CHAPTER 3 :
CHAPTER 3 :THE MOVEMENT OF SUBSTANCES ACROSS THE PLASMA
MEMBRANE3.1MOVEMENT ACROSS THE PLASMA MEMBRANE
3.1.1NECESSITY FOR MOVEMENT OF SUBSTANCES ACROSS THE PLASMA
MEMBRANE
1.To provide all substances required by the cells eg: oxygen,
digested food like glucose, amino acids and fatty acids, vitamins
and minerals.2.To excrete the waste products eg: urea and carbon
dioxide from the cells.3.To maintain the concentration of ions that
are needed for the activities of cells eg: potassium ions (K+) and
sodium ions (Na+) are needed to transmit impulses.
4.To maintain the optimal pH value by regulating the
concentration of hydrogen ions (H+), hydroxyl ions (OH-) and
hydrogen carbonate ions (or bicarbonate ions HCO3-)3.1.2THE
STRUCTURE OF THE PLASMA MEMBRANE
1.The plasma membrane is the boundary which separates the
protoplasm of the each cell from its surroundings.
2.S.J.Singer and G.Nicholson proposed the fluid mosaic model to
describe the structure of the plasma membrane in 1972.
3.Based on the fluid mosaic model, the plasma membrane :a- is
comprised of phospholipid bilayer with various types of protein
molecules embedded in it in which it is pictured as a mosaic.
b- has the fluidity characteristics as as the phospholipid
bilayer, protein and other components are dynamic and flexible and
all proteins molecules can float about in the phospholipid bilayer
to form the mosaic pattern that always changes.c- has amphiphatic
phospholipid molecules. Each phospholipid molecule consists of two
regions ie :
i- hydrophilic polar head which is attracted to water and
ii- hydrophobic non-polar tail which is attracted to lipids.
d- has cholesterol molecules to stabilize and strengthen the PM,
making it more flexible and less permeable to water soluble
substances eg: ions.
e- has various types of embedded protein molecules such as :-i -
the pore or channel protein molecules that provide passages to
allow certain molecules to pass throughii- the carrier protein
molecules which can carry specific molecules across the plasma
membrane
iii- the identification protein molecules which give the
identification to the cells.f- has the semi-permeability
characteristic which allows certain molecules to move across it and
not to the other molecules.4. There are two types of transportation
of substances across the plasma membrane ie:
i passive transport
ii active transport5.The passive transport is classified into
three types ie:
i-simple diffusion
ii-fascilitated diffusion
iii-osmosis
phospholipid
a phospholipid pore / channel bilayer
carrier protein
molecule protein
carrier protein
ATP molecule cholesterol pore / channelThe structure of plasma
membrane3.1.3COMPARISON BETWEEN PASSIVE TRANSPORT AND ACTIVE
TRANSPORT ACROSS THE PLASMA MEMBRANE
Comparison aspectPassive transportActive transport
Simple diffusionFacilitated diffusionOsmosis
Definition Movement of the molecules from a higher concentration
region to a lower concentration region without using energy until
the two regions reach the equilibrium.Movement of the molecules
from a higher concentration region to a lower concentration region
by the aid of the carrier protein or pore/channel protein without
using energy until the two regions reach the equilibrium.Movement
of ( the solvent molecules eg:) water molecules from a
dilute/hypotonic solution to a concentrate/hypertonic solution
(across a semi-permeable membrane (S-PM)) without using energy
until the two regions (which are separated by the S-PM) reach the
equilibrium.Movement of ions from a lower concentration region to a
higher concentration region across the membrane plasma of living
cells using energy based on the ion's requirement of the cells
Concentration gradient (CG)Follows the CG
Follows the CG
Follows the CG
Against the CG
Type of substances transportedSmall, uncharged, water soluble,
lipid soluble molecules
1- Large and uncharged 2- ion (small charged molecules)
Solvent molecules Ion/small charged molecules
Example of substancesO2, CO2, water soluble vitamins ie Vit B
& C, lipid soluble vitamins ie Vit A, D, E & K
Glucose, amino acids and ionsWater moleculesSodium ions,
potassium ions
Means of transportMolecules move freely through the phospholipid
bilayer across the plasma membrane (PM)
Molecules move through pore/channel protein or carrier protein
molecules across the PMMolecules move freely through the
phospholipid bilayer across the PMMolecules move through specific
carrier protein molecules across the PM
Energy requirementDoes not require energyDoes not require
energyDoes not require energyRequires a lot of energy in for of ATP
produced by the mitochondria
Involvement of semi-permeable membrane (S-PM)
Does not involveInvolves the S-PMInvolves the S-PMInvolves the
plasma membrane of living cells
Involvement of pore proteinDoes not involveInvolves the pore
protein which allows ions or polarized molecules to pass through
it
Does not involveDoes not involve
Involvement of carrier proteinDoes not involveInvolves the
carrier protein to carry such large molecules eg: glucose and amino
acids across the PM
Does not involveInvolves the carrier protein
Requirement to reach equilibrium The two regions reach
equilibrium
The two regions reach equilibriumThe two regions reach
equilibriumDepends on cell requirement eg: if a cell requires a lot
of ion A so, ion A will be taken into the cell continuously.
Example based on daily lifeDiffusion of oxygen and carbon
dioxide in the alveoli
Absorption of glucose in the small intestine1- Absorption of
water by the root hair cell2- Preservation of food
Absorption of iodide ions by the sea food
3.2THE MOVEMENT OF SUBSTANCES ACROSS THE PM IN EVERYDAY LIFE
(A)CONCEPT OF PASSIVE TRANSPORT IDetermine the substance to be
transported across the PM
IIDetermine, state and compare the two regions with different
concentration of the particular substance
IIIState the direction of diffusion of the substance ie : from
the higher concentration region to the lower concentration region
IVstate the method of diffusion ie: by simple diffusion or
facilitated diffusion.Examples :-(1) The gaseous exchange at the
alveoli
a- Diffusion of OxygenThe concentration of oxygen is higher in
the alveoli as compared to the concentration of oxygen in the blood
capillaries.
Oxygen (dissolves in the moist of the alveoli) diffuses out of
the alveoli into the blood capillaries by simple diffusion.
b-Diffusion of Carbon dioxide
The concentration of carbon dioxide is higher in the blood
capillaries as compared to the concentration of carbon dioxide in
the alveoli.
Carbon dioxide (dissolves in the blood plasma) diffuses out of
the blood capillaries into the alveoli by simple diffusion.
A longitudinal section of an alveoli
(2) The absorption of digested food in the small intestine
a- Absorption of glucose / amino acids
The concentration of glucose / amino acid is higher in the lumen
of small intestine as compared to the concentration of glucose /
amino acids in the blood capillaries of the villi.
Glucose / Amino acid diffuses from the lumen of the small
intestine into the blood capillaries of the villi by facilitated
diffusion.
b- Absorption of fatty acids and glycerol
The concentration of fatty acids and glycerol is higher in the
lumen of the small intestine as compared to the concentration of
the fatty acids and glycerol in the lacteals of the villi.
Fatty acids and glycerol diffuse from the lumen of the small
intestine into the lacteals of the villi by simple diffusion.
A longitudinal section of a villus
THE ROLE OF PROTEIN MOLECULES IN FACILITATED DIFFUSION
Carrier proteinPore protein / channel protein
- can change its shape so that it fits the shape of a specific
large and uncharged molecules eg : glucose and amino acids.
Mechanism of transport :1-molecules move to the binding site of
the specific CP.2- molecules bind to the CP at the binding site.3-
CP changes its shape and carry the molecules across the PM follows
the concentration gradient.4- CP returns to its normal
shape.(reference : Bio Form 4 Textbook, figure 3.5, pg 46)- has
pore / channel to provide a passage for a non-specific small
molecules and ion eg : mineral ions to pass through follows the
concentration gradient.Mechanism of transport :
1-molecules/ ions move to the pore/chanel protein2-
molecules/ions move via the pore/chanel provided across the PM
follows the concentration gradient.(reference : Bio Form 4
Textbook, figure 3.6, pg 46)
(B)CONCEPT OF OSMOSISIBear in mind that only water molecules can
be transported across the PM by osmosis. IIDetermine, state and
compare the two solutions (that are separated by a SP-M) with
different tonicity ie : distilled water is hypotonic as compared to
the cytoplasm of the RBCIIIState the direction of diffusion
(movement) of water molecules ie : water molecules diffuse from the
hypotonic solution to the hypertonic solution (across the PM or
other semi- permeable membrane) by osmosis.IVState the effects of
the solution on cells/tissues.
TONICITY OF THE SOLUTION
TONICITY OF THE SOLUTIONDEFINITION
Hypotonic solutionThe solution with a lower concentration of
solute than the cytoplasm / cell sap eg : Distilled water (dH2O),
sodium chloride solution < 0.85%, sucrose solution < 16%
Isotonic solutionThe solution of equal solute concentration with
the cytoplasm / cell sap eg : sodium chloride solution 0.85%,
sucrose solution 16%
Hypertonic solutionThe solution with a higher concentration of
solute than the cytoplasm / cell sap eg : Sodium chloride solution
> 0.85%, sucrose solution > 16%
EFFECTS OF HYPOTONIC, ISOTONIC AND HYPERTONIC SOLUTIONS ON PLANT
AND ANIMAL CELLS AND PLANT TISSUES (1) Plant cell eg : epidermal
cell of scale leaf of onionSituation : A plant cell is immersed in
the stated solution for 10 minutes and observation is carried
out.
(a) Hypotonic solution eg : distilled water
A normal cell A turgid cellDistilled water is hypotonic to the
(cell sap of a) plant cell.
Water molecules diffuse into (the cell sap of) the plant cell by
osmosis.
Volume of the vacuole increases so it expands (and exerts
pressure outwards on the cell wall). Cell becomes turgid.
(b) Isotonic solution eg : Sucrose solution 16%
The treated cell shows no changes in shape and volume.
Sucrose solution 16 % is isotonic to the (cell sap of a) plant
cell.
Water molecules diffuse into and out (of the cell sap) of the
plant cell at the same rate by osmosis. (Nett water movement is
zero)
The cells shape and volume remains the same.
Cell maintains its turgidity.
(a) Hypertonic solution eg : Sucrose solution 25%
A normal cell A flaccid cell (or a plasmolysed cell)Sucrose
solution 25% is hypertonic to the (cell sap of a) plant cell.
Water molecules diffuse out of( the cell sap of) the plant cell
to the sucrose solution 25% by osmosis.
Volume of the vacuole decreases and it shrinks. The PM is pulled
away from the cell wall.
The cell undergoes plasmolysis and becomes flaccid
/plasmolysed.
(a) Immersing a plasmolysed cell in distilled water
A plasmolysed cell A turgid cellDistilled water is hypotonic to
the( cell sap of a) plasmolysed cell.
Water molecules diffuse from distilled water into the
plasmolysed cell by osmosis.
Volume of the vacuole increases rapidly and it expands (and
exerts pressure outwards on the cell wall) .
The plasmolysed cell undergoes deplasmolysis.
(2) Animal cell eg : red blood cell (RBC)
Situation : RBCs are immersed in the stated solution for 10
minutes and observation is carried out.
(a) Hypotonic solution eg : distilled water
Normal RBC A haemolysed RBCDistilled water is hypotonic to the
(cytoplasm of the) RBC.
Water molecules diffuse into the (cytoplasm of the) RBC by
osmosis.
Volume of the cytoplasm increases. The PM of the RBC expands and
finally burst.
RBC undergoes haemolysis.
(b) Isotonic solution eg : sodium chloride solution 0.85%
The treated RBC shows no changes in shape and volume.
