cell membrane

[MIA6]

On my book, it says Cell membrane consists of two layers of lipid molecules. Protein molecules are embedded in this lipid bilayer. My question is: What is on the inner memberane surface and outer membrane surface. i think lipid molecules are on the both surfaces because of the first sentence i wrote, but on my book, it also says, the protein molecules may be on the inner membrane surface, on the outer membrane surface, or may extend entirely through both membrane surfaces. so i am totally confused, i don't know where the protein molecules should be on?! Also, my teacher told us that 'P L L P' which means protein molecules are on the two surfaces, lipid molecules are between.
Hope you can give me idea, i am just so confused! Thanks so much!

[asutoshsahu]

You just see there are transmembrane proteins which go from one surface to other surface and integral proteins one the out side of both surface.
So Int. Protein - Phospholipid - Phospholipid - Int. protein (PLLP)

[MrMistery]

also there are some sugars that form the glycocalix on the outside.

[MIA6]

Hi, asutoshsahu. But as you can see the picture you posted, the two surfaces are phospholipid.!! the protein just travels around. THey just appear like a point, not the whole surface.

[MrMistery]

ok, so here is what the book means by " the protein molecules may be on the inner membrane surface, on the outer membrane surface, or may extend entirely through both membrane surfaces".
There are two types of membrane proteins: peripheral and transmembrane. Transmembrane proteins are the ones that can easly be seen in asutoshsahu's picture. They span the whole membrane.
Peripheral proteins, on the other hand, do not span the whole plasma membrane, but in stead they are attached to one of the two sides: either interior or exterior.
What your teacher said is kinda true, but not exactly accurate. the lipids and proteins form a complex structure, they are not stacked like a sandwich. More than 50 years ago two researchers, named Davidson and Danielli proposed a model for the cell membrane that was kinda like a sandwich: protein-lipids-protein. This model was proved to be inaccurate and replaced with the fluid mosaic model in subsequent years.

Hope i cleared it up a little bit

[MIA6]

Hi, MrMistery. I think according to the picture above and on my book, i think the two surfaces must be lipids, and proteins are just like some points stand out. What i mean is they don't cover the whole surface as lipids shown above do. THat's my point. So even protein span the whole membrane or not, they just occupy a point, not the whole two 'bread'.

[sachin]

Hi, MrMistery. I think according to the picture above and on my book, i think the two surfaces must be lipids, and proteins are just like some points stand out. What i mean is they don't cover the whole surface as lipids shown above do. THat's my point. So even protein span the whole membrane or not, they just occupy a point, not the whole two 'bread'.

Yes, these are Intrinsic and Extrinsic proteins.... Covers 50% of surface area of cell membrane and useful for passive and active transport...