Download THE CELL (III)

THE CELL (III)
TRANSPORT IN AND OUT OF
CELL
Passive diffusion
• net movement of a substance (liquid or gas)
from an area of higher concentration to one
of lower concentration.
• Passive transport requires no energy from the cell.
• Examples include the diffusion of oxygen and
carbon dioxide, osmosis of water, and facilitated
diffusion.
• Diffusion of a dye in a
beaker of water
Diffusion across cell membrane
• across the lipid bilayer
• via membrane channels (pores)
• via carrier proteins ( facilitated diffusion )
• Diagram of a cell membrane
• Structure of a
phospholipid, spacefilling model (left) and
chain model (right).
Diagram of a
phospholipid bilayer.
Movement of selected molecules across the cell membrane.
Diffusion across cell membrane
 across the lipid bilayer
The rate of transport depends on :
• The concentration gradient across the
membrane.
• Temperature.
• molecular size - small molecules tend to
penetrate more rapidly.
• Polarity of molecules - non-polar
molecules pass through readily.
 Diffusion via carrier proteins ( facilitated
diffusion )
• The solute molecules are bound to a
specific protein carrier (permease),
which carries them across
• is driven by potential energy of a
concentration gradient.
e.g. movement pf glucose into red blood cells, which is
not inhibited by respiratory inhibitors
Facilitated diffusion through a carrier protein e.g.
movement of glucose into a cell down a conc gradient
Facilitated diffusion through a protein carrier
 Diffusion via membrane channels (pores)
• Many small ions and polar
molecules, e.g. Na+, K+,
diffuse across the
membrane readily
through some channels
• The channels are protein
in nature and specific to
one type of molecule or
ion.
Diffusion through membrane channel
Active transport
• against a concentration gradient.
• expenditure of metabolic energy, usually in form of
ATP (inhibited by respiratory poison, e.g. cyanide)
• asymmetric; one direction only.
Active
Transport
Substance
Low conc
Carrier
Protein
ATP
ADP + Pi
Substance
High conc
Cells and tissues carrying out active transport
are characterized by :
• numerous mitochondria
• A high concentration of ATP
• A high respiratory rate.
Examples of active transport:
 Transport of sugars and amino acids across the
epithelial cells during absorption in the intestine
and kidney.
 Sodium pump in the nerve cell.
 Active transport of calcium into the sarcoplasmic
reticulum in muscle cells
Endocytosis and Exocytosis
Endocytosis
• infolding or extension of the plasma membrane
to form a vesicle or vacuole.
 Phagocytosis (cell eating)
 Pinocytosis (cell drinking)
Phagocytosis (cell eating)
•
•
•
•
Invagination of membrane surrounding the particles.
depression then pinched off to form phagocytic vacuole.
Lysosomes release their enzymes
Useful products are absorbed and undigested matter is expelled
by exocytosis.
Pinocytosis (cell drinking)
• in liquid form
• Vesicle are extremely small
Endocytosis (pinocytosis) by a capillary endothelial cell. Note the
desmosome connection between opposing cell membranes
Phagocytosis and pinocytosis are active
process and require energy
• inhibited by respiratory poison and other
metabolic poison
• Endocytosis is not an alternative process of
active transport, but rather a supporting one.
• It provided a much larger interior interface
where passive and active transport are carried
out more efficiently than at surface membrane
Exocytosis
• the reverse of endocytosis
• two main functions :
 replace the plasma membrane that have been
removed by endocytosis or to add new membrane
 Provide a route for releasing the molecules
synthesized by the cell, e.g. secretion, hormones.
Movement of water across the
plasma membrane
Movement of water across the plasma
membrane
Osmosis - This is the passage of solvent
(water) molecules from a regions of
their high concentration to a region of
their low concentration through a
differentially permeable membrane
• Free energy - is defined as the
energy available (without change in
temperature) to do work. It depends
on the number and activity of the
molecules
• Molecules will move from regions of
higher free energy to regions of lower
free energy.
• Chemical potential - of a substance is
the free energy per mole of that
substance.
• Water potential (  ) - It is the chemical
potential of water. The water potential of pure
water at 25 C and one atmospheric pressure is set
arbitrarily as zero.
• The water potential of a system can be defined
as the difference in chemical potential of water
in this system and that of pure water at the
same temperature and pressure.
• All solution have lower water potentials than pure
water and therefore have negative values of .
The water potential of a plant cell has
two components : osmotic potential and
pressure potential.
water potential = osmotic potential + pressure
potential.
 =   (  s) +  p
• Osmotic potential (  ) / Solute potential
( s)
- is defined as the component of water
potential that is due to the presence of
solute. Solute particles decrease the free
energy and hence the chemical potential of
solvent molecules.
• always negative
• The more concentrated a solution is, the lower is
its osmotic potential ( more negative ).
• Osmotic potential is a measure of the tendency of
a solution to pull water into it.
• The presence of a solute decreases the water
potential of a substance. Thus there is more
water per unit of volume in a glass of freshwater than there is in an equivalent volume of
sea-water.
• In a cell, which has so many organelles and
other large molecules, the water flow is
generally into the cell.
