LECTURE 10:
TRANSMEMBRANE
TRANSPORT
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Cell Membrane Transport
Outline
⚫ Passive
⚫ Diffusion
⚫ Facilitated
Diffusion
⚫ Osmosis
⚫ Water
potential
⚫ Tonicity
2
⚫ Active
⚫ Carrier
Protein
mechanisms
⚫ Coupled
channels:
Na/K pump
⚫ Bulk Transport
⚫ Endocytosis
⚫ Exocytosis
Objectives- At the end of this
lesson you should be able to:
1. Describe the structure of the
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cell membrane
2. Explain factors which affect
permeability of the
membrane to water and other
substances.
3. Describe the various
mechanisms involved in
transport of substances across
Cell membranes
⚫ Only about 7 nm wide
⚫ Present barriers to movement
of ions and molecules.
Semipermeable.
⚫ E.g. Lipid bilayer prevents polar
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molecules (like glucose and
amino acids) from passing, but
allows lipid soluble molecules to
Cell membrane fluidity
⚫ Saturated FAs (C-C bonds are all single)
are regularly shaped.
⚫ Phosphoglycerides with saturated FAs are
densely packed together resulting in low
membrane fluidity.
⚫ Unsaturated FAs (at least one C-C bond is
double) are irregularly-shaped.
⚫ Phosphoglycerides with unsaturated FAs are less
densely packed together resulting in high
membrane fluidity.
⚫ Cholesterol
⚫ Binds weakly to adjacent phosphoglycerides,
making the lipid bilayer less fluid, less flexible
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and more viscous.
Cell membrane proteins
Proteins provide a means for polar
substances to pass.
1.
2.
Peripheral (attached to membrane
surface).
Integral (embedded within membrane or
transmembrane)
⚫ Transmembrane (span the lipid bilayer) or
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fully or partially embedded in the membrane.
⚫ Amphipathic
⚫ Nonpolar portions within hydrocarbon core
of bilayer;
⚫ Polar portions protrude from core into
aqueous intra- or extra-cellular solutions.
Cell membranes
⚫ Offers some control over what
is allowed into the cell.
⚫ Referred to as being
selectively permeable because
it allows some molecules to
pass easily but restricts others.
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Cell Membrane Structure
Hydrophillic
Hydrophobic
Phospholipid bilayer
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http://www.people.virginia.edu/~rjh9u/cellmemb.html
Transport Across Cell
Membranes
Transport across cell membranes must
occur because the cell needs to:
Obtain nutrients
Excrete waste substances
Secrete useful substances
Generate the ionic gradients essential
for nervous and muscular activity
Maintain a suitable internal pH and
ionic concentration for enzyme
activity.
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Transport Across Cell
Membranes
Four basic mechanisms by which
substances cross the membrane
⚫ Passive (do not require ATP)
1. Diffusion + Facilitated diffusion
2. Osmosis
⚫ Active (energy consuming)
3. Active transport
4. Bulk transport (endocytosis
and exocytosis).
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Passive Transport - Diffusion
1. Diffusion
⚫ Passive movement of molecules
or ions from a region where
they are in high concentration
to a region where they are in low
concentration. ie. down/along a
diffusion gradient.
⚫ May be facilitated
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Passive Transport - Diffusion
Diffusion
⚫ Rate affected by
a. steepness of the gradient
b. surface area of membrane
c. distance to be diffused
d. size and charge of diffusing
molecule
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Factors affecting the rate of diffusion
a. The steepness of the diffusion
gradient is the difference in
concentration of the molecule
between the its point of origin and the
final destination.
⚫ The steeper the gradient, the
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faster the rate of diffusion
because there are more
molecules available for
Factors affecting the rate of
diffusion
b. The greater the surface
area of the membrane
through which diffusion
takes place, the greater the
rate of diffusion.
⚫ Microvilli increase the surface
area of animal cells for
absorption purposes.
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Factors affecting the rate of
diffusion
c. The rate of diffusion
decreases rapidly with
distance over which
diffusion occurs.
