bio chapter 3
TRANSCRIPT
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CHAPTER 3 :
MOVEMENT OF
SUBSTANCES ACROSS
THE PLASMAMEMBRANE
Athirah Badrulsham
4 Setia (2010)
5 Setia (2011)
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NECESSITY FOR MOVEMENT OF
SUBSTANCES ACROSS THE PLASMA
MEMBRANE
To provide nutrients for metabolism and
growth To supply oxygen for respiration;
To regulate solute concentration and suitable
pH for maintaining a stable internalenvironment for optimal enzymatic activities
To maintain an ion concentration gradient
required for nerve and muscle cell activities
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Substances can move into or out of a cell by :
Passive transport :a. Simple diffusion
b. Osmosis
c. Facilitated diffusion
Active transport
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Movement of substances across the plasma
membrane would depend on :
Selectivity of the partially permeable
membrane; The difference in concentration between the
cell & extracellular fluid
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Structure of plasma membrane
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Structure of the Plasma
Membrane
All cells are covered by a thin plasma
membrane.
It separates the cell contents from thesurrounding
1972, S.J. Singer & G.L. Nicolson proposed the
fluid-mosaic model of plasma membrane.
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The plasma membrane is dynamic and fluid.
The phospholipid molecules can move thus
giving the membrane its fluidity & flexibility
The proteins are scattered in the membrane
giving it a mosaic appearance
Thickness : 7.0 8.0 nm
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The membrane consists of a phospholipid bilayer(2 molecules thick)
The polar hydrophilic head
outer layer faceoutwards, chemicallyattracted to the waterysurrounding
The non-polar hydrophobic
hydrocarbon fatty acid tails face inwards, away fromwater.
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Structure of the Plasma
Membrane
There are proteins on the outer and inner
surfaces of the plasma membrane.
Some proteins penetrate partially through themembrane, others penetrate completely.
The phospholipid bilayer is permeable to
diffusion of small uncharged molecules such
as O and CO .
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Structure of the Plasma
Membrane
Two types of transport protein :
Pore proteins have pore to facilitate
diffusion of particular ions / molecules across the plasmamembrane.
Some carrier proteins have binding sites that bind to
specific molecules such as glucose or amino acids
alter their shape to facilitate the diffusion of solutes.Other carrier proteins function in active transport
an energized carrier protein actively pumps the soluteacross the cell membrane against the concentrationgradient.
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Cholesterol makes the bilayer stronger,more
flexible, and less permeable to water-soluble
substances such as ions.
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PASSIVE TRANSPORT
The movement of particles (molecules/ions)
within a gas or liquid across the plasma
membrane from a region of higher concentration
to a region of lower concentration.
Does not require expenditure of energy fromATP.
The substances move down their concentration
gradient through different ways :
-Phospholipid bilayer
-Pore protein
-Carrier protein
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PASSIVE TRANSPORT
SIMPLE DIFFUSION
OSMOSIS
FACILITATED DIFFUSION
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Simple diffusion
The net movement of molecules / ions from aregion of higher concentration to a region oflower concentration until an equilibrium is
reached. Substances :
-Small non-polar molecules O and CO
-Lipid-soluble substances vitamins ADEK,steroids & alcohols
-Water molecules
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The bigger the concentration gradient thefaster the rate of diffusion.
These substances will diffuse down the
concentration gradient if there is a
concentration gradient. (until an equilibrium is
reached).
Examples : gaseous exchange between the
alveolus & the blood capillaries, bloodcapillaries & body cells
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EXAMPLES:
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Osmosis
The diffusion of water molecules (solvent) from aregion of higher water concentration (dilutedsolution)to a region of lower water concentration(concentrated solution) through a semi-permeable membrane until an equilibrium isreached.
A special type of diffusion.
Examples :
-Absorption of water from soil solution by plantroot hairs
-Reabsorption of water by kidney tubules
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Facilitated Diffusion
The movement of molecules / ions down theirconcentration gradient assisted by transportproteins(pore protein) across the plasma membrane
withoutusing energy.
The transport proteins facilitate & increase the rate
of diffusion across the plasma membrane.
Not require energy
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The rate of facilitated diffusion depends on
the number of transport protein molecules in
the membrane & how fast they can move
their specific solute.
Only allows small charged molecules such asmineral ions to pass through the pore protein.
Carrier protein : allows larger uncharged polar
molecules glucose and amino acids to crossthe membrane
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THE MECHANISM
The solute moves to the binding site of thespecific carrier protein.
The solute binds to the carrier protein at the
binding site & triggers the carrier protein tochange its shape.
The carrier protein changes its shape & moves
the solute across the membrane. The carrier protein returns back to its original
shape
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Active transport
The movement of substances across the plasmamembrane from a region of low concentration to aregion of high concentration (against the concentration
gradient) by usingmetabolic energy.
The substances move across a membrane against theconcentration gradient, using metabolic energy
Perform by a specific protein embedded in the plasmamembrane.
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ACTIVE TRANSPORT
Require energy to change the shape of theprotein such that the substance can be pumpedacross the membrane.
Example : absorption of potassium ions frompond water by algae Nitella sp. against aconcentration gradient, the intake of mineral ionsby the plant roothairs, Na+/ K+ protein pumps in the plasmamembrane of neurones transport Na+ & K+against their concentration gradients
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Comparison between passive
transport and active transport
PASSIVE TRANSPORT ACTIVE TRANSPORT
The movement of substances follows the
concentration gradient
Substances move against the
concentration gradient
Cellular energy is not required Cellular energy (ATP) is required
The process continues until a dynamic
equilibrium is reached
The process results in the accumulation
and elimination of substances from the
cell.