• Pressure potential ( p )
- This is the component of water potential that
is due to hydrostatic pressure. This can be
regarded as the capacity of the existing
hydrostatic pressure of a cell to drive water
out of it. The pressure is often a positive
inside the living cell, and has a positive
value.
p = 0
s = - 4 MPa
Placed in
distilled
water
p = ?
s = ?
cell = ?
cell = ?
Cell at equilibrium
Cell at start
p = ?
s = ?
cell = ?
Then placed in
solution of s =
- 3MPa
p = +8 MPa
p = +2 MPa
s = - 12 MPa
s = - 14 MPa
cell = ?
cell = ?
Cell A
Cell B
•
Calculate the water potential in cell A and cell B.
•
What is the direction of water flow ?
•
Calculate the p , s and cell at equilibrium.
Xylem
cell = -1 MPa
Water will move from the
root cells into the xylem.
Explain.
A continuous
column of water
under tension,
i.e. = -3MPa
Water relations and cell shape in blood cells
Water relations in a plant cell
Determination of the osmotic
potential and water potential
Determination of water potential of a plant
tissue
• Pieces of tissues carefully measured length or
weight are placed in a range of solutions of
different concentration.
• The tissue samples are allowed to come to
equilibrium with the surrounding solution.
• The change in size or weight is measured after
the tissue reaches equilibrium.
• In the solution in which no size change ( or
weight change ) takes place, the water potential
of the cell is equal to that of the solution.
Conc. of sucrose
solution / M
0.3
0.35
0.4
0.45
0.5
0.55
0.6
Volume of distilled
water / cm3
14
13
12
11
10
9
8
Volume of sucrose
solution / cm3
6
7
8
9
10
11
12
Table 1 : Dilution table of sucrose solution
• Lengths of beetroots strips left in different concentrations of
sucrose solution for 24 hours.
Molarity of
sucrose solution /
M
Initial length of
beetroot strip/ cm
Final length of
beetroot strip/ cm
Percentage
change in length /
%
0.00
4.8
5.0
4.2
0.10
5.1
5.3
3.9
0.20
5.1
5.2
2.0
0.30
4.9
4.9
0.0
0.40
4.9
4.9
0.0
0.50
5.0
4.8
-4.0
0.60
4.8
4.6
-4.2
0.70
4.9
4.6
-6.1
• Potato tissue takes up
water in hypotonic solution
(0.1-0.2M) causing it to
increase in length.
• However, the potato tissue
releases water in hypertonic
solution (0.4-0.6) causing it
to decrease in length.
• There would be no change
in length in isotonic solution
(0.27M).
WP of potato is equivalent to the
WP of 0.27 M sucrose solution
• Hypertonic solutions are those in which more
solute (and hence lower water potential) is present.
• Hypotonic solutions are those with less solute
(again read as higher water potential).
• Isotonic solutions have equal (iso-) concentrations
of substances.
• Water potentials are thus equal, although there will
still be equal amounts of water movement in and
out of the cell, the net flow is zero.
Determination of the mean
osmotic potential of the cell sap
Determination of the mean osmotic potential of
the cell sap
Principle
WP = OP + PP
If PP = 0, then WP = OP
PP = 0 > wall pressure drop to 0 > cytoplasm / cell
membrane just touches the cell wall, i.e. Incipient
plasmolysis
Method
•
•
•
•
a graded series of solution of different concentration
Small pieces of tissues are placed in each solution
examined microscopically after equilibrium
some of them show signs of plasmolysis
• when 50 % of cells have plasmolysed. 50 % of cells are
unplasmolysed, 50% of cells will have higher OP than
the solution and 50% will have lower OP than the
solution. Therefore the solution at this point has
approximately the same osmotic potential as the median
osmotic potential of tissue.
Percentage of cells plasmolysed in different concentrations of sucrose
solution
Molarity of Sucrose 0.3
solution / M
0.35 0.4
0.45 0.5
0.55
0.6
Percentage of
plasmolysed cell
2.9
13.5 74.0
100
100
2.5
3.5
Percentage of plasmolysed
cells/ %
Percentage of cells plasmolysed in different
concentration of sucrose solution.
120
100
80
60
40
20
0
1
2
3
4
5
6
Molarity of sucrose solution/ M
7
% of
plasmolysis
Conc (M)
If all the cells have the same OP, all the cells would
become plasmolysed at certain conc.
But, OP of cells show variations.
No. of
cells
Osmotic potential
At 0.2 M, 8% of cells
plasmolysed
8 % of cell OP > OP of
0.2M sucrose
At 0.4 M, 78 % of cells
plasmolysed
78 % of cell OP > OP of
0.4 M sucrose
At 0.65 M, 100 % of
cells plasmolysed
100 % of cell OP > OP
of 0.65 M sucrose
At 0.35 M, 50 % of cells
plasmolysed
50 % of cell OP > OP of
0.35 M sucrose
The mean OP of cells is
equivalent to that of
0.35 M sucrose solution
Comment on the statements found in a
student’s report
• In sucrose solutions of 0 M to 0.2 M, below
50 % of cells are plasmolysed. This means
that the solution is hypotonic to the cell sap.
• From the graph, the osmotic potential of cell
sap is 0.38 M.