⚫ Therefore diffusion is
only effective over very
short distances. (membrane
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approx. 7nm wide)
Factors affecting the rate of
diffusion
d. The size of the diffusing
molecule matters
⚫ Smaller molecules/ions move
at a faster rate.
e. The charge of diffusing
molecule matters
⚫ The larger the charge the
slower the ions move.
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Transmembrane Transport by
Diffusion
⚫ The molecules that can be
transported across the
membrane by diffusion
include
⚫ The respiration gases oxygen
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and carbon dioxide which can
diffuse rapidly across the
membranes.
⚫ Water molecules (although
Transmembrane Transport by
Diffusion
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Molecules that diffuse
include
⚫ Ions and larger polar
molecules such as amino
acids, sugars, fatty acids
and glycerol diffuse across
slowly since they are
repelled by the
Diffusion
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Diffusion - Facilitated
Facilitated diffusion
⚫ Special transport proteins in the
membrane aid the diffusion of
some ions and polar molecules
⚫ Since diffusion would not be
possible with out the proteins,
the process is known as
facilitated diffusion.
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Diffusion - Facilitated
⚫ Channel proteins and carrier
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proteins contain water-filled
hydrophobic channels or pores
whose shape is specific for a
particular ion or molecule.
⚫ Channel proteins have a fixed
shape
⚫ Carrier proteins undergo rapid
changes in shape.
Diffusion - Facilitated
⚫ Alternatively several proteins can
combine to form a channel
between them.
⚫ Diffusion can occur through the
channel in either direction.
⚫ Transport proteins which allow
the passage of ions are called ion
channels
⚫ Ion channels are usually gated
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Facilitated Diffusion
molecule
Channel
protein
Closed
Open
Passage through protein channel
closed
open
open
Carrier
protein
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closed
open
closed
Facilitated Diffusion
Channel protein
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Carrier protein
Passive Transport - Osmosis
2. Osmosis
⚫ Passive movement of free
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water molecules (solvent in
the cell) from a region of
their high concentration to
a region of their low
concentration through a
partially/selectively
permeable membrane.
Osmosis - Water potential
Water potential (psi – Ψ)
⚫ The tendency of water molecules
to move from one place to
another (leave or enter a system).
⚫ Water always moves from a
region of higher water potential
to a region of lower water
potential.
⚫ The highest value for Ψ is zero
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Osmosis - Water potential
⚫ Pure water (Ψ =0) will move
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to a region that has solutes
dissolved in the water (Ψ is
negative).
⚫ It is therefore the
differences in the water
potential that causes water
to move across membranes.
Osmosis - Solute potential (Ψs)
Solute potential (Ψs)
⚫ The presence of solute molecules in water
lowers Ψ (makes it more negative).
⚫ The presence of solute molecules prevent
water from moving. Water surrounds the
solutes so less free water is available for
movement. Eg. H on Cl , O on Na
⚫ The extent to which solute molecules lower
the Ψ is called solute potential (Ψs)
⚫ Water molecules are attracted to solutes, so
Ψs pulls in water molecules. Ψs has a
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negative value.
+
-
-
+
Osmosis - Pressure Potential (Ψp)
Pressure potential (Ψp)
⚫ As water molecules enter a plant
cell, the wall pushes back on the
cytoplasm as it expands.
⚫ This pressure is called the
pressure potential and tends to
force water out of a cell.
⚫ Ψp is given a positive value.
Water potential (Ψ) = solute
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Osmosis - Cell status
-hyperosmotic
⚫ Hyperosmotic cell - has a higher
solute concentration than that
found in the solution outside the
cell.
⚫ The outside of the cell would
therefore be hypotonic, having less
solute (= more solvent [Water]
molecules) that inside the cell
30⚫ The cell would gain water as it would
Osmosis - Cell status
Hypotonic solution
⚫ If an animal cell is placed in
a hypotonic solution
⚫ Water would leave such a
solution and enter the cell.
⚫ The cell would increase in
volume and eventually burst.
