Membranes: Structure, Function, and Chemistry – Study Guide

Membranes: Structure, Function, and Chemistry

Overview of Membrane Structure

Cell membranes are essential for defining cellular boundaries and compartmentalizing functions. The modern understanding of membranes is encapsulated in the Fluid Mosaic Model, which describes membranes as a dynamic sea of phospholipids with embedded proteins.

• Fluid Mosaic Model: Membranes are composed of a lipid bilayer with proteins floating within, allowing flexibility and diversity.

• Membrane Diversity: Variations in lipid and protein composition create a wide array of membrane types across organisms and cell compartments.

Functions of Membranes

Membranes perform multiple critical roles in cellular life, including:

• Boundary Definition: Separate the cell from its environment and compartmentalize organelles.

• Permeability Barrier: Hydrophobic interior prevents free passage of polar molecules.

• Biological Function Sites: Host processes like electron transport (mitochondria) and protein processing (ER).

• Transport Regulation: Membrane proteins control movement of substances.

• Signal Detection: Receptors detect and transmit external signals (signal transduction).

• Cell Adhesion & Communication: Mediate cell-to-cell contact, adhesion, and communication via junctions.

Membrane Lipids: The "Fluid" Component

Lipids are fundamental to membrane fluidity and structure. The main classes are phospholipids, glycolipids, and sterols.

• Phospholipids: Most abundant; amphipathic with a polar head and two non-polar fatty acid tails. Backbone can be glycerol or sphingosine.

• Glycolipids: Lipids with carbohydrate groups; include cerebrosides and gangliosides (sphingosine-based).

• Sterols: Cholesterol (animals), phytosterols (plants), ergosterol (fungi); stabilize membranes and modulate fluidity.

• Hopanoids: Bacterial analogs to sterols.

Fatty Acids in Membranes

• Chain Length: Typically 12–20 carbons, optimal for bilayer formation.

• Saturation: Saturated (no double bonds) vs. unsaturated (one or more double bonds). Unsaturated fatty acids increase fluidity.

• Polyunsaturated Fatty Acids: Multiple double bonds; e.g., linoleate (18:3), arachidonate (20:4), omega-3 fatty acids.

Membrane Asymmetry

• Asymmetry: Lipids are distributed unequally between the two monolayers; glycolipids are often in the outer layer.

• Maintenance: Asymmetry is established during synthesis and maintained; flip-flop (transverse diffusion) is rare and catalyzed by flippases.

Lipid Mobility and Fluidity

• Lateral Diffusion: Lipids move rapidly within their monolayer.

• Rotation: Phospholipids rotate about their axes.

• Phase Transition: Membranes have a transition temperature ($T_m$) where they shift from gel to fluid state.

• Measurement: Differential scanning calorimetry measures $T_m$.

Factors Affecting Membrane Fluidity

• Fatty Acid Chain Length: Longer chains increase $T_m$ (less fluid).

• Degree of Unsaturation: More double bonds lower $T_m$ (more fluid).

• Sterols: Cholesterol acts as a fluidity buffer, decreasing fluidity at high temperatures and preventing gel formation at low temperatures.

• Homeoviscous Adaptation: Organisms adjust membrane lipid composition to maintain fluidity, especially poikilotherms.

• Desaturase Enzyme: Introduces double bonds into fatty acids, increasing unsaturation and fluidity.

Lipid Microdomains

• Lipid Rafts: Microdomains that sequester proteins and lipids for specific functions; dynamic and transient.

• Nanodomains: Small, short-lived groupings of select lipids.

Techniques for Lipid Analysis

• Thin-Layer Chromatography (TLC): Separates lipids based on polarity using a silicic acid-coated plate and nonpolar solvents.

Membrane Proteins: The "Mosaic" Component

Proteins are the main functional components of membranes, existing in various forms and orientations.

• Freeze-Fracture Microscopy: Reveals protein distribution and supports the fluid mosaic model.

• Protein/Lipid Ratio: Varies among cell types and membranes.

Classes of Membrane Proteins

Class	Location	Extraction Method
Integral	Embedded in bilayer	Detergents
Peripheral	Surface of bilayer	pH/ionic strength changes
Lipid-Anchored	Attached via lipid	Similar to peripheral

• Integral Membrane Proteins: Hydrophobic regions embedded in bilayer; include monotopic (one side) and transmembrane (span both sides) proteins.

