Cell Membrane

Introduction

Cell membrane: essential cellular boundary controlling molecular exchange, signal reception, and structural integrity. Composed primarily of lipids, proteins, carbohydrates; dynamic and selectively permeable. Central to cell communication, transport, and homeostasis.

"The cell membrane is not merely a barrier but a complex, dynamic system that integrates cellular functions." -- Singer & Nicolson, 1972

Structure

Bilayer Arrangement

Phospholipids form a bilayer: hydrophilic heads outward, hydrophobic tails inward. Thickness: ~7-8 nm. Amphipathic nature enables barrier and fluidity.

Lipid Rafts

Microdomains enriched in cholesterol, sphingolipids. Function: organize signaling molecules, trafficking platforms.

Asymmetry

Outer leaflet: glycolipids, phosphatidylcholine. Inner leaflet: phosphatidylserine, phosphatidylethanolamine. Maintained by flippases, floppases, scramblases.

Composition

Lipids

Phospholipids (~50% mass): phosphatidylcholine, phosphatidylserine, phosphatidylethanolamine. Cholesterol modulates fluidity and permeability.

Proteins

Integral and peripheral proteins account for ~50% of membrane mass. Functions: transport, enzymatic activity, signal reception.

Carbohydrates

Glycolipids and glycoproteins present on extracellular side. Roles: cell recognition, adhesion, protection.

Fluid Mosaic Model

Historical Context

Proposed 1972 by Singer and Nicolson. Describes membrane as fluid lipid bilayer with embedded proteins.

Fluidity

Lipids and proteins move laterally. Fluidity essential for function: transport, signaling, growth.

Mosaic Nature

Proteins vary in size, shape, function. Some span bilayer, others anchored peripherally. Dynamic organization.

Membrane Proteins

Integral Proteins

Span bilayer, hydrophobic regions inside membrane. Examples: channels, carriers, receptors.

Peripheral Proteins

Bind to membrane surface or integral proteins. Roles: signaling complexes, cytoskeletal attachment.

Lipid-Anchored Proteins

Covalently attached to lipids within bilayer. Function in signal transduction, membrane trafficking.

Membrane Fluidity

Factors Affecting Fluidity

Temperature: higher increases fluidity. Cholesterol: stabilizes by preventing packing. Fatty acid saturation: unsaturated increase fluidity.

Biological Implications

Fluidity modulates protein mobility, fusion, endocytosis, signal transduction.

Regulation Mechanisms

Cells adjust lipid composition via desaturases, cholesterol content to maintain optimal fluidity.

Selective Permeability

Barrier Function

Hydrophobic core restricts polar molecule passage. Controls ion, nutrient, waste transport.

Permeability to Molecules

Small nonpolar molecules diffuse freely. Ions, large polar molecules require transport proteins.

Role in Homeostasis

Maintains ionic gradients, osmotic balance, cellular environment stability.

Transport Mechanisms

Passive Transport

Diffusion: movement down concentration gradient. Facilitated diffusion via channels, carriers.

Active Transport

Energy-dependent transport against gradient. Examples: Na+/K+ ATPase, proton pumps.

Endocytosis and Exocytosis

Vesicle-mediated bulk transport. Endocytosis internalizes extracellular material. Exocytosis secretes molecules.

Transport Types Summary:- Simple diffusion: no energy, small nonpolar molecules- Facilitated diffusion: no energy, carrier/channel proteins- Primary active transport: ATP-driven pumps- Secondary active transport: gradient-driven co-transport- Bulk transport: vesicle-mediated

Signal Transduction

Receptor Proteins

Bind ligands (hormones, neurotransmitters). Initiate intracellular signaling cascades.

Second Messengers

Examples: cAMP, Ca2+, IP3. Amplify and propagate signals inside cell.

Membrane Microdomains

Lipid rafts concentrate receptors, facilitate signal specificity and efficiency.

Membrane Dynamics

Endocytosis Types

Phagocytosis: uptake of large particles. Pinocytosis: fluid-phase uptake. Receptor-mediated endocytosis: selective cargo internalization.

Membrane Recycling

Vesicles fuse with membrane, recycle proteins/lipids, maintain composition.

Membrane Fusion and Fission

Essential for vesicle trafficking, exocytosis, cell division.

Membrane Biosynthesis

Lipid Synthesis

Occurs in smooth ER. Phospholipids synthesized and inserted into cytoplasmic leaflet.

Protein Integration

Rough ER synthesizes integral membrane proteins. Translocated and folded into bilayer.

Membrane Expansion

Vesicular transport delivers lipids/proteins to plasma membrane. Maintains size and composition.

Membrane Biosynthesis Pathway:1. Lipid synthesis in smooth ER2. Protein synthesis in rough ER3. Vesicle budding from Golgi4. Vesicle fusion with plasma membrane

Membrane Disorders

Genetic Defects

Mutations in membrane proteins cause cystic fibrosis (CFTR), familial hypercholesterolemia (LDL receptor).

Membrane Permeability Alterations

Infections, toxins disrupt barrier leading to edema, inflammation.

Autoimmune Disorders

Antibodies target membrane proteins: myasthenia gravis (acetylcholine receptor).

Experimental Techniques

Electron Microscopy

Visualizes membrane ultrastructure. Freeze-fracture reveals protein distribution.

Fluorescence Recovery After Photobleaching (FRAP)

Measures lateral mobility of lipids and proteins.

Patch Clamp

Records ion channel activity in membranes.

References

• Singer, S.J., Nicolson, G.L. The fluid mosaic model of the structure of cell membranes. Science, 175, 1972, 720-731.
• Alberts, B. et al. Molecular Biology of the Cell. 6th ed., Garland Science, 2014, pp. 287-320.
• Lodish, H. et al. Molecular Cell Biology. 8th ed., W.H. Freeman, 2016, pp. 123-145.
• Simons, K., Ikonen, E. Functional rafts in cell membranes. Nature, 387, 1997, 569-572.
• Janmey, P.A., Kinnunen, P.K.J. Biophysical properties of lipids and dynamic membranes. Trends Cell Biol, 21, 2011, 596-603.