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