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cell membrane

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cell membrane

Postby MIA6 » Sun Jan 07, 2007 3:08 am

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!
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Postby asutoshsahu » Sun Jan 07, 2007 3:19 am

Image

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)
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Postby MrMistery » Sun Jan 07, 2007 10:42 am

also there are some sugars that form the glycocalix on the outside.
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Postby MIA6 » Sun Jan 07, 2007 5:11 pm

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.
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Postby MrMistery » Sun Jan 07, 2007 7:11 pm

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
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Postby MIA6 » Mon Jan 08, 2007 3:08 am

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'.
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Postby sachin » Mon Jan 08, 2007 4:43 am

MIA6 wrote: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...
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Postby bellyjelly » Tue Jul 12, 2011 7:33 am

The inner membrane of the nuclear envelope is connected to the outer nuclear envelope membrane through nuclear pores. It contains a number of proteins involved in the structural organization of the nucleus and the attachment of chromatin to the nuclear envelope. In metazoan cells, the inner nuclear membrane contains proteins of the nuclear lamina, a protein meshwork underlying the nuclear envelope and providing structural support. Mutations in inner nuclear envelope proteins can cause nuclear envelopathies, a number of genetic disorders in humans.
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Postby nebeyuasrat » Fri Jul 22, 2011 1:42 am

glycerol is on the outside and the fatty acid tail is on the inside

the reason is because the fatty acidis soluble in water but the glycerol doesn't dissolve
that is why there are inner membrane proteins to check and allow whatever the cell needs (its is one of the reason it is called a semi permiable membrane)
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Postby Jastina13 » Mon Aug 01, 2011 7:28 am

While the plant cell has a rigid cell wall, an animal cell membrane is a flexible lipid bilayer. The lipid molecules (mostly phospholipids) that make up the membrane have a polar, hydrophilic head and two hydrophobic hydrocarbon tails. When the lipids are immersed in an aqueous solution the lipids spontaneously bury the tails together and leave the hydrophilic heads exposed. Thus this is a handy membrane to use, because it can automatically fix itself when torn. There are three different major classes of lipid molecules - phospholipids, cholesterol, and glycolipids. Different membranes have different ratios of the three lipids.
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Re:

Postby JackBean » Mon Aug 01, 2011 8:08 am

nebeyuasrat wrote:glycerol is on the outside and the fatty acid tail is on the inside

the reason is because the fatty acidis soluble in water but the glycerol doesn't dissolve
that is why there are inner membrane proteins to check and allow whatever the cell needs (its is one of the reason it is called a semi permiable membrane)


you're little bit confused, aren't you?
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Cis or trans? That's what matters.
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