Transcript BACK TO MAIN AS BIOLOGY MENU Membranes and phospholipids The fluid mosaic model Roles of cell membrane parts Diffusion and facilitated diffusion Osmosis and water potential Osmosis.

BACK TO MAIN AS
BIOLOGY MENU
Membranes and phospholipids
The fluid mosaic model
Roles of cell membrane parts
Diffusion and facilitated diffusion
Osmosis and water potential
Osmosis in animal cells
Osmosis in plant cells
Active transport
Bulk transport
Macro exchange surfaces -alveoli
Macro exchange surfaces - root hairs
Review
1
Membrane structure
Membranes and phospholipids
Although very thin, membranes regulate very precisely indeed the substances
that enter and leave the cell.
Membranes also have receptors to enable hormones to influence certain cells
Phospholipids
When mixed with water, phospholipid molecules spontaneously assemble to
form membrane-like structures.
Their polar heads point outwards towards the surrounding charged water
molecules, and their non-polar tails point inwards.
CHECK OUT
BIOLOGICAL
MOLECULES PHOSPHLIPIDS
Under certain conditions they form bilayers, the basis of cell membranes
Here phospholipid
molecules have
formed a spherical
‘micelle’
PHOSPHOLIPID MOLECULE
The molecules move around by
diffusion (driven by kinetic energy).
HYDROPHOBIC TAIL
These face inwards forming a nonpolar hydrophobic interior
Here phospholipid
molecules have
formed a bilayer
DETAILED VIEW
OF BILAYER
HYDROPHILIC HEAD
These face the aqueous (watercontaining) medium around the
membrane.
2
Membrane structure
Features of the fluid mosaic model
The double line seen at very
high power is thought to be
the 2 phospholipid layers. The
bilayer is about 7 nm wide.
High power TEM, cell membrane
x 100000
The phospholipid bilayer is the
fluid part because phospholipid
molecules can move around
Membranes also contain
proteins and the model of
membranes accepted at
present is called the ‘fluid
mosaic’ model.
The protein molecules form a
mosaic pattern set in the
phospholipid bilayer
3
Membrane structure
Features of the fluid mosaic model
outside
Carbohydrate (polysaccharide ) part
of a glycoprotein, or gylcolipid
Some phospholipids tails are
unsaturated. The more unsaturated
they are, the more fluid the membrane,
because bent tails fit together more
loosely. Recheck unsaturated
phospholipid structure
Some proteins are
embedded in the
outer layer
(extrinsic) and
some in the inner
layer (intrinsic).
Hydrophobic
protein areas are
anchored in the
hydrophobic inner
part of the
membrane.
PHOSPHOLIPID
LAYER
3 INTRINSIC PROTEINS
inside
EXTRINSIC
PROTEIN
Most protein molecules
are mobile, moving
around freely. Others
are fixed like islands to
structures in the
membrane and do not
move
4
Membrane structure
Roles of the components of cell membranes
Glycolipids and
glycoproteins These lipids
and proteins have
carbohydrate chains which
jut out in from the membrane
They stabilise the
membrane and
also act as
receptor
molecules for
hormones and
neurotransmitters
Transport
proteins provide
hydrophilic
channels for the
passage of ions
and polar
molecules
Membrane enzymes are sometimes
present. e.g. small intestine cell
membranes have enzymes which
hydrolyse disaccharides.
Phospholipids: Because their tails
are non-polar, water soluble
molecules such as ions, cannot
pass through them.
Cholesterol molecules:
These too have hydrophilic
heads and hydrophobic tails.
They fit neatly between
phospholipid molecules and
help maintain the fluidity
and stability of the
membrane.
Cholesterol molecules, being hydrophobic, help
prevent ions or polar molecules from passing
through. This is especially important in myelin
sheaths around nerves, where ion leakage would
slow down impulse transmission
5
Transport across the plasma membrane
Diffusion and facilitated diffusion
Factors affecting the rate of diffusion
across membranes include:
The steepness of the
concentration gradient. The
greater the difference in
concentration, the faster the rate.
Temperature: Molecules have
more kinetic energy at high
temperatures and diffuse
faster
Diffusion is the net movement
of molecules down a
concentrated gradient.
The type of molecule or ion: Large
molecules diffuse more slowly than
small ones. Non-polar molecules
diffuse faster
Surface area: the
greater the surface
area, the more
molecules or ions
that can cross it.
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Transport across the plasma membrane
Diffusion and facilitated diffusion
PERMEABLE
Oxygen is uncharged and non-polar. It
passes across the phospholipid bilayer
quickly
PERMEABLE
Carbon dioxide is polar but small
enough to pass through rapidly
PERMEABLE
Water molecules, despite being polar,
can diffuse across rapidly because they
are so small
Amino acids
glucose
nucleotides
IMPERMEABLE
H+, Na+, K+,
Mg+, Ca2+,
Cl-, HCO3-
IMPERMEABLE
PHOSPHOLIPID
BILAYER
All these can only cross the
membrane through hydrophilic
channels created by protein
molecules. Diffusion through
these channels is called
facilitated diffusion, because it
is ‘made easy’, or ‘made
possible.
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Transport across the plasma membrane
Diffusion and facilitated diffusion
Plasma membranes contain many
different types of protein channel,
each type allowing only 1 kind of
molecule or ion to diffuse through it.
In cystic fibrosis a protein channel in lung and gut
epithelial cells which normally allows sodium
chloride to move out of the cells is faulty. As a
result chloride ions cannot move out.
The rate of facilitated
diffusion depends on how
many appropriate channels
there are, and whether they
are open.
CHECK OUT
CYSTIC
FIBROSIS
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Transport across the plasma membrane
Osmosis – water potential and solute potential
Terms you need to know include solute, solvent and solution.
e.g. in sugar solution the sugar is the solute and the water is the solvent.
The solute molecules (red) are too
large to pass through the
selectively permeable membrane.
Cell membranes
are semipermeable. They
only allow certain
molecules (small)
through.
A
B
High solute
concentration
The symbol for water
potential is Ψ
Water always moves from a region of high
water potential to low water potential.
Before osmosis starts
There is a net
movement of water
molecules from A to
B until an
equilibrium is
reached where
solution A has the
same water
concentration as B
Water potential is the tendency of
water to move from 1 place to another
A
B
Pure water has the highest water potential.
The effect of solute molecules is to lower
water potential
By convention the water potential of pure
water is zero. Increasing solute
concentrations produce increasingly
negative values for water potential.
Osmosis finished
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Transport across the plasma membrane
Osmosis in animal cells – red blood cells
Movement of water into or out of red blood cells by osmosis in solutions
of different concentration
Red cell bursts
Red cell remains
normal
Red cell shrinks
In (hypotonic) pure water
or dilute solution
In a (isotonic) solution with
the same concentration as
the red cell
In a (hypertonic) more
concentrated solution
Low concentration
of solute molecules,
high concentration
of water molecules
High concentration
of solute molecules,
low concentration of
water molecules
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Transport across the plasma membrane
Osmosis in plant cells - turgidity
Pressure potential is
particularly important in plant
cells.
In a hypertonic
solution
They have a strong and rigid cell wall and if water
enters the plant cell protoplast by osmosis and
increases the protoplast volume, the confining cell
wall causes a pressure build-up.
In an isotonic
solution
vacuole
cell undergoing plasmolysis
The cell wall prevents the cell
from bursting, as would happen
with an animal cell under these
conditions
vacuole
normal
In a hypotonic
solution
vacuole
turgid
This is the pressure potential,
and it increases the water
potential of the cell inside until
it equals the external water
potential. At this point water
entry stops. The cell is now
described as turgid
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Transport across the plasma membrane
Osmosis in plant cells - plasmolysis
Water leaves the
cell by osmosis.
In a hypertonic
solution
e.g. concentrated sucrose
As it does so the
protoplast gradually
shrinks until there is no
pressure on the cell wall
In an isotonic
solution
In a hypotonic
solution
vacuole
vacuole
vacuole
At this point the pressure
potential is zero and the
water potential is equal to
the solute potential
cell undergoing plasmolysis
The point at which the
pressure potential has
just reached zero and
plasmolysis is about to
occur is referred to as
incipient plasmolysis.
As the protoplast
continues to shrink it pulls
away from the cell wall.
This process is called
plasmolysis and the cell is
said to be plasmolysed
(protoplast not touching
the cell wall).
normal
turgid
For plant cells the water potential is thus
a combination of solute potential and
pressure potential, as follows:
Ψ =
ΨS +
ΨP
ΨS
Solute potential
ΨP
Pressure potential
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Transport across the plasma membrane
Osmosis in plant cells
Osmotic changes in plant
cells can be easily observed
using a light microscope
Rhubarb epidermal strips or
the swollen storage leaves of
onion bulbs contain a red
pigment which highlights the
protoplasts in sharp contrast
to the cell walls.
Light micrograph of plasmolysed red onion cells
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Transport across the plasma membrane
Active transport
Active transport is the pumping of ions
across membranes against a diffusion
gradient, using energy from ATP.
high concentration
Membrane
protein
pump
ADP
ATP
Like facilitated transport it is achieved
by special transport proteins but in
active transport ATP is required to
change the 3D shape of the protein
and therefore move the ‘bound’ ion or
molecule across.
Most cells have active transport
pumps, Check out some examples of
active transport:
Glucose reabsorption in the kidney
low concentration
Nerve cell resting potential
Ion uptake by plant root hairs
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Transport across the plasma membrane
Bulk transport - endocytosis
Diffusion, osmosis and active transport refer
to the movement of individual particles
across membranes
Adherence
Ingestion
Mechanisms also exist for the bulk
transport of materials in and out of
cells (endo- and exocytosis).
Formation of
phagosome
Phagolysosome
Lysosome
Release of
microbial
debris
Fusion
Destruction of
microbe
Stages in phagocytosis of a
bacterium by a white blood cell
Phagocytosis or ‘cell eating’. The
bulk uptake of solid materials. Cells
which do this are phagocytes, e.g.
some white blood cells
Pinocytosis or ‘cell drinking’. The
bulk uptake of liquid.
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Transport across the plasma membrane
Bulk transport - exocytosis
EM of pancreatic
acinar cell secreting
protein
Exocytosis is the reverse of
endocytosis
It happens, for example, in the
secretion of digestive enzymes from
the pancreas
Secretory vesicles from the Golgi body
carry the enzymes to the cell surface
and release them to the outside of the
cell
Golgi apparatus
Secretory vesicle
containing secretory
product, e.g. enzyme
Diagram of Golgi
apparatus secretion
Plant cells use exocytosis to get their
cell wall building materials to the
outside of the plasma membrane
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WHAT YOU SHOULD
KNOW AT THE END OF
THIS UNIT
describe and explain the fluid mosaic model of
membrane structure, including an outline of the
roles of phospholipids, cholesterol, glycolipids,
proteins and glycoproteins;
outline the roles of membranes within cells and
at the surface of cells;
describe and explain the processes of diffusion, osmosis, active
transport, facilitated diffusion, endocytosis and exocytosis
(terminology described in the IOB’s publication Biological Nomenclature should be
used; no calculations involving water potential will be set);
*investigate the effects on plant cells of immersion in
solutions of different water potential;
use the knowledge gained in this section
in new situations or to solve related
problems.
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Name ____________________
1.
PHOSPHOLIPID MOLECULE
HYDROPHOBIC TAIL
LABEL THE
PARTS
SHOWN
(3)
HYDROPHILIC HEAD
Carbohydrate (polysaccharide ) part
of a glycoprotein, or gylcolipid
outside
2.
(3)
channel protein
inside
extrinsic protein
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3.
Glycolipids and They stabilise the membrane and also act as receptor
glycoproteins molecules for hormones and neurotransmitters
GIVE THE
FUNCTIONS OF
THESE
MEMBRANE
COMPONENTS
Transport provide hydrophilic channels for the passage of ions and
proteins polar molecules
(3)
Cholesterol help maintain the fluidity and stability of the membrane. help
molecules: prevent ions or polar molecules from passing through
4.
Na+
IMPERMEABLE
Carbon dioxide
PERMEABLE
Oxygen
PERMEABLE
Amino acids
IMPERMEABLE
Water
PERMEABLE
glucose
IMPERMEABLE
H+
IMPERMEABLE
STATE WHETHER
THE MEMBRANE
IS PERMEABLE
OR IMPERMEABLE
TO EACH OF
THESE
SUBSTANCES
(6)
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cystic fibrosis a _______________
channel protein in lung and gut
5. Inepithelial
cells which normally allows
Fill in the gaps
______________
sodium
chloride ions to move out of the cells is faulty.
tendency of water to move from 1 place to another
6. Water potential is the ___________________________________________:
The effect of solvent molecules is to ___________
decrease the water potential
(2)
(2)
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7.
In a _____________
hypertonic
solution
In an_________________
isotonic
solution
vacuole
vacuole
cell undergoing plasmolysis
8.
normal
In a _______________
hypotonic
solution
vacuole
turgid
Fill in the
missing
words
(3)
Write an equation showing the relationship between solute potential,
pressure potential and water potential. Use the correct symbols
Ψ =
ΨS +
ΨP
ΨS
Solute potential
ΨP
Pressure potential
(4)
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9.
vacuole
The point at which the
pressure potential has
just reached zero and
plasmolysis is about to
occur is referred to as
incipient plasmolysis
_________________.
(1)
This cell is in concentrated
sucrose solution
10.
Active transport is the:
pumping of ions across membranes
against a diffusion gradient, using
energy from ATP.
11.
Give 2 examples of
active transport:
Glucose reabsorption in the kidney
(3)
Nerve cell resting potential
Ion uptake by plant root hairs
(2)
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12.
13.
Exocytosis
bulk transport out of a cell
Endocytosis
bulk transport into a cell
Phagocytosis
membrane transport of solids
Pinocytosis
membrane transport of liquids
Define
these
terms
(4)
EM of pancreatic
acinar cell secreting
protein
Golgi
apparatus
Secretory vesicle
containing
secretory product,
e.g. enzyme
Total
Label
the
parts
(2)
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