Cell Biology: Plasma Membrane, Transport, and Endomembrane System

Comprehensive practice flashcards covering cell biology topics including plasma membrane architecture, lipid fluidity, transport kinetics, primary and secondary active transport, rough and smooth endoplasmic reticulum functions, and Golgi processing.

Amphipathic Lipid

A lipid molecule possessing both a polar hydrophilic head and nonpolar hydrophobic fatty acid hydrocarbon tails, which orient facing each other in the lipid bilayer.

Phosphoglycerides

Glycerol ester-based phospholipids composed of a glycerol backbone, two fatty acid tails (saturated or unsaturated), a phosphate bridge, and an organic head residue.

Sphingolipids

Membrane phospholipids built upon an amino alcohol backbone (sphingosine) rather than glycerol, exemplified by sphingomyelin.

Cholesterol Fluidity Buffering

The dual regulatory action where cholesterol intercalates into the membrane to increase fluidity below the transition temperature ($T_m < 20^\circ\text{C}$) and rigidify the bilayer above $20^\circ\text{C}$ by restricting the movement of fatty acid polar heads.

Lipid Rafts

Dynamic, semisolid microdomains formed by the aggregation of cholesterol and sphingolipids that concentrate specific proteins to serve as cell signaling platforms.

Membrane Lipid Asymmetry

The unequal distribution of phospholipids across bilayers: the outer leaflet (E face) is enriched in phosphatidylcholine, sphingomyelin, and glycolipids, while the inner leaflet (P face) contains phosphatidylethanolamine, phosphatidylserine (negatively charged), and phosphatidylinositol (negatively charged).

Glycocalyx

A carbohydrate coat on the extracellular surface of the plasma membrane made of oligosaccharides covalently bound to proteins and lipids that provides mechanical protection, selective barrier function, adhesion, and cell recognition.

Hereditary Elliptocytosis

An erythrocyte membrane disorder caused by deficiencies in membrane skeletal proteins (such as spectrin and Protein 4.1), leading to impaired membrane deformability and anemic syndrome.

Simple Diffusion

The passive, non-carrier-mediated transport of small, lipid-soluble, or nonpolar molecules (such as $O_2$, $CO_2$, benzene, and $H_2O$) down their concentration gradient directly through intermolecular spaces of the bilayer.

Aquaporins

Tetrameric membrane water channels forming an hourglass-shaped pore lined with NPA tripeptide motifs that mediate rapid water reabsorption under the regulation of antidiuretic hormone (ADH).

Gated Ion Channels

Selective integral pore proteins exhibiting transitory, regulated opening driven by electrical membrane potential changes (voltage-gated), chemical binding (ligand-gated), or physical tension (mechanically gated).

GLUT4

An insulin-regulated glucose carrier protein found in adipocytes and skeletal muscle cells that mobilizes from intracellular vesicles to the cell surface to increase glucose uptake.

F-Class Proton Pumps

Reversible ATPases found in the bacterial plasma membrane, inner mitochondrial membrane, and thylakoid membranes that couple ATP hydrolysis to $H^+$ pumping or utilize a transmembrane proton gradient to synthesize ATP.

V-Class Proton Pumps

ATP-powered proton pumps located in vacuolar, endosomal, and lysosomal membranes that pump $H^+$ into lumens to maintain an acidic interior without forming a phosphorylated intermediate.

ABC Superfamily Transporters

A major class of ATP-binding cassette pumps that transport amino acids, peptides, lipids, bile salts, and lipophilic drugs, including flippases/floppases, MDR pumps, and CFTR.

P-Class Pumps

ATP-dependent ion pumps that undergo transient phosphorylation of an aspartate residue during transport, including the plasma membrane $Na^+/K^+$ pump and the sarcoplasmic reticulum $Ca^{2+}$ pump.

CFTR (Cystic Fibrosis Transmembrane Conductance Regulator)

An ABC superfamily chloride channel; mutations in this transporter cause cystic fibrosis, an inherited disease affecting $1/2500$ Caucasian newborns leading to dehydrated airway mucus.

Secondary Active Transport

The uphill movement of a solute (such as glucose via SGLT1 cotransport) coupled to the downhill electrochemical gradient of another ion (such as $Na^+$) previously established by primary ATPases.

Dictyosome

The functional morphological unit of the Golgi apparatus, consisting of a stack of parallel, flattened, interconnected cisternae surrounded by transport vesicles.

ERGIC (ER-Golgi Intermediate Compartment)

A dynamic tubulovesicular cluster structure located between the rough endoplasmic reticulum and the cis-Golgi network that mediates intermediate sorting and transport.

KDEL Sequence

A carboxyl-terminal tetrapeptide signal (Lys-Asp-Glu-Leu) responsible for the retrieval of soluble ER resident proteins from the ERGIC and cis-Golgi back to the ER lumen.

Mannose-6-Phosphate (M6P) Receptor Pathway

The lysosomal enzyme trafficking mechanism where acid hydrolase precursors receive an M6P tag in the cis-Golgi, bind M6P receptors in the trans-Golgi network, and are packaged into clathrin-coated vesicles destined for endosomes and lysosomes.

O-Glycosylation

The post-translational attachment of sugars one-by-one to the hydroxyl groups of serine or threonine residues, occurring primarily in the trans-Golgi cisternae.

Golgi Sulfation

A modification catalyzed by sulfotransferases in the trans-Golgi network (TGN) that transfers sulfate groups to tyrosine residues and glycosaminoglycans, increasing the negative charge density of extracellular proteoglycans.

Lowe Syndrome

A human disease caused by defective Golgi and endosomal trafficking that clinically affects the eyes, nervous system, and kidneys.

Signal Recognition Particle (SRP)

A cytosolic ribonucleoprotein that binds the hydrophobic ER signal sequence of a nascent polypeptide, pauses translation, and targets the ribosome-nascent chain complex to the SRP receptor on the rough ER membrane.

Translocon

The Sec61 protein channel complex in the rough ER membrane through which nascent polypeptides are translocated co-translationally into the lumen or integrated into the membrane.

Protein Disulfide Isomerase (PDI)

An ER luminal chaperone enzyme that catalyzes the oxidation, reduction, and isomerization of disulfide bridges ($-S-S-$) to ensure proper tertiary protein folding.

ERAD (ER-Associated Degradation)

A quality-control pathway wherein terminally misfolded ER proteins are retrotranslocated to the cytosol, deglycosylated, polyubiquitinated, and destroyed by 26S proteasomes.

Unfolded Protein Response (UPR)

A cellular adaptive pathway triggered by stress-induced accumulation of unfolded proteins in the ER lumen, sensed by IRE1, PERK, and ATF6 to upregulate chaperones like BiP and ERAD components.

Primary Hyperoxaluria

A metabolic disorder causing severe kidney stones in children, caused by a defective peroxisomal targeting signal on alanine:glyoxylate aminotransferase that mistakenly directs the enzyme to mitochondria.

Eukaryotic 80S Ribosome

The cytosolic protein synthesis complex composed of a 60S large subunit ($28S$, $5.8S$, $5S$ rRNAs and 50 proteins) and a 40S small subunit ($18S$ rRNA and 33 proteins).

ER Scramblase

An energy-independent translocase in the smooth ER membrane that randomly flips newly synthesized phospholipids from the cytosolic monolayer to the luminal monolayer to maintain symmetric bilayer growth.

Cytochrome P450 (CYP450)

A family of monooxygenase enzymes located on the smooth ER membrane of hepatocytes that catalyzes the oxidative detoxification of lipophilic drugs and endogenous toxins.

Microsomal Ethanol Oxidizing System (MEOS)

An inducible smooth ER enzymatic pathway in hepatocytes that metabolizes excess ethanol into acetaldehyde, generating reactive oxygen species (ROS) when cytochrome P450 systems become saturated.

Glucose-6-Phosphatase ($\text{G6Pase-}\alpha$)

A smooth ER luminal enzyme in liver cells that dephosphorylates glucose-6-phosphate to free glucose during glycogenolysis, allowing glucose export into circulation via GLUT2.

Sarcoplasmic Reticulum (SR)

A modified smooth endoplasmic reticulum in muscle fibers specialized for calcium sequestration via SERCA pumps and rapid release through ryanodine receptors to drive sarcomere contraction.

Chylomicrons

Large lipoprotein complexes synthesized in enterocyte smooth ER and Golgi that package dietary triglycerides, cholesterol, and apolipoproteins (such as apoB-48) for secretion into lymphatic lacteals.

Phagophore

An isolation membrane formed during autophagy initiation via activation of the ULK1 complex (inhibited by mTOR) and ATG/LC3 proteins, which sequesters damaged cytoplasmic components into an autophagosome.

COPII-Coated Vesicles

Cytoplasmic coat protein complexes that assemble at ER exit sites to transport secretory proteins and lipids in an anterograde direction from the rough ER to the ERGIC and cis-Golgi.

COPI-Coated Vesicles

Vesicle coat complexes that mediate retrograde transport from the Golgi apparatus and ERGIC back to the endoplasmic reticulum, retrieving escaped ER resident proteins.