2. Movement of Molecules

8/8/2019 2. Movement of Molecules

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Passive transport is the cellular process of moving moleculesand other substances across membranes.

Passive transport differs from active transport in that it doesnot involve any chemical energy. Rather, passive transport relieson the permeability of the cell membrane and its componentproteins and lipids.

There are 3 main types of passive transport :a)Diffusionb)Facilitated diffusionc)osmosis

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y D iffusion is the random movement of ions ormolecules from an area of higher concentrationto an area of lower concentration i.e. down theconcentration gradient until equilibrium is

reached.y O nce this happens, molecules or ions still tends to

move between the 2 areas but with no net gain orloss from one side or the other.

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y Diffusion



y The phospholipid bilayer is permeable to very smalluncharged mole cules like oxygen and carbon dioxide .These diffuse freely in and out of the cell through thephospholipid bilayer.Hydrophobic substances, for example, steroids can alsodiffuse through the membrane.The phospholipid bilayer is not permeable to chargedions such as Na +, K+, Cl-, HCO3 -, and hydrophylicmolecules and macromolecules.

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Rate Of Diffusion Depends Upon:

The

concentrationgradient. Surface area.

Distanceover whichdiffusion

takes place.

Size andnature of

the diffusingmolecule

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F acilitated diffusion is the carrier-

mediated transport of large moleculesthrough the cell membraneusing transport proteins :2 types of transport proteins :a)Protein channel

b) Carrier proteinembedded within the cell membrane.The large molecules would not be able topass through the cell membrane throughthe phospholipid bilayer , but the

transport proteins effectively "transport the molecules through themembrane from a higher concentration(outside the cell) to a lowerconcentration (inside the cell) withoututilizing any chemical energy .

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Facilitated Diffusion

Of Ions & Molecules

y Transport proteins form channels or act ascarrier to facilitate and to increase the rateof diffusion across cell membranes.

y Protein Channelsy Certain transport proteins form different

specific water filled hydrophylic channelsto permit diffusion of various charged ionssuch as K +, Na+, Ca2-, Cl- and HC O3 -.

The protein channels that can open or close and are called gatedchannel.There are also specialised channels for water known as aquaporinsfound in both plant and animal cells.These aquaporins speed up the rate of diffusion of water moleculesdown its water potential gradient .

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Carrier Proteiny

Some small hydrophilic organic moleculesfor example, glucose and amino acids canpass through cell surface membranes by facilitated diffusion using carrierprotein . Carrier proteinstransport substances out of or into the cellby facilitated diffusion and active transport .

y Each carrier protein is designed to recognizeonly one substance or one group of very similar substances.

y The molecule or ion to be transported (thesubstrate) must first bind at a binding site atthe carrier molecule, with a certain bindingaffinity.

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osmosisIn biological terms,osmosis is the diffusion

or movement of water molecules through asemi-permeablemembrane from a regionof higher concentration

of water molecules to aregion with a lower concentration of water molecules.

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W ater potential is the potentialenergy of water relative to pure free water (e.g.deionized water) in reference conditions.It quantifies the tendency of water to move from one

area to another due to osmosis ,Water potential is measured in units of pressure andis commonly represented by the Greek letter (Psi ).This concept has proved especially useful inunderstanding water movement within plants , animals ,and soil .Typically, pure water at standard temperature andpressure (or other suitable reference condition) isdefined as having a water potential of 0.

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The addition of solutes to water lowers its potential (makes it

more negative), just as the increase in pressure increases itspotential (makes it more positive).

If possible, water will move from an area of higher water potential to an area that has a lower water potential.

One very common example is water that contains a dissolvedsalt, like sea water or the solution within living cells.These solutions typically have negative water potentials, relativeto the pure water reference.If there is no restriction on flow, water molecules will proceedfrom the locus of pure water to the more negative water potentialof the solution.



Pr essur epotential( p)P ressure potential is based on mechanical pressure,

and is an important component of the total water potential within plant cells

P ressure potential is increased as water enters acell.As water passes through the cell wall andcell membrane , it increases the total amount of water present inside the cell, which exerts an outwardpressure that is retained by the structural rigidity of

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The pressure potential in a living plant cell is usually positiveIn plasmolysed cells, pressure potential is almost zero.

