Biological membranes Flashcards
What are the properties of biological membranes?
Act as a barrier
Continuous
Self-repairing (repair after damage)
Flexible
Selectively permeable (only allow passage of certain molecules in and out the cell)
Made of lipids, proteins and carbohydrates
What is the composition of biological membranes?
Lipids (phospholipids and cholesterol/other sterol)
Proteins (channel proteins and carrier proteins)
Carbohydrates(bonds with proteins or lipids to give glycoproteins or glycolipids)
(composition is pretty much the same for all membranes)
What are oligosaccharides and what do they do?
They are a carbohydrate and they covalently bond with proteins (to give glycoproteins) or lipids (to give glycolipids)
They are made up of sugar residues that is linked covalently in branched chains
What is the role of membranes?
Provides a barrier between the environment and cells
What is a plasma membrane?
Provides a cell boundary and is selective with what enters and leaves the cell.
Prevent material movement in and out the cell
What is an organelle membrane?
Provides a similar boundary
Divide the cytoplasm into compartments
Provide a boundary within the boundary. Inside environment is different to outside environment
What are 3 examples of biological membranes?
- Schwaan cell which have specialised plasma membranes to insulate the axon
- Plasma membrane of epithelial cells
- Plasma membrane of rod and cone cells in the eye, these have a stack of membranes
What is a glycosidic bond?
Linkage between 2 glucose molecules between the alpha-1 carbon of one glucose to the alpha-2 position of the other glucose molecule
This gives a alpha 1,4-glycosidic bond
In general the glycosidic bond is between one carbohydrate molecule to another molecule (which can be a carbohydrate or something else)
What is glycosylation?
the reaction where a carbohydrate is attacked to a hydroxyl or another functional group of another molecule
What are phospholipids?
1x10^9 lipid molecules that form a bilayer around a small cell
Held by NON-COVALENT bonds
They are AMPHIPATHIC by nature
they provide structure and permeability barrier of membranes
have an important role in cell signalling and membrane interaction
What does amphipathic mean?
hydrophilic and hydrophobic components in the same molecule
How are phospholipids amphipathic?
Have a polar headgroup and 2 hydrophobic tails
What are the fatty acid tails in phospholipids?
They have a linear hydrocarbon chain of 12-22 carbon atoms long
They have an even number of C atoms
They can be saturated or unsaturated
(If unsaturated then the C=C double bond is always CIS)
Shape of fatty acid tail
Saturated HC chain = linear fatty acid
Introducing just ONE cis double bond will intorduce a significant kink in the chain
Fatty acyl chains differ in length and degree of unsaturated therefore varied shaped of fatty acid tails
Name the 4 major membrane phospholipids
- Phosphatidylthanolamide (neutral charge)
- Phosphatidylserine (-ve charge)
- Phosphatidylcholine (neutral charge)
- Sphingomylin (neutral charge)
Membrane phospholipid structure
Have a polar head which is linked by a -ve phosphate to a glycerol backbone then a fatty acyl tail
The fatty acyl chains associate with each other in a bilayer structure and the polar headgroup interact with the aqueous environment on the outside
Structure of cholesterol
polar head group
rigid steroid ring structure
non-polar hydrocarbon tail (short chain)
the structure is similar to that of phosphatidylserine and phosphatidylcholine
Hydrophobicity
hydrophobic and hydrophilic molecules react differently in aqueous solution
Acetone = partially polar, hydrophilic, dissolves well due to partial polarity which allows it to interact with H2O molecules and form electrostatic interactions
2-methylpropane = hydrophobic, cannot interact with H2O, forces water into ice crystals which requires high energy so fully insoluble in water
What structures does forcing lipids in aqueous solution give?
- Micelles = spherical structure, hydrophobic tail on inside and hydrophilic tail on outside
- Lipid bilayer = form 2 layers, each layer called a leaflet of the plasma membrane. Polar headgroup on outside and hydrophobic fatty acid chain facing in
The lipid bilayer
Have tendency to form closed spheres when in aqueous solution
Lipid bilayer (and biological membranes) prefer to form sealed compartments as it is energetically unfavourable to have a planar phospholipid bilayer where the edges are exposed to water
What happens when the lipid bilayer is damaged?
When damaged it forms sealed compartments (as lipids are self-repairing) so they self-repair to a more energetically favourable structure which is a spherical structure
This contributes to the repair and flexibility of biological membranes
What are liposomes?
Spherical vesicle of at least 1 lipid bilayer
Aqueous hydrophilic on inside and hydrophobic on outside
Uses of liposomes
Drug therapy
Genetic engineering
Delivery DNA or RNA into cells
Cosmetics
