Transport

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Membranes
and
Transport
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
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Membranes
Membrane structure
Transport
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
The molecules move around by diffusion (driven
by kinetic energy).
These face inwards forming a non-polar
hydrophobic interior
These face the aqueous (water-containing) medium
around the membrane.
DETAILED VIEW OF BILAYER
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Membranes
Membrane structure
Transport
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
Membranes also contain proteins and the model of
membranes accepted at present is called the
fluid mosaic model.
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Membranes
Membrane structure
Transport
Features of the fluid mosaic model
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.
Most protein molecules are mobile, moving around
freely. Others are fixed like islands to
structures in the membrane and do not move
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Membranes
Membrane structure
Transport
Roles of the components of cell membranes
Phospholipids Because their tails are non-polar,
water soluble molecules such as ions, cannot pass
through them.
Glycolipids and glycoproteins These lipids and
proteins have carbohydrate chains which jut out
in from the membrane
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.
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
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
Membrane enzymes are sometimes present. e.g.
small intestine cell membranes have enzymes which
hydrolyse disaccharides.
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Membranes
Transport across the plasma membrane
Transport
Diffusion and facilitated diffusion
Factors affecting the rate of diffusion across
membranes include
Diffusion is the net movement of molecules down a
concentrated gradient.
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Membranes
Transport across the plasma membrane
Transport
Diffusion and facilitated diffusion
PHOSPHOLIPID BILAYER
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Membranes
Transport across the plasma membrane
Transport
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.
The rate of facilitated diffusion depends on how
many appropriate channels there are, and whether
they are open.
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.
CHECK OUT CYSTIC FIBROSIS
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Membranes
Transport across the plasma membrane
Transport
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.
Water potential is the tendency of water to move
from 1 place to another
The solute molecules (red) are too large to pass
through the selectively permeable membrane.
A
B
The symbol for water potential is ?
High solute concentration
Water always moves from a region of high water
potential to low water potential.
Pure water has the highest water potential. The
effect of solute molecules is to lower water
potential
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
A
B
By convention the water potential of pure water
is zero. Increasing solute concentrations
produce increasingly negative values for water
potential.
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Membranes
Transport across the plasma membrane
Transport
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 (hypertonic) more concentrated solution
In a (isotonic) solution with the same
concentration as the red cell
Low concentration of solute molecules, high
concentration of water molecules
High concentration of solute molecules, low
concentration of water molecules
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Membranes
Transport across the plasma membrane
Transport
Osmosis in plant cells
- turgidity
Pressure potential is particularly important in
plant cells.
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 a hypertonic solution
In an isotonic solution
In a hypotonic solution
vacuole
vacuole
vacuole
cell undergoing plasmolysis
normal
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
The cell wall prevents the cell from bursting, as
would happen with an animal cell under these
conditions
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Membranes
Transport across the plasma membrane
Transport
Osmosis in plant cells
- plasmolysis
Water leaves the cell by osmosis.
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
In a hypertonic solution
vacuole
vacuole
e.g. concentrated sucrose
vacuole
At this point the pressure potential is zero and
the water potential is equal to the solute
potential
normal
turgid
For plant cells the water potential is thus a
combination of solute potential and pressure
potential, as follows
cell undergoing 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).
The point at which the pressure potential has
just reached zero and plasmolysis is about to
occur is referred to as incipient plasmolysis.
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Membranes
Transport across the plasma membrane
Transport
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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Membranes
Transport across the plasma membrane
Transport
Active transport
Active transport is the pumping of ions across
membranes against a diffusion gradient, using
energy from 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.
high concentration
Membrane protein pump
Most cells have active transport pumps, Check out
some examples of active transport
ATP
ADP
low concentration
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Membranes
Transport across the plasma membrane
Transport
Bulk transport
- endocytosis
Diffusion, osmosis and active transport refer to
the movement of individual particles across
membranes
Adherence
Mechanisms also exist for the bulk transport of
materials in and out of cells (endo- and
exocytosis).
Formation of phagosome
Ingestion
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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Membranes
Transport across the plasma membrane
Transport
Bulk transport
- exocytosis
Exocytosis is the reverse of endocytosis
EM of pancreatic acinar cell secreting protein
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
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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Membranes
WHAT YOU SHOULD KNOW AT THE END OF THIS UNIT
REVIEW
Transport
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 IOBs 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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Membranes
TEST
Name ____________________
Transport
1.
LABEL THE PARTS SHOWN
(3)
2.
outside
(3)
inside
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Membranes
TEST
Transport
3.
Glycolipids and glycoproteins
They stabilise the membrane and also act as
receptor molecules for hormones and
neurotransmitters
GIVE THE FUNCTIONS OF THESE MEMBRANE COMPONENTS
provide hydrophilic channels for the passage of
ions and polar molecules
Transport proteins
(3)
Cholesterol molecules
help maintain the fluidity and stability of the
membrane. help prevent ions or polar molecules
from passing through
4.
Carbon dioxide
Na
STATE WHETHER THE MEMBRANE IS PERMEABLE OR
IMPERMEABLE TO EACH OF THESE SUBSTANCES
PERMEABLE
IMPERMEABLE
Amino acids
Oxygen
IMPERMEABLE
PERMEABLE
glucose
Water
IMPERMEABLE
PERMEABLE
H
IMPERMEABLE
(6)
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Membranes
TEST
Transport
5.
In cystic fibrosis a _______________ in lung and
gut epithelial cells which normally allows
______________ to move out of the cells is
faulty.
channel protein
Fill in the gaps
sodium chloride ions
(2)
6.
Water potential is the ___________________________
________________
tendency of water to move from 1 place to another
(2)
The effect of solvent molecules is to ___________
water potential
decrease the
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Membranes
TEST
Transport
7.
In a _____________ solution
In an_________________ solution
In a _______________ solution
hypertonic
isotonic
hypotonic
Fill in the missing words
vacuole
vacuole
vacuole
cell undergoing plasmolysis
normal
turgid
(3)
8.
Write an equation showing the relationship
between solute potential, pressure potential and
water potential. Use the correct symbols
(4)
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Membranes
TEST
Transport
9.
The point at which the pressure potential has
just reached zero and plasmolysis is about to
occur is referred to as _________________.
vacuole
incipient plasmolysis
(1)
This cell is in concentrated sucrose solution
pumping of ions across membranes against a
diffusion gradient, using energy from ATP.
10.
Active transport is the
(3)
11.
Give 2 examples of active transport
(2)
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Membranes
TEST
Transport
12.
bulk transport out of a cell
Exocytosis
Define these terms
Endocytosis
bulk transport into a cell
Phagocytosis
membrane transport of solids
(4)
membrane transport of liquids
Pinocytosis
13.
EM of pancreatic acinar cell secreting protein
Golgi apparatus
Label the parts
(2)
Secretory vesicle containing secretory product,
e.g. enzyme
Total /38