Fluid Mosaic Model of Cell Membranes Flashcards
Membrane Functions
- Site of biochem functions e.g. ETC in respiration
- A permeable barrier, allowing certain molecules in
- Organisation + localisation in the cell
-Transport in/out of cell through proteins that regulate movement of substances - Detect + transmit chemical + electrical signals with proteins acting as receptors
- Cell adhesion
The Fluid Mosaic Model for Membrane Structure - Brief Summary
By Singer and Nicolson (1973)
- some proteins are embedded into the membrane so has an irregular structure
- is very fluid
How did Ernst Overton conclude cells have a selectively permeable barrier?
- Discovered that lipid-soluble substances crossed easily while water-soluble substances didn’t
=> Concluded that lipids were present in the barrier
How did Katharine Blodgett and Irving Langmuir discovered lipids are hydro -phobic/ -philic
1910s/1920s
- Added clean water to a trough
- Then added lipids, forming a layer on top with fatty acid tails orienting away from water
What did Gorter and Grendel do to discover membranes are bilayers?
- Used red blood cells as they only have plasmid membrane
- Mixed them with benzene to extract lipids, added then to trough so solvent evaporates
- Area occupied were used to find surface area of cell
=> was double surface area of RBCs
Errors in Gorter and Grendel’s Methods
- Assummed solvent would extract all lipids - was only 2/3
- Ended up underestimating surface area
Bilayer organization confirmed by electron microscopy in the 1950s
Problems with the idea of a biological membrane being a simple lipid bilayer
- Surface tension, electrical resistance, and solute permeability were different from a lipid bilayer
- Couldn’t explain how selective a cell is e.g galactose can’t enter but glucose cant
Model for membrane structure proposed by Hugh Davson and James Danielli (1935)
- Due to emerging concept (at the time) of enzyme-substrate specificity
- Theoretical model of a bilayer of lipids coated in protein (‘Sandwich Model’)
Fluid Mosaic Model of Membrane Structure - In detail
- Membranes containing lipids and proteins
- Phospholipids form a bilayer due to their amphipathic nature
- Phospholipids move freely causing membrane fluidity
- Some proteins move, some are restricted
- Proteins aren’t uniformly distributed (not an even coating)
- Membranes are asymmetrical + selectively permeable
Membrane Topology (they have 2 sides)
Exoplasmic side faces the surrounding
Cytoplasmic side faces inside the cell
Membrane lipids
Predominant lipids are phospholipids (have glyceride backbone)
e.g. phosphatidylcholine, phosphatidylserine, phosphatidyletanolamine
Typical membrane phospholipid structure
- Hydrophilic head: phosphate, glycerol + e.g. choline
- Hydrophobic (fatty acid) tails: hydrocarbon tails (2)
Are amphipathic molecules - drives formation of lipid bilayers
Hydrophobic forces
Water forces hydrophobic groups together to minimise their disruptive effects on the hydrogen-bonded network
Formation of sealed spherical compartment
- It is energetically unfavourable to have a planar phospholipid bilayer
- The aqueous environment in and out is sealed by the bilayer => energetically favourable
Vanderwaals Forces in Membranes
- At a short distance, 2 atoms show weak bonding interaction due to fluctuating electrical charge
- If too close together, they repel each other
Individually, interactions are very weak but can form strong forces when macromolecules come close together
