Fluid Mosaic Model and Freeze Fracture technique

How does the Fluid Mosaic Model describe the structure of the cell membrane

• dynamic

• flexible structure

• made up of a mosaic of different components

‘Fluid’ - why

• phospholipids are constantly moving

‘mosaic’ - why

• protein molecules are scattered throughout the phospholipids like tiles in a mosaic

Functions of Membrane Proteins

• transport across membrane

• cell recognition

• signal reception

• enzymatic activity

  • can change position unless they are structural proteins

Intrinsic Membrane Proteins

• they are embedded

• transmembrane

Extrinsic/Peripheral Membrane Proteins

• located in the inner/outer surface of the phospholipid bilayer

• temporarily bound to phospholipid molecule

Asymmetry of the bilayer

• INNER

  • Fewer cholesterol molecules and more unsaturated fats, making it more flexible for movement.

  • Actin filaments interact with membrane proteins for structure and motility.

  • Intermediate filaments indirectly support membrane stability through connections to desmosomes and other junctions

• OUTER

  • more cholesterol and saturated fats making it more stable for protection

  • contains glycolipids/proteins to form glycolyx (signalling,recognition)

Hydrophobic/Hydrophilic - Integral proteins

• amphipathic

  • hydrophobic regions are embedded in lipid layer - lock protein in place

Peripheral Proteins Hydrophobic/Philic

• Hydrophilic (found in the exterior)

How is the membrane electrically polarized

• separates ions which creates a separation of charge

• membrane potential

what regulates fluidity in animal cells

• cholesterol

What has provided evidence supporting fluid mosaic model

TEM and SEM microscopes

Freeze fracture vs. Freeze etching

Freeze fracture - breaking a frozen specimen to reveal internal structures

Freeze etching - sublimation of surface ice

Describe the Freeze Fracture Technique

1. Cell tissue is embedded in a medium and frozen with the use of liquid nitrogen

2. 2. Microtome is tapped on the frozen tissue splitting the membrane in half

3. Proteins carbs pits etc will pull apart on one side or the other

4. Freeze etching - ice is sublimed away in a vacuum to reveal the etched surface

5. Surface of fractured sample is coated with carbon then platinum vapor - creates a replica

6. Organic material is digested by an acid - leaves the replica

7. Replica is viewed by a TEM - “ see bumps and holes from where the proteins where attached”

Cell membranes are assembled from 4 components…

1. phospholipid bilayer

2. Membrane proteins - embedded or floating on lipid layer

3. Interior protein network

4. Cell surface markers

how are cell surface markers produced and what is their function

1. Membrane components are assembled in the er

2. Transferred to golgi

3. Golgi tags proteins and adds chains of sugar molecules to membrane proteins or lipids

4. Glycoproteins and glycolipids are located on the outside of the cell and act as cell identity markers

5. Serve as recognition sites for other cells and molecules

Glycolipids

Found on red blood cells - antigens

Glycoproteins

Role is self recognition - self non self

MHC recognized by immune system

Proteoglycans

Class of glycoproteins -

signaling

hydration

structural support

make up ecm, fills intracellular spaces