Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong)
The Cell ...... Page 41
Transport of substances in and out of the cell
Syllabus : The selective permeability of membranes. The destruction of membranes at high temperatures and by
some chemicals, e.g. chloroform, ethanol.
The processes of diffusion, osmosis and active transport. The processes of pinocytosis and
phagocytes. Turgor and plasmolysis in plant cells with reference to water potential, solute potential
and pressure potential.
Selective permeability of cell membrane :
The plasma membrane only allow some substances to pass through but not others, so it can
control the entrance and exit of molecules and ions.
Factors affecting permeability of cell membrane :
1. Natures of molecules passing through :
a) Water : almost all natural membrane are freely permeable to water. The concept of
‘solvent drag’ can be used in explain the passive entry of water through the
plasma membrane. The idea is that as the water molecules pass through the
membrane they exert a dragging force on those behind to draw the other water
molecules into the cell.
b) Water soluble neutral substances : they can pass through the membrane freely by the
dragging force when the water moves into the cells
and possibly taking solute molecules with it together.
c) Fat soluble neutral substances : they pass more quicker than water soluble substances
because the core of the cell membrane is lipid.
d) Electrolytes : the entry of electrolytes into cells is generally found to be slower than that
of non- electrolytes of comparable size. Weak electrolytes which dissociate
into ions, enter more rapidly than strong ones. The higher the valency of the
ions, the slower is its penetration.
2. Size of the molecules passing through:
Too large molecules will be unable to pass through natural membranes except through
endocytosis (phagocytosis and pinocytosis).
3. Chemicals :
Organic solvent such as chloroform and ethanol will dissolve the membrane, so destroy the
selective permeability of it.
4. Temperature :
High temperature will denature the protein that make up the membrane, so as destroy its
selective permeability.
Exercise : (98 I 4a)
An experiment was set up to study the effects of chemicals on the permeability of the cell
membrane of beet root. Beet root discs were prepared, rinsed with water and blotted dry. The
same number of beet root discs were placed into three test- tubes labelled A, B and C containing
equal volumes of fluids : tube A contains water,
tube B contains 30 % ethanol, and
tube C contains 0 % sucrose solution.
Indicate from which test tube(s) Beet root discs sank. The liquid in the test- tube looked
intensely red.Give your reasons.
[2½ marks]
Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong)
The Cell ...... Page 42
Transmembrane transport :
Diffusion
Definition :Diffusion is the process by which a substance moves from a region of high
concentration of that substance to a region of low concentration of the same
substance.
It is the dispersion of atoms, ions or molecules by random thermal motion (kinetic energy)
until these particles are equally distributed in the available space.
For example , if some particles concentrated in the left side of a vessel, they will collide with
each others, some may spread to the right side. Since initially, there is no such particles in the
right, so the movement is in one direction only - - - there is a large concentration gradient and
diffusion is rapid. However, after a short time, the particles have spread themselves more
evenly, particles now may move from right to left as well as from left to right. But with a
higher concentration of particles in the left, there is a greater probability of a particle
moving to the right - - - there is a smaller concentration gradient and diffusion is slower. Some
time later, the particles will be evenly distributed throughout the vessel and the concentrations
will be equal on each side. The system is in equilibrium. The particles are not in static but remain
in random motion. i.e. the number of particles move from left to right equal to those move from
right to left. This situation is called a dynamic equilibrium - - - there is no concentration gradient
and no net diffusion.
Free diffusion through solutions is a very slow process, hence the development of circulatory
system in most multicellular organisms.
UB p50 fig 4.14
Fig. 46 Processes of Diffusion.
Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong)
The Cell ...... Page 43
Factors affecting the rate of diffusion :
1. Concentration gradient : the greater the difference in concentration between two regions
of a substance the greater the rate of diffusion.
2. Distance : the shorter the distance between two regions of different concentration the
greater the rate of diffusion.
3. Surface area : the greater the surface area the greater the rate of diffusion. Diffusion
surfaces frequently have structures for increasing their surface area and hence the rate at
which they exchange materials. e.g. villi and microvilli.
4. Nature of structure across which diffusion occurs : e.g. the greater the number and size of
pores in cell membranes the greater the rate diffusion.
5. Size and nature of the diffusion molecules : smaller molecules diffuse faster than large ones.
Fat- soluble ones diffuse more rapidly than water- soluble ones when passing through the
cell membrane.
Osmosis
Definition : The movement of solvent molecules (water) from a pure solvent or solution with
higher water potential to a region with lower water potential through a
semi- permeable membrane which separated the two regions.
UB p50 fig 4.15
[Note] Free energy = it includes the velocity or kinetic
motions of the particles and
their rotational and vibrational
energies
at
a
certain
temperature.
Chemical potential = it is the free energy per mole. It
is a measure of the energy
content of particles in a system
and therefore of their tendency
to diffuse spontaneously from
one place to another.
Fig. 47 Processes of Osmosis.
Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong)
The Cell ...... Page 44
Terms to know :
Water potential (ψ )
Definitions: (1) It is the chemical potential of water.
(2) For practical purpose, the water potential of a system is defined as the
differences in chemical potential of water in this system and that of pure water
at the same temperature and pressure. (The water potential of pure water at
STP is set arbitrarily as zero.)
(3) In the cell situation, the water potential of a cell is the difference in the chemical
potential of water inside the cell and the chemical potential of pure water.
In plants, water potential is the controlling factor for water movement among cells. Since water
always flows down an energy gradients, water passed from cells with higher water potential to
cells with lower water potential.
Water potential has 2 main components, osmotic potential (ψ s ) and pressure potential (ψ p ).
i.e. ψ = ψ s + ψ p
Osmotic potential (ψ s )
Definition : It is the component of water potential that is due to the presence of solute, i.e. a
measure of the presence of solute.
For example : sucrose, the solute particles decrease the chemical potential of solvent molecules.
Thus in a cell system an increase in the solute concentration would lower its water
potential. Thus it is always negative (- ve) in sign.
The osmotic potential of a plant cell has the following characteristics :
[1] It is always - ve, apart from the osmotic potential of pure water which has a value of zero.
[2] The - ve value of osmotic potential of a solution means if a solution is separated from pure
water by a semi- permeable membrane, water will always enter the solution as the water
potential of a pure water higher.
[3] The value of osmotic potential of a solution is determined by the concentration of the
solution. The more concentrated a solution is, the lower is its osmotic potential (more - ve)
and vice versa.
Pressure potential (ψ p )
Definitions : (1) It is the component of water potential that is due to hydrostatic pressure.
(2) It is the capacity of the existing hydrostatic pressure of a cell to drive water
out of it.
For example : when a plant is immersed in hypotonic solution (solution of higher water
potential). there is a net influx of water. As water enters the cell, the protoplast
enlarges and creates a hydrostatic pressure. This hydrostatic pressure is used to
force water out from the expanding cell. (it is usually a positive force, then it is
(+ ve) in sign)
Plasmolysis and deplasmolysis in plant cells :
In plant, the rigid cell wall maintain the original form of the cell. Shrinkage of cytoplasm due
to osmotic loss of water is termed plasmolysis. Plasmolysis involves;
[1] outward diffusion of water,
[2] reduction of turgor pressure,
[3] separation cell wall and plasma membrane,
[4] decrease in size of vacuole and cytoplasm,
[5] decrease in water potential (less - ve value)
Water will re- enter into a plasmolyse cells as they are immersed in a solution of higher water
potential (hypotonic). This is known as deplasmolysis.
Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong)
The Cell ...... Page 45
Fig. 48 Chart to show differences between cells place in external solutions of different water potential.
UB p487 fig 33.3
Fig. 49 The graph below shows the changes in water potential, osmotic potential and pressure potential of an artificially
induced, highly plasmolysed cell when it is placed into pure water.
Functional approach p54 fig 4.6
Stage A
Stage B
Stage C
Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong)
The Cell ...... Page 46
Stage A :
[1] In this stage, the cell is still plasmolysed, there is no hydrostatic pressure, then the
pressure potential (ψ p ) is zero.
[2] Thus, the water potential of the cell is equal to the osmotic potential of the cell. The value
of the water potential is very negative. i.e. very low (ψ p = 0, ψ = ψ s )
[3] If such a cell is put into pure water (ψ = 0), water will move in down the water potential
gradient.
[4] As water enters the cell, it dilutes the solvents and thus the osmotic potential increases
(ψ s ↑ / becomes less negative.)
Stage B :
Eventually the cell membrane touches the cell wall and the condition of incipient plasmolysis
(the point at which the cell membrane just touches the cell wall) is reached.
[1] A pressure potential at that time starts to build up and gradually increase as more water
enters (ψ p ↑).
[2] The osmotic potential continues to increase as the cell content is still diluted (ψ s ↑).
[3] The water potential of the cell further increase, at a faster rate than osmotic potential
as the value of the pressure potential is positive (ψ ↑).
Stage C :
As water potential reaches zero, i.e. equal to the water potential of water outside, no more
water enter the cell and the cell is said to be fully turgid.
Exercise :
(97 I 7)
(i) Give a word equation to illustrate the relationship between the water potential (Ψ), the
osmotic potential (Ψs) and the pressure potential (Ψp) in a plant cell. [1 mark]
(ii) Sketch a graph to show the changes in Ψ, Ψs and Ψp when a fully plasmolysed plant cell
is placed in pure water and regains full turgor in 40 minutes (given : pressure potential at full
turgor is +400 kPa and incipient plasmolysis occurs at 20 minutes).
[5 marks]
(98 I 4b)
An experiment was set up to study the effects of chemicals on the permeability of the cell
membrane of beet root. Beet root discs were prepared, rinsed with water and blotted dry. The
same number of beet root discs were placed into three test- tubes labelled A, B and C containing
equal volumes of fluids : tube A contains water,
tube B contains 30 % ethanol, and
tube C contains 30 % sucrose solution.
Indicate from which test tube(s) Beet root discs floated initially and then sank. The liquid in the
test tubes looked the same in colour as that in the control test- tube. Give your reasons.
[3½ marks]
Osmosis and the animal cells :
Animal cells do not have a cell wall so pressure potential is not a factor affecting their water
potential. The osmotic potential of cytoplasm is the main factor which determines the water
potential of animal cells.
If an animal cell is placed in pure water, or very dilute solutions (hypotonic), swells until its
plasma membrane ruptures (cell lysis).
Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong)
The Cell ...... Page 47
Advanced biology p11 fig1.11
Fig. 50 Osmotic behaviour of a red blood cell following immersion in different solution. Rupture of red blood cells in
hypotonic solutions is known as haemolysis.
Active Transport
Definition : It is an energy- consuming transport of molecules or ions across a membrane against
a concentration gradient. Movement of the molecules is usually unidirectional.
Factors affecting active transport :
1. Concentration : The higher the concentration difference, the more the energy needed (as
greater concentration gradient should be overcome)
2. Electrical charge : Ions usually attracted towards a region of opposite charge, and move
away from a region of similar charge. Thus active transport of ions is their
movement against an electrochemical gradient.
3. Energy availability : The lack of energy decrease the rate of active transport.
Characteristics of cells carrying out active transport :
1. The presence of numerous mitochondria
2. A high concentration of ATP
3. A high respiratory rate :
So that any factor which increase the rate of respiration, e.g. higher temperature or increased
concentration of oxygen, will increase the rate of active transport. Any factor reducing the rate
of respiration or causing it to cease, e.g. the presence of cyanide, will cause active transport
to slow to stop altogether.
Example : Sodium-potassium pumps
In the recent years it has been shown that the plasma membrane of most cells possess
N a- pumps that actively pump sodium ions out of the cell. Usually the Na- pump is coupled with
a K - pump which actively accumulated K + ions from the external medium and passes them into
the cell. The pump is essentially a protein which spans the membrane from one side to the other.
On the inside it accepts Na+ and ATP, while on the outside it accepts K.
Exercise : (93 II 2a)
Distinguish between diffusion and active transport.
[4 marks]
Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong)
The Cell ...... Page 48
Advanced biology p104 fig 6.11b
Fig. 51 Transmembrane transport mechanisms.
Endocytosis
Phagocytosis
It is the process by which the cell can obtain particles which are too large to be absorbed
by diffusion or active transport.
The cell invaginates to form a cup- shaped depression in which the particle is contained. The
depression is then pinched off to form a vacuole. Lysosomes fuse with the vacuole and their
enzymes break down the particle, the useful contents of which may be absorbed. The process
only occurs in a few specialized cells (phagocytes), such as white blood cells or Amoeba.
Pinocytosis
It may called ‘cell drinking’. It is similar to phagocytosis except that the vesicles produced
are smaller. The process is used for the intake of liquids rather than solids.
BSI 3rd Ed. P147 fig 5.23
Fig. 52 Endocytosis and exocytosis.
[Note ] The reverse process of endocytosis is the exocytosis, in which materials are removed from the cell