Cell Biology: Lipids, Membranes, Transport, and Protein Sorting

Vocabulary flashcards covering lipid structure, membrane fluidity, transport proteins, photosystems, nuclear import/export, and ER protein translocation.

Triacylglycerol (Triglyceride)

A lipid molecule composed of a three-carbon glycerol molecule covalently bound to three fatty acid chains, serving as the primary form of energy storage.

Saturated Fat

A fat containing fatty acid chains without double bonds, allowing the chains to pack tightly together and exist as a solid at room temperature.

Unsaturated Fat

A fat containing fatty acid chains with one or more double bonds that create kinks, preventing close packing and keeping the fat liquid at room temperature.

Amphipathic Molecule

A molecule that possesses both hydrophobic (water-repelling) and hydrophilic (water-attracting) regions, such as phospholipids and sterols.

Phospholipid

An amphipathic lipid composed of a glycerol backbone attached to two fatty acid tails, a phosphate group, and a polar head group such as choline.

Phosphatidylserine

A membrane phospholipid carrying a net negative charge that is maintained on the cytosolic leaflet of the lipid bilayer by flippases.

Flippase

A membrane-bound enzyme that transfers specific phospholipids from one leaflet of the lipid bilayer to another to establish and maintain bilayer asymmetry.

Lateral Diffusion

The rapid unassisted movement of lipids and proteins side-to-side within the same leaflet of a membrane lipid bilayer.

Cholesterol

A rigid amphipathic sterol containing four linked carbon rings with multiple double bonds that modulates membrane fluidity by stiffening the bilayer structure.

Transmembrane Protein

An integral membrane protein that extends entirely through the lipid bilayer, typically crossing as one or more hydrophobic $\alpha$-helices or a $\beta$-barrel.

Porin

A transmembrane protein formed by a $\beta$-barrel structure that creates a large aqueous channel across outer membranes.

Tight Junction

A protein barrier in polarized epithelial cells that prevents the lateral diffusion of membrane lipids and proteins between apical and basolateral domains.

Passive Transport

The exergonic movement of a solute down its concentration or electrochemical gradient across a membrane without energy input.

Active Transport

The endergonic movement of a solute against its concentration or electrochemical gradient, requiring coupling to an exergonic energy source.

Electrogenic Pump

A transport protein that creates an electrical charge imbalance across a cell membrane during active transport.

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

An electrogenic enzyme pump that hydrolyzes ATP to export $3\,\text{Na}^+$ ions and import $2\,\text{K}^+$ ions per cycle, maintaining cellular polarization.

Symporter

A secondary active transporter that moves two different solutes across a membrane in the same direction, such as the $\text{Na}^+$/glucose symporter.

Selectivity Filter

A narrow region of an ion channel lined with carbonyl groups that strip hydration shells from specific ions to allow selective passage.

Gated Channel

An ion channel that undergoes conformational changes to open or close in response to voltage changes, mechanical stress, or ligand binding.

Rubisco

The enzyme ribulose-1,5-bisphosphate carboxylase that catalyzes carbon fixation by joining $\text{CO}_2$ to ribulose-1,5-bisphosphate in the chloroplast stroma.

Water-Splitting Enzyme

An enzyme complex in Photosystem II that oxidizes $\text{H}_2\text{O}$ to extract electrons, releasing $\text{O}_2$ gas and pumping $\text{H}^+$ into the thylakoid space.

Glyceraldehyde-3-Phosphate (G3P)

The three-carbon sugar carbohydrate synthesized and exported during the light-independent reactions of photosynthesis.

Cytosol

The protein-rich, aqueous fluid inside the cytoplasm that surrounds all membrane-enclosed organelles.

Nuclear Envelope

A double-membrane system enclosing the nucleus that is contiguous with the endoplasmic reticulum and supported by the nuclear lamina.

Nucleolus

A non-membrane-bound region within the nucleus where rRNA genes cluster and ribosomal subunits are assembled.

Nuclear Pore Complex (NPC)

A large structure composed of approximately 30 different proteins that regulates gated transport of folded macromolecules into and out of the nucleus.

Nuclear Localization Signal (NLS)

A surface sequence of 5 basic amino acids on prospective nuclear proteins that directs import through nuclear pores without being cleaved.

Ran GTPase

A small GTP-binding protein that cycles between GTP-bound and GDP-bound states to drive nucleocytoplasmic transport.

Ran-GAP

A cytosolic GTPase-activating protein that triggers Ran to hydrolyze its bound GTP into GDP, switching Ran to its off state.

Ran-GEF

A nuclear guanine nucleotide exchange factor that catalyzes the exchange of GDP for GTP on Ran, converting it to its active state.

Rough Endoplasmic Reticulum

A region of the endoplasmic reticulum studded with cytosolic ribosomes, serving as the major site of protein translation and translocation.

Smooth Endoplasmic Reticulum

A tubular region of the endoplasmic reticulum lacking ribosomes that is dedicated primarily to lipid synthesis.

ER Signal Sequence

A peptide motif containing 6 to 15 hydrophobic residues preceded by arginine or lysine near the N-terminus that targets a protein to the ER.

Signal Recognition Particle (SRP)

A cytosolic protein-RNA complex that binds an ER signal sequence on a nascent peptide, pauses translation, and targets the ribosome to the ER membrane.

Translocon

A protein-lined aqueous channel in the ER membrane through which unfolded polypeptide chains pass into the ER lumen.

Stop-Transfer Sequence

A hydrophobic $\alpha$-helical peptide segment that halts translocation through the translocon and embeds the polypeptide into the membrane bilayer.

N-Linked Glycosylation

The covalent attachment of a preformed oligosaccharide to the nitrogen atom of an asparagine side chain within an Asn-X-Ser/Thr motif in the ER lumen.

Unfolded Protein Response (UPR)

A cellular stress pathway activated by misfolded protein accumulation in the ER that expands ER size and increases chaperone protein synthesis.