2.Movement of Molecules in & Out of Farizah

8/14/2019 2.Movement of Molecules in & Out of Farizah

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2.2 MOVEMENT OF

MOLECULES/

SUBSTANCES

IN AND OUT OF CELL


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Substances move in & out of cells by thefollowing processes:

a) Passive transport- Diffusion

- Facilitated diffusion

- osmosisb) Active transport

c) Endocytosis

- pinocytosis- phagocytosis

d) Exocytosis


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a) PASSIVE TRANSPORT

Passive transport is the

movement of ions & molecules

down their concentrationgradient. The process does not

require ATP & energy

expenditure.


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DiffusionDiffusion

Diffusion is the random movement of ionsDiffusion is the random movement of ionsor moleculesor molecules down the concentrationdown the concentrationgradient

gradient (from the region of higher(from the region of higherconcentration to a region of lowerconcentration to a region of lower

concentration) until an equilibrium isconcentration) until an equilibrium isreached.reached.

The phospholipid bilayer is permeable toThe phospholipid bilayer is permeable to

very small uncharged molecules like Overy small uncharged molecules like O22

&&

COCO22. These diffuse freely in & out of the. These diffuse freely in & out of thecell through the phospholipid bilayer.cell through the phospholipid bilayer.Hydrophobic substances, for eg. steroidsHydrophobic substances, for eg. steroids

can also diffuse through.can also diffuse through.


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The phospholipid bilayer is not permeable toThe phospholipid bilayer is not permeable to

charged ions such as Nacharged ions such as Na++, K, K++, Cl, Cl, HCO, HCO33, &, &

hydrophilic molecules like glucose &hydrophilic molecules like glucose &macromolecules.macromolecules.

The rate of diffusion depends uponThe rate of diffusion dep

ends upon::

a. The concentration gradient.a. The concentration gradient.

b. Surface areab. Surface areac. Distance over which diffusion takesc. Distance over which diffusion takes

placeplace

d. Size & nature of the diffusing molecules.d. Size & nature of the diffusing molecules.


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Facilitated DiffusionFacilitated Diffusion

Facilitated diffusion is the movement of solutesFacilitated diffusion is the movement of solutes(such as ion or polar molecules) across a(such as ion or polar molecules) across a

membrane, with the help ofmembrane, with the help oftransport proteinstransport proteins..

This process follows the concentration gradientThis process follows the concentration gradient

& no energy is needed.& no energy is needed. An eg. of facilitated diffusion is theAn eg. of facilitated diffusion is the movementmovement

of glucose & amino acids molecules into theof glucose & amino acids molecules into the

cellcell..


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A transport protein in theA transport protein in the

membrane binds a solutemembrane binds a solute

particle.particle.

The transport protein changesThe transport protein changesits shape, opening a channelits shape, opening a channel

through the membrane,through the membrane,

releasing the diffusingreleasing the diffusing

molecules at the other side ofmolecules at the other side of

thethe membrane.membrane.


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OSMOSISOSMOSIS

Osmosis is the movement ofOsmosis is the movement ofwater molecules from awater molecules from a

region of higher waterregion of higher water

potential to a region of lowerpotential to a region of lower

water potential across awater potential across a

selectively permeableselectively permeablemembrane until equilibriummembrane until equilibrium

is reached.is reached.


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Water PotentialWater Potential

Water potential is the term givenWater potential is the term givento the tendency for waterto the tendency for watermolecules to enter or leave amolecules to enter or leave a

solution by osmosis.solution by osmosis. Water always moves from an areaWater always moves from an area

of higher water potential to anof higher water potential to an

area of lower water potential.area of lower water potential. Water potential is affected by twoWater potential is affected by two

factors: pressure and the amountfactors: pressure and the amount

of solute.of solute.


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Pure water has the highest water potentialPure water has the highest water potential

which is set zero (which is set zero ( ww = 0 kPa ).= 0 kPa ).

The addition of solutes lowers the waterThe addition of solutes lowers the water

potential. The water potential of solutionspotential. The water potential of solutionstherefore is lower than pure water & has atherefore is lower than pure water & has a

negative value (negative value ( solutionsolution < 0 kPa ).0 kPa ).


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The greater the concentration ofThe greater the concentration of

solutes, the more negative is thesolutes, the more negative is thewater potential.water potential.

In comparing 2 solutions, theIn comparing 2 solutions, the

hypotonichypotonic solution is the solutionsolution is the solutionwith the lower solute concentration,with the lower solute concentration,

& the& the hypertonichypertonic solution is thesolution is the

solution with the higher solutesolution with the higher soluteconcentration. Anconcentration. An isotonicisotonic solutionsolution

has the same solute concentration ashas the same solute concentration as

the other solution.the other solution.

C l l ti W t P t ti lC l l ti W t P t ti l


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Calculating Water PotentialCalculating Water PotentialWater potential is calculated using theWater potential is calculated using the

following formula:following formula:Water potential ( ) = pressure potential( p) + solute

potential ( s )

Pressure potential

( p ):

In a plant cell, pressure exerted

by the rigid cell wall that limitsfurther water uptake.

Solute potential (( s )

The effect of soluteconcentration. Pure water atatmospheric pressure has asolute potential of zero. As soluteis added, the value for solutepotential becomes morenegative. This causes waterpotential to decrease also. In

sum, as solute is added, thewater potential of a solution


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Osmosis in Plant CellsOsmosis in Plant Cells When aWhen a plant cell is placed in aplant cell is placed in a

hypotonic solutionhypotonic solution, water enters, water entersthe cell by osmosis. The vacuolethe cell by osmosis. The vacuoleexpands and the cell contentsexpands and the cell contentspress against the cell wall.press against the cell wall.

As more water enters, the pressureAs more water enters, the pressurepotential produced by the cell wallpotential produced by the cell wallincreases until the pressureincreases until the pressurepotential equals the solutepotential equals the solutepotential.potential.

There is no net movement of waterThere is no net movement of waterin either direction. The plant cell isin either direction. The plant cell issaid to besaid to be turgidturgid..

Water pressure

within the cell

Pressure from

cell wall


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== ss ++ pp

whenwhen ss == pp

= 0= 0

Plant cell do not burst becausePlant cell do not burst because

they are surrounded by theirthey are surrounded by their

strong cellulose cell walls.strong cellulose cell walls.

InIn isotonic solutionsisotonic solutions, there is, there is nono

net movement of waternet movement of water moleculesmolecules& no change in the volume of the& no change in the volume of the

cell the cell become flaccid.cell the cell become flaccid.


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In hypertonic solutions, there is aIn hypertonic solutions, there is a

net outflow of water by osmosis fromnet outflow of water by osmosis from

the cell.the cell.

The cell vacuole shrinks & theThe cell vacuole shrinks & the

plasma membrane pulls away fromplasma membrane pulls away fromthe cell wall.the cell wall.

PlasmolysisPlasmolysis of the cell occurs causesof the cell occurs causes

the plant to wilt & can lead to plantthe plant to wilt & can lead to plantdeath.death.

Turgid cells give support toTurgid cells give support to

herbaceous plants.herbaceous plants.


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Example of questionExample of question1.1. The table below shows 2 adjacent cells & their valuesThe table below shows 2 adjacent cells & their values

s &s & p are given in kPa.p are given in kPa.

a) State the direction of water flow between thea) State the direction of water flow between the

two cells & give the reason for your answer.two cells & give the reason for your answer.

b) Calculate, the values of the pressure potentialb) Calculate, the values of the pressure potential

pp

&&

the water potential,the water potential, of cell A & cell B, whenof cell A & cell B, when

equilibrium is reached. Assume that changes in soluteequilibrium is reached. Assume that changes in solute

potential are negligible.potential are negligible.

Cell A Cell B

s =

-2200

s =

-1600 p = 1000 p = 800


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a) Cell Aa) Cell A

== s +s + pp

= - 2200 kPa + 1000 kPa= - 2200 kPa + 1000 kPa= - 1200 kPa= - 1200 kPa

Cell BCell B

== s +s + pp

= - 1600 kPa + 800 kPa= - 1600 kPa + 800 kPa

= - 800 kPa= - 800 kPa

Water flows from cell B to cell A. In osmosis,Water flows from cell B to cell A. In osmosis,

water flows down a water potential gradientwater flows down a water potential gradientfrom a region of high water potential to afrom a region of high water potential to aregion of lower water potential.region of lower water potential.


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b) At equilibrium,b) At equilibrium, cell A =cell A = cell Bcell B

Cell A,Cell A, = - 1000 kPa= - 1000 kPa

p = - 1000kPa + 2200kPap = - 1000kPa + 2200kPa

= 1200kPa= 1200kPa

Cell B,Cell B, = - 1000 kPa= - 1000 kPa

p = - 1000 kPa + 1600 kPap = - 1000 kPa + 1600 kPa= 600 kPa= 600 kPa


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Osmosis in animal cellsOsmosis in animal cells

When anWhen an animal cellanimal cell ( eg. red blood cell) is( eg. red blood cell) isplaced in aplaced in a hypotonic solutionhypotonic solution, water enters, water enters

the cell by osmosis.the cell by osmosis.

The cell expends & the thin plasmaThe cell expends & the thin plasma

membrane bursts, releasing cell contents.membrane bursts, releasing cell contents. The red blood cell is said to beThe red blood cell is said to be haemolysedhaemolysed..

In isotonic solutions, there is no netIn isotonic solutions, there is no net

movement of water molecules & no change inmovement of water molecules & no change in

the shape or volume of the cell.the shape or volume of the cell.


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InIn hypertonic solutionshypertonic solutions, there is a, there is a

netnet

outflow of water by osmosis fromoutflow of water by osmosis from

thethe

cell.cell.

TheThe cell shrinkscell shrinks & the plasma& the plasmamembrane has a crinkledmembrane has a crinkled

appearance.appearance.

The cell is said to beThe cell is said to be crenatedcrenated..


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b.b.ACTIVE TRANSPORTACTIVE TRANSPORT

Active transport is the movement of ions orActive transport is the movement of ions ormolecules across a cell membrane againstmolecules across a cell membrane against

their concentration gradient, The processtheir concentration gradient, The process

requires energy expenditure provided by ATP.requires energy expenditure provided by ATP.

This process also involves carrier proteins inThis process also involves carrier proteins inmembranes.membranes.

Active transport can be slowed down orActive transport can be slowed down or

inhibited by respiratory poisons such asinhibited by respiratory poisons such as

cyanide or a lack of oxygen.cyanide or a lack of oxygen.

EE S di t iS di

t i


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Eg.Eg. Sodium-potassiumSodium-potassium

pumppump

The carrier pumps NaThe carrier pumps Na++& K& K++ ionsions

across the cell membrane againstacross the cell membrane against

their concentration gradients.their concentration gradients. For every 3 NaFor every 3 Na++ pumped out of thepumped out of the

cell, 2 Kcell, 2 K++ are pumped into the cell.are pumped into the cell.

ATP is used directly in thisATP is used directly in thisexample.example.


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1.1. Binding of cytoplasmic NaBinding of cytoplasmic Na++ to the proteinto the proteinstimulates phosphorylation by ATP.stimulates phosphorylation by ATP.

2.2. Phosphorylation causes the protein toPhosphorylation causes the protein to

change its conformation.change its conformation.3.3. The conformational change expels NaThe conformational change expels Na++ toto

the outside, & extracellular Kthe outside, & extracellular K++ binds.binds.

4.4. KK++ binding triggers release of a phosphatebinding triggers release of a phosphate

group.group.5.5. Loss of phosphate restores originalLoss of phosphate restores original

conformation.conformation.

6.6. KK++ is released & Nais released & Na++ sites are receptivesites are receptive

again; the cycle repeats.again; the cycle repeats.


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Cytoplasm

Extracellular

fluid

P

ADP

ATP

i

Na+

Na+

Na+

Na+

Na+

Na+

(1)

Na+Na+

Na+

K+

K+

K+

K+

K+

K+

(2)

(3)

(4)(5)

(6)


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c)c)ENDOCYTOSISENDOCYTOSIS

In endocytosis, the cell takes inIn endocytosis, the cell takes in

macromolecules & particulatemacromolecules & particulate

matter by forming new vesiclesmatter by forming new vesicles

from the plasma membrane.from the plasma membrane. There are 3 types of endocytosis:There are 3 types of endocytosis:

1. Phagocytosis ( cell eating )1. Phagocytosis ( cell eating )

2. Pinocytosis ( cell drinking )2. Pinocytosis ( cell drinking )3. Receptor-mediated endocytosis3. Receptor-mediated endocytosis


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1. Phagocytosis1. Phagocytosis In phagocytosis, the cell ingests largeIn phagocytosis, the cell ingests large

solid particles such as bacteria & food.solid particles such as bacteria & food. Phagocytosis is a mechanism used byPhagocytosis is a mechanism used by

certain protists & by several types ofcertain protists & by several types ofvertebrate white blood cells to ingestvertebrate white blood cells to ingestparticles ( eg. bacteria).particles ( eg. bacteria).

During ingestion, folds of the plasmaDuring ingestion, folds of the plasmamembrane enclose the particle, whichmembrane enclose the particle, which

has bound to surface of the cell,has bound to surface of the cell,forming a large membranous sac, orforming a large membranous sac, orvacuole.vacuole.


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When the membrane hasWhen the membrane hasencircled the particle, it fusesencircled the particle, it fuses

at the point of contact.at the point of contact.

The vacuole then fuses withThe vacuole then fuses with

lysosomes, & the ingestedlysosomes, & the ingested

material is degraded.material is degraded.This requires energy.ATPThis requires energy.ATP


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2. Pinocytosis2. Pinocytosis The cell takes in dissolved materials.The cell takes in dissolved materials.

Tiny droplets of fluid are trapped by folds inTiny droplets of fluid are trapped by folds in

the plasma membrane, which pinch off intothe plasma membrane, which pinch off into

the cytosol as tiny vesicles.the cytosol as tiny vesicles.

The liquid contents of these vesicles are thenThe liquid contents of these vesicles are then

slowly transferred into the cytosol; theslowly transferred into the cytosol; the

vesicles may become progressively smaller, tovesicles may become progressively smaller, to

the point that they appear to vanish.the point that they appear to vanish.


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d)d) EXOCYTOSISEXOCYTOSIS This is the reverse of endocytosis.This is the reverse of endocytosis. In exocytosis, the cell secretesIn exocytosis, the cell secretes

macromolecules by the fusion ofmacromolecules by the fusion of

vesicles with the plasma membrane.vesicles with the plasma membrane.

Many secretory cells use exocytosis toMany secretory cells use exocytosis toexport their products. For eg:export their products. For eg:

1.1. Certain cells in the pancreasCertain cells in the pancreas

manufacture the hormone insulin &manufacture the hormone insulin &

secrete it into the blood by exocytosis.secrete it into the blood by exocytosis.

2.2. Neuron cells which uses exocytosis toNeuron cells which uses exocytosis to

release chemical signals that stimulaterelease chemical signals that stimulate

another neurons or muscle cells.another neurons or muscle cells.


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2.Movement of Molecules in & Out of Farizah

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