Negative pressure potentials occur when water is pulledthrough an open system such as a plant xylem vessel.W ithstanding negative pressure potentials (frequentlycalled tension ) is an important adaptation of xylem vessels.

.



S olute potential is the potential or force of attraction towards water molecules caused bydissolved substance ( solute ) inside the cell.

P ure water is usually defined as having a solute

potential ( ) of zero, and in this case, solutepotential can never be positive.

For example, when a solute is dissolved in water,water molecules are less likely to diffuse awayvia osmosis than when there is no solute.A solution will have a lower and hence morenegative water potential than that of pure water.

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Furthermore, the more solute molecules present, the morenegative the solute potential is.S olute potential has important implication for manyliving organisms .

If a living cell with a smaller soluteconcentration/hypotonic solution is surrounded by a

more concentrated or hypertonic solution,the cell will tend to lose water to the more negative water potential of the surrounding environment.This is often the case for marine organisms living in sea

water and halophytic plants growingin saline environments.In the case of a plant cell, the flow of water out of the cell

may eventually cause the plasma membrane to pull awayfrom the cell wall, leading to plasmolysis .



W ater potential of plant cell is the sum of its solute potential andpressure potential

cell = s + p W ater solute pressure potentialPotential potential

In comparing 2 solutions the hypotonic solution is the solution withthe lower solute concentration.

And the hypertonic solution is the solution with higher soluteconcentration.

An isotonic solution has the same solute concentration as the othersolution

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Plant cells are surrounded by rigid cellulose walls,(unlike animal cells), but plant cells still take in water by osmosis when placed in pure water.

However, plant cells do not burst because theircellulose cell walls limit how much water can move

in.The cell walls exert pressure, called turgor pressure, asthe cells take up water.

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These physical forces can be described by a

simple mathematical equation: = s + p where ("Psi") is thewater potential, a measure of the overalltendency of water to move into a cell;

s is the pressure potential, a measure of the turgor pressure exerted by the cell walls;and p is the osmotic potential (see above).

Water always moves from regions of higher to areas of lower .



W hen a plant cell is placed in a hypertonic solution, the water in the cells movesout to hypertonic solution or surrounding , and the cell shrinks and sobecomes plasmolysed - the cell membrane has completely pulls away from the

cell wall due to lack of water pressure on it; the opposite of turgid .) Also, osmosis is responsible for the ability of plant roots to draw water from thesoil.Since there are many fine roots, they have a large surface area, and water entersthe roots by osmosis.

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S uppose an animal cell is

placed in a solution of sugar or salt in water. Eg.Red blood cellIf the mediumis hypotonic a dilutesolution, with a higher waterconcentration than the cell

the cell will gain waterthrough osmosis , expandthen finally burst.This process is calledhaemolysis

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If the medium is isotonic a solution withexactly the same water potential as the cell

there will be no net movement of water across the cell membrane of the cell .If the medium is hypertonic aconcentrated solution, with a lower waterpotential than the cell the cell will lose

water by osmosis and undergo crenation where the cell shrink /shriveled up and thecell surface membrane has a crinkled

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Active transport is the movement of ions ormolecule across the semi permeable membraneagainst concentration gradient .The process requires energy expenditureprovided by A TP.O

ther characteristic includes:1) Carrier protein is required.2)Each type of carrier molecules are specificfor 1 type of substance they carry.

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Movement is against concentration gradient.The process will stop only when there is no more substance totransport.

Example of active transpor t:1) Mineral ions are actively transported into the root hair

cells or young epidermal cell of root.2) Sodium potassium pump that maintain a potential

difference between the inside and outside of a nerve cell.

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Differences between passive and active transport



Endocytosis is the case whena molecule is taken into thecell by the invagination of cell membrane forming a

vesicle.Phagocytosis is the type of endocytosis where acytoplasmic projection areformed that extend around

solid particles. Egpseudopodia in amoeba.The projection then fusedtogether trapping the solidparticles within the vacuoles.

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E xocytosis occurs when a vesicle membrane fuses withthe plasma membrane. Molecules (e.g. insulin, serotonin,proteases) stored in the vesicles are then ³released´ fromthe lumen of the vesicle to the exterior of the cell.

E xocytosis

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2. Movement of Molecules

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