Plasma Membrane: Structure, Composition, and Function

Comprehensive vocabulary flashcards reviewing the molecular composition, lipid dynamics, protein organization, glycocalyx, and pathology associated with the plasma membrane.

E Face (Outer Leaflet)

The external hemilayer of the plasma membrane bilayer facing the extracellular space, measuring approximately $5\text{ nm}$ in thickness and enriched in phosphatidylcholine, sphingomyelin, and glycolipids.

P Face (Inner Leaflet)

The cytosolic (protoplasmic) hemilayer of the plasma membrane bilayer facing the intracellular environment, measuring approximately $5\text{ nm}$ in thickness and enriched in phosphatidylethanolamine, phosphatidylserine, and phosphatidylinositol.

Phosphoglycerides

Glycerol-based phospholipids consisting of a glycerol backbone esterified to two fatty acid tails, a phosphate bridge, and a polar organic residue (such as choline in lecithin or ethanolamine in cephalin).

Sphingolipids

Membrane lipids built on an amino alcohol backbone termed sphingosine attached to a single fatty acid tail, a phosphate group, and an organic residue such as choline.

Phosphatidylserine

A negatively charged phosphoglyceride strictly concentrated in the inner leaflet (P face) of the plasma membrane under normal physiological conditions.

Phosphatidylinositol

A negatively charged phosphoglyceride possessing an inositol ring that is localized primarily to the cytosolic leaflet (P face) of the membrane.

Flip-Flop (Transbilayer Diffusion)

The slowest and rarest form of lipid motion, involving the movement of a phospholipid molecule from one monolayer of the bilayer to the other, regulated by translocase enzymes such as flippases and scramblases.

Cholesterol (Membrane Fluidity Buffer)

A eukaryotic sterol intercalating between phospholipids that buffers membrane fluidity; it increases membrane fluidity below the transition temperature ($T_m < 20^\circ\text{C}$) and decreases fluidity by rigidifying the membrane above $20^\circ\text{C}$.

Hepatocarcinoma Mitochondrial Alteration

A pathological state in which an increase in cholesterol within the outer mitochondrial membrane alters its permeability, contributing to chemotherapy resistance.

Cerebrosides

Neutral glycolipids composed of a hydrophobic ceramide backbone linked covalently to a single simple sugar (monosaccharide) residue.

Gangliosides

Complex glycolipids containing a branched oligosaccharide chain with one or more sialic acid residues, serving as cell markers such as the ABO blood group antigens.

H Antigen

The basal carbohydrate structure on erythrocyte membranes composed of N-acetylglucosamine, galactose, and fucose, serving as the common precursor for the A and B antigens.

A Antigen

An erythrocyte surface glycolipid antigen produced when an N-acetylgalactosamine residue is enzymatically appended to the terminal galactose of the precursor H antigen.

B Antigen

An erythrocyte surface glycolipid antigen formed by the addition of an extra galactose residue to the H antigen precursor.

Lipid Asymmetry

The unequal, non-random distribution of lipid species across the bilayer leaflets, with neutral choline-containing lipids and glycolipids situated in the outer leaflet, and negatively charged aminophospholipids restricted to the inner leaflet.

Lipid Rafts

Dynamic, small, semisolid membrane microdomains formed by assemblies of cholesterol and sphingolipids that compartmentalize specific proteins to function as cell signaling platforms.

Integral Membrane Proteins

Amphipathic proteins firmly embedded within or traversing the hydrophobic core of the lipid bilayer, which can only be released by disrupting the bilayer.

Peripheral Membrane Proteins

Proteins bound loosely to the hydrophilic exterior of the membrane or to integral proteins through electrostatic and non-covalent interactions, without inserting into the hydrophobic interior.

Prenylation

A covalent post-translational lipid modification that anchors integral proteins to the cytosolic leaflet of the lipid bilayer through a thioether linkage between a cysteine residue and a farnesyl group.

GPI Anchor (Glycosylphosphatidylinositol)

A glycolipid anchor covalently attached to the carboxy-terminus of certain proteins, anchoring them strictly to the outer leaflet (E face) of the plasma membrane.

Spectrin and Protein 4.1

Cytoskeletal peripheral proteins that assemble beneath the erythrocyte plasma membrane to bind glycophorin and actin, conferring structural integrity and elastic deformability to red blood cells.

Hereditary Elliptocytosis

An anemic syndrome and erythrocyte membrane pathology characterized by oval-shaped red blood cells due to genetic defects or deficiencies in membrane skeletal proteins like spectrin or protein 4.1.

Glycocalyx

The carbohydrate coat on the extracellular face of the plasma membrane formed by oligosaccharides linked to glycoproteins and glycolipids, performing mechanical protection, selective barrier, recognition, and adhesion functions.

Fluid Mosaic Model (Singer & Nicolson, 1972)

A structural model describing the plasma membrane bilayer as a two-dimensional fluid wherein proteins and lipids can move laterally, but do not mix completely freely, being organized into distinct domains.

Plasma Membrane

A thin, flexible laminar semipermeable barrier composed of a bilayer of lipids, proteins, and carbohydrates that defines the intracellular and extracellular compartments and controls the selective exchange of substances.

