1. CELL MEMBRANES AND
TRANSPORT
The fluid mosaic model of
membrane structure
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2. Phospholipids
• Form a single layer with heads in water
and tails projecting out of the water
• Can form stable structures called micelles
if shaken up with water
• Two-layered structures can from in sheets
(bilayers)
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5. Structure of membranes
• The phospholipid bilayer is visible using the
electron microscope (> x100 000)
• 7nm wide
• Also contains proteins
• Model for the structure is called fluid mosaic
model
• ‘fluid’ – the individual phospholipid and protein
molecules move around within their layer
• ‘mosaic’ – describes the pattern produced by the
scattered protein molecules when the surface of
the membrane is viewed from above
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7. Features of the fluid mosaic model
• Membrane is bilayer of phospholipid molecules that
move about by diffusion within their own monolayer
• Phospholipid tails point inwards and form a non-polar
hydrophobic interior. Heads face the aqueous medium
surrounding membrane
• Some phospholipid tails are saturated and some
unsaturated. The more unsaturated, the more fluid the
membrane
• Most protein molecules float in phospholipid layers
• Some proteins are embedded in the outer layer, some in
the inner layer and some span the whole membrane
• Thickness = 7nm
• Many proteins and lipids have short , branching
carbohydrate chains attached to the external surface,
forming glycoproteins and glycolipids
• presence of cholesterol
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8. Features of the fluid mosaic model
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10. Roles of components of cell
membranes
• Phospholipids
– Form bilayer (basic structure of membrane)
– Act as a barrier to most water-soluble substances
• Cholesterol
– Have hydrophilic heads and hydrophobic tails
– Regulates the fluidity of the membrane, preventing it
from becoming too fluid or too rigid
– Important for the mechanical stability of membranes
– Hydrophobic regions help to prevent ions or polar
molecules from passing through the membrane
(myelin sheath around nerve cells – leakage of ions
would slow down nerve impulses)
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11. • Proteins
– Transport proteins: provide hydrophilic channels or
passageways for ions and polar molecules to pass
through the membrane
– Enzymes: catalyse the hydrolysis of molecules such
disaccharides (small intestines)
– Protein in membranes of organelles
(mitochondria and chloroplasts): involved in the
processes of respiration and photosynthesis
• Glycolipids and glycoprotein
– Protein and lipid molecules with short carbohydrate
chains
– Form hydrogen bonds with the water molecules and
so help to stabilise the membrane structure
– Act as receptor molecules, binding with hormones
or neurotransmitters (chemicals that enable nerve
impulses to pass from one nerve cell to another)
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