Transcript Transmembrane transport

Chapter 4
Movement of substances across the
membrane
Fluid mosaic model
of membrane
The fluid nature is due to the lateral movement of the
phosopholipid layers or the break and build up nature of these
layers at some points.
The mosaic appearance is due to the scattering pattern of the
protein molecules.
Selective permeability of cell membrane
Pores of channel proteins allow ions, water soluble molecules of
small size to pass through. (Organic or fat soluble molecules
cannot pass through these pores.)
Phospholipid layers allow fat, fat soluble or organic molecules
to pass through. (Ions or water soluble molecules cannot pass
across the phospholipid layers.)
Carrier molecules transport specific ions or molecules across
the membrane.
Large molecules are unable to pass through either the channel
protein, phospholipid or by the carrier molecules.
Factors affecting membrane permeability
1. Temperature: If substances diffuse across the cell
membrane, the permeability increases with temperature.
But very high temperature destroys the cell membrane so
that it will be freely permeable.
2. Organic chemicals such as ether, chloroform and
alcohol dissolve the bimolecular layer of phospholipids.
They break the cell membrane and hence it becomes
freely permeable.
3. Charges of protein channels OR transmembrane
potential can affect the rate of diffusion of cations/ anions.
4. Solubility of fat soluble molecules in the phospholipid
layers: The higher its solubility, the easier it can pass across
the membrane.
Diffusion
Movement of fluid particles along concentration gradient.
Does it need metabolic energy?
Does it need enzyme?
Does it need carrier protein?
Does it take place in living cell only?
All answers above: No! No! No! No!
Factors affecting the rate of diffusion
1. Concentration gradient between two regions;
2. Distance and surface area of diffusion;
3. Size and nature of molecules;
4. Temperature;
5. Nature of the barrier????
+
+
+
+
+
High conc. of NaCl
membrane
-
-
-
-
-
Low conc. of NaCl
Compare the rate of downward diffusion of Na+ and Cl- across
the membrane.
Facilitated diffusion
Along a concentration gradient
Across the cell membrane
Requires specific channels or carrier molecules
Does not require the expenditure of energy
Example: absorption of glucose from plasma into the red
blood cell by facilitated diffusion.
Osmosis
The net movement of water molecules along the
water potential gradient through a selectively
permeable membrane.
Water potential
It is the measure of the free energy of water
molecules. It is related to the motion of the water
molecules.
Pure water at standard atmospheric pressure and
25oC has its water potential arbitrarily set at zero.
The water potential of a solution is the difference
in free energy between the water molecules of the
solution and the water molecules of pure water at
the standard conditions.
Water potential has two components. They are solute
potential and pressure potential.
Water potential = Solute potential + pressure potential
Solute potential is due to the dissolved substances. Solute
attracts water molecules and make them less mobile.
Therefore the solute potential is always negative in value.
Pressure potential is due to the hydrostatic pressure other
than the standard atmospheric pressure. Hydrostatic
pressure usually renders the water molecules more mobile.
Therefore the pressure potential is usually positive in value.
Plant cell A
Plant cell B
Solute potential is – 2 atm
Solute potential is – 3 atm
Pressure potential is + 1 atm
Pressure potential is +1atm
What is the direction of the net movement of water?
Plant cell A
Plant cell B
Solute potential is – 2 atm
Solute potential is – 2 atm
Pressure potential is + 2 atm
Pressure potential is +1atm
What is the direction of the net movement of water?
Xylem vessel
Cortex cell of root
Solute potential is –1atm
Solute potential is –1.5 atm
Pressure potential is –2atm
Pressure potential is +1atm
What is the direction of the net movement of water?
Plant cell A is added to a beaker of distilled water.
Plant cell A
Solute potential = -2 atm
Distilled water
Pressure potential = + 1 atm
What is the direction of the net movement of water?
Plant cell A is added to a beaker of distilled water.
Plant cell A
Solute potential = -2 atm
Distilled water
Pressure potential = + 1 atm
What will the final water potential of plant cell be?
Its solute potential is getting less negative; its pressure
potential is getting more positive. Finally, the water potential
will approach zero. The cell will be fully turgid.
No net water movement occurs.
Plant cell A is added to a beaker of sugar solution with solute
potential of – 3 atm.
Plant cell A
Solute potential = -2 atm
Sugar solution
Pressure potential = + 1 atm
What is the direction of the net movement of water?
Plant cell A is added to a beaker of sugar solution with solute
potential of – 3 atm.