E Face (Outer Leaflet)

The external hemilayer of the plasma membrane lipid bilayer facing the extracellular space, measuring approximately $5\,\text{nm}$ in thickness.

P Face (Inner Leaflet)

The protoplasmic or cytosolic hemilayer of the plasma membrane lipid bilayer facing the cytosol, measuring approximately $5\,\text{nm}$ in thickness.

Phosphoglycerides

Glycerol-based phospholipids composed of a glycerol backbone esterified to two fatty acid tails, a phosphate bridge, and an organic residue such as choline (lecithin) or ethanolamine (cephalin).

Sphingolipids

A class of membrane lipids built on an amino alcohol (sphingosine) backbone rather than glycerol, such as sphingomyelin.

Lipid Movements of the Plasma Membrane

The four distinct modes of motion exhibited by membrane lipids: flexion of hydrocarbon tails, rotation around their long axis, lateral diffusion, and transbilayer diffusion (flip-flop).

Transbilayer Diffusion (Flip-Flop)

The slowest type of lipid motion, involving the transfer of a lipid molecule from one leaflet to the opposite leaflet, facilitated by translocase enzymes such as flippases and scramblases.

Cholesterol (Membrane Sterol)

An amphipathic eukaryotic sterol that intercalates between phospholipids; it acts as a fluidity buffer by increasing fluidity below $T_m$ ($< 20^\circ\text{C}$) and reducing fatty acid head movement to rigidify membranes above $20^\circ\text{C}$.

Hepatocarcinoma Mitochondrial Alteration

A pathological condition where elevated cholesterol levels in the outer mitochondrial membrane reduce permeability, contributing to chemotherapy resistance and suggesting the therapeutic potential of statins.

Cerebrosides

Neutral glycolipids characterized by a polar head group containing a simple monosaccharide unit.

Gangliosides

Complex glycolipids containing branched oligosaccharide chains with one or more sialic acid residues, serving as cell-surface markers such as ABO blood group determinants.

ABO Blood Group Antigens

Erythrocyte cell-surface glycolipids featuring an $H$ antigen core (fucose, galactose, and N-acetylglucosamine) modified with an additional N-acetylgalactosamine ($A$ antigen) or galactose ($B$ antigen).

Lipid Asymmetry

The non-random, unequal distribution of lipids across the bilayer: phosphatidylcholine, sphingomyelin, and glycolipids populate the outer (E) leaflet, whereas phosphatidylethanolamine and negatively charged phosphatidylserine and phosphatidylinositol populate the inner (P) leaflet.

Transition Temperature ($T_m$)

The critical temperature threshold at which a biological membrane changes from a rigid, crystalline gel phase to a fluid, liquid-like phase.

Membrane Fluidity Modulators

Key factors regulating membrane viscosity: increased temperature, shorter fatty acid chain lengths, and higher degrees of unsaturation (double bonds) promote fluidity, while cholesterol stabilizes and rigidifies membranes at temperatures above $20^\circ\text{C}$.

Lipid Rafts

Dynamic, semisolid microdomains enriched in cholesterol and sphingolipids that aggregate in the plasma membrane to organize and concentrate specific proteins for cell signaling.

Lipid Rafts in Alzheimer's Disease

A pathological state in which altered composition and organization of lipid rafts create an environment favorable for the enzymatic cleavage that generates neurotoxic amyloid-$\beta$ peptides.

Integral Membrane Proteins

Amphipathic proteins that penetrate into or span the hydrophobic core of the lipid bilayer, or are firmly anchored to it by covalently attached lipid moieties.

Peripheral Membrane Proteins

Proteins that do not penetrate the hydrophobic core of the bilayer, instead binding non-covalently to hydrophilic lipid heads or the exposed domains of integral membrane proteins.

Lipid Anchors

Covalently linked lipid groups that anchor proteins to the membrane, including myristic acid (myristoylation), palmitic acid (palmitoylation), and farnesyl groups (prenylation) on the cytosolic leaflet.

Glycosylphosphatidylinositol (GPI) Anchor

A glycolipid anchor attached to the carboxyl terminus of certain proteins, anchoring them strictly to the outer (extracellular) leaflet of the plasma membrane.

Erythrocyte Membrane Skeleton

A submembranous structural network composed of spectrin, actin, protein 4.1, and ankyrin tethered to transmembrane band 3 and glycophorin, conferring elasticity and reversible deformability to erythrocytes.

Hereditary Elliptocytosis

A clinical erythrocyte disorder caused by genetic deficiencies in membrane skeletal proteins (such as spectrin or protein 4.1), leading to abnormal elongated cell shapes and anemic syndrome.

Glycocalyx

The carbohydrate-dense external cell coat composed of oligosaccharide chains projecting outward from membrane glycolipids and glycoproteins.

Glycocalyx Functions

Eight primary functions: mechanical protection, selective permeability barrier, chemical protection, enzymatic adsorption, nonspecific adhesion, specific adhesion, cell-cell recognition, and cell migration.

Modified Fluid Mosaic Model

A structural model formulated by S. J. Singer and G. L. Nicolson (1972) describing biological membranes as two-dimensional fluids wherein lipids and proteins diffuse laterally but are organized and constrained rather than homogeneously mixed.