Download Membrane. Mechanism of transport charge and non charge partial

Membrane. Mechanism of
transport charge and non
charge partial throw
membrane structure of cell.
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Plan
• Biological membrane
• Transport Across Cell Membranes
– Passive Transport
– Active Transport
– Endocytosis/Exocytosis
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All living cells and many of the
tiny organelles internal to cells are
bounded by thin membranes. These
membranes are composed primarily
of phospholipids and proteins and are
typically described as phospholipid
bi-layers.
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Phospholipids self assemble into different
structures because their hydrophobic and
hydrophilic ends repel each other
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In this sketch, the spheres represent the
phosphate end, which is polar and water
soluble (hydrophilic). The twin extensions
represent the fatty acid components which are
not water soluble (hydrophobic).
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Cell membranes also contain cholesterol in the
phospholipid bilayer. In some membranes there are only a
few cholesterol molecules. Cholesterol makes the bilayer
stronger, more flexible but less fluid, and less permeable to
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water-soluble substances such as ions and monosaccharides.
• A biological membrane or biomembrane
is an enclosing or separating membrane
that acts as a selective barrier, within or
around a cell. It consists of a lipid bilayer
with embedded proteins that may
constitute close to 50% of membrane
content. The cellular membranes should
not be confused with isolating tissues
formed by layers of cells, such as mucous
and basement membranes.
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Transport Across Cell Membranes
Essential and continuous parts of the life of a cell are the taking
in of nutrients and the expelling of wastes. All of these must pass
through the cell membrane.
Transport may occur by diffusion and osmosis across the
membrane. It can also occur when a vescicle attaches to the cell
membrane from the inside and then opens to form a pocket, expelling
its contents to the outside. This may be called exocytosis. The cell
membrane may also envelope something on the outside and surround it,
taking it into the cell. This may be called endocytosis or phagocytosis.
There are also examples where molecules move across a
membrane from a region of low concentration to an region of high
concentration, and this requires a source of energy to "pump" the
molecules uphill in concentration. Such processes are called active
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transport.
Transport Across Cell
Membranes
• 3 Types
• 1. Passive Transport
• 2. Active Transport
• 3. Endocytosis/Exocytosis
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Transport Across Cell
Membranes
• 1. Passive Transport
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Brownian Motion and
Concentration Gradients
 Brownian Motion: matter is made up of
tiny particles that are in constant motion
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Molecules always move
1) randomly
2) from areas of high
concentration to areas of
low concentration
This is called DIFFUSION
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Concentration Gradient
The difference in concentration between the
high and low concentration areas.
The greater the difference in concentration
– the faster the particles move
Eventually equilibrium is reached
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Why does this happen?
• At high concentrations the molecules
ricochet off of each other and move
towards the area of low concentration
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Equilibrium
• Remember: at equilibrium the particles DO
NOT stop moving, they continue to move
back and forth across the concentration
gradient.
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Passive Transport
NO ENERGY REQUIRED
• Small molecules move by diffusion
(water, oxygen, carbon dioxide)
• When WATER molecules move by
diffusion across a membrane we call it
OSMOSIS
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Diffusion
Diffusion refers to the process by
which molecules intermingle as a result
of their kinetic energy of random
motion. Consider two containers of gas
A and B separated by a partition. The
molecules of both gases are in constant
motion and make numerous collisions
with the partition. If the partition is
removed as in the lower illustration, the
gases will mix because of the random
velocities of their molecules. In time a
uniform mixture of A and B molecules
will be produced in the container.
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Steady-State Diffusion
Flux proportional to concentration gradient =
dC
dx
Fick’s first law of diffusion
C1 C1
C2
x1
x
C2
dC
J  D
dx
x2
dC C C2  C1
if linear


dx
x
x2  x1
D  diffusion coefficient
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Facilitated Transport (also called
facilitated diffusion)
• Another form of passive transport
• Used for molecules that are too large
to cross the membrane by diffusion
(i.e. glucose), and for charged
molecules
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Facilitated Diffusion
• Carrier proteins bind to larger molecules,
and change their shape so molecules can
diffuse through.