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Osmosis
⚫ Hypoosmotic cell - has a lower
solute concentration than that found in
the solution outside the cell.
⚫ The surrounding solution
would therefore be
hypertonic (more solutes).
⚫ Water would move out of the
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cell to the area of lower water
potential.
Osmosis
Hypertonic Solution
⚫ If an animal cell was placed
in a hypertonic solution
⚫ There would be more free
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water molecules in the cell
than outside and so water
would move out of the cell.
⚫ The cell would therefore
Osmosis
⚫ Isosmotic cell /Isotonic
solution
(iso = equal) - the outer and
internal environment of a cell
are osmotically equal.
⚫ The movement of water into
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and out of a cell is equal;
therefore there is no net gain
Control of Osmosis
Animal cells
⚫ Develop osmoregulatory
mechanisms to maintain the
balance of water to prevent
excessive shrinkage or bursting
when the concentration of
surrounding solutions change.
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Control of Osmosis
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Plant cells
⚫ Have a rigid cell wall which
resist swelling if place in a
hypotonic solution.
⚫ However, they can lose water if
placed in a hypertonic
environment.
⚫ As their volume decreases,
they shrink and the
cytoplasm moves away from
the cell wall (plasmolysis).
Transmembrane Transport
-Active
3. Active Transport
⚫ Movement of molecules or ions
across a membrane against a
concentration gradient (from a
region of lower concentration to
one of high concentration) I
⚫ Requires chemical energy since
it involves movement against the
natural tendency to diffuse in the
opposite direction.
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Active Transport
⚫ Energy is directly or
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indirectly supplied by the
hydrolysis of ATP.
⚫ Movement is usually in one
direction only, unlike
diffusion which is
reversible.
⚫ Active transport in cells is
achieved by carrier proteins
Active Transport
⚫ One of the most widely
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occurring active transport
proteins is the
sodium-potassium pump, a
protein found in all types of
eukaryotes (mostly animal
cells).
⚫ The pump is a carrier
protein which spans the
Active Transport
⚫ But it is more than a protein - it is
also an enzyme, ATPase,
(Adenosine triphosphatease) which
hydrolyzes ATP and utilizes the
energy to move the ions against
their concentration gradient.
⚫ The protein transports sodium
ions out of the cells and potassium
ions into the cells, both against
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Active Transport
⚫ Cells are high in potassium and
low in sodium.
⚫ For every 2 K+ taken into the cell 3
Na+ are removed
⚫ On the inside of the cell the
protein accepts sodium and ATP;
while on the outside it accepts
potassium.
+
+
⚫ The transfer of Na and K across
the membrane is brought about by
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Active Transport
⚫ This builds up a potential
difference across the
membrane, with the inside of
the cells being negative.
⚫ Positively charged ions
(cations) therefore tend to be
attracted to the inside of the
cell and the entry of
negatively charged ions
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Sodium Potassium Pump
Sodium high
Potassium low
Potassium high
Sodium low
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Active Transport
+
+
⚫ The Na /K pump is essential
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in controlling the osmotic
balance of animal cells
(osmoregulation).
⚫ If the pump is inhibited the
cells swell up and burst
because a build up of sodium
ions results in excess water
entering the cells by osmosis.
⚫ Bacteria, fungi and plants have
cell walls and do not need the
pump.
Transport
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Transmembrane Transport
Endocytosis and Exocytosis
4.Bulk transport (Endocytosis
& Exocytosis )
⚫ Active process involving the
bulk transport of materials
through membranes whether
into cells (endocytosis) or out
of cells (exocytosis).
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Transmembrane Transport Endocytosis
Endocytosis
⚫ Large molecular weight materials
are enclosed within infoldings or
extensions of the cell surface
membrane to form a vesicle.
⚫ There are two types phagocytosis and pinocytosis
leading to the formation co
vesicles called phagosomes and
pinosomes respectively
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Active transport - Endocytosis
Types of endocytosis
⚫ Phagocytosis (cell eating) – material
taken up is in the solid form.
⚫ Cells specializing in phagocytosis
are called phagocