University:
Felician University
Course:
BIO-103 | General Biology I
Academic year:

2022
Views:

63

Pages:

1
Author:

5CelestialChronicle

membrAne Structure Phospholipid Bilayer Structure of Phospholipids: • Contain a polar (hydrophilic) head composed of phosphate (+ glycerol) • Contain two non-polar (hydrophobic) tails, each composed of a fatty acid chain • Hence, phospholipids are amphipathic (have hydrophilic and hydrophobic parts) Arrangement in Membranes: • Phospholipids spontaneously arrange into a bilayer • The hydrophilic phosphate heads face out into the surrounding solution, while the hydrophobic fatty acid tails face inwards and are shielded from the polar fluids Properties of the Phospholipid Bilayer: • The bilayer is held together by weak hydrophobic interactions between the tails • Individual phospholipids can move within the bilayer (fluidity and flexibility) • Amphipathic properties restrict passage of certain substances (semi-permeable) Polar head Hydrophilic Non-polar tail Hydrophobic Phospholipid Bilayer Cholesterol Membrane Proteins Cholesterol is a fundamental component of animal cell membranes • It is not present in plant cell membranes (as they have a rigid cell wall) Membrane proteins are diverse in terms of their structure and position in a membrane Cholesterol reduces membrane fluidity and permeability to some solutes • It also anchors certain peripheral proteins and prevents crystallization Membrane proteins serve many functions: Cholesterol (amphipathic) • • • • • • Junctions Enzymes Transport Recognition Anchorage Transduction Fluid Mosaic Model Membrane Models Cell membranes are represented as a fluid-mosaic model • Fluid – membrane components can move position • Mosaic – phospholipid bilayer is embedded with protein Membranes appear trilaminar when viewed with an electron microscope (trilaminar = three distinct layers) This model was proposed by Singer-Nicolson in 1972, following the falsification of the Davson-Danielli model integral protein phospholipid peripheral protein cholesterol Davson-Danielli proposed a model whereby a phospholipid bilayer was flanked by two protein layers (sandwich model) This model was falsified based on the following findings: • Fluorescent tagging showed the proteins are mobile • Not all membranes have a constant lipid : protein ratio • Freeze fracturing identified transmembrane proteins Trilaminar appearance Sandwich Model