• Channel proteins provide water filled
pores for charged ions to pass through
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Osmosis
If two solutions of different concentration
are separated by a semi-permeable membrane
which is permeable to to the smaller solvent
molecules but not to the larger solute
molecules, then the solvent will tend to diffuse
across the membrane from the less
concentrated to the more concentrated
solution. This process is called osmosis.
Osmosis is of great importance in
biological processes where the solvent is
water. The transport of water and other
molecules across biological membranes is
essential to many processes in living
organisms. The energy which drives the
process is usually discussed in terms of
osmotic pressure.
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3. Selective Filter – SEMIPERMEABLE –
only allows
certain substances in or out
• Regulated by particle size and by
selective transport by membrane
proteins
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Osmosis
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Active Transport Across Cell Membranes
There are numerous situations in living organisms when
molecules move across cell membranes from an area of lower
concentration toward an area of higher concentration. This is
counter to what would be expected and is labeled "active
transport".
There is a very strong tendency for molecules to move
from higher concentration to low, just based on thermal
energy. Molecules at normal temperatures have very high
speeds and random motions. For example, water molecules at
20°C have an effective or rms speed of over 600 m/s or over
1400 miles/hr! This motion from areas of high concentration
to low is called diffusion. There are times when membranes
are impermeable to some molecules because of their size,
polarity, etc. and only the smaller solvent molecules like water
molecules will move across the membrane. This is called
osmosis, and the tendency to transport the solvent molecules
is
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quantified in terms of osmotic pressure.
Active Transport
• Molecules move against the concentration
gradient (low to high)
• Energy must be provided (even when we are
resting, 40% of our energy is spent on active
transport!)
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Active Transport
• Uses specialized transport proteins
and protein pumps
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Why spend so much energy on
active transport?
Maintains internal cell environments (i.e.
cell’s electrical gradient, roots pull in
minerals from soil, filtering blood in your
kidneys)
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If a molecule is to be transported from an area of
low concentration to an area of high concentration,
work must be done to overcome the influences of
diffusion and osmosis. Since in the normal state of a
cell, large concentration differences in K+, Na+ and
Ca2+ are maintained, it is evident that active transport
mechanisms are at work.
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Many crucial processes in the life of
cells depend upon active transport.
Included in the illustration above is the
sodium-potassium pump which is a vital
cell process. Active transport mechanisms
may draw their enegy from the hydrolysis
of ATP, the absorbance of light, the
transport of electrons, or coupling with
other processes that are moving particles
down their concentration gradients.
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The Sodium-Potassium Pump
The process of moving sodium and
potassium ions across the cell membrance is an
active transport process involving the hydrolysis
of ATP to provide the necessary energy. It
involves an enzyme referred to as Na+/K+ATPase. This process is responsible for
maintaining the large excessof Na+ outside the
cell and the large excess of K+ ions on the
inside. A cycle of the transport process is
sketched below. It accomplishes the transport of
three Na+ to the outside of the cell and the
transport of two K+ ions to the inside. This
unbalanced charge transfer contributes to the
separation of charge across the membrane. The
sodium-potassium pump is an important
contributer to action potential produced by
nerve cells. This pump is called a P-type ion
pump
because
the
ATP
interactions
phosphorylates the transport protein and causes
a change in its conformation.
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The Na-K pump
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The Na-K pump
cycle
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The
sodiumpotassium
pump
moves toward an
equilibrium state with
the
relative
concentrations
of
Na+ and K+ shown
at left.
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Functions of the Cell Membrane
1. Barrier – keeps “baddies out, cell
organelles in
Bacteria
pollution resistant
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Functions of the Cell Membrane
2. Organization – membranes surround
and package materials in
vessicles,
lysosomes
reticulum
endoplasmic
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2. Organization – membranes
organize complex reactions like
photosynthesis and cellular respiration
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Two components of an electrochemical gradient
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