Plant cell A
Solute potential = -2 atm
Sugar solution
Pressure potential = + 1 atm
What will the final water potential of plant cell be?
Its solute potential is getting more negative; its pressure potential
is getting less positive. Eventually, the solute potential
becomes –3atm; its pressure potential becomes zero. In other
words, its water potential becomes –3atm. The cell is fully
plasmolysed. No more net movement of water occurs.
Incipient plasmolysis is the condition at which the protoplast just
detaches from the cell wall.
Full plasmolysis is the condition at which the protoplast detaches
from the cell wall. No more exosmosis occurs.
Plant cell = Protoplast + cell wall
It is placed in a
hypertonic soln
Condition of
ordinary plant cell
It is placed in
distilled water
Sketch a graph to show the change in
water potential, solute potential and
pressure potential of the sucrose solution
with time. (Assume that the dialysis
tubing is not permeable to sucrose
molecule.)
Length of
the liquid
column
time
Sketch a graph to show the change in
water potential, solute potential and
pressure potential of the sucrose solution
with time. (Assume that the dialysis
tubing is not permeable to sucrose
molecule.)
positive
Pressure potential
time
0
negative
Water potential
Sketch a graph to show the change in
water potential, solute potential and
pressure potential of the sucrose solution
with time. (Assume that the dialysis
tubing is not permeable to sucrose
molecule.)
Pressure potential
positive
0
time
Water potential
negative
Solute potential
A plant cell is added to a sucrose solution.
Plant cell:
Solute potential: - 3 units
Pressure potential: +1 unit
Sucrose solution:
Solute potential: - 5 units
Sketch a graph to show the change in water potential, solute
potential and pressure potential of the plant cell with time.
Active transport
It can transport the fluid particles across the cell
membrane against the concentration gradient.
It needs energy supply.
The fluid particles are transported by carrier
proteins which are specific in function.
The active transport is uni-directional.
e.g.
epithelial cell of ileum villus
epithelial cell of proximal and distal tubule of
kidney nephron.
Active Transport
Active Transport
Active Transport
Active Transport
Active Transport
Can active transport carries
molecules along concentration
gradient?
Yes!
Active Transport along
concentration gradient
Active Transport along
concentration gradient
Active Transport along
concentration gradient
Active Transport along
concentration gradient
Active Transport along
concentration gradient
Active Transport along
concentration gradient
Phagocytosis and pinocytosis
They are bulk transport (i.e. swallowing or drinking) of
particles into a cell .
They are active process involved expenditure of energy.
The particles transported into a cell can be large
particles. Some of them can be seen under light
microscope.
Intracellular digestion is common accompanying
pinocytosis and phagocytosis. Therefore they are
associated with the activities of lysozomes.
Phagocytosis
It is the bulk transport of solid particles into a cell.
It is cell “eating”
e.g.
Amoeba ingests food particles
Mammalian phagocytes ingest pathogens
Kupfer cell of mammalian liver ingests aged
erythrocytes.
Phagocytosis
When a phagocytic cell approaches a solid particle, its plasma
membrane pushes out to form pseudopodia to enclose the solid.
The tips of the pseudopodia then fuse, forming a phagocytic
vesicle enclosing the solid food. Lysozomes then fuse with it to
form a vesicle in which intracellular digestion of the solid is
carried out.
The end product of digestion is absorbed into cytoplasm by
active transport and diffusion.
Pinocytosis
It is bulk transport of liquid into a cell.
It is cell “drinking”.
Examples:
Epithelial cell of ileum villus incorporate the
digestive end products from the ileum lumen.
Epithelial cell of nephron proximal tubule reabsorb the
fluid from the glomerular filtrate.
Pinocytosis
Plasma membrane of such cell invaginates forming a
pinocytic channel. The channel is then cut off from the
plasma membrane to give a pinocytic vesicle. Lysozome
approaches and carries out intracellular digestion as
mentioned in phagocytosis.
Exopinocytosis and Exophagocytosis
They are reverse process of pinocytosis and phagocytosis
respectively.
Exopinocytosis is related to secretion of a cell.
Exophagocytosis is related to the elimination of the
undigested solid from a cell after phagocytosis and
intracellular digestion.
Phagocytosis, exophagocytosis, pinocytosis and
exopinocytosis are good examples to show the fluid nature
of cell membrane.
The fluidity is shown by breaking and immediate rebuilding
of the cell membrane.
End of Chapter