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
Membrane Asymmetry
Differs based on what side of membrane
Biological membranes are asymmetrical with respect to phospholipids
- Exoplasmic: phosphatidylcholine (PC) and sphingomyelin (SM)
- Cytosolic: phosphatidylserine (PS) and phosphatidylethanolamine (PE)
- Asymmetry is established and maintained by scramblases and flippases
Lipid bilayer synthesis: the role of Scramblases
- New phospholipids are only added to cytosolic side
- Transfers them from one side of bilayer to another to ensure equal growth of membrane
- Not specific to certain phospholipids
Lipid bilayer synthesis: the role of Flippases
Are head-group specific phospholipid translocators to transfer PS + PE from exoplasmic side to cytosolic side
- Operate continuously to counteract scramblases in plasma membrane - ensure 2 sides of bilayer are balanced
Cholesterol
Are abundant in eukaryotic cells - stabilises bilayer and thus regulates membrane fluidity
Structure
- Hydrophilic polar head group
- Rigid planar steroid ring structure
- Non-polar hydrocarbon tail
