Lecture 3
Structure and function of the Plasma Membrane:
Membrane Structure:
Cellular membranes are very dynamic
Plasma Membrane:
Performs functions essential for life
- Protective barrier of 7.5-10 nm thick. separates living cell from its surroundings,
- Composed of lipids (Bilayer), proteins and carbohydrates
- Important functions such as
• cell identity and function, ext surface - glycoproteins give rise to cell identity
• (signal transduction) - cell communication
• cell composition - controlling the movement of ions, nutrients and proteins etc in and out of
cell
Features of biological membrane:
Membranes are sheet like structures a few molecules thick that form closed boundaries
Consist of lipid, proteins and carbohydrates - linked to lipids or proteins
Are non covalent assemblies, held by es interactions
Are fluid structures
Most are asymmetric in composition , (phospholipids are either inside or outside pm ?)
Most electrically polarised, generated by permeability properties of membrane
Composition:
Membrane lipids:
, - A membrane lipid is an amphipathic molecule containing a hydrophilic and a hydrophobic
moiety
Lipid bilayer formation:
- Amphipathic molecules spontaneously form bilayers in aqueous solutions and as non
polar group cant H bond with water so H bonds form around it so more ordered and this is
not preferred so instead polar phosphate hydrophilic head points outward to the aqueous
solution while non polar hydrophobic tails point inwards away from water.
- 2 molecules thick, forms closed boundaries
- Co-operative structures - es forces between HP fa tails, H bonds
- Tendency to be extensive in size, can make cells of diff sizes
- Tend to close in on themselves to prevent exposed hydrocarbon chains)
- Self sealing damage
Liposomes - synthetic lipids mixed with water to introduce components to aqueous
phase of cell
There is actually a dramatic asymmetry in composition of lipids in outer and inner part of
membrane
The phospholipids can flip, which is a difficult and slow process, and can be catalysed by
enzyme flippase
Lipids do different things in inner and outer part of membrane
What are the 3 major membrane lipids?
Phospholipids
Glycolipids
Cholesterol
Structure of phospholipids:
- Phosphoglycerates (glycerol backbone) of phospholipid
- 2 fatty acid chains can be: saturated = linear and unsaturated = kink (for most eukaryotic
molecules, principal unsaturated fatty acid type is mono unsaturated = single C bond)
- Can have unsaturated or saturated or mixture.
- Phosphorylated alcohol: choline, ethanolamine, serine, inositol
- Or sphingomyelin (sphingosine backbone)
Structure of glycolipids:
Structure and function of the Plasma Membrane:
Membrane Structure:
Cellular membranes are very dynamic
Plasma Membrane:
Performs functions essential for life
- Protective barrier of 7.5-10 nm thick. separates living cell from its surroundings,
- Composed of lipids (Bilayer), proteins and carbohydrates
- Important functions such as
• cell identity and function, ext surface - glycoproteins give rise to cell identity
• (signal transduction) - cell communication
• cell composition - controlling the movement of ions, nutrients and proteins etc in and out of
cell
Features of biological membrane:
Membranes are sheet like structures a few molecules thick that form closed boundaries
Consist of lipid, proteins and carbohydrates - linked to lipids or proteins
Are non covalent assemblies, held by es interactions
Are fluid structures
Most are asymmetric in composition , (phospholipids are either inside or outside pm ?)
Most electrically polarised, generated by permeability properties of membrane
Composition:
Membrane lipids:
, - A membrane lipid is an amphipathic molecule containing a hydrophilic and a hydrophobic
moiety
Lipid bilayer formation:
- Amphipathic molecules spontaneously form bilayers in aqueous solutions and as non
polar group cant H bond with water so H bonds form around it so more ordered and this is
not preferred so instead polar phosphate hydrophilic head points outward to the aqueous
solution while non polar hydrophobic tails point inwards away from water.
- 2 molecules thick, forms closed boundaries
- Co-operative structures - es forces between HP fa tails, H bonds
- Tendency to be extensive in size, can make cells of diff sizes
- Tend to close in on themselves to prevent exposed hydrocarbon chains)
- Self sealing damage
Liposomes - synthetic lipids mixed with water to introduce components to aqueous
phase of cell
There is actually a dramatic asymmetry in composition of lipids in outer and inner part of
membrane
The phospholipids can flip, which is a difficult and slow process, and can be catalysed by
enzyme flippase
Lipids do different things in inner and outer part of membrane
What are the 3 major membrane lipids?
Phospholipids
Glycolipids
Cholesterol
Structure of phospholipids:
- Phosphoglycerates (glycerol backbone) of phospholipid
- 2 fatty acid chains can be: saturated = linear and unsaturated = kink (for most eukaryotic
molecules, principal unsaturated fatty acid type is mono unsaturated = single C bond)
- Can have unsaturated or saturated or mixture.
- Phosphorylated alcohol: choline, ethanolamine, serine, inositol
- Or sphingomyelin (sphingosine backbone)
Structure of glycolipids:
