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Membrane Structure
All biological membranes are composed mainly
of lipid and protein molecules
The three major types of lipids in cell membranes are:
• PHOSPHOLIPIDS – the most abundant
• CHOLESTEROL – responsible for stabilising the
membrane
• GLYCOLIPIDS – found at the external surface of the
membrane
All of the lipids are described as being AMPHIPATHIC
as they have a HYDROPHILIC (‘water-loving’) end and
a HYDROPHOBIC (‘water-hating’) end to the molecule
The PROTEINS within the membrane are largely concerned
with the transport of molecules across the membrane
The phospholipid molecule
has a polar phosphate –
containing head group and
two hydrophobic fatty acid
tails
The tails vary in length and
may have one or more double
bonds
Each double bond creates
a kink in the tail
The differences in tail length
and the presence of double
bonds are important for
influencing the FLUIDITY
of the membrane
Kink due to the presence
of a double bond
The hydrophilic head
consists of a phosphate
and glycerol group
Two non-polar
hydrophobic tail groups
are bonded to the
hydrophilic head group
The FLUID MOSAIC MODEL
proposes a double layer of
phospholipids with PROTEINS
penetrating this layer to
different extents
The proteins are globular in nature
and form a MOSAIC in
the fluid-like lipid bilayer
Lipid
bilayer
Extrinsic proteins
(are partially
embedded
in the bilayer)
Intrinsic proteins
(extend right across
the bilayer)
The FLUID MOSAIC MODEL envisages the membrane as a sea of
phospholipids within which a mosaic of proteins float like icebergs
The following two dimensional view of the cell membrane illustrates additional
chemical components of the membrane
The lipid bilayer
is asymmetrical in
that certain protein
and lipid molecules
at the external surface
contain carbohydrate
chains as part of
their structure
glycolipid
carbohydrate
group
Glycolipids and
glycoproteins form
part of the external
structure of the
membrane
glycoprotein
Bimolecular
phospholipid
layer
Extrinsic
Glycolipids play a protein
part in
communication
between cells and
cell to cell recognition
Many glycoproteins
function as surface antigens
enabling cells to distinguish
self from ‘non-self’
Intrinsic
protein
cholesterol stabilising
the membrane
Cholesterol molecules are positioned within the
bilayer close to the fatty acid chains;
these molecules partially immobilise these chains
and help to stabilise the membrane
Transport across Membranes
The transfer of relatively small molecules across cell
membranes may occur in a variety of ways
The PASSIVE TRANSPORT of materials occurs in two ways:
1. SIMPLE DIFFUSION where molecules diffuse across the
the lipid bilayer or through channel proteins in the direction
of their concentration gradient and
2. FACILITATED DIFFUSION where protein carrier molecules
within the membrane assist the passage of substances across
the membrane in the direction of their concentration gradient
Cellular energy is NOT required for passive methods of transport
and relies largely on the random movement
of molecules and ions
energy
energy
The ACTIVE TRANSPORT of materials also involves carrier proteins
assisting the molecules across the membrane. In this case molecules
are transported against their concentration gradient and cellular
energy is required for this to be achieved
Passive and active methods of transport are used by cells
for the transfer of molecules and ions across membranes
simple
diffusion
facilitated
diffusion
Passive Transport
Active Transport
Surface Area and Simple Diffusion
The rate of diffusion is affected by a variety of factors that include:
• Temperature
• Surface area
• Steepness of the concentration gradient
• Distance over which diffusion is taking place
Three of these factors are expressed in FICK’S LAW, which
states that:
Rate of diffusion =
surface area x steepness of concentration gradient
thickness of membrane
The larger the surface area
The steeper the concentration gradient
The thinner the membrane or diffusion barrier
The faster is the
rate of diffusion
Facilitated diffusion is a carrier-assisted transport mechanism
in which molecules are transferred across membranes
along their concentration gradients
Hydrolysis of ATP provides the energy
for the protein carriers to change shape
and transport the molecules across
the membrane
ATP
Active transport is an energy –
requiring transport system that
is able to transfer material
against a concentration gradient
Intrinsic globular proteins
within the membrane
function as carriers for the
active transport of molecules
across membranes
ADP
energy
Many different ions are actively
transported across membranes
as is glucose when being absorbed
from the gut into the blood stream
The Bulk Transfer of Materials
Many of the substances that enter cells
are too large to be transported through
the bilayer or by transport proteins
The bulk transfer of materials INTO the cell
occurs by ENDOCYTOSIS
The bulk transfer of materials OUT OF the cell
occurs by EXOCYTOSIS
Osmosis is a special kind of diffusion by which water molecules
are transported across partially permeable membranes
Partially permeable membrane with
pores created by channel proteins
solute molecules
too large to pass
through pores
in the membrane
The term WATER POTENTIAL (Y) is used when describing the tendency of
water molecules to move from one place to another
To move requires energy; water potential is a measure of the free energy available
for water molecules to move
The presence of dissolved solutes hinders the ability of water molecules to move
The free energy available for water molecules in PURE WATER to do work and
move is GREATER than that for water molecules in a
SOLUTION (water plus dissolved solutes)
As pure water contains no dissolved solutes, the water potential of pure water is
defined as ZERO – zero is the highest water potential possible
The presence of any substances dissolved in water
LOWERS THE WATER POTENTIAL
THE WATER POTENTIAL OF A SOLUTION IS
ALWAYS LESS THAN ZERO AND IS THEREFORE A NEGATIVE VALUE
Water potential is measured in units called pascals and the definition of osmosis
in terms of water potential is:
Osmosis is the net movement of water molecules through a partially permeable
membrane from a region of high water potential to a region of lower
water potential i.e. movement DOWN a water potential gradient
In the situation shown below, two solutions with different
water potential values are separated by a membrane
Pure water has a water potential
of ZERO
The presence of solute molecules in this
solution lowers the water potential, e.g. -4
This fully turgid plant
cell has been placed in
a hypotonic solution
A hypotonic solution is
one that is less concentrated
than the protoplast of the
plant cell and thus has a
higher water potential than
the cell
The net movement of water by
osmosis is into the cell and the
protoplast swells and presses
against the cell wall
This plant cell has been
placed in an isotonic solution
This plant cell has been
placed in a hypertonic solution
An isotonic solution is
one that has the same
concentration as the protoplast
of the plant cell and thus has the
same water potential
A hypertonic solution is one
that is more concentrated than
the protoplast of the plant cell
and thus has a lower water
potential than the cell
The cell displays incipient
plasmolysis where the membrane
is just beginning to pull away
from the cell wall
The net movement of water
by osmosis is out of the cell
and the protoplast shrinks
There is no net movement of
water in this case and no
pressure potential as the
protoplast ceases to press
against the cell wall
The protoplast is pulled
completely away from the
cell wall and the cell is
fully plasmolysed (flaccid)
Consider two adjacent plant cells (A and B) in a tissue
where each cell possesses a different water potential
In which direction will water move as a consequence of osmosis?
CELL B has the higher water potential (less negative) and therefore water
will move down the water potential gradient from Cell B to CELL A