Topic 2d: Lipids and Membranes

Comprehensive practice vocabulary flashcards covering Topic 2d (Lipids and Membranes) from BIOL 225, including lipid structures, membrane models, thermodynamics, fluidity, microscopy, transport mechanisms, and optogenetics.

Amphipathic (Amphiphilic)

Molecules that possess both polar (hydrophilic) and nonpolar (hydrophobic) domains.

Water Dipole

The asymmetric charge distribution in an $H_2O$ molecule produced by the high electronegativity of oxygen and a bent bond angle of $104.5^\circ$.

Hydrogen Bonding (in Water)

Non-covalent electrostatic attractions between the partial negative charge on an electronegative oxygen atom and the partial positive charge on a hydrogen atom of an adjacent water molecule.

Flickering Clusters

Short-lived, highly dynamic networks of hydrogen bonds that continuously form and break among liquid water molecules in the bulk aqueous phase.

Specific Heat and Heat of Vaporization of Water

Unique thermal properties resulting from water's extensive hydrogen-bonding network that require high energy inputs to raise temperature or convert liquid water into vapor.

Hydration Shell

An organized sphere of water molecules oriented electrostatically around a dissolved ion, with partial negative oxygens facing cations such as $Na^+$ and partial positive hydrogens facing anions such as $Cl^-$.

Hydrophobic Effect

The thermodynamic tendency of nonpolar molecules to aggregate in aqueous solution to minimize hydrophobic interactions and maximize the overall entropy of surrounding water.

Clathrate Cage

A highly ordered, cage-like structure of hydrogen-bonded water molecules forced to form around nonpolar solutes or hydrophobic alkyl chains, resulting in an unfavorable decrease in entropy.

Lipids

A diverse group of generally hydrophobic, water-insoluble organic molecules that are substantially more soluble in nonpolar solvents (e.g., benzene, toluene) than in polar solvents such as water.

Adipose Tissue

Fat tissue specialized for energy storage, cushioning internal organs, and providing thermal insulation by decreasing thermal conductivity.

Anandamide

A lipid signalling molecule that functions as the endogenous ligand for cannabinoid receptors.

Fatty Acid

A compound consisting of a hydrophilic carboxyl group ($-COOH$) and an unbranched hydrophobic hydrocarbon chain that behaves as an acid upon losing an $H^+$ ion.

Saturated vs. Unsaturated Fatty Acids

Fatty acids lacking double bonds along their hydrocarbon chains versus those containing one or more double bonds that create kinks in the chain.

Monounsaturated vs. Polyunsaturated Fatty Acids

Fatty acids possessing exactly one carbon-carbon double bond along their hydrocarbon tail versus those containing two or more double bonds.

Myristic Acid

A $14$-carbon saturated fatty acid denoted as $14:0$, whose ionized form is named myristate.

Palmitic Acid

A $16$-carbon saturated fatty acid denoted as $16:0$, whose ionized form is named palmitate.

Stearic Acid

An $18$-carbon saturated fatty acid denoted as $18:0$, whose ionized form is named stearate.

Oleic Acid

An $18$-carbon monounsaturated fatty acid containing one cis double bond at carbon $9$, denoted as $18:1^{\text{cis-}\Delta 9}$, with an ionized form called oleate.

Linoleic Acid

An $18$-carbon polyunsaturated fatty acid containing two cis double bonds at carbons $9$ and $12$, denoted as $18:2^{\text{cis,cis-}\Delta 9, \Delta 12}$, with an ionized form called linoleate.

Elaidic Acid

The trans isomer of oleic acid containing a trans double bond at carbon $9$ that retains a straight, unbent conformation.

Cis vs. Trans Fatty Acids

A stereochemical variation where hydrogens attached to a double bond lie on the same side, causing a sharp bend, versus opposite sides, resulting in a linear hydrocarbon chain.

Triacylglycerol (Triglyceride)

A neutral storage lipid composed of three fatty acids esterified to a single glycerol backbone, containing roughly $2\times$ as much energy per gram as polysaccharides.

Ester Bond (in Lipids)

The covalent linkage formed between a hydroxyl group of glycerol and the carboxyl group of a fatty acid via a condensation reaction that releases an $H_2O$ molecule.

Glycerol

A three-carbon polyol with three hydroxyl groups that serves as the molecular backbone for triacylglycerols and phosphoglycerides.

Phosphatidic Acid

The simplest phosphoglyceride, consisting of a glycerol backbone esterified to two fatty acid chains and a single terminal phosphate group ($-PO_4$).

Phosphoglyceride

A major phospholipid class consisting of phosphatidic acid covalently joined to a polar head group through a phosphodiester bond.

Phospholipid Polar Head Groups

Small hydrophilic organic molecules—such as serine, ethanolamine, choline, and inositol—that attach to phosphatidic acid to define different phosphoglyceride classes.

Phosphatidylcholine (PC)

A predominant phosphoglyceride in eukaryotic plasma membranes formed by esterifying choline to phosphatidic acid.

Phosphatidylethanolamine (PE)

A common membrane phosphoglyceride possessing an ethanolamine head group, abundant in the plasma membrane of $E.\,coli$ and the inner leaflet of eukaryotic membranes.

Phosphatidylserine (PS)

A negatively charged phosphoglyceride with a serine head group that is typically restricted to the inner (cytosolic) leaflet of healthy eukaryotic plasma membranes.

Phosphatidylinositol (PI)

A membrane phosphoglyceride possessing an inositol sugar head group, capable of being phosphorylated to mediate intracellular signaling cascades.

Cardiolipin (Diphosphatidylglycerol)

A specialized double phospholipid consisting of two phosphatidic acid molecules linked by a central glycerol bridge, characteristic of inner mitochondrial and thylakoid membranes.

Sphingosine

An eighteen-carbon amino alcohol containing an unsaturated hydrocarbon chain that forms the structural backbone of sphingolipids.

Sphingomyelin

A phosphosphingolipid consisting of a sphingosine backbone linked to a fatty acid via an amide bond and a phosphocholine polar head group.

Glycolipid

A specialized membrane lipid featuring a hydrophobic lipid tail (typically sphingosine-based) linked directly to a carbohydrate head group rather than a phosphate.

Galactocerebroside

A neutral glycosphingolipid containing a single galactose residue attached to sphingosine, found abundantly in neural membranes and myelin.

Ganglioside

A complex glycosphingolipid featuring an oligosaccharide head group with one or more negatively charged sialic acid residues.

Sterols

Lipids characterized by a rigid four-ring hydrocarbon nucleus, a small polar hydroxyl group, and a nonpolar hydrocarbon tail.

Cholesterol

The predominant sterol in animal plasma membranes that intercalates between fatty acyl chains to regulate membrane fluidity.

Phytosterols

Plant-derived sterols—including campesterol, sitosterol, and stigmasterol—that serve as membrane fluidity regulators in plant cells.

Ergosterol and Hopanoids

Sterol equivalents serving as fluidity modulators in fungal membranes (ergosterol) and bacterial membranes (hopanoids).

Fluidity Buffer

The role played by sterols (e.g., cholesterol) to decrease membrane fluidity at elevated temperatures and prevent rigid phase crystallization at lower temperatures.

Steroid Hormones

Hydrophobic signaling molecules derived from cholesterol—such as estradiol, testosterone, cortisol, and aldosterone—that cross membranes to bind intracellular receptors.

Basic Lipid Structures in Water

Self-assembling macromolecular structures formed by amphipathic lipids in aqueous solutions, including spherical micelles, inverted micelles, planar bilayers, and closed vesicles.

Micelle

A spherical monolayer formed in water by single-tailed lipids or soap molecules, with hydrophilic head groups on the surface and hydrophobic tails sequestered inside.

