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.