Membrane Structure 1.3

Essential Idea: The structure of biological membranes makes them fluid and dynamic 1.3 Membrane Structure Understandings: Phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules   Membrane proteins are diverse in terms of structure, position in the membrane and functions Cholesterol is a component of animal cell membranes Applications: Cholesterol in mammalian membranes reduces membrane fluidity and permeability to some solutes Skills: Draw the fluid mosaic model Analyze evidence from electron microscopy that led to the proposal of the Davson-Danielli model Analyze the falsification of the Davson-Danielli model that led to the Singer-Nicolson model

I. Nature of Science Davson-Danielli Model – proposed 1935 1. When viewed under a transmission electron microscope, membranes exhibit a characteristic ‘trilaminar’ appearance (3 layers - two dark outer layers and a lighter inner region)

2. Danielli and Davson proposed a model whereby two layers of protein flank a central phospholipid bilayer. 3. The model was described as a ‘lipo- protein sandwich’, as the lipid layer was sandwiched between two protein layers. 4. The dark segments seen under electron microscope were identified (Wrongly) as representing the two protein layers.

Fluid Mosaic Model – current model 1. Seymour Singer and Garth Nicolson – 1972 2. Proteins embedded into bilayer a. Realized membranes are not identical or symmetrical b. Knew different functions required different structures c. Deduced a protein layer would be largely non-polar

3. Supporting evidence from electron microscopy Light Microscope Electron Microscope Pass light through sample Pass electrons through sample Inexpensive Very Expensive Simple specimen preparation Complex specimen preparation Magnifies up to 2000x Magnifies up to 500,000x Can use live or dead specimens Must use dead specimens (prep would kill them)

II. Fluid Mosaic Model

Phospholipids 1. Glycerol backbone with 2 fatty acid chains and 1 phosphate group with organic alcohol 2. Fatty acids compose non-polar, hydrophobic tails 3. Phosphate-based group composes polar, hydrophilic heads 4. Amphipathic – part of the molecule is polar, part non- polar

5. Align into a bilayer in aqueous environments a. Attraction between fatty acid tails in very weak  flexibility

6. Properties of the Phospholipid Bilayer a 6. Properties of the Phospholipid Bilayer a. The bilayer is held together by weak hydrophobic interactions between the tails b. Hydrophilic/ hydrophobic layers restrict the passage of many substances c. Individual phospholipids can move within the bilayer, allowing for membrane fluidity and flexibility.

Cholesterol 1. Regulates flexibility and fluidity 2. Only found in animal cells (lack of cell wall) 3. Disrupts the regular packing of tails – increases flexibility 4. Restricts molecular motion because more bonds are formed – decreases flexibility 5. Reduces permeability 6. Helps form vesicles

7. Cholesterol is also an amphipathic molecule a. Cholesterol's hydroxyl (-OH) group is hydrophilic and aligns towards the phosphate head of phospholipids. b. The remainder of the molecule (steroid ring and hydrocarbon tail) is hydrophobic and associates with the phospholipid

Proteins Responsible for diversity of membrane function The amino acids of membrane proteins are located based on their polarity (non-polar amino acids adjacent to hydrophobic tails, polar amino acids next to aqueous interior and exterior of cell and hydrophilic heads)

3. Two major types a. Integral – go through head and tail regions of bilayer b. Peripheral proteins – only associate with polar region of bilayer i. Often anchored to an integral protein

3. Glycoproteins – have carbohydrates attached a 3. Glycoproteins – have carbohydrates attached a. Act in cell-to-cell recognition

4. Functions - Membrane proteins can serve a variety of key functions: Junctions-Serve to connect and join two cells together. Enzymes –fixing to membranes localizes metabolic pathways Transport- Responsible for facilitated diffusion and active transport. Recognition- May function as markers for cellular identification Anchorage- Attachment points for cytoskeleton and extracellular matrix Transduction – function as receptors for peptide hormones. *JET RAT