Molecular Biology of the Cell: Comprehensive Exam Practice

Comprehensive set of 300 rigorous vocabulary flashcards covering the entire breadth of Molecular Biology of the Cell (Alberts 6th ed.) and the p53 review, designed for deep active recall and examination preparation.

Amphipathic lipid

A lipid molecule possessing both a hydrophilic (water-interacting) polar region and a hydrophobic (water-avoiding) nonpolar hydrocarbon region.

Glycerophospholipid

A major structural membrane lipid built on a glycerol backbone esterified to two fatty-acid tails, with its third carbon linked to a phosphate-containing head group.

Phosphatidylcholine (PC)

A major glycerophospholipid containing a choline head group that is commonly abundant in the outer (noncytosolic) leaflet of the plasma membrane.

Phosphatidylethanolamine (PE)

A glycerophospholipid bearing an ethanolamine head group that is preferentially enriched in the cytosolic leaflet of the plasma membrane.

Phosphatidylserine (PS)

A negatively charged glycerophospholipid containing serine that is normally restricted to the cytosolic leaflet of the plasma membrane.

Phosphatidylinositol (PI)

A minor membrane phospholipid whose inositol head group can be phosphorylated at specific positions to generate regulatory signalling and targeting lipids.

Sphingolipid

A class of membrane lipids built on a sphingosine backbone rather than glycerol, containing an attached fatty acid and a polar head group.

Sphingomyelin

A phospholipid built on sphingosine containing a fatty acid linked to an amino group and a phosphate-containing head group, prominent in animal plasma membranes.

Glycolipid

A lipid bearing one or more covalently linked carbohydrate groups, concentrated exclusively on the noncytosolic/extracellular leaflet for cell recognition and protection.

Cholesterol

A sterol with a rigid four-ring structure, a small polar hydroxyl group, and a short hydrocarbon tail that modulates membrane packing, fluidity, and permeability.

Saturated fatty-acid tail

A straight hydrocarbon chain lacking double bonds that can pack tightly with neighbours, reducing membrane fluidity.

Ununsaturated fatty-acid tail

A hydrocarbon chain containing one or more cis double bonds that introduce kinks, preventing tight packing and increasing membrane fluidity.

Micelle

A spherical lipid aggregate formed by a single layer of amphipathic molecules with a hydrophobic core and polar heads facing water, favoured by single-tailed lipids.

Lipid bilayer

A two-layered sheet of amphipathic lipids where hydrophobic tails face inward toward one another and polar heads face the opposing aqueous surroundings.

Liposome

A closed, spherical lipid bilayer vesicle enclosing an aqueous compartment, formed spontaneously when a bilayer sheet seals to eliminate exposed hydrophobic edges.

Lateral diffusion

The rapid sideways movement of lipid and protein molecules within the plane of their own membrane monolayer.

Transbilayer flip-flop

The spontaneous or enzyme-catalysed movement of a lipid molecule from one membrane leaflet to the opposite leaflet across the hydrophobic core.

Lipid raft

A dynamic, transient microdomain in the membrane enriched in cholesterol and sphingolipids that helps organise signalling complexes and transport machinery.

Lipid droplet

An organelle storing neutral lipids (triacylglycerols and cholesterol esters) delimited by a single phospholipid monolayer rather than a typical bilayer.

Flippase

An ATP-dependent, selective lipid-translocating enzyme that moves specific phospholipids from the noncytosolic leaflet to the cytosolic leaflet.

Floppase

An ATP-dependent transporter that selectively moves specific lipids from the cytosolic leaflet outward to the noncytosolic leaflet.

Scramblase

An enzyme that facilitates non-specific, bidirectional movement of phospholipids between leaflets, rapidly dissipating membrane asymmetry upon activation.

Simple diffusion

Passive, non-protein-mediated net movement of a solute down its concentration gradient directly across the hydrophobic core of the lipid bilayer.

Facilitated diffusion

Passive transport of a substance down its electrochemical gradient mediated by a transmembrane channel or carrier protein without metabolic energy input.

Osmosis

The net movement of water across a selectively permeable membrane toward the region with a higher effective solute concentration.

Primary active transport

Transport of a solute against its electrochemical gradient directly driven by an energy-yielding chemical reaction, most commonly ATP hydrolysis.

Secondary active transport

Transport of a solute against its electrochemical gradient powered by the downhill movement of another solute along a pre-existing electrochemical gradient.

Uniporter

A carrier protein that moves only one type of solute across a membrane down its electrochemical gradient or via coupled energy input.

Symporter

A coupled transporter that moves two or more different solutes simultaneously across a membrane in the same direction.

Antiporter

A coupled transporter that moves two or more different solutes across a membrane in opposite directions.

Alternating-access model

A transport mechanism in which carrier binding sites are alternately exposed to opposite sides of the membrane through conformational changes without opening a continuous pore.

P-type ATPase

A class of ATP-driven ion pumps that becomes transiently phosphorylated on a conserved aspartate residue during its transport cycle.

