Phospholipids are the major structural components of the liposome lipid bi-layer. Phospholipids are amphiphilic molecules. They consist of a glycerol backbone with two long fatty-acid chains (R-groups), and a polar portion of the molecule. The polar portion, a phosphate ester group, can be attached to a phosphatidyl ethanolamine, a phosphatidyl choline, or a phosphatidyl inositol. The most commonly used phospholipid is phosphatidyl choline, also known as lecithin. It can be readily obtained from egg yolk and soya bean, less readily from bovine heart and spinal cord (R.R.C. New 1990).
Apart from phospholipids, sterols are also commonly inserted into the membrane. They do not by themselves form bilayer structures, but their incorporation can bring about major changes in the properties of these membranes. They can be inserted in very high concentrations, up to 1:1 or even 2:1 molar ratios of sterol to phospholipid. Examples of sterols are: cholesterol in mammals, stigmasterol and sitosterol from plants and ergosterol from yeasts, fungi and some protozoa.
In addition, we are able to insert various lipophilic or amphiphilic molecules into the lipid membrane. These include proteins/peptides, glycopeptides and carbohydrates. These have various functions such as affecting the stability of the membrane, or targeting of the liposome. Since the means of their incorporation is entirely physical, there is little restriction on the types of molecules to be inserted. Only 2 types of molecules are unsuitable: molecules that are insoluble in either aqueous or organic media, and molecules that are highly soluble in both.What is ';The Phospholipid Bi-Layer';?
i have no idea
I'm assuming you are at least A level if not an undergrad
:
OK the phospholipid bilayer is pretty much as it sound a double layer of lipids with phosphate head groups. Because lipid molecules are hydrophobic they pack together so that they are not exposed to water (it's a hydrophobic interaction, look it up if you need to understand the thermodynamics). Phospholipids also contain a phosphate group, the head group. The phosphate is a polar molecule so likes the water. Basically what you get is the phosphate group making a layer on the outside and attached to two lipid tails. These face the lipid groups of from the second layer and the phosphate group from that is on the outside.
That's the basic explanation of the structure and if you are A-level that should be enough.
Don't forget though that there are other molecules that sit in the bilayer, mainly proteins. Also you can have carbohydrate attachments to the lipids to make glycolipids and to the proteins to make glycoproteins. Some proteins run right through the membrane and some sit on one side or the other
If you are an undergrad:- then you will need to look up the different types of lipids etc. cis vs trans double bonds in the lipids change the melting point of the lipid and so the fluidity of the system (look up mesomorphism). You will also need to look at flipping which occurs in the membrane, where a phospholipid from one layer can flip into the next. All of this is part of the fluid mosaic model.
OK functions, obviously a barrier, but it is also used for cytoskeletal attachment and signalling (mainly through the proteins etc embedded into the membrane). Sorry that is all I can think of off the top of my head.
Hope this helps.
No comments:
Post a Comment