Cell Biology"
5 µ
✗
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MEMBRANE STRUCTURE
A plasma membrane is about five nanometers thick.
It's functions as a barrier to prevent te contents
of the cell form escaping and mixing with the
molecules in the surrounding environment but also
let the nutrients inwards and the wast product
outwards. The main constituents of plasma
membrane are lipid molecules.
THE LIPID BILAYER
Lipids consist of a hydrophilic head
and a hydrophobic tail. The most
abundant lipids in the cell Fats like triglycerides coalesce into fat droplets in
membrane are phospholipids, water because their purely hydrophobic molecules.
which have a phosphate-
containing, hydrophilic head linked
to a hydrophobic tail. Lipids are
amphipathic, the molecules consist
of both hydrophilic and
hydrophobic parts.
Cholesterol and glycolipids have sugars as part of
their hydrophilic head.
Having both hydrophilic and hydrophobic parts plays a
crucial part in driving the lipid molecules into bilayers Lipids can move by random
in an aqueous environment. thermal motions which
Amphipathic molecules are subject to two conflicting causes exchange of lipids
forces: the hydrophilic head is attracted to water, within the same monolayer.
while the hydrophobic tails shun water and seek to Lipids very rarely tumble
aggregate with other with other hydrophobic from one half of the bilayer
molecules; hydrophobic molecules repel water to the other without help of
molecules, by clustering together, the contacts with proteins, this process is
surrounding water molecules is limited and minimises called flip-flop.
the free energy. The formation of a lipid bilayer
resolves this conflict and is energetically most
favourable. The hydrophilic heads face water both The fluidity of the lipid bilayer at a given
surfaces of the bilayer and the hydrophobic tails are temperature depends on its phospholipid composition
shielded from the water within the bilayer interior. and the nature of the hydrocarbon tails: the closer
(This is as well suitable for the formation of closed and more regular the packing of the tails, the more
vesicles, organelles and even the whole cell) viscous and less fluid the bilayer will be.
A shorter chain length reduces the tendency of the
hydrocarbon tails to interact with on another and
therefore increases the fluidity of the bilayer.
If a lipid bilayer contains a large portion of
unsaturated (= contain one or more double bonds)
hydrocarbon tails are more fluid than those with
lower proportion. Each double bond in an
unsaturated tail creates a small kink in the tail,
which makes it difficult for the tails to pack against
one another.
5 µ
✗
☒
MEMBRANE STRUCTURE
A plasma membrane is about five nanometers thick.
It's functions as a barrier to prevent te contents
of the cell form escaping and mixing with the
molecules in the surrounding environment but also
let the nutrients inwards and the wast product
outwards. The main constituents of plasma
membrane are lipid molecules.
THE LIPID BILAYER
Lipids consist of a hydrophilic head
and a hydrophobic tail. The most
abundant lipids in the cell Fats like triglycerides coalesce into fat droplets in
membrane are phospholipids, water because their purely hydrophobic molecules.
which have a phosphate-
containing, hydrophilic head linked
to a hydrophobic tail. Lipids are
amphipathic, the molecules consist
of both hydrophilic and
hydrophobic parts.
Cholesterol and glycolipids have sugars as part of
their hydrophilic head.
Having both hydrophilic and hydrophobic parts plays a
crucial part in driving the lipid molecules into bilayers Lipids can move by random
in an aqueous environment. thermal motions which
Amphipathic molecules are subject to two conflicting causes exchange of lipids
forces: the hydrophilic head is attracted to water, within the same monolayer.
while the hydrophobic tails shun water and seek to Lipids very rarely tumble
aggregate with other with other hydrophobic from one half of the bilayer
molecules; hydrophobic molecules repel water to the other without help of
molecules, by clustering together, the contacts with proteins, this process is
surrounding water molecules is limited and minimises called flip-flop.
the free energy. The formation of a lipid bilayer
resolves this conflict and is energetically most
favourable. The hydrophilic heads face water both The fluidity of the lipid bilayer at a given
surfaces of the bilayer and the hydrophobic tails are temperature depends on its phospholipid composition
shielded from the water within the bilayer interior. and the nature of the hydrocarbon tails: the closer
(This is as well suitable for the formation of closed and more regular the packing of the tails, the more
vesicles, organelles and even the whole cell) viscous and less fluid the bilayer will be.
A shorter chain length reduces the tendency of the
hydrocarbon tails to interact with on another and
therefore increases the fluidity of the bilayer.
If a lipid bilayer contains a large portion of
unsaturated (= contain one or more double bonds)
hydrocarbon tails are more fluid than those with
lower proportion. Each double bond in an
unsaturated tail creates a small kink in the tail,
which makes it difficult for the tails to pack against
one another.
