaving phagocytes unable to kill ingested microbes and leading to recurrent infections and granuloma formation.

How can pinocytic vesicles and phagosomes be distinguished from one another under transmission electron microscopy (TEM)?

Pinocytic coated pits and early endosomes are coated by clathrin and exhibit electron-lucent (transparent or white) luminal content, whereas phagosomes are large vacuoles displaying dense, electron-opaque (black) content.

What is the structural subunit composition of a clathrin triskelion?

A clathrin triskelion consists of $3$ clathrin heavy chains (each around $190\,\text{kDa}$) and $3$ clathrin light chains (each around $30\text{--}40\,\text{kDa}$) radiating from a central trimerization vertex to assemble geodesic polyhedral cages.

Which four specific protein subunits assemble to form the AP-2 adaptor complex in clathrin-mediated endocytosis?

$\alpha$-adaptin, $\beta 2$-adaptin, $\mu 2$-chain, and $\sigma 2$-chain.

What is the diameter and biochemical architecture of a low-density lipoprotein (LDL) particle?

LDL particles measure $225\text{--}275\,\text{\AA}$ in diameter, containing an amphipathic shell of a single apolipoprotein B-100 (apo B-100) molecule and a phospholipid monolayer interspersed with unesterified cholesterol, enclosing a hydrophobic core of cholesterol esters and triglycerides.

Who discovered transcytosis, and what is its classic physiological demonstration in neonates?

George Palade discovered transcytosis in 1953; a classic demonstration is the receptor-mediated endocytosis and basolateral exocytosis of maternal milk antibodies across neonatal gut epithelial cells into the infant bloodstream.

What molecular components constitute the transmembrane core and cytoplasmic linkage of zonula adherens (belt desmosomes)?

Transmembrane adhesion is mediated by $Ca^{2+}$-dependent homophilic binding of E-cadherin molecules, which are linked to actin microfilaments via an intracellular plaque containing catenins, vinculin, and $\alpha$-actinin.

What cadherins and cytoskeletal filaments form the structural basis of macula adherens (spot desmosomes)?

Transmembrane adhesion is mediated by the specialized non-classical cadherins desmoglein and desmocollin, which anchor into an intracellular attachment plaque connected to intermediate filaments (keratin tonofilaments).

What is the quaternary structure, channel diameter, and intercellular gap width of a gap junction?

A gap junction consists of opposing hexameric hemichannels (connexons) made of $6$ connexin subunits each, narrowing the intercellular space to $2\text{--}3.5\,\text{nm}$ and forming a continuous hydrophilic pore approximately $1.5\,\text{nm}$ in diameter that permits direct transfer of ions and small molecules under $1\text{--}1.5\,\text{kDa}$.

What cellular components sediment across fractions 1 through 4 during differential centrifugation of a tissue homogenate?

Fraction 1 yields nuclei; Fraction 2 yields mitochondria; Fraction 3 yields the Golgi apparatus; Fraction 4 yields microsomes (vesicles derived from the fragmented endoplasmic reticulum).

How does nuclear position vary between glandular acinar cells and mature white adipocytes?

In glandular acinar cells, nuclei are situated at the basal pole of the cell beneath apical secretory granules, whereas in mature adipocytes, the nucleus is compressed into an eccentric, peripheral position against the plasmalemma by a central lipid droplet.

What are the three principal types of lipid and protein movements that occur within the fluid mosaic lipid bilayer of cell membranes?

1) Basculement or flip-flop (transverse diffusion from one leaflet to another, which is very slow and rare); 2) Rotation around their own longitudinal axis; 3) Translation or lateral diffusion within the same leaflet (which is rapid).

What fundamental property of membrane proteins was demonstrated by the fusion of a human cell and a mouse cell to create a heterokaryon?

It demonstrated that intrinsic membrane proteins undergo lateral diffusion, allowing the proteins from both species to mix throughout the continuous hybrid membrane.

In freeze-fracture electron microscopy of cell membranes, what do the P-face and E-face designate?

The P-face (protoplasmic face) represents the interior fracture surface of the inner membrane leaflet facing the cytosol, while the E-face (extoplasmic face) represents the interior fracture surface of the outer membrane leaflet facing the extracellular space.

What are the four primary physiological functions of the cell glycocalyx?

1) Participating in cell adhesion; 2) Ensuring cell identity, where terminal glucides on glycoproteins and glycolipids serve as cell-type-specific antigens; 3) Serving in immunity and receptor signaling for hormones and chemical messengers; 4) Storing cations like $Ca^{2+}$ bound to sialic acid residues.

What are the three distinct structural domains of a transmembrane cell-surface receptor?

1) Ligand-binding domain located on the external surface of the cell; 2) Transmembrane portion spanning the hydrophobic lipid bilayer; 3) Signal transduction domain exposed to the cytoplasm.

What is the pharmacological mechanism of action of the antidepressant Prozac (fluoxetine) at the neuronal synapse?

Prozac selectively blocks the reabsorption (reuptake) of serotonin by the presynaptic neuron, prolonging serotonin presence and receptor stimulation in the synaptic cleft.

What is the molecular structure of calmodulin, and how is its target-binding activity triggered?

Calmodulin is an $\alpha$-helix-loop-helix protein with four EF-hand domains (EF1 to EF4); binding of four $Ca^{2+}$ ions induces a conformational shift that allows calmodulin to bind and activate its target proteins.

Which two scientists were awarded the 2012 Nobel Prize in Chemistry for their studies concerning G-protein-coupled receptors (GPCRs)?

Robert Lefkowitz and Brian Kobilka.

By what cellular mechanism does the Vibrio cholerae enterotoxin produce life-threatening watery diarrhea?

The cholera toxin binds ganglioside receptors on intestinal epithelial cells, allowing subunit A to permanently activate adenylyl cyclase, causing elevated $\text{cAMP}$ levels that drive massive secretion of $Cl^-$ and $Na^+$ into the lumen, followed osmotically by water ($H_2O$).

How is a solute's oil/water partition coefficient related to its permeability constant across the plasma membrane?

There is a direct positive correlation: solutes with a higher oil/water partition coefficient (greater lipid solubility) exhibit a higher permeability constant ($K$) and traverse the bilayer faster by simple diffusion.

How do the transport mechanisms of the ionophores gramicidin and valinomycin differ?

Gramicidin is a channel-forming ionophore that forms a fixed pore allowing $Na^+$ diffusion, whereas valinomycin is a mobile carrier ('shuttle' or navette) that encapsulates $K^+$ in a hydrophobic shell and diffuses across the lipid bilayer.

What is the transport function and domain architecture of the erythrocyte Band 3 protein?

Band 3 is an anion exchanger mediating the electroneutral facilitated exchange of $HCO_3^-$ and $Cl^-$; it consists of a $40\,\text{kDa}$ cytoplasmic domain (carrying the N-terminus) and membrane-spanning domains of $17\,\text{kDa}$ and $38\,\text{kDa}$.

How does Aquaporin 1 (AQP1) achieve selective rapid water transport without dissipating the membrane proton gradient?

AQP1 forms a narrow water channel with internal positive charges that electrostatically repel protons ($H^+$ and hydronium ions) while permitting neutral $H_2O$ molecules to pass in single file.

What are the four major classes of ATP-driven transport pumps depicted across biological membranes?

1) P-type pumps (autophosphorylated during ion transport); 2) ABC transporters (hydrolyze ATP to transport small molecules); 3) V-type proton pumps (transport $H^+$ into organelles); 4) F-type ATP synthases (normally operate in reverse to synthesize ATP using a proton gradient).

What is the stoichiometry of the $Na^+/K^+$ ATPase per molecule of ATP hydrolyzed, and what specific toxin inhibits it?

The pump transports $3\,Na^+$ out of the cell and $2\,K^+$ into the cell per molecule of ATP hydrolyzed; its activity is specifically inhibited by ouabain.

What type of transporter is CFTR, and what ion does it conduct?

CFTR (Cystic Fibrosis Transmembrane Conductance Regulator) belongs to the ABC transporter family and functions as an ATP-regulated channel conducting chloride ($Cl^-$) ions.

How does the intestinal $Na^+$/glucose symporter transport glucose into cells against its concentration gradient?

It utilizes secondary active transport (coupled transport), driving glucose influx against its gradient by pairing it with the thermodynamically favorable downhill influx of $Na^+$ maintained by the primary active $Na^+/K^+$ pump.

How does the light-driven bacteriorhodopsin pump generate an electrochemical proton gradient?

Upon absorption of light, its retinal prosthetic group undergoes a conformational change that drives the directional translocation of protons ($H^+$) from the cytosol across the membrane to the extracellular space.

What are the names of the protein translocation complexes located in the outer and inner mitochondrial membranes?

TOM (Translocase of the Outer Membrane) and TIM (Translocase of the Inner Membrane).

What are five key cellular roles of exocytosis?

1) Evacuation of cellular secretion products (hormones, enzymes); 2) Evacuation of neurotransmitters into the synaptic cleft; 3) Release of melanin granules from skin melanocytes; 4) Elimination of undigested residues from lysosomes (cellular defecation); 5) Release of lysosomal enzymes physiologically (in osteoclasts) or pathologically (in cellular regurgitation and shock).

Which four types of immune cells are classified as professional phagocytes?

Neutrophils, macrophages (activated monocytes), eosinophils, and dendritic cells.

What intracellular lipid-signaling pathway links chemoattractant receptor activation to neutrophil cytoskeletal reorganization during chemotaxis?

Chemoattractant GPCR activation frees $G\beta\gamma$ subunits, which activate $PI(3)K\gamma$ to convert $PtdIns(4,5)P_2$ to $PtdIns(3,4,5)P_3$; $PtdIns(3,4,5)P_3$ recruits PH domain-containing proteins that activate small GTPases to regulate actin remodeling.

What are the four sequential steps of leukocyte extravasation into inflamed tissue?

1) Activation (induced by factors like histamine and thrombin); 2) Rolling along the endothelium (mediated by L-selectin); 3) Firm adhesion (integrin binding to endothelial receptors); 4) Diapedesis / Transmigration through the blood vessel wall toward chemoattractants (leukotriene D4, C5a, N-formyl methionine).

What is opsonization and how does it enhance pathogen recognition by macrophages?

Opsonization is the binding of antibodies or complement fragments to pathogen surface antigens, allowing the pathogen to be targeted and engulfed via macrophage Fc receptors that bind the antibody heavy-chain constant regions.

What biochemical defect causes Chronic Granulomatous Disease (CGD)?

A genetic defect in the NADPH oxidase complex (responsible for the respiratory/oxidative burst), preventing phagocytes from producing reactive oxygen species ($\text{ROS}$: $O_2^-\rightarrow H_2O_2\rightarrow HOCl$) to kill engulfed microbes, leading to persistent intracellular survival and granuloma formation.

What is the subunit composition and structural organization of a clathrin triskelion?

A clathrin triskelion is composed of three heavy chains ($\approx 190\,\text{kDa}$ each) and three light chains ($\approx 30\text{--}35\,\text{kDa}$ each) assembled at a central trimerization domain, radiating outward as three legs that self-assemble into a geodesic polyhedral cage.

What four polypeptide subunits compose the AP-2 adaptor complex involved in clathrin-coated vesicle formation?

$\alpha$-adaptin, $\beta_2$-adaptin, $\mu_2$-chain, and $\sigma_2$-chain.

What are the primary structural components and physical dimensions of a low-density lipoprotein (LDL) particle?

With a diameter of $225\text{--}275\,\text{Å}$, an LDL particle consists of a hydrophobic core of cholesteryl esters and triglycerides, surrounded by a monolayer of phospholipids and unesterified cholesterol embedded with a single Apolipoprotein B-100 (apo B-100) molecule.

What is the primary cellular defect responsible for familial hypercholesterolemia and premature atherosclerosis?

A genetic deficiency or defect in cell-surface LDL receptors, which prevents receptor-mediated endocytosis of LDL cholesterol from the blood, causing elevated serum LDL levels and arterial plaque formation.

What is transcytosis, and what classic neonatal transport process was identified by George Palade in 1953?

Transcytosis is receptor-mediated vesicular transport of macromolecules across a cell from one membrane surface to the opposite membrane surface; Palade documented the transfer of maternal antibodies from breast milk across the neonatal intestinal epithelium into the bloodstream.

What transmembrane adhesion molecules and intracellular anchor proteins form the zonula adherens (belt desmosome)?

Transmembrane E-cadherins (which require $Ca^{2+}$ to bind homophilically) anchor internally to actin microfilaments via catenins, vinculin, and $\alpha$-actinin, along with myosin and tropomyosin.

What specific cadherin proteins and cytoskeletal elements are found in spot desmosomes (maculae adherentes)?

They utilize the desmosomal cadherins desmoglein and desmocollin across the intercellular space, which link through intracellular attachment plaques to intermediate filaments (such as keratin tonofilaments).

What is the molecular architecture and pore size of a gap junction channel?

A gap junction channel is formed by the end-to-end alignment of two hemichannels (connexons), each formed by 6 connexin protein subunits; this forms a central hydrophilic channel of approximately $1.5\,\text{nm}$ across an intercellular gap of $2\text{--}3.5\,\text{nm}$.

What classes of molecules can pass through gap junction channels, and what molecular weight threshold governs their permeability?

Inorganic ions ($Na^+$, $K^+$, $Ca^{2+}$), second messengers (such as $\text{cAMP}$), small metabolites, and low-molecular-weight fluorescent dyes (such as fluorescein) can pass freely, with an exclusion limit generally below $1000\text{--}1500\,\text{Da}$.

In differential centrifugation of cell homogenates, which organelle fractions sediment in sequence from 1 to 4?

Fraction 1: Nuclei; Fraction 2: Mitochondria; Fraction 3: Golgi apparatus; Fraction 4: Microsomes (vesicles derived from endoplasmic reticulum fragments).

How do nuclear positions and shapes characteristic of acinar glandular cells, adipocytes, and neutrophils compare?

Acinar glandular cells have spherical nuclei located at the basal cell pole; mature adipocytes have flattened nuclei pushed to the cell periphery by a lipid droplet; and neutrophils possess multi-lobed (segmented) nuclei.