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

An electrogenic P-type pump that hydrolyses $1\,\text{ATP}$ to pump $3\,\text{Na}^+$ out of the cell and $2\,\text{K}^+$ into the cell.

$\text{Ca}^{2+}$ pump

An ATP-driven pump that clears cytosolic calcium by transporting $\text{Ca}^{2+}$ into the sarcoplasmic/endoplasmic reticulum lumen or outside the cell.

V-type ATPase

A multi-subunit proton pump that uses ATP hydrolysis to pump protons into intracellular organelles such as lysosomes and endosomes, acidifying their interiors.

ABC transporter

A transport protein containing ATP-binding cassette domains that hydrolyses ATP to pump varied substrates (including lipids, ions, and drugs) across membranes.

Ion channel

A transmembrane protein forming an aqueous pore that permits selected ions to diffuse rapidly down their electrochemical gradient without hydrolysing ATP.

Voltage-gated ion channel

An ion channel whose opening or closing probability is regulated by changes in the electrical potential difference across the membrane.

Ligand-gated ion channel

An ion channel that opens or closes in response to the specific binding of an extracellular or intracellular chemical messenger.

Mechanically gated ion channel

An ion channel that opens or closes in response to physical tension, deformation, or mechanical stress applied to the membrane.

Aquaporin

A channel protein that selectively facilitates rapid water movement in single file across biological membranes while completely excluding ions and protons.

Membrane potential

The electrical voltage difference across a biological membrane, measured as the electrical potential inside the cell relative to the outside.

Electrochemical gradient

The combined thermodynamic driving force acting on an ion across a membrane, composed of its concentration gradient and the electrical potential difference.

$\text{K}^+$ leak channel

An ion channel that randomly flickers between open and closed states, maintaining high resting permeability to $\text{K}^+$ and establishing resting membrane potential.

Cytosol

The aqueous gel-like compartment of the cytoplasm outside membrane-enclosed organelles, containing dissolved proteins, metabolites, and cytoskeletal structures.

Cytoplasm

The contents of a cell that are contained within its plasma membrane, excluding the nucleus in eukaryotic cells.

Endomembrane system

The interconnected network of eukaryotic organelles (including ER, Golgi, endosomes, and lysosomes) that communicate via transport vesicles.

Topologically equivalent compartments

Compartments or spaces that can communicate directly through vesicle budding and fusion without crossing a lipid bilayer, such as the ER lumen and the extracellular space.

Gated transport

The movement of macromolecules between the cytosol and the nucleus through nuclear pore complexes without crossing a membrane bilayer.

Transmembrane transport

The directed translocation of a protein across a membrane bilayer from one distinct compartment to another through a protein translocator channel.

Vesicular transport

The movement of proteins and lipids between organelles via membrane-enclosed transport vesicles that bud from a donor and fuse with a target membrane.

Signal sequence

A continuous stretch of amino acids on a newly synthesised protein that directs it to its correct subcellular destination or organelle.

Signal recognition particle (SRP)

A cytosolic ribonucleoprotein complex that binds the hydrophobic ER signal sequence as it emerges from the ribosome and temporarily arrests translation.

SRP receptor

An integral membrane protein on the cytosolic face of the ER that binds the SRP-ribosome complex, targeting it to the translocator.

Sec61 translocator

The core protein-conducting channel in the ER membrane through which nascent polypeptides pass into the lumen or integrate into the bilayer.

Cotranslational translocation

The import of a polypeptide into the endoplasmic reticulum that occurs simultaneously with ongoing protein synthesis by a membrane-bound ribosome.

ER-associated degradation (ERAD)

A quality-control pathway that exports terminally misfolded ER proteins into the cytosol for ubiquitination and degradation by the proteasome.

Nuclear pore complex (NPC)

A large protein assembly perforating the nuclear envelope that creates an aqueous channel regulating traffic between cytosol and nucleus.

Nucleoporins

The family of approximately $30$ distinct structural and channel proteins that assemble into the nuclear pore complex.

FG-repeat nucleoporins

Channel nucleoporins containing unstructured phenylalanine-glycine repeats that form a selective barrier within the central pore of the NPC.

Nuclear localisation signal (NLS)

A short sequence rich in positively charged amino acids (lysine and arginine) that tags a cytosolic protein for nuclear import.

Importin

A soluble transport receptor that recognises an NLS on cargo proteins and mediates their passage through the nuclear pore complex.

Exportin

A nuclear transport receptor that binds nuclear export signals and Ran-GTP in the nucleus to transport cargo out into the cytosol.

Ran GTPase

A small monomeric GTPase that provides directionality to nuclear transport through its steep compartmental gradient of Ran-GTP in the nucleus and Ran-GDP in the cytosol.

Coated pit

A specialised region of a donor membrane where cytosolic coat proteins assemble to concentrate cargo and mechanically bend the membrane into a bud.

Clathrin

A coat protein that forms a polyhedral lattice around budding transport vesicles during endocytosis and transport from the trans-Golgi network to endosomes.

