TheCell7e Ch14 Lecture

The Plasma Membrane

Introduction

  • All cells are surrounded by a plasma membrane.

  • Functions of the plasma membrane:

    • Defines the cell boundary and separates it from its environment.

    • Serves as a selective barrier, determining the composition of the cytoplasm.

    • Mediates interactions between the cell and its environment.

Structure of the Plasma Membrane

Phospholipid Bilayer

  • The fundamental structure is the phospholipid bilayer.

  • Proteins embedded in the bilayer perform specific functions:

    • Selective transport of molecules.

    • Cell-cell recognition.

Models and Studies

  • Mammalian red blood cells (erythrocytes) are valuable models for studying membrane structure due to their lack of nuclei and internal membranes.

Morphology

  • Electron micrographs show bilayer structure:

    • Polar head groups appear as dark lines; hydrophobic fatty acid chains are lightly stained.

Phospholipid Composition

  • Mammalian plasma membranes have five major phospholipids:

    • Outer leaflet: Phosphatidylcholine and sphingomyelin.

    • Inner leaflet: Phosphatidylethanolamine, phosphatidylserine, and phosphatidylinositol.

Lipid Composition Table (Mole percent)

  • Phosphatidylcholine: 20%

  • Phosphatidylethanolamine: 11%

  • Phosphatidylserine: 4%

  • Phosphatidylinositol: 2%

  • Cholesterol: 49%

  • Sphingomyelin: 13%

  • Glycolipids: 1%

Properties of the Bilayer

  • The bilayer is a viscous fluid:

    • Fatty acids contain double bonds which create kinks, preventing tight packing.

    • Lipids and proteins can diffuse laterally within the membrane.

  • Cholesterol influences membrane fluidity and forms lipid rafts with sphingolipids.

Fluid Mosaic Model

  • Proposed by Singer and Nicolson (1972):

    • Membranes are two-dimensional fluids with proteins embedded in lipid bilayers, capable of lateral diffusion.

  • Lateral movement demonstration: Human and mouse cells fused in culture showed intermixing of membrane proteins within 40 minutes.

Membrane Proteins

Peripheral Membrane Proteins

  • Associate with the membrane through protein-protein interactions, primarily ionic bonds.

  • Can be disrupted by polar reagents.

  • Often part of the cortical cytoskeleton (e.g., spectrin, actin).

Integral Membrane Proteins

  • Inserted into the lipid bilayer and can only be dissociated by agents disrupting hydrophobic interactions (detergents).

Transmembrane Proteins

  • Span the lipid bilayer, with portions exposed on both sides; coherent structures visible via freeze-fracture electron microscopy.

Examples of Transmembrane Proteins

  • Glycophorin: Single transmembrane α helix.

  • Band 3: Transporter for bicarbonate and chloride ions with 14 transmembrane α helices.

Protein Anchoring

  • Some proteins are anchored by covalently attached lipids (e.g., GPI anchors) or by myristic acid, prenyl groups, or palmitic acid.

Glycocalyx and Membrane Domains

Glycocalyx

  • Formed by oligosaccharides of glycolipids and glycoproteins.

  • Protects cell surface from ionic and mechanical stress and forms barriers to microorganisms.

Membrane Domains

  • Many epithelial cells are polarized, with plasma membranes divided into apical and basolateral domains.

  • Tight junctions separate these domains, allowing movement of proteins within domains but preventing cross-movement.

Transport of Small Molecules

Selective Permeability

  • Plasma membranes selectively permit small molecules to pass through.

  • Transport proteins mediate the passage of glucose, amino acids, and ions.

Facilitated Diffusion

  • Movement is determined by concentration gradients; does not require energy. Transport is facilitated by proteins allowing polar/charged molecules across the membrane.

Carrier Proteins

  • Bind molecules on one side, undergoing conformational changes to release them on the other side.

Channel Proteins

  • Create open pores allowing free diffusion of appropriately sized and charged molecules, exemplified by aquaporins for water molecules.

Rapid Transport

  • Ion channels allow for rapid transport of ions, with specific channels for Na+, K+, Ca2+, and Cl–, often gated by signals or changes in membrane potential.

Action Potentials

  • Hodgkin and Huxley pioneered the study of ion currents in nerve signaling, demonstrating how Na+ and K+ channels affect membrane potential changes during action potentials.

Active Transport

Sodium-Potassium Pump

  • Active transport mechanism powered by ATP hydrolysis; 3 Na+ are pumped out for every 2 K+ pumped in.

Other Active Transport Mechanisms

  • Additional pumps, such as Ca2+ pumps, maintain low intracellular Ca2+ concentrations, critical for cell signaling.

ABC Transporters

  • Use ATP hydrolysis to transport molecules in one direction; crucial in various cellular processes, including detoxification in cancer cells.

Endocytosis

Types of Endocytosis

  • Allows cells to uptake large particles and molecules:

    • Phagocytosis: "Cell eating" involving extension of pseudopodia.

    • Clathrin-mediated endocytosis: Specific uptake mechanism for macromolecules, involving receptor binding and vesicle formation.

Clathrin-Mediated Endocytosis

  • Involves LDL uptake and mutational studies informing on receptor function in hypercholesterolemia.

Significance

  • Endocytic processes such as receptor-mediated endocytosis are important for nutrient uptake and cellular response to environmental changes.