AQA AS Biology Unit 1: The Cell Surface Membrane

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  • Created on: 22-08-13 16:13
Preview of AQA AS Biology Unit 1: The Cell Surface Membrane

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The Cell Surface Membrane
All membranes around and within cells have the same basic structure and are known as
plasma membranes.
The cell-surface membrane is the plasma membrane that surrounds cells and forms a
boundary between the cell cytoplasm and the environment.
It allows different conditions to be established inside and outside a cell.
It controls the movement of substances in and out of the cell.
The cell surface membrane is approximately 7.5 nm thick and consists of a bilayer of
lipids along with a highly variable component of protein.
Some of these proteins are embedded on the surface of the membrane whilst others
span the entire width of the membrane.
This is known as the fluid mosaic model.

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Phospholipids form a bilayer sheet.
They make up 75% of the lipid.…read more

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Glycolipids make up 5% of membrane lipids.
They occur on the external surface of the cell surface membrane and the carbohydrate
portion of the glycolipid extends into the intercellular space and is called a glycocalyx. These
are important cell to cell recognition.
A steroid which makes up 20% of lipids in animal membranes but is rarely found in plant
cell membranes.
All lipids can move sideways (laterally) within the membrane and exchange position with
each other.…read more

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The proteins of the cell surface membrane are arranged more randomly than the regular
pattern of phospholipids.
They are embedded in the phospholipid bilayer in two main ways:
Extrinsic proteins
Occur either on the surface of the bilayer or only partly embedded in it.
They never extend completely across it.
They act either to give mechanical support to the membrane or, in conjunction with
glycolipids, as cell receptors for molecules such as hormones.…read more


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