Movement of molecules-2013.pptx
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Learning objectives
At todays lecture we will cover the main
mechanisms through which cells exchangematerials with their environment.
Different types of movement of moleculesthrough the cell membrane will be elucidated
and examples of each discussed.
The importance of each mechanism will also beoutlined.
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Learning outcomes
By the end of todays lecture
you should:
1) Understand the main mechanisms of movementof molecules through the cell membranediffusion; osmosis; facilitated diffusion; active
transport and cytosis.2) Be able to comparethese mechanisms.
3) Understand their requirementsand significance
for the functioning of the cells.
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The state of matter
As the temperature rises the kinetic energy of the
molecules increases and they move faster.
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Second Law of Thermodynamics
All things tend towardsentropy(randomness).
Molecules move (diffuse)
from an area of highconcentration to areas of
low concentration.
Eventually moleculesbecome randomly
distributedunless acted
on by something else.
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Random Movement of Molecules
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Movement along a concentration
gradient
Molecules of water move in every direction.
As a result the lump of salt disappears and the
molecules of salt spread throughout the water.
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Movement down a concentration
gradient
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Movement across cell membrane
Movement of molecules across the cell
membrane has two main features:
Allows uptake and release of molecules
according to the biological needsof the cell
(eg. uptake and release of oxygen by red
blood cells).
Transport can be regulated (eg. increased
transport of glucose into muscles during
physical activity).
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The nature of phospholipid bilayers
Hydrophilic phospholipid heads interact with the
aqueous media inside and outside of the cell.
Hydrophobic tails repel aqueous media and prevent
passage through the membrane.
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The three characteristics of a molecule that
determine the phospholipid bilayerpermeability to that species are . . .
1) polarity - (Hydrophobic vs Hydrophylic)
2) charge - (charged vs uncharged)
3) size - (large vs small)
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Molecules that pass through the phospholipid bilayer
easily
hydrophobic molecules i.e. O2, N2, steroids. Nonpolar - benzene
Small uncharged polar molecules - H2O, Urea, glycerol,
CO2
Molecules that dontpass through the phospholipid bilayer
easily...
Large uncharged - Glucose
Polar molecules - Sucrose
Ions (charged) - H+, Na+, HCO3-, K+, Ca2+ , Cl-, Mg2+
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The cell membrane is more than just
a phospholipid bilayer
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Control of molecule transport in and
out of the cell
Cell membrane is selectively permeableallows the passage of certain substances but
not that of others.
Property depends on both protein andphospholipid components of membrane
Mechanisms of movement include:
1. Diffusion.
2. Osmosis.
3. Facilitated Diffusion.
4. Active Transport.
5. Cytosis.
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Diffusion
A physical process characterised by the net
movement of particles from a place of high
concentration to a place of lower concentration.
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The rate of diffusion
depends on a number of factors such as:
Particle sizeinversely proportional
Temperaturedirectly proportional
Concentration gradient sizedirectlyproportional
Path lengthinversely proportionalSurface areadirectly proportionalDensityinversely proportional
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Ficks Law
In many living organisms
temperatureand densityare constant.
Cells and tissues adapt to
diffusion by short pathlengths, large surface
areas and high
concentration gradients.
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The rate of exchange of substances
depends on the organism's surface area
that's in contact with the surroundings. Requirements for materials depends on
the volumeof the organism.
The ability to meet the requirementsdepends on the surface area : volume
ratio.
Surface area and volume
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The problem of getting large
As organisms get larger their volume andsurface area both increase, but volume
increases much more than surface area.
ORGANISM LENGTH SA (M) VOL. (M) S/A:VOL
bacterium 1 mm 6 x 10-12 10-18 6,000,000:1
amoeba 100 mm 6 x 10-8 10-12 60,000:1
fly 10 mm 6 x 10-4 10-6 600:1
dog 1 m 6 x 100 100 6:1
whale 100 m 6 x 104
106
0.06:1
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Osmosisdiffusion of water through
a partially permeable membrane
http://www.google.kz/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=wf_GGFUAVhoF_M&tbnid=A1NjleX7nfEZzM:&ved=0CAUQjRw&url=http://keepinapbiologyreal.wikispaces.com/Osmosis&ei=lTtVUta6EceJ4ATo0YDoAg&bvm=bv.53760139,d.bGE&psig=AFQjCNE-XLqslF3M2a3Bi1nKeiAZwK8bIA&ust=1381403875243652 -
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Water potential
The force pushing water to move from one side of amembrane to another is measured by the water
potential which has the units of pressure
The water potential () of pure water is 0.The pressure on a cell will increase its .
cell = s+ p
ssolute potential
ppressure potential
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Animal cells and osmosis
cell < outside cell = outside cell > outside
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Plant cells and osmosis
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Facilitated diffusion
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Passive diffusion of glucose -
permease
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Active transport - principles
Creates internal environment different from theexternal one. Different from diffusion where theend state is equilibrium between internal andexternal environment.
Represents the movement of particles across thecell membrane often from an area of low to anarea of high concentration.
Would not be possible according to 2ndLaw of
Thermodynamics unless cellular energy (ATP) isexpended to transport against concentrationgradient.
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Types of active transport
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Uniportproton pumps
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Antiportsodium/potassium pump
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Co-transport
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Secondary active transport
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Cytosis
Some things are too big to diffuse fast enough
or pass through a pore or a pump. These are
moved in vesicles via cytosis.
Vesicles and other large complexes are moved
around the cell by motors attached to
microtubules (dynein and kinesins) or
microfilaments (myosins).
Import of particles is endocytosisexport of
particles is exocytosis.
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Phagocytosis
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Pinocytosis
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Clathrin (receptor) mediated
endosytosis
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Exocytosis - secretion
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2013 Nobel prize in Physiology or
Medicine
Awarded jointly to James E. Rothman, RandyW. Schekman and Thomas C. Sdhof "for their
discoveries of machinery regulating vesicle
traffic, a major transport system in our cells".
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Summary
The following mechanisms of transport through thecell membrane were elucidated:
1. Diffusion.
2. Osmosis.3. Facilitated diffusion.
4. Active transport.
5. Cytosis.The subtypes and their importance for the cellswere also discussed.