• Transmembrane Proteins: Singlepass (one segment) or multipass (multiple segments); often α-helical or β-barrel (porins).

• Peripheral Proteins: Bound to membrane surfaces via weak interactions; easily removed.

• Lipid-Anchored Proteins: Covalently attached to lipids (fatty acids, isoprenyl groups, GPI anchors).

Isolation and Analysis of Membrane Proteins

• Solubilization: Detergents disrupt hydrophobic interactions for integral proteins; peripheral and lipid-anchored proteins are easier to extract.

• Gel Electrophoresis: Separates proteins by size and charge; polyacrylamide or agarose media.

• Western Blotting: Identifies proteins post-electrophoresis.

• Affinity Labeling: Uses radioactive molecules to identify functional proteins.

• Membrane Reconstitution: Proteins mixed with phospholipids to form liposomes for functional studies.

Determining Protein Structure

• X-ray Crystallography: Determines 3D structure; challenging for membrane proteins.

• Hydropathy Analysis: Predicts transmembrane segments using hydrophobicity plots.

• Molecular Biology: DNA sequencing and site-specific mutagenesis reveal protein function and structure.

Functions of Membrane Proteins

• Enzymes: Localize specific functions to membranes; serve as organelle markers.

• Transport Proteins: Facilitate movement of solutes; channel proteins and ATPases.

• Receptors: Mediate signal transduction; respond to hormones, neurotransmitters, etc.

• Cell Communication: Connexons (gap junctions), plasmodesmata (plants).

• Endocytosis/Exocytosis: Uptake and secretion of substances.

• Structural Support: Cytoskeletal meshwork stabilizes membrane shape.

Protein Orientation and Mobility

• Asymmetric Orientation: Proteins are oriented consistently across the bilayer; do not flip-flop.

• Labeling Techniques: Radioactive labeling distinguishes inner and outer surface proteins.

• Glycosylation: Addition of carbohydrate chains (N-linked or O-linked) in ER and Golgi; important for cell recognition.

Glycoproteins and Glycolipids

• Glycoproteins: Carbohydrate chains attached to proteins; prominent in plasma membranes for cell-cell recognition.

• Lectins: Plant proteins used to study glycoproteins.

• Glycolipids: Carbohydrate groups attached to lipids; ABO blood group system depends on glycolipid structure.

• Glycocalyx: Surface coat formed by glycoproteins and glycolipids; functions in recognition, adhesion, protection, and permeability.

Protein Mobility and Membrane Domains

• Mobility: Some proteins move freely; others are restricted by anchoring or aggregation.

• Experimental Evidence: Cell fusion (Frye and Edidin) and FRAP demonstrate protein mobility and diffusion rates.

• Membrane Domains: Areas with distinct protein composition and function; tight junctions restrict diffusion.

• Anchoring: Proteins anchored to cytoskeleton or extracellular structures limit mobility.

Erythrocyte Membrane Structure

• Meshwork: Peripheral and integral proteins support membrane and maintain cell shape.

• Main Peripheral Proteins: Spectrin, ankyrin, band 4.1 protein; provide mechanical support.

• GAPDH: Peripheral protein involved in glucose catabolism.

Summary Table: Major Classes of Membrane Lipids

Lipid Class	Structure	Function	Location
Phospholipids	Glycerol/sphingosine backbone, fatty acid tails, phosphate head	Structural, permeability barrier	All membranes
Glycolipids	Lipid + carbohydrate	Cell recognition	Outer plasma membrane
Sterols	Four-ring structure (cholesterol, etc.)	Fluidity modulation	Eukaryotic membranes

Key Equations

• Transition Temperature ($T_m$): The temperature at which a membrane shifts from gel to fluid state.

• Hydropathy Index: Used in hydropathy plots to predict transmembrane segments.

$T_m \text{ increases with chain length and saturation; decreases with unsaturation and shorter chains}$

$\text{Hydropathy Index} = \sum_{i=1}^{n} \text{hydrophobicity of residue}_i$

Additional info:

• Trans fats, created by hydrogenation, increase cardiovascular risk due to their packing properties similar to saturated fats.

• Lectins combined with ferritin allow visualization of glycoproteins in electron microscopy.

• Homeoviscous adaptation is especially important for organisms unable to regulate body temperature (poikilotherms).