1. Cell Membrane Structure
All membranes throughout the cell have the same structure e.g.
Cell surface membrane, endoplasmic reticulum, golgi apparatus….

2. Membrane structure The cell membrane is composed of:
a phospholipid bilayer:
Hydrophilic heads face outwards into the surrounding solution
Hydrophobic tails turn away from the solution towards the centre of the membrane,
proteins, scattered throughout the membrane:
intrinsic proteins, are found in the membrane, either in one layer or across the 2 layers (transmembrane proteins)
Extrinsic proteins are attached to the periphery of the membrane (external to the phospholipid bilayer).

3. Cholesterol molecules lie between the phospholipid tails.
Glycocalyx extends from the outer surface of the membrane and contains polysaccharides attached to lipids (glycolipid) or membrane proteins (glycoprotein).
The structure is called THE FLUID MOSAIC MODEL:
Fluid as phospholipids can move sideways throughout the membrane
Mosaic because of the appearance of the different molecules in the structure.

4. Component
Function
Further info:
Phospholipid bilayer
Forms backbone of membrane
Selectively permeable
Cholesterol
Increases stability at high temperature
Restricts sideways movement of phospholipids, making them less fluid
Maintains stability at low temperatures
Acts as a wedge between adjacent phospholipids, preventing them sticking together, making them more fluid
Proteins
Stability & support
Anchors phospholipids
Enzymes e.g. in ileum
Prevents them being lost in secretion
Adhesion sites
Holds adjacent cells together
Transport
Channel protein create hydrophilic channels allowing polar molecules to pass, open or gated
Carriers carry specific ions/molecules across membrane, either because it has charged groups (glucose) or moves them against a concentration gradient.
Cell recognition (see glycocalyx)
Receptors
Antigens
Glycocalyx
Cell-cell recognition
Similar cells recognise each other and bind to form tissues.
This identifies individual cell types so also act as antigens.
Receptor sites
Complementary in shape to specific molecules. Important in hormone action (eg insulin only recognised by liver & muscle cells) & passage of neurotransmitters between neurones.
Stabilise the membrane
Glycoproteins & glycolipids can form hydrogen bonds with H2O molecules outside the membrane.