Module 3: Membrane Structure

Comprehensive vocabulary flashcards covering membrane structure, lipid composition and asymmetry, membrane proteins, and diffusion dynamics from Module 3.

Glycerophospholipid

A primary membrane lipid built on a glycerol backbone with two hydrophobic fatty acid chains esterified to positions $1$ and $2$, and a hydrophilic phospho-head group attached to position $3$.

Sphingophospholipid

A class of phospholipids based on a sphingosine backbone rather than a glycerol backbone.

Amphiphilic

A property describing molecules that possess both hydrophilic (water-loving, polar) and hydrophobic (water-fearing, nonpolar) regions.

Phosphatidylcholine (PC)

A cylindrically shaped, electrically neutral phospholipid that facilitates tight molecular packing within a bilayer.

Phosphatidylethanolamine (PE)

A cone-shaped phospholipid featuring a relatively small head group that promotes membrane curvature.

Phosphatidylserine (PS)

A negatively charged phospholipid normally restricted to the inner leaflet of the plasma membrane; its translocation to the outer surface serves as an apoptosis signal.

Phosphatidylinositol (PI)

A phospholipid possessing an inositol phosphate head group that plays an essential role in intracellular signaling cascades.

Cholesterol

A major membrane sterol consisting of a planar, rigid, hydrophobic steroid ring that intercalates between phospholipid fatty acid chains to buffer fluidity across temperatures and decrease water permeability.

Micelle

A spherical aggregate formed in aqueous solution by cone-shaped, single-tailed lipids in which hydrophobic hydrocarbon tails pack inward away from water.

Liposome

An artificial spherical lipid bilayer vesicle enclosing an aqueous center, commonly utilized in scientific research and drug delivery.

Lateral Diffusion

The rapid and energetically favorable two-dimensional movement of lipid molecules or proteins within the plane of the same membrane leaflet.

Transbilayer Diffusion (Flip-Flop)

The rare, slow, and energetically unfavorable movement of a phospholipid from one leaflet of the membrane bilayer to the opposite leaflet.

Lipid Rafts

Specialized, thicker microdomains within the membrane bilayer enriched in cholesterol and saturated lipids that pack closely together via enhanced Van der Waals attractions.

Integral Membrane Proteins

Proteins permanently associated with the lipid bilayer that penetrate or span the hydrophobic membrane interior and require detergents for dissociation.

Peripheral Membrane Proteins

Proteins attached to membrane surfaces through non-covalent interactions (such as ionic and hydrogen bonds) with integral proteins or lipid heads, dissociable by alterations in pH or salt concentration.

Lipid-Anchored Proteins

Membrane proteins tethered to the bilayer through covalent attachment to lipid anchors, such as myristoyl, palmitoyl, or farnesyl groups.

Hydropathy Plot

A graphical index measuring the free energy required to transfer successive amino acid segments of a protein from an oily interior to water, used to predict hydrophobic transmembrane segments.

Bacteriorhodopsin

A bacterial transport protein composed of seven transmembrane $\alpha$-helices covalently bound to retinal, which captures light energy to pump $H^+$ out of the cell to drive ATP production.

Glycocalyx

The carbohydrate coat on the non-cytosolic surface of the cell membrane formed by oligosaccharide side chains of glycoproteins and glycolipids, functioning in cell-cell recognition and adhesion.

Fluorescence Recovery After Photobleaching (FRAP)

A microscopy technique that assesses membrane fluidity and protein lateral mobility by bleaching a defined fluorescent patch with a laser beam and monitoring recovery over time.

Tight Junction

A dense junctional structure in epithelial cells that seals adjacent cells together, preventing the mixing of apical and basolateral membrane proteins.

Spectrin

A dimeric and tetrameric fibrous cytoskeletal protein forming a meshwork under the plasma membrane of red blood cells, linked to Band $3$ and glycophorin to maintain cell shape and flexibility.

Membrane Corrals

Submembrane compartments ($200\text{--}400\,\text{nm}$) formed by the cortical cytoskeleton and associated proteins that transiently restrict the lateral diffusion of membrane proteins.

Amphiphilic (Amphipathic)

A molecular property characterized by having both a hydrophilic (water-loving, polar) region and a hydrophobic (water-fearing, nonpolar) region, which drives the self-assembly of lipid bilayers.

Glycerophospholipids

A major class of membrane phospholipids built on a glycerol backbone with two hydrophobic fatty acid chains esterified to positions $1$ and $2$, and a hydrophilic phospho-head group linked to position $3$.

Sphingophospholipids

A primary class of membrane phospholipids that utilizes a sphingosine backbone rather than a glycerol backbone.

Phosphatidylcholine (PC)

A cylinder-shaped, electrically neutral glycerophospholipid whose cylindrical geometry promotes tight packing in planar bilayers.

Phosphatidylethanolamine (PE)

A cone-shaped glycerophospholipid possessing a small polar head group relative to its tails, which introduces curvature into biological membranes.

Phosphatidylserine (PS)

A negatively charged phospholipid predominantly confined to the inner cytosolic leaflet of healthy plasma membranes, whose transbilayer exposure on the cell exterior acts as an apoptosis signal.

Phosphatidylinositol (PI)

A glycerophospholipid bearing an inositol phosphate head group that serves essential regulatory roles in intracellular signaling cascades.

