Chapter 10 | Membrane Structure

Functions of cell membranes

• Compartmentalization

• Selective permeability

• Connection

• Metabolic activity

Components of cell membranes

A lipid bilayer composed of amphipathic lipids (phospholipids, sphingolipids, cholesterol) with embedded proteins

Glycerolipid/phospholipid

Glycerol + two fatty acid chains (one saturated, one unsaturated) + hydrophilic head (phosphate group attached to an alcohol)

Sphingolipid

Lipids with a sphingosine backbone that replace one fatty acid

Sphingomyelin

Sphingosine backbone + fatty acid + phosphate group

Glycosphingolipid

Sphingosine backbone + fatty acid + sugar (oligosaccharide chain of ~15–20 monosaccharides); mainly faces the extracellular matrix on the plasma membrane

Cholesterol

4 hydrocarbon rings + hydrocarbon tail + 1 hydroxyl (-OH) group

Cholesterylester when attached to a fatty acid

Micelle

Single layer of lipids with hydrophilic heads facing outward and hydrophobic tails facing inward

Lipid bilayer

Biomolecular sheets or closed vesicles called liposomes

Membrane fusion

Electrostatic interactions and hydrogen bonding between the hydrophilic lipid heads and water create a stable barrier; breaking these interactions to fuse membranes requires energy input

Unilocular membranes

Storage fat (e.g., in adipose tissue) uses monolayers (unilocular membranes) where hydrophobic tails contact inner lipid droplets

Membrane fluidity & temperature

Gel-like/paracrystalline state under low temperatures

Liquid disordered state under high temperatures

Degree of saturation in fatty acids

Unsaturated fatty acids have kinked tails that decrease fluidity

Saturated fatty acids become tightly packed to decrease fluidity

Cholesterol’s role in membranes

Decreases permeability with hydrophilic heads

Disrupts tight packing of saturated fatty acids

Stiffens unsaturated fatty acid areas to reduce excess movement

Phospholipid flip-flops

Spontaneous flip-flops are rare because moving hydrophilic heads across the hydrophobic core is energetically unfavorable. Flippases catalyze this using ATP

Membrane assymetry

Cytosolic side: More negatively charged lipids + bioactive signaling lipids

Extracellular side: Contains glycolipids that form a protective carbohydrate coat

Lipid domains

Lipid Rafts: Transient areas with longer fatty acids & high cholesterol that recruit specific proteins

Caveolae: Indentations in the membrane showing topological variation

Peripheral proteins

Noncovalently attached to surface; transient

Lipid-linked proteins

Covalently bound to lipid anchors (palmitoyl/myristoyl for cytosolic; GPI anchor for extracellular)

Integral/embedded proteins

Monotropic (one monolayer) or transmembrane (spans full bilayer)

Kyte-Doolittle Hydropathy Plots

Usually hydrophobic alpha-helices (single-pass or multi-pass)

Calculates DG for moving amino acids to a hydrophilic environment to predict membrane spanning proteins

Exception: Channels/transporters or beta-barrels (porins) alternate hydrophobic/hydrophilic residues and fail simple hydropathy plots

Glycocalyx

A carbohydrate layer on the extracellular side (glycoproteins/glycolipids). Functions include:

1. Cell identification (e.g., blood groups)

2. Cell-cell interactions & communication

3. Physical separation/enclosure of cells

Studying transmembrane proteins

Isolation: Extracted using mild detergents (SDS, Triton), then reconstituted into liposomes or nanodiscs.

Tracking: Fluorescently labeled genetically and tracked in vivo using FRAP (Fluorescence Recovery After Photobleaching).

Fluid Mosaic Model

A model describing biological membranes as dynamic, fluid lipid bilayers embedded with a mosaic of mobile proteins and lipids

Limited movement of membrane proteins

Physical barriers (tight junctions), structural attachments, lipid domains/rafts, cell cortext

Cell cortext

A matrix of proteins underneath the plasma membrane (such as cytoskeletal proteins like spectrin cross-linked with actin) that provides structure and shape

Transmembrane protein attachment to cell cortex

Anchored via glycophorin (a single-pass protein) and ankyrin.