Liposome

An artificial spherical lipid bilayer surrounding an aqueous interior, used as an experimental model or a delivery vehicle for therapeutic drugs and mRNA.

Doxil™

A clinical liposomal formulation designed to encapsulate doxorubicin to improve targeted delivery to tumor cells while minimizing systemic toxicity.

Overton and Langmuir Membrane Studies

Early membrane discoveries showing that lipid-soluble nonpolar molecules penetrate cells easily (Overton, 1890s) and that purified phospholipids form a monolayer on water (Langmuir, 1917).

Gorter and Grendel (1925)

Researchers who measured the monolayer surface area of lipids extracted from red blood cells and found a $2:1$ ratio relative to cell surface area, establishing that membranes are lipid bilayers.

Davson and Danielli (1935)

The scientists who proposed the sandwich model of biological membranes, envisioning a central lipid bilayer coated on both outer surfaces by continuous sheets of globular proteins.

J. David Robertson (1960)

The electron microscopist who identified a trilaminar "railroad-track" pattern in osmium-stained membranes, proposing the "unit membrane" model with an approximate thickness of $5\text{–}8\,\text{nm}$.

Singer and Nicolson (1972)

The scientists who introduced the fluid mosaic model, portraying the biological membrane as a two-dimensional fluid lipid bilayer with embedded integral proteins and associated peripheral proteins.

Unwin and Henderson (1975)

Structural biologists who utilized electron crystallography on bacteriorhodopsin to demonstrate that integral membrane proteins traverse lipid bilayers via hydrophobic transmembrane $\alpha$-helices.

Lipid Raft

A small, dynamic, tightly packed membrane microdomain enriched in cholesterol, sphingolipids, and glycolipids that coordinates protein trafficking and cell signaling.

Membrane Protein Topology Classes

The structural classification of membrane proteins into integral transmembrane (single or multi-pass $\alpha$-helices, $\beta$-barrels), monolayer-associated, and peripheral proteins.

Integral Membrane Protein

A membrane protein embedded directly within the hydrophobic core of the lipid bilayer, which can only be released through detergent disruption.

Transmembrane Alpha-Helix

A recurring membrane-spanning protein motif typically consisting of $20\text{--}30$ hydrophobic amino acids that shields backbone peptide bonds from the nonpolar lipid interior.

Beta-Barrel Membrane Protein

An integral membrane protein architecture composed of antiparallel $\beta$-sheets rolled into a cylinder to form a central hydrophilic pore, typical of outer membrane porins.

Monolayer-Associated Protein

An integral protein anchored into the membrane via an amphipathic $\alpha$-helix that associates exclusively with one leaflet of the bilayer without spanning it.

Peripheral Membrane Protein

A water-soluble protein associated with the membrane surface through weak non-covalent electrostatic interactions and hydrogen bonds with lipid head groups or integral proteins.

Lipid-Anchored Protein

A protein located entirely outside the lipid bilayer but covalently attached to an embedded lipid molecule, such as a fatty acid or prenyl group.

Membrane Asymmetry

The unequal, non-random distribution of specific lipid species and carbohydrate modifications between the exoplasmic (outer) and cytosolic (inner) leaflets of a membrane.

Transverse Diffusion (Flip-Flop)

The rare and energetically unfavorable migration of a phospholipid molecule from one leaflet of the bilayer to the other across the hydrophobic core.

Lateral Diffusion

The rapid and spontaneous two-dimensional movement of lipids and proteins within the plane of a single membrane leaflet.

Gel-to-Fluid Transition Temperature ($T_m$)

The characteristic temperature at which a lipid bilayer converts from an ordered gel phase to a disordered fluid phase, recorded by a peak in heat absorption in a calorimeter.