Cholesterol

A membrane sterol containing a rigid, planar, hydrophobic steroid ring, a nonpolar hydrocarbon tail, and a tiny polar hydroxyl head group ($-\text{OH}$) that intercalates between phospholipid fatty acid chains.

Membrane Fluidity Buffering by Cholesterol

The ability of cholesterol to modulate bilayer physical state across temperatures by increasing fluidity at lower temperatures (preventing tight crystalline packing) and decreasing fluidity at higher temperatures above $T_m$.

Liposomes

Artificial spherical lipid bilayers enclosing an aqueous core that form spontaneously to eliminate exposed hydrophobic bilayer edges, used widely in biological research and drug delivery.

Lateral Diffusion (Lipids)

The rapid and energetically favorable two-dimensional translation of lipid molecules along the plane of a single bilayer leaflet.

Transbilayer Diffusion (Flip-Flop)

The extremely slow, rare, and energetically unfavorable movement of a phospholipid molecule across the hydrophobic membrane core from one leaflet to the opposite leaflet.

Lipid Movements in a Bilayer

The four characteristic modes of phospholipid dynamic motion: lateral diffusion, tail flexion, molecular rotation, and transbilayer flip-flop.

Lipid Rafts

Specialized, non-uniform microdomains within biological membranes enriched in saturated lipids and cholesterol that associate stably via Van der Waals forces.

Lipid Asymmetry

The unequal distribution of specific lipid species across the bilayer leaflets, with phosphatidylcholine and sphingomyelin enriched on the outer leaflet, and phosphatidylethanolamine and phosphatidylserine localized to the inner cytosolic leaflet.

Antiphospholipid Syndrome

An autoimmune condition (seen in lupus erythematosus) caused by genetic mutations or defects that impair the asymmetric restriction of phosphatidylserine, leading to its abnormal exposure on viable cells.

Integral Membrane Proteins

Proteins that embed hydrophobic amino acid segments directly into the hydrophobic interior of the lipid bilayer and can only be dissociated using detergents.

Peripheral Membrane Proteins

Proteins that associate with membranes through weak, non-covalent ionic or hydrogen bonds with integral proteins or lipid head groups, releasable by shifts in $\text{pH}$ or ionic strength.

Lipid-Anchored Proteins

Proteins that are covalently linked to lipid groups (such as fatty acids, prenyl groups, or GPI anchors) that insert into a bilayer leaflet to tether the protein to the membrane.

Myristoyl Anchor

A lipid anchor attached to the cytosolic side of a membrane protein via an amide linkage between the protein's terminal amino group and myristic acid.

Palmitoyl Anchor

A lipid modification formed via a thioester linkage between an internal cysteine residue of a membrane protein and a palmitic acid group.

Farnesyl Anchor

A prenyl lipid modification joined to a protein by a thioether linkage between a cysteine residue and a prenyl group.

Transmembrane Domain

A hydrophobic segment of a polypeptide—typically an $\text{α}$-helix of $20\text{--}30$ nonpolar amino acids—that spans the entire hydrophobic thickness ($\thickapprox 3\text{ nm}$) of the lipid bilayer.

Hydropathy Plot

A quantitative bioinformatic graph measuring the free energy required to transfer sequential amino acid segments from hydrophobic solvent to water, where positive peaks identify hydrophobic transmembrane segments.

Hydropathy Plot of Bacteriorhodopsin

A hydropathy index graph exhibiting $7$ distinct positive peaks above the zero baseline, reflecting its $7$ transmembrane $\text{α}$-helices.

Bacteriorhodopsin

A bacterial integral membrane transport protein consisting of $7$ transmembrane $\text{α}$-helices and a retinal prosthetic group that uses light energy to pump protons ($\text{H}^+$) out of the cell to drive $\text{ATP}$ synthesis.

Glycocalyx

A protective carbohydrate-rich surface coat on eukaryotic plasma membranes formed by the oligosaccharide chains of transmembrane glycoproteins, glycolipids, and adsorbed proteoglycans.

Membrane Protein Disulfide Bonds

Covalent sulfur-to-sulfur bridges that form between cysteine residues exclusively in the extracellular environment to stabilize protein conformation, but fail to form in the reducing cytosolic environment.

Frye-Edidin Experiment

A landmark 1970 cell biology experiment demonstrating membrane protein lateral diffusion by fusing mouse and human cells with a virus and observing the progressive mixing of distinct surface proteins over time.

FRAP (Fluorescence Recovery After Photobleaching)

An optical technique in which fluorescently labeled membrane molecules in a defined area are bleached by an intense laser pulse, followed by measuring the rate of fluorescence recovery to calculate lateral mobility.

Tight Junction

A specialized continuous belt of intercellular sealing proteins in epithelial cells that blocks the lateral diffusion of membrane proteins and lipids between the apical and basolateral plasma membrane domains.

Spectrin

A long, flexible cytoskeletal protein formed by $\text{α}$ and $\text{β}$ chains that crosslinks into a meshwork beneath the erythrocyte plasma membrane, maintaining biconcave red blood cell morphology.

Membrane Corrals (Cytoskeletal Fences)

Submembrane compartments ($200\text{--}400\text{ nm}$ wide) formed by the cortical cytoskeleton and associated anchor proteins that temporarily constrain the lateral diffusion of integral membrane proteins.