Membrane Structure and Function (I) Flashcards
What are the characteristics of membranes?
Flexible
Self-sealing
Selectively permeable
What makes up the composition of membrane bilayers?
Membrane bilayers are composed largely of lipids and proteins
The lipids are amphipathic meaning they have a hydrophilic part and a hydrophobic part
Assemble spontaneously
Non-covalent assemblies
Describe the lipid composition of a cellular membrane?
The phospholipids are the main component of the membrane and these are based upon a glycerol backbone with two fatty acids (hydrophobic)
The glycerol is then linked to a polar head group through a phosphate bridge (hydrophilic)
The polar head group can take many forms e.g. choline, serine etc.
Is this is a choline molecule the name would be phosphatidylcholine
Another component is cholesterol- a large flat molecule with a small hydrophilic OH head
Another type of components is based upon sphingosine
What is sphingosine?
Another type of component of the membrane is based upon the molecule sphingosine
Sphingosine attached to a fatty acid becomes a ceramide molecule
This molecule can wither be combined with a carbohydrate in which case it becomes a glycolipid
Or it can be linked via a phosphate group to a choline molecule to form sphingomyelin; an important molecule present in significant quantities in neuronal tissues
List all of the components of the cell membrane
Phosphatidylcholine Phosphatidylserine Phosphatidylethanolamine Phosphatidylinositol Cholesterol Sphingomyelin Sphingomyelin is a phospholipid where the glycerol backbone is replaced by sphingomyelin
What happens to synthesise phospholipids?
Phospholipid synthesis takes place in the cytosolic surface of the endoplasmic reticulum
Three enzymes are involved:
Floppase- moves phospholipids from the inner to the outer leaflet, requires ATP
Flippase- moves phospholipids from the outer to the inner leaflet, requires ATP
Scramblase- bidirectional movement (driven more by concentration)
This allows the transport of these highly charged head groups across this very hydrophobic region in the core of the membrane
What is the movement like for phospholipids?
Lateral movement of lipids in the membrane is rapid
Transverse movement is slow and requires the action of three enzymes
What are the factors that alter fluidity?
Temperature Fatty acid composition: Chain length Degree and extent of saturation Saturated fatty acids can get closer together and the non-covalent bonds between the adjacent phospholipids will be higher therefore low fluidity Cholesterol content
How is membrane fluidity affected by temperature and cholesterol content?
At low temperature the phospholipid molecules can pack together more tightly reducing membrane fluidity
As we increase the amount of cholesterol at low temperature this will reduce the interactions between the phospholipids which will help increase the fluidity at a low temperature
AT high temperatures the molecules will have more energy associated with them and they will be more motile
Cholesterol in the membrane at high temperatures will help stabilise the membrane by
maintaining some of the interactions that will help keep the membrane intact
What causes spur cell anaemia?
Cholesterol content increased by 25-65% leading to decreased membrane fluidity
This means the red blood cells get trapped in the capillaries and some rupture due to spurs
How is the membrane presented in the Fluid Mosaic model?
Flexible
Self-sealing
Selectively permeable
They define boundaries
Divide the internal spaces
In this model it is envisaged that the proteins will interact in a dynamic way in relation to the phospholipids membrane
As with the membrane it is believed that the movement of proteins through that membrane is restricted and in most cases prohibited by the energy barrier involved
What are integral membrane proteins?
They traverse the entire membrane Single or multi-pass Strong non-covalent bonds Trans-membrane domain often an alpha helix can be predicted from sequence
What are peripheral membrane proteins?
Located on both the extracellular cytosolic membrane
Associated by non-covalent bonds but may be more transient
Association within the cell probably transient
3 types of interactions
What are lipid anchored membrane proteins?
Covalently linked to a lipid molecule such as glycosyl-phosphatidylinositol or through association with a fatty acid
What are carbohydrates like in the membrane?
Carbohydrates 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 RBCs 8% of the weight is carbohydrate
Selectins are an important group of membrane glycoproteins
