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Part of the phospholipid molecule hydrophilic, other part is hydrophobic (amphipathic)
The hydrophilic part is the phosphate group
The hydrophobic part consist of two hydrocarbon chains
Arranged in double layer (bilayer) where phosphate heads are in the outer part and hydrocarbon
tails in the inner
They are stable structures and they form the basis of all cell membranes

Davson-Danielli model
• Proposed layers of protein adjacent to the phospholipid bilayer on both sides of the membrane
• Explained how membranes despite of being vey thin are so resistant
• Proteins appear darker and phospholipid lighter

Singer and Nicolson model
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Fluid mosaic model
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• Primary function: form a barrier through which ions and hydrophilic molecules can’t pass
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easily
Integral proteins:
Hydrophobic on at least one part of their surface and therefore embedded in the hydrocarbon
chains
They can extend across the membrane with hydrophilic parts projecting
Peripheral proteins:
Hydrophilic on at least one part of their surface so are not embedded in the membrane
Attached to the surface of integral proteins and this attachment is often reversible
Single hydrocarbon chain that anchors them to the membrane

Functions of membrane proteins
Hormone binding sites
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g insulin receptors

Functions of membrane proteins
Immobilised enzymes with the active site on the outside
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g the small intestine
Cell adhesion to form tight junctions between groups of cells in tissues and organs
Cell-to-cell communication
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g receptors for neurotransmitters at synapses
Channels for passive transport to allow hydrophilic particles across by facilitated diffusion
Pumps for active transport which use ATP to move particles across the membrane

Cholesterol
Cholesterol is a component of animal cell membranes

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It is a component of animal cell membranes
It is a type of lipid that belongs to the group of steroids
It is mostly hydrophobic so it's attached to hydrocarbon tails
One molecule of cholesterol contains hydroxyl (OH) which is hydrophilic

Role of cholesterol in membranes
Cholesterol in mammalian membranes reduces membrane fluidity and permeability to some solutes

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