Exam Terms: Model Definitions

Comprehensive vocabulary flashcards covering key cellular biology terms, membrane transport mechanisms, organelle functions, and signaling pathways from the Exam Terms lecture notes.

ABC transporter

A family of transmembrane pumps with highly conserved ATP-binding cassettes (two transmembrane subunits of 6 helices each and two cytosolic ATP-binding domains) found in plasma membranes that use ATP hydrolysis to perform primary active transport, importing nutrients in bacteria and exporting toxic substances or drugs in eukaryotes.

Acid lysosomal hydrolase

A degradative enzyme (comprising about 50 types such as proteases, lipases, phosphatases, and nucleases) localized in the lysosome lumen that operates exclusively at an acidic pH to digest macromolecules ingested via endocytosis, phagocytosis, or autophagy.

Adenylyl cyclase

A membrane-bound enzyme located on the cytosolic side of the plasma membrane that is activated by a G protein following GPCR activation to synthesize the second messenger cAMP from ATP.

Aquaporin

A family of 13 or more channel proteins localized in the plasma membrane that selectively permit rapid facilitated diffusion of water across biological membranes while remaining completely impermeable to ions.

ATP synthase

A reversible proton pump in the inner mitochondrial and thylakoid membranes consisting of a membrane-embedded $F_0$ rotor and a stationary $F_1$ head (stator) that synthesizes ATP from $ADP + P_i$ driven by an electrochemical $H^+$ gradient.

Autophagy

A cellular recycling process in the cytoplasm wherein Atg9 vesicles and Atg2 lipids generate a phagophore that engulfs cytosol or damaged organelles into a double-membrane autophagosome, which then fuses with a lysosome to form an autolysosome for enzymatic degradation.

cAMP (cyclic AMP)

A small cytosolic second messenger synthesized from ATP by adenylyl cyclase that activates protein kinase A (PKA) by binding to its regulatory subunits and releasing its active catalytic subunits.

Carrier

A transmembrane transport protein in cell membranes that binds specific solutes (such as sugars, amino acids, or nucleosides) and undergoes a conformational change to transfer them across the membrane via facilitated diffusion as a uniport, symport, or antiport.

COPII vesicle

A transport vesicle coated by the COPII complex assembled via the small GTPase Sar1 that buds from transitional ER exit sites to carry newly synthesized secretory and membrane proteins anterograde to the ERGIC and Golgi.

Cytosolic chaperone

A cytosolic protein that assists nascent polypeptides in proper folding, prevents aberrant aggregation, and maintains mitochondrial and peroxisomal precursor proteins in an unfolded state required for post-translational import.

ER stress / Unfolded Protein Response (UPR)

A stress response initiated in the ER membrane by sensors IRE1, PERK, and ATF6 (governed by BiP) upon accumulation of unfolded proteins, resulting in decreased general translation, increased chaperone expression, and retro-translocation of misfolded proteins for proteasomal degradation.

Fluid mosaic model

The structural membrane model proposed by Singer and Nicolson in 1972 describing membrane proteins floating as a dynamic mosaic within a fluid lipid bilayer, supported experimentally by cryofracture electron microscopy.

Glucose transporter (GLUT1)

A 12-transmembrane α-helix carrier protein in the plasma membrane that mediates the passive, facilitated diffusion of glucose down its concentration gradient via uniport transport.

GPI-anchored protein

A lipid-anchored protein whose C-terminus is covalently attached to glycosylphosphatidylinositol in the ER lumen, directing it exclusively to the extracellular leaflet of the plasma membrane, where it clusters in lipid rafts.

H⁺ pump (proton pump)

A transmembrane primary active transporter that couples ATP hydrolysis to the translocation of $H^+$ against an electrochemical gradient, such as the gastric $H^+/K^+$-ATPase or the lysosomal V-ATPase.

Ligand-gated channel

An ion channel located in the plasma membrane (such as postsynaptic membranes) that opens upon binding a specific chemical ligand, converting chemical stimuli into electrical signals via passive ion flux.

Lipid raft

A dynamic, thickened, and less fluid microdomain in the outer leaflet of the plasma membrane enriched in cholesterol, sphingolipids, GPI-anchored proteins, and receptors that coordinates cellular signaling and macropinocytosis.

Mannose-6-phosphate mark (M6P)

A phosphate tag added to mannose residues of N-linked glycans in the cis Golgi that serves as a sorting signal recognized by M6P receptors in the trans Golgi network to target acid hydrolases to lysosomes.

MAP kinase cascade

A sequential phosphorylation cascade consisting of serine/threonine kinases ($\text{Raf} \rightarrow \text{MEK} \rightarrow \text{ERK}$) acting downstream of RTKs and Ras that translocates signals from the cytosol into the nucleus to regulate cell proliferation, differentiation, and gene expression.

Membrane domain

A distinct, specialized region of the plasma membrane segregated by tight junctions (such as the apical and basolateral domains of an enterocyte) containing a characteristic protein set tailored for localized physiological functions.

Mitochondrial matrix

The internal compartment enclosed by the inner mitochondrial membrane containing Krebs cycle enzymes, ribosomes, and mtDNA, where pyruvate and fatty acid-derived acetyl-CoA are oxidized to $CO_2$ to generate NADH and $FADH_2$.

N-glycosylation (N-linked glycosylation)

A cotranslational modification initiated in the RER lumen where a preassembled oligosaccharide is transferred from dolichol to an asparagine (Asn) residue of a nascent polypeptide, followed by trimming and maturation in the Golgi.

Peripheral membrane protein

A protein temporarily associated with the extracellular or cytosolic surface of a biological membrane via non-covalent interactions with integral membrane proteins or lipid polar head groups, easily dissociated without disrupting the lipid bilayer.

Peroxisome

A single-membrane metabolic organelle devoid of its own DNA that imports nuclear-encoded cytosolic proteins to carry out oxidative breakdown of fatty acids, producing $H_2O_2$ which catalase decomposes to water.

Phagocytosis

A specialized form of endocytosis executed by professional phagocytes (macrophages and neutrophils) involving actin-driven pseudopodia that engulf large particulate matter or pathogens into an intracellular phagosome.

Phagolysosome

A digestive cytoplasmic hybrid organelle formed by the fusion of a phagosome with a lysosome, exposing internalized particles or microorganisms to acid hydrolases for complete degradation.

PKA (protein kinase A)

A cAMP-dependent serine/threonine kinase composed of two regulatory and two catalytic subunits that becomes activated when cAMP binds the regulatory subunits, releasing catalytic subunits to phosphorylate downstream targets.