Why are biological membranes bilayers?
Biological membranes are composed of lipids and proteins
The lipids are amphipathic meaning they have a hydrophilic head and hydrophobic tail.
key facts of membranes
- membrane lipids are amphipathic
- major lipid classes are phospholipids, glycolipids and cholesterol.
- lipids spontaneously form bilayers in aqueous solutions
- phospholipid synthesis takes place on the cytosolic surface of the endoplasmic reticulum.
- distribution of newly formed lipid requires the enzymes flippase and floppase
membrane fluidity
lateral movement of lipids in the membrane is rapid.
transverse movement is slow and requires the action of 3 enzymes.
what factors affect fluidity?
- temperature
- fatty acid composition
- chain length
- degree and extent of saturation
- cholesterol content
what happens at low temperatures of cholesterol?
-molecules pack tightly together. this reduces the interactions between the phospholipid fatty acid chains. this helps to maintain and increase the fluidity
what happens at high temperatures of cholesterol?
molecules of the membrane will have more energy associated and motile. Cholesterol will act to decrease fluidity. this means organisms will be able to live in extreme conditions
what is spur cell anemia?
cholesterol content is increased by 25%-65% leading to decreased membrane fluidity. Red blood cells get trapped as they pass through the capillaries some cells will rupture leading to anemia.
The fluid mosaic model
it is:
- flexible
- self sealing
- selectively permeable
- they define boundaries
- they divide the internal spaces
integral membrane proteins
- single/multi pass
- strong non covalent bonds
- trans membrane domain often an alpha helix
- can be predicted from the sequence
Peripheral membrane proteins
it is located on both the extracellular cytosolic membrane and is associated by non covalent bonds
lipid anchored membrane proteins
it is covalently linked to a lipid molecule such as glycosyl-phosphatidylinositol
2 major types of membrane proteins: integral and peripheral
- integral proteins interact extensively with the lipid bilayer
- peripheral proteins interact with integral proteins or lipid polar head groups
Membrane Carbohydrates
Carbs are associated with both membrane lipids and proteins
Form 2-10% of the membrane weight
The majority of glycolipids and glycol proteins are externally facing
They are often involved in cell-cell interactions or cellular recognition
The blood group antigens are glycolipids
In red blood cells 8% of the weight is carbohydrate
Selectins are an important group of membrane glycoproteins.
describe the role of cholesterol in membrane fluidity?
cholesterol acts as a bidirectional regulator of membrane fluidity because at high tempertaures, it stabilizes the membrane and raisesx its m.p. whereas at low temperatures it intercalates between the phospholipids and prevents them from clustering together and stiffening
describe the major molecular components of biological membranes and their distribution
biological membranes consist of a double sheet (bilayer) of lipid molecules. - phospholipid bilayer.