Clathrin triskelion

The basic structural unit of clathrin coats, composed of three clathrin heavy chains and three light chains radiating from a central vertex.

Adaptor protein 2 (AP2)

A multi-subunit adaptor complex that links cargo receptors to clathrin and binds $\text{PI}(4,5)\text{P}_2$ at the plasma membrane.

Dynamin

A large GTPase that forms a helical collar around the neck of a budding clathrin-coated vesicle and mediates membrane fission upon GTP hydrolysis.

BAR-domain protein

A crescent-shaped protein dimer that binds to and induces or stabilizes curvature in cellular membranes through electrostatic interactions.

COPI coat

A protein coat that mediates retrograde transport of vesicles within the Golgi apparatus and retrieval from the Golgi back to the ER.

COPII coat

A protein coat that mediates anterograde export of newly synthesised proteins and lipids from the endoplasmic reticulum to the Golgi apparatus.

Phosphoinositide

A phosphorylated derivative of phosphatidylinositol whose distinct phosphorylation pattern acts as a compartment-specific marker to recruit regulatory trafficking proteins.

$$\text{PI}(4,5)\text{P}_2$$

A phosphoinositide species enriched at the plasma membrane that recruits endocytic adaptor proteins including AP2 during vesicle formation.

$$\text{PI}3\text{P}$$

A phosphoinositide characteristic of early endosomal membranes that recruits proteins containing specific lipid-binding domains required for endosomal function.

Rab GTPase

A family of monomeric GTPases that act as molecular switches on vesicle and target membranes, coordinating membrane identity and tethering specificity.

Rab effector

A downstream protein or tethering complex recruited by active Rab-GTP to capture transport vesicles and position them near target membranes.

SNARE protein

A family of membrane-associated proteins that mediate vesicle fusion by forming a stable four-helix bundle that draws opposing bilayers together.

v-SNARE

A SNARE protein incorporated into the transport vesicle membrane during budding that interacts specifically with t-SNAREs on target membranes.

t-SNARE

A SNARE protein or complex localized on the target membrane that pairs with a complementary v-SNARE to catalyze membrane fusion.

trans-SNARE complex

A stable four-helix coiled-coil bundle formed between opposing v-SNAREs and t-SNAREs that pulls membranes into close proximity to drive fusion.

NSF

An ATP-dependent chaperone that uses ATP hydrolysis to disassemble stable four-helix SNARE complexes following membrane fusion, recycling the SNAREs.

Constitutive exocytosis

The continuous, unregulated fusion of transport vesicles with the plasma membrane, supplying newly synthesized lipids, membrane proteins, and secreted factors.

Regulated exocytosis

The signal-triggered fusion of specialized secretory vesicles with the plasma membrane, typically activated by a rise in cytosolic $\text{Ca}^{2+}$.

Cytoskeleton

A dynamic network of protein filaments in eukaryotic cells comprising actin filaments, microtubules, and intermediate filaments that provides mechanical support and drives cell movement.

Actin filament (F-actin)

A two-stranded helical polymer of globular actin monomers with a diameter of approximately $7\,\text{nm}$, possessing structural polarity and dynamic turnover.

G-actin

The globular monomeric form of actin that binds ATP and polymerizes into filamentous F-actin.

Actin plus end

The faster-growing barbed end of an actin filament that exhibits higher rates of subunit addition and dissociation.

Actin minus end

The slower-growing pointed end of an actin filament with a higher critical concentration for monomer addition.

Treadmilling (actin)

A steady-state dynamic behaviour where actin subunits assemble at the plus end and dissociate from the minus end at identical rates, keeping filament length constant.

Formin

An actin-nucleating protein that remains attached to the growing barbed end and promotes the rapid assembly of long, unbranched actin filaments.

Arp2/3 complex

A protein complex that binds to the side of an existing actin filament and nucleates a daughter filament at a characteristic $70^\circ$ angle, creating branched networks.

Profilin

An actin-binding protein that binds G-actin, promotes ADP-to-ATP exchange, and channels monomers selectively onto the growing plus end.

Thymosin

An actin-sequestering protein that binds ATP-G-actin and prevents it from polymerizing onto filament ends, maintaining a monomer reservoir.

Cofilin

An actin-binding protein that preferentially binds ADP-actin filaments, introduces torsional strain, and accelerates filament severing and subunit disassembly.

Tropomyosin

A rod-shaped protein that binds along the grooves of actin filaments, stabilizing them and regulating the access of other actin-binding proteins.

Gelsolin

A calcium-activated actin-severing protein that fragments actin filaments and caps the newly created plus end to rapidly disassemble actin networks.

Cell cortex

A dense, specialised actin-rich protein layer located directly beneath the plasma membrane that maintains cell shape and generates surface tension.

Microvilli

Finger-like plasma-membrane extensions whose structural cores consist of bundled parallel actin filaments cross-linked by fimbrin and villin.

Lamellipodium

A broad, sheet-like cellular protrusion containing a dense, branched actin network nucleated by Arp2/3 at the leading edge of a crawling cell.