1.3 Membrane structure

Nature of science: * Using models as representations of the real world; there are alternative models of membrane structure * Falsification of theories with one theory being superseded by another: evidence falsified the Davson-Danielli model Understandings: * Phospholipids form bilayers in water due to amphipathic properties * Membrane proteins are diverse * Cholesterol is a component of animal cell membranes Applications: Cholesterol in mammalian membranes reduces membrane fluidity and permeability Skills: Drawing the fluid mosaic model Analysis of evidence from electron microscopy that led to proposal of Davson-Danielli model Analysis of falsification of the Davson-Danielli model that led to the Singer-Nicolson model

Understanding: Phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules. Hydrophilic = water-loving (attracted to water) Hydrophobic = water-fearing (not attracted to water) Amphipathic = both (part of molecule attracted to water, other part not) Ambi- and Amphi- = two or both Name a word that starts with ambi- or amphi-….??? Phospholipid = phosphate head + hydrocarbon tail

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1920s – Gorter & Grendel = bilayer model Davson-Danielli model = Sandwich model Nature of Science: Using models as representations of the real world: there are alternative models of membrane structure

1970s – Danielli = protein lining the pores in the membrane model 1970s – Singer & Nicolson = Fluid Mosaic Model

Fluid Mosaic model Fluid = phospholipids & proteins can move laterally within each layer Mosaic = protruding proteins appear dark in electron microscope & are like tiles in a mosaic.

Nature of Science: Falsification of theories: evidence falsified the Davson-Danielli model Freeze-etched electron micrographs – showed transmembrane proteins Structure of membrane proteins – found proteins that were globular in shape and hydrophobic (attracted to hydrocarbon tails inside membranes) Fluorescent antibody tagging – mixed throughout membrane, showing proteins can move within membrane; not fixed on top

Oliver Cromwell’s Rule “Think it possible that you might be mistaken.”

Skill: Evidence for and against the Davson-Danielli model of membrane structure For homework! P 28, purple box, #1-4 Due Monday!

Understanding: Membrane proteins are diverse in terms of structure, position in the membrane & function. 1.Hormone-binding sites (hormone receptors, e.g. insulin receptor) 2.Immobilized enzymes with active site on outside 3.Cell adhesion to form tight junctions between cells 4.Cell-to-cell communication (e.g. receptors for neurotransmitters at synapses) 5.Channels for passive transport (to allow hydrophilic molecules across membrane by facilitated diffusion) 6.Pumps for active transport (use ATP to move molecules across membrane)

Understanding: Cholesterol is a component of animal cell membranes. In animal cell membranes Cholesterol = lipid, but not fat or oil… it is a STEROID. Mostly hydrophobic – attracted to tails of phospholipid Hydroxyl group is hydrophilic – attracted to head Located between phospholipids

Application: Cholesterol in mammalian membranes reduces membrane fluidity & permeability to some solutes. Membrane is fluid – it’s free to move Fluidity is controlled Cholesterol disrupts regular packing of tails, so prevents them crystallizing and being solid Restricts motion and controls fluidity Reduces permeability to hydrophilic molecules such as sodium and hydrogen ions Helps membranes curve, to form vesicles during endocytosis 4 fused rings; yellow

Cell Membrane 3-D Pop-Up Model Create a 3-D model (sort of like a page in a kids’ pop-up book) of the phospholipid bilayer of an animal cell. Use any materials available. Label each of these structures: Phospholipid bilayer Hydrophilic head Hydrophobic tail Integral protein Peripheral protein Glycoprotein Cholesterol Protein channel