Sodium chloride solution 0.85% is isotonic to the (cytoplasm of)
RBC.
Water molecules diffuse into and out of the (cytoplasm of the)
RBCs at the same rate by osmosis. (Nett water movement is zero)
The shape and volume of the RBC remains the same.
Hypertonic solution eg: sodium chloride solution 2%.
Normal RBC A crenated RBCSodium chloride solution 2% is
hypertonic to the (cytoplasm of the) RBC.
Water molecules diffuse out of the (cytoplasm of the) RBC to the
sodium chloride solution 2% by osmosis.
Volume of the cytoplasm decreases and the PM of the RBC shrinks.
The RBC undergoes crenation.
(3) Plant tissue eg : potato stripSituation : A plant tissue is
immersed in the stated solution for 30 minutes and observation is
carried out
(a) Hypotonic solution eg : distilled water Before immersion =
mm
After immersion = .. mm
Distilled water is hypotonic to the (cell sap of each cell of a)
potato strip.
Water molecules diffuse from distilled water into the (cell sap
of each cell of the) potato strip by osmosis.
(Volume of the vacuole of each cell in the potato strip
increases so vacuole expands and becomes turgid). The cells become
turgid. The mass, size, length of potato strip increases.
(b) Isotonic solution eg : sucrose solution 16%
Before immersion = ..mm
After immersion = mm
Sucrose solution 16 % is isotonic to the (cell sap of each cell
of a) potato strip.
Water molecules diffuse into and out of the (cell sap of each
cell of the) potato strip at the same rate by osmosis. (Nett water
movement is zero)
(Each cells shape and volume remains the same).
The turgidity of cell remains the same.
The mass, size and length of the potato strip remains the
same.
(c) Hypertonic solution eg : sucrose solution 25%
Before immersion = .mm
After immersion = ..mmSucrose solution 25% is hypertonic to the
(cell sap of each cell of a) potato strip.
Water molecules diffuse out of the (cell sap of each cell of
the) potato strip to the sucrose solution 25% by osmosis.
(Volume of the vacuole of each cell decreases due to water loss
and it shrinks).
The cells are flaccid/plasmolysed.
The potato strip wilts / softens and the mass, size and length
decreases.
(4) Waxy cuticle covered-plant tissue eg : spinach stem
Situation : A waxy cuticle covered-plant tissue is immersed in
the stated solution for 30 minutes and observation is carried
out.
(a) Hypotonic solution eg : distilled waterBefore immersion
After immersion
Distilled water is hypotonic to the spinach stem.Water molecules
diffuse into the cell of cut region(which is not covered with waxy
cuticle layer) by osmosis. Water molecules cannot diffuse into the
waxy cuticle covered-cells as the waxy cuticle layer is water
proof.Cells at the cut region are turgid and increase in length,
whereas waxy cuticle covered-cells show no change in length so the
spinach stem curved outwards.
(b)Isotonic solution eg : sucrose sol. 16%
Before immersion After immersion
Sucrose solution 16 % is isotonic to the spinach stem.
Water molecules diffuse into and out of the cell of the cut
region at the same rate by osmosis.
Water molecules cannot diffuse into or out of waxy cuticle
covered-cells as the waxy cuticle layer is water proof.
The turgidity of cell, mass, size and length of the spinach stem
remains the same.
Hypertonic solution eg : sucrose sol. 25%
Before immersion After immersion Sucrose solution 25% is
hypertonic to the spinach stem.
Water molecules diffuse out of the cell of the cut region to the
sucrose solution 25% by osmosis.
Water molecules cannot diffuse out of waxy cuticle covered-cells
as the waxy cuticle layer is water proof.
Cells at the cut region decrease in length whereas waxy cuticle
covered-cells show no change in length so the spinach stem curved
inwards.
The table below summarizes the effects of solutions of different
tonicity on cells and tissueTonicity of the solutionEffect of the
solution on
Plant cellAnimal cell (RBC)Plant tissue
HypotonicBecomes more turgidHaemolysis occurs. Cell is
haemolysed.Length / mass / size increases
IsotonicNo changesNo changesNo changes
HypertonicPlasmolysis occurs. Cell is plasmolysed /
flaccidCrenation occurs. Cell is crenated.Length / mass / size
decreases
The example of osmosis process in living organisms is the
absorption of water by root hair cells in plants.
The soil water is hypotonic to (the cell sap of) the root hair
cell. Water molecules diffuse into (the cell sap of) the root hair
cell by osmosis. Now, the root hair cell is hypotonic to the
adjacent cells so water molecules diffuse (from the root hair cell)
into the adjacent cells by osmosis. Next, the adjacent cells are
hypotonic to the vessel xylem so water molecules diffuse (from the
adjacent cell) into the vessel xylem by osmosis.(C)CONCEPT OF
ACTIVE TRANSPORTIDetermine and state the ion to be transported
across the PM of living cellsIIDetermine, state and compare the two
regions with different concentration of the ion.IIIState the
direction of movement of the ion i.e: from the lower concentration
region to the higher concentration region using energy / ATP by
active transportExample :-
1Absorption of iodide ions by the seaweed / seafoodThe
concentration of iodide ion in the sea water is lower than the
concentration of iodide ion in the seaweed / seafood.Iodide ion
diffuses from the sea water into the seaweed/ seafood using
energy/ATP by active transport.
2Absorption of nitrate ions by the root hair cellThe
concentration of nitrate ion in the soil water is lower than the
concentration of nitrate ion in the cell sap of the root hair
cell.Nitrate ion diffuses from the soil water into the root hair
cell using energy/ATP by active transport.THE ROLE OF CARRIER
PROTEIN IN TRANSPORTATION ACROSS THE PM BY ACTIVE TRANSPORT1CP has
shape that fits the shape of a specific mineral ions eg : sodium
ion. 2Mechanism of transport by the CP in active transport :-
a- Ions move to the binding site of the specific CP
b- CP changes its shape by using energy/ATPs
c- Ions bind to the CP at the binding site and forms the CP-Ion
complexd- Energy/ATP is supplied so that the CP-Ion complex can
across the PM against the concentration gradient.
e- CP releases ions at the higher concentration region.
f- CP changes back to normal shape and returns to its initial
position.
APPLICATION OF THE TRANSPORT ACROSS THE PM
1Phenomenon of wilting in plants due to excessive use of
chemical fertilizers
- Excess fertilizers dissolve in soil water.
- Soil water becomes hypertonic to the cell cap of the root hair
cell.
- Water molecules diffuse out of the root hair cells to the soil
water
by osmosis
Concept of
- Each plant cell undergoes plasmolysis and becomes flaccid due
to
osmosis
water loss / dehydration and vacuole shrinks-cause the plant to
wilt.2(a)Preservation of food - Salting- Food can be preserved
using concentrated salt solution (CSS) eg: salted fish, salted
cuttlefish etc.- Food is immersed in the concentrated salt solution
(1 3 weeks)
- CSS is hypertonic to the food.
Concept of- Water molecules diffuse out of the food to the CSS
by osmosis.
osmosis
- Food dehydrates due to water loss.
- Water molecules diffuse out of the microorganisms by osmosis
too
.-Microorganisms dehydrate due to water loss
- The food is being preserved as microorganisms cannot grow in
dehydrated
food and microorganisms are dehydrated too
- Salted food can last longer. (b)Preservation of food -
Pickling
- Food eg vegetables and young mango fruits can be preserved
using vinegar ..
- Food is immersed in the vinegar
- Vinegar is acidic.
Concept of
- Vinegar diffuses into the food by simple diffusion follows the
CG
Simple diffusion
- The food turns acidic
- Microorganisms cannot survive in acidic condition.
- Food is being preserved because microorganisms cannot grow in
acidic condition, so the pickled food is last longer. PAGE 22