Chapter 7: Cell Membranes

Introduction to Cell Membranes

Cell membranes are fundamental structures that define the boundaries of cells and their internal compartments. They play critical roles in maintaining cellular integrity, mediating communication, and regulating the movement of substances.

• Membrane-bound organelles subdivide eukaryotic cells into functional domains.

• The plasma membrane surrounds the cell, while internal membranes enclose organelles.

Functions of Membranes

• Boundary Definition: Membranes delineate the cell and organelles, acting as selective permeability barriers.

• Sites for Biological Functions: Membranes host essential processes such as electron transport (mitochondria) and protein processing (endoplasmic reticulum).

• Transport Regulation: Membranes contain proteins that control the movement of substances into and out of cells and organelles.

• Signal Transduction: Membrane proteins receive and transmit external signals.

• Cell-Cell Interaction: Membranes facilitate contact, adhesion, and communication between cells.

Membrane Structure: The Fluid Mosaic Model

Overview of the Fluid Mosaic Model

The fluid mosaic model, proposed by Singer and Nicolson, describes membranes as a mosaic of proteins embedded in a fluid lipid bilayer. This model emphasizes both the diversity of membrane proteins and the dynamic nature of the lipid matrix.

• Lipid Bilayer: Provides the fluid matrix for membrane proteins.

• Proteins: Attached to or embedded within the bilayer, contributing to membrane function.

Membrane Lipids: The "Fluid" Component

Membrane lipids are crucial for the fluidity and structural integrity of membranes. The three major classes are:

• Phospholipids

• Glycolipids

• Sterols

Phospholipids

• Most abundant membrane lipids.

• Composed of a backbone (glycerol or sphingosine), two fatty acids, a phosphate group, and a charged head group.

• Amphipathic: Contain hydrophilic heads and hydrophobic tails, enabling bilayer formation.

Glycolipids

• Formed by the addition of carbohydrates to lipids.

• Can be glycerol-based (glycoglycerolipids) or sphingosine-based (glycosphingolipids).

• Cerebrosides and gangliosides are prominent in nerve and brain tissue; gangliosides also function as antigens on the plasma membrane surface.

Sterols

• Present in most eukaryotic membranes.

• Cholesterol is the main sterol in animal cells; phytosterols in plants; ergosterol in fungi.

• Stabilize and maintain membrane fluidity.

Fatty Acids in Membrane Structure

• Fatty acids (except in sterols) are essential for membrane structure, providing a hydrophobic barrier to polar solutes.

• Typical chain length: 12–20 carbons (optimal for bilayer formation; membrane thickness ~6–8 nm).

• Saturation: Fatty acids may be saturated (no double bonds) or unsaturated (one or more double bonds).

• Polyunsaturated fatty acids (e.g., linolenate 18:3, arachidonate 20:4) are important for membrane fluidity and human health (e.g., omega-3 fatty acids).

Fatty Acid Saturation and Membrane Fluidity

• Saturated fatty acids pack tightly, reducing membrane fluidity and increasing melting temperature ().

• Unsaturated fatty acids (with double bonds) introduce kinks, preventing tight packing, increasing fluidity, and lowering .

• Shorter chains and more unsaturation both increase fluidity.

Effects of Sterols on Membrane Fluidity

• Cholesterol intercalates between phospholipids, decreasing fluidity at high temperatures and preventing gelling at low temperatures.

• Acts as a fluidity buffer to maintain optimal membrane consistency.

Regulation of Membrane Fluidity

• Organisms adjust membrane lipid composition to maintain fluidity, especially poikilotherms (organisms with variable body temperature).

• Homeoviscous adaptation: Altering fatty acid length and saturation in response to temperature changes.

• Desaturase enzymes introduce double bonds as needed; oxygen availability at low temperatures facilitates this process in plants and yeasts.

Membrane Asymmetry

• Lipids are distributed unequally between the two monolayers of the bilayer.

• Most glycolipids are found in the outer leaflet of the plasma membrane.

• Asymmetry is established during membrane synthesis and maintained by specific enzymes.

Phospholipid Movement Within Membranes

• Phospholipids can rotate and diffuse laterally within their monolayer (rapid and random).

• Transverse diffusion (flip-flop): Rare, but catalyzed by enzymes such as scramblases and flippases.

• Scramblases transfer phospholipids between leaflets randomly; flippases move specific lipids to the cytosolic side.

• The same membrane leaflet always faces the cytosol during vesicle trafficking.

Membrane Proteins: The "Mosaic" Component

Types of Membrane Proteins

• Integral membrane proteins: Embedded in the lipid bilayer due to hydrophobic regions; difficult to remove; include monotopic and transmembrane proteins.

• Peripheral membrane proteins: Hydrophilic, bound to membrane surfaces via electrostatic interactions and hydrogen bonds; easily removed by changing pH or ionic strength.

• Lipid-anchored proteins: Hydrophilic proteins covalently attached to lipid molecules embedded in the bilayer.

Transmembrane Proteins

• Span the membrane one or more times (singlepass or multipass).

• Transmembrane segments are typically α-helices (20–30 hydrophobic amino acids) or, less commonly, β-barrels (in pore-forming proteins).

• Orientation: C-terminus and N-terminus may be on opposite sides of the membrane.

Glycosylation of Membrane Proteins

• Glycosylation: Addition of carbohydrate chains to proteins, either N-linked (to asparagine) or O-linked (to serine, threonine, or modified lysine/proline).

• Carbohydrate chains can be straight or branched, 2–60 sugar units long.

• Common sugars: galactose, mannose, N-acetylglucosamine, sialic acid.

Functions of Glycoproteins

• Prominent in plasma membranes, especially for cell-cell recognition.

• Carbohydrate groups protrude from the outer surface of the membrane.

• Lectins: Plant proteins that bind specific sugars, used to study glycoproteins.

Experimental Evidence for Membrane Fluidity

Frye and Edidin Cell Fusion Experiments

• Human and mouse cells were fused and labeled with fluorescent antibodies (green for mouse, red for human).

• After fusion, red and green membrane proteins intermixed, demonstrating lateral mobility of membrane proteins.

Membrane Models and Artificial Membranes

Detergents and Lipid Structures

• Detergents: Amphipathic molecules that form micelles and can solubilize membranes.

• Liposomes: Artificial vesicles formed from phospholipids in aqueous solution.

• Detergents can incorporate lipids and proteins into micelles, sometimes preserving protein function.

Lipid Nanoparticles and mRNA Vaccine Delivery

• Lipid nanoparticles (LNPs) are used to deliver mRNA in vaccines.

• Components include cationic lipids, helper lipids, cholesterol, and RNA.

• LNPs protect mRNA and facilitate its entry into cells.

Table: Major Classes of Membrane Lipids

Table: Types of Membrane Proteins

Key Equations

• Melting temperature (): The temperature at which a membrane transitions from a gel to a fluid state.

Summary

• Cell membranes are dynamic, complex structures essential for cellular function.

• The fluid mosaic model describes the organization of lipids and proteins in membranes.

• Membrane composition and fluidity are tightly regulated to maintain proper function.

• Experimental evidence supports the lateral mobility of membrane components.