Cell Membrane Lipid Bilayers

Vocabulary practice flashcards covering cell membrane lipid bilayer structures, classes, asymmetry, lipid-protein interactions, and clinical relevance based on lecture notes.

Lipids

Carbon-containing organic molecules defined by being soluble in non-polar organic solvents according to the principle "like dissolves like", and assembled enzymatically by protein complexes.

Hydrophobic Lipids

Entirely water-insoluble lipids, such as triglycerides.

Amphipathic Lipids

Molecules containing both hydrophobic and hydrophilic regions, such as fatty acids, phospholipids, and cholesterol.

Fatty Acid Structure

Lipids consisting of a carboxyl group attached to a long hydrophobic tail containing an even number of carbon atoms ($16$ to $20$) with carbon-carbon and carbon-hydrogen bonds only.

Unsaturated Fatty Acid

A fatty acid containing one or more double bonds, which typically occur in the $cis$ configuration.

Phospholipids

Amphipathic lipids composed of two hydrophobic fatty acid tails, a glycerol backbone ($\text{glycerol} = 3\,\text{OH's}$), and a polar head group containing phosphate.

Phospholipid Bilayer Spontaneous Assembly

The process by which phospholipids spontaneously form bilayers in aqueous environments driven by the hydrophobic effect, minimization of exposed hydrophobic surface area, and maximization of system entropy.

Overton's Membrane Observation (1899-1925)

The observation that membrane permeability was determined by polarity, suggesting that cell membranes were "oil-like".

Gorter and Grendel Hypothesis

The proposal that lipids in cell membranes are arranged in a "bilayer" structure.

Unit Membrane Model (1935-1959)

The model proposed by Davson suggesting that proteins coat the surface of lipid bilayers, which was later confirmed by Robertson using electron microscopy.

Fluid Mosaic Model

The membrane model developed by Singer in the 1970s describing cell membranes as dynamic lipid bilayers with embedded proteins.

Modern Membrane Concept

The present membrane model recognizing lateral heterogeneity, restricted diffusion, and complex lipid-protein interactions.

Glycerolipids

Amphipathic membrane lipids based on a glycerol backbone with various head groups and fatty acyl chains, constituting approximately $50\%$ of membrane lipids.

Sphingolipids

Amphipathic membrane lipids containing a sphingoid long-chain base with $N$-acylated fatty acids, constituting approximately $30\%$ of membrane lipids.

Sterols

Amphipathic membrane lipids consisting of a tetracyclic hydrocarbon scaffold with a polar hydroxyl head group, constituting approximately $20\%$ of membrane lipids.

Sphingomyelin Bilayer Thickness Effect

Pure sphingomyelin (SM) bilayers are thicker than phosphoglyceride or phosphatidylcholine (PC) bilayers, and cholesterol increases phosphoglyceride bilayer thickness via a lipid-ordering effect.

Phosphatidylcholine (PC)

A membrane lipid with a cylindrical shape whose head is about the same width as the tails, forming flat monolayers and enriching the exoplasmic leaflet.

Phosphatidylethanolamine (PE)

A membrane lipid with a conical shape due to its smaller head group, causing the lipid layer to curve and enriching the cytosolic leaflet.

Neurons vs Glia Ceramide Fatty Acids

In brain cells, neurons have $\text{C18}$ as the dominant ceramide fatty acid, whereas glia have $\text{C24}$.

Yeast Secretory Pathway Lipid Composition

The percentage of sphingolipid and sterol increases along the secretory pathway in yeast from $27\%$ in the trans-Golgi network to $53\%$ in secretory vesicles and $82\%$ in the plasma membrane.

Cardiolipin

A unique class of lipids contained within mitochondria that is not found in other organelles.

Lipid Rafts

Small membrane domains enriched in cholesterol and sphingolipids, first observed in liposomes, that create areas of the membrane with distinct composition and properties.

Lateral Heterogeneity

The organization of different membrane areas with distinct lipid compositions and properties, driven by hydrogen bonding, head group size, and acyl chain saturation.

Ceramide Membrane Effects

The addition of $0.5\%$ ceramide to SM membranes forms detergent-resistant membranes, while ceramides with greater than $12$ carbon chains ($>12$) create rigid domains.

Lipid Flip-Flop

The translocation of a lipid from one leaflet to the other, which is very slow ($t_{1/2} = \text{hours to days}$) and non-spontaneous due to the high energy required for the polar head group to cross the hydrophobic core.

Membrane Leaflet Asymmetry

The unequal distribution of lipids between leaflets, where the outer leaflet contains SM and PC, and the inner leaflet contains PE, PS, PA, and PI.

Asymmetry in Lipid Saturation

The state where the inner leaflet is more fluid (averaging $3.4$ double bonds per lipid and $12\%$ inner PC saturation) while the outer leaflet packs more tightly (averaging $1.6$ double bonds per lipid and $2\%$ outer PC saturation).

Flippases

P4-ATPases that consume ATP to translocate lipids toward the inner leaflet.

Floppases

ABC transporters that use ATP to move lipids toward the outer leaflet.

Scramblases

GPCRs that passively allow lipids to move in either direction, establishing equal distribution between leaflets.

Phosphatidylserine (PS) Exposure

The exposure of PS on the outer leaflet as an irreversible phagocytotic signal regulating blood coagulation, engulfment of apoptotic cells, brain synaptic remodeling, and phagocytosis of photoreceptors.

Transmembrane Protein Abundance

Approximately $30\%$ of the human transcriptome encodes transmembrane proteins, and the plasma membrane ratio is approximately $1$ protein per $50$ lipid molecules.

Specific Lipid-Protein Binding

Direct or allosteric interaction occurring at conserved structural sites on membrane proteins that affects protein shape and activity.

Examples of Lipid-Protein Interactions

Specific functional couplings including skeletal muscle $\text{Ca}^{2+}\text{-ATPase}$ activity in PC, $\text{Na}^+/\text{K}^+\text{-ATPase}$ peak activity in cholesterol/PC bilayers, $\text{PIP}_2$ activation of inward-rectifying $\text{K}^+$ channels, and COPI protein p24 binding to SM.

Transmembrane Domain (TMD) Asymmetry

The surface area of single-pass TMDs is higher in the inner leaflet than in the outer leaflet, matching the tighter lipid packing of the outer leaflet and higher fluidity of the inner leaflet.

Lipid Anchoring Types

Three mechanisms attaching water-soluble proteins to lipids: acylation (attaches to cytosolic protein/side, e.g., v-Src), prenylation (attaches to cytosolic protein/side, e.g., Ras, Rab G), and GPI anchor (attaches protein to exterior/extracellular side).

Liposomes

Lipid vesicles that encapsulate drugs and deliver them to specific tissues.

Familial Hypercholesterolemia

A genetic disorder resulting from mutations in LDL receptor genes that causes high cholesterol levels and leads to premature cardiovascular disease.

Tay-Sachs Disease

A fatal genetic disorder affecting lipid metabolism caused by mutations in the HEXA gene, leading to the accumulation of gangliosides in brain cells.

Cystic Fibrosis

A membrane disorder caused by mutations in CFTR, a membrane protein affecting chloride ion transport across cell membranes, resulting in thick, sticky mucus production.