Fatty Acid Saturation and Membrane Fluidity

The relationship whereby saturated fatty acids increase packing and raise $T_m$ (e.g., stearate-enriched $T_m = 70^\circ\text{C}$), while cis-unsaturated fatty acids disrupt packing to lower $T_m$ and enhance fluidity (e.g., oleate-enriched $T_m = 16^\circ\text{C}$).

Fluorescence Recovery After Photobleaching (FRAP)

An optical microscopy method used to quantify the lateral mobility and diffusion coefficients of fluorescently labeled lipids or proteins within living cell membranes.

Photobleaching

The irreversible photochemical destruction of a fluorophore's emission capabilities caused by prolonged exposure to high-intensity excitation light.

Jablonski Energy Diagram

A schematic showing electronic energy states (ground state $S_0$, excited singlets $S_1, S_2$, triplet $T_1$) and transitions including excitation ($10^{-15}\,\text{s}$), vibrational relaxation ($10^{-14}\text{--}10^{-11}\,\text{s}$), fluorescence ($10^{-9}\text{--}10^{-7}\,\text{s}$), intersystem crossing, and phosphorescence ($10^{-3}\text{--}10^{2}\,\text{s}$).

Green Fluorescent Protein (GFP)

An $11$-stranded $\beta$-barrel bioluminescent protein originally isolated from the jellyfish Aequorea victoria that fluoresces green upon excitation.

GCaMP

A genetically encoded biosensor created by fusing GFP to calmodulin and an M13 peptide, engineered to emit fluorescence upon binding $\text{Ca}^{2+}$.

Postsynaptic Density (PSD)

An electron-dense proteinaceous specialization anchored to the postsynaptic membrane of dendritic spines, containing scaffold proteins (PSD-95, Shank, Homer), neurotransmitter receptors, and actin.

SNARE Complex

A molecular machine mediating synaptic vesicle fusion with the presynaptic membrane, consisting of synaptobrevin, syntaxin, SNAP-25, and the $\text{Ca}^{2+}$ sensor synaptotagmin.

Erythrocyte Membrane Skeleton

A structural network of spectrin tetramers and short actin filaments anchored to integral membrane proteins (Band 3 and Glycophorin) via adapter proteins ankyrin, Band 4.1, and Band 4.2.

Mechanisms Restricting Membrane Protein Mobility

Physical barriers to protein diffusion in membranes, including tethering to the actin cytoskeleton, anchoring to intracellular organelles (e.g., ER), binding to the extracellular matrix, or self-assembly into large protein clusters.

Membrane Permeability Spectrum

The selective permeability hierarchy of synthetic lipid bilayers: highly permeable to hydrophobic gases and steroids, slowly permeable to small uncharged polar molecules ($H_2O$, urea), essentially impermeable to large polar molecules (glucose), and completely impermeable to ions ($Na^+$, $K^+$, $Cl^-$).

Simple Diffusion

The unassisted, non-saturating movement of a solute directly across the lipid bilayer from an area of higher concentration to lower concentration, driven thermodynamically by entropy.

Osmosis

The passive net diffusion of water molecules across a selectively permeable membrane from a region of lower solute concentration (higher free water) to one of higher solute concentration.

Osmotic Pressure

The precise hydrostatic pressure that must be applied to a concentrated solution to prevent the inward osmotic flow of pure water across a semipermeable partition.

Cellular Responses to Osmolarity

The morphological changes occurring when animal cells shrivel in hypertonic, remain normal in isotonic, or lyse in hypotonic solutions, compared to plant cells which become plasmolyzed, flaccid, or turgid.

Facilitated Diffusion

Protein-mediated passive transport of polar or charged solutes down an electrochemical gradient across a biological membrane without the input of metabolic energy.

Saturation Kinetics (in Carrier Transport)

The hyperbolic relationship between solute concentration and rate of transport observed in carrier-mediated diffusion, which plateaus at a maximum velocity ($V_{\max}$) when binding sites become saturated.

Carrier Protein (Permease)

A transmembrane transport protein that binds specific solute molecules and shifts between alternating conformations to transfer the solute across the membrane.

Channel Protein

A transmembrane protein that forms an open, water-filled hydrophilic pore across the lipid bilayer, facilitating rapid passive transport of ions or solutes without alternating conformational changes.

Uniport vs. Coupled Transport

Transport of a single solute across a membrane versus the simultaneous or sequential transport of two distinct solutes in the same direction (symport) or opposite directions (antiport).

GLUT1 Glucose Transporter

A classic uniport carrier protein in erythrocyte membranes that alternates between an outward-facing $T_1$ conformation and an inward-facing $T_2$ conformation to transport glucose down its concentration gradient.

Porin and Aquaporin

A large outer-membrane $\beta$-barrel pore permitting passage of water and larger solutes versus an $\alpha$-helical channel selectively transporting up to $10^9\,\text{H}_2\text{O}/\text{second}/\text{channel}$.

Direct Active Transport

Membrane transport in which the uphill translocation of a solute against its electrochemical gradient is coupled directly to an exergonic chemical reaction, most commonly ATP hydrolysis.

Indirect Active Transport

A secondary active transport mechanism that couples the uphill movement of a solute to the downhill movement of an ion (e.g., $Na^+$ or $H^+$) down a gradient established by direct active transport.

P-type ATPase

A family of cation-transporting pumps characterized by transient autophosphorylation of an aspartate residue during the catalytic cycle; humans possess approximately $70$ distinct P-type pumps.

Sodium-Potassium Pump ($\text{Na}^+/\text{K}^+$ ATPase)

A P-type transport ATPase that hydrolyzes one ATP molecule to export $3\,\text{Na}^+$ ions and import $2\,\text{K}^+$ ions, maintaining an inside-negative resting membrane potential of $-60\,\text{mV}$.

V-type ATPase

A vacuolar proton-pumping ATPase that hydrolyzes ATP to pump $H^+$ into organelles (e.g., lysosomes and plant vacuoles) to lower internal pH without forming a phosphorylated intermediate.

F-type ATPase ($\text{F}_o\text{F}_1$ ATP Synthase)

A reversible rotary multi-subunit complex comprising a membrane proton channel ($F_o$) and a catalytic stalk/head ($F_1$) that utilizes a transmembrane proton gradient to synthesize ATP.

ABC Transporter

A transport protein containing four core domains (two transmembrane domains and two ATP-binding cassettes) that utilizes ATP binding and hydrolysis to transport diverse substrates across membranes.

Cystic Fibrosis Transmembrane Conductance Regulator (CFTR)

An atypical ABC-type protein that functions as an ATP-gated ion channel, regulating the passive efflux of $Cl^-$ and $HCO_3^-$ to hydrate airway mucus.

Sodium-Glucose Symporter ($\text{Na}^+/\text{glucose}$ Symporter)

An indirect active transport carrier in intestinal epithelial cells that harnesses the inward electrochemical gradient of two $Na^+$ ions to drive the uphill import of one glucose molecule.

Bacteriorhodopsin

A light-driven proton pump found in Halobacterium salinarum whose covalently bound retinal chromophore photoisomerizes from all-trans to 13-cis upon absorbing light, expelling protons across the plasma membrane.

Channelrhodopsin-2 (ChR2)

A light-gated cation channel from the green alga Chlamydomonas reinhardtii that opens in response to blue light to allow $Na^+$ and $Ca^{2+}$ influx, enabling optical excitation of excitable cells.

Halorhodopsin

A light-driven inward chloride ion pump that uses light energy to transport $Cl^-$ into cells against an electrochemical gradient, inducing cellular hyperpolarization.

Optogenetics

A biological technique utilizing light and light-sensitive microbial proteins (such as channelrhodopsin, halorhodopsin, and bacteriorhodopsin) to optically control the electrical activity of specific genetically targeted cells.