1. CELL MEMBRANES AND
CELL MEMBRANES AND
TRANSPORT
TRANSPORT
The fluid mosaic model of
The fluid mosaic model of
membrane structure
membrane structure
ALBIO9700/2006JK
2. Phospholipids
Phospholipids
• Form a single layer with heads in water
Form a single layer with heads in water
and tails projecting out of the water
and tails projecting out of the water
• Can form stable structures called
Can form stable structures called micelles
micelles
if shaken up with water
if shaken up with water
• Two-layered structures can from in sheets
Two-layered structures can from in sheets
(
(bilayers
bilayers)
)
ALBIO9700/2006JK
5. Structure of membranes
Structure of membranes
• The phospholipid bilayer is visible using the
The phospholipid bilayer is visible using the
electron microscope (> x100 000)
electron microscope (> x100 000)
• 7nm wide
7nm wide
• Also contains proteins
Also contains proteins
• Model for the structure is called
Model for the structure is called fluid mosaic
fluid mosaic
model
model
• ‘
‘fluid’ – the individual phospholipid and protein
fluid’ – the individual phospholipid and protein
molecules move around within their layer
molecules move around within their layer
• ‘
‘mosaic’ – describes the pattern produced by the
mosaic’ – describes the pattern produced by the
scattered protein molecules when the surface of
scattered protein molecules when the surface of
the membrane is viewed from above
the membrane is viewed from above
ALBIO9700/2006JK
7. Features of the fluid mosaic model
Features of the fluid mosaic model
• Membrane is bilayer of phospholipid molecules that
Membrane is bilayer of phospholipid molecules that
move about by diffusion within their own monolayer
move about by diffusion within their own monolayer
• Phospholipid tails point inwards and form a non-polar
Phospholipid tails point inwards and form a non-polar
hydrophobic interior. Heads face the aqueous medium
hydrophobic interior. Heads face the aqueous medium
surrounding membrane
surrounding membrane
• Some phospholipid tails are saturated and some
Some phospholipid tails are saturated and some
unsaturated. The more unsaturated, the more fluid the
unsaturated. The more unsaturated, the more fluid the
membrane
membrane
• Most protein molecules float in phospholipid layers
Most protein molecules float in phospholipid layers
• Some proteins are embedded in the outer layer, some in
Some proteins are embedded in the outer layer, some in
the inner layer and some span the whole membrane
the inner layer and some span the whole membrane
• Thickness = 7nm
Thickness = 7nm
• Many proteins and lipids have short , branching
Many proteins and lipids have short , branching
carbohydrate chains attached to the external surface,
carbohydrate chains attached to the external surface,
forming
forming glycoproteins
glycoproteins and
and glycolipids
glycolipids
• presence of cholesterol
presence of cholesterol
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8. Features of the fluid mosaic model
Features of the fluid mosaic model
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10. Roles of components of cell
Roles of components of cell
membranes
membranes
• Phospholipids
Phospholipids
– Form bilayer (basic structure of membrane)
Form bilayer (basic structure of membrane)
– Act as a barrier to most water-soluble substances
Act as a barrier to most water-soluble substances
• Cholesterol
Cholesterol
– Have hydrophilic heads and hydrophobic tails
Have hydrophilic heads and hydrophobic tails
– Regulates the fluidity of the membrane, preventing it
Regulates the fluidity of the membrane, preventing it
from becoming too fluid or too rigid
from becoming too fluid or too rigid
– Important for the mechanical stability of membranes
Important for the mechanical stability of membranes
– Hydrophobic regions help to prevent ions or polar
Hydrophobic regions help to prevent ions or polar
molecules from passing through the membrane
molecules from passing through the membrane
(myelin sheath around nerve cells – leakage of ions
(myelin sheath around nerve cells – leakage of ions
would slow down nerve impulses)
would slow down nerve impulses)
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11. • Proteins
Proteins
– Transport proteins
Transport proteins: provide hydrophilic channels or
: provide hydrophilic channels or
passageways for ions and polar molecules to pass
passageways for ions and polar molecules to pass
through the membrane
through the membrane
– Enzymes
Enzymes: catalyse the hydrolysis of molecules such
: catalyse the hydrolysis of molecules such
disaccharides (small intestines)
disaccharides (small intestines)
– Protein in membranes of organelles
Protein in membranes of organelles
(mitochondria and chloroplasts): involved in the
(mitochondria and chloroplasts): involved in the
processes of respiration and photosynthesis
processes of respiration and photosynthesis
• Glycolipids
Glycolipids and
and glycoprotein
glycoprotein
– Protein and lipid molecules with short carbohydrate
Protein and lipid molecules with short carbohydrate
chains
chains
– Form hydrogen bonds with the water molecules and
Form hydrogen bonds with the water molecules and
so help to stabilise the membrane structure
so help to stabilise the membrane structure
– Act as
Act as receptor molecules
receptor molecules, binding with
, binding with hormones
hormones
or
or neurotransmitters
neurotransmitters (chemicals that enable nerve
(chemicals that enable nerve
impulses to pass from one nerve cell to another)
impulses to pass from one nerve cell to another)
ALBIO9700/2006JK
