Structure and Function of Membranes Chapter 7

What you need to know! Why membranes are selectively permeable. The role of phospholipids, proteins, and carbohydrates in membranes.

Plasma Membrane (PM) of animal/plant cells and organelles Barrier between interior and exterior Maintains homeostasis Allow for compartmentalization (eukaryotes) PM is selectively permeable for gas, H2O, entering nutrients, exiting waste Blocks harmful substances, microorganisms Controls ion exchange Made from phospholipids

Phospholipids Phospholipids form a bilayer in aqueous solutions the heads (phosphate groups) are polar (hydrophilic) and will form the two outer faces the fatty acid tails are hydrophobic and point toward the inside

Fluid Mosaic Model Structure can be observed with EM Mosaic of floating phospholipids with cholesterol, proteins, glycoproteins, and glycolipids embedded Held together by hydrophobic/hydrophilic interaction and cytoskeleton attached to desmosomes. Flexible and in constant motion –Think of a soap bubble skin rather than saran wrap kuIhttp:// kuI

Fluidity: Phospholipid molecules move around constantly Fluidity regulated by different kinds of fatty acid (FA) tails: More unsaturated FA, membrane stays fluid at lower temp (winter) More saturated FA, membrane is more stable at high temperatures (summer) Cholesterol embedded in animal membranes, keeps FA tails from twisting together

Membrane Molecules Integral – proteins that are embedded in the bilayer –contain hydrophilic ends and hydrophobic mid- sections to mimic the phospholipids –Transmembrane proteins span the entirety of the bilayer (stick out on both ends) Peripheral proteins: not embedded but loosely attached (usually to integral proteins)

Transmembrane Proteins 1.Transport 2.Enzymatic 3.Signal transduction 4.Cell-cell recognition 5.Intercellular joining 6.Attachment to the cytoskeleton and extracellular matrix (ECM)

Membrane Molecules Peripheral Proteins loosely connected on only one side of the membrane Membrane bound pigments in plants Chlorophylls Carotenoids Both sides of the membrane are not identical: Cytoplasmic side Extracellular side Animals: Extracellular Matrix (ECM) Glycoproteins = carbohydrates attached to proteins for self recognition/immune system Collagen fibers for connectivity Plants: Cell wall (1 st, middle lamella, 2 nd )

Intrercellular Connections: Animals: Tight Junctions: allowing movement of material across cell layer/preventing movement of material between cells: digestive system, epithelial cells Desmosomes: tight connection of adjacent cells under high physical stress (muscle, cartilage), attached to cytoskeleton Gap-Junctions: intercellular ion and small molecules transfer

Intrercellular Connections: Plants: Plasmodesma: channel between plant cells where ER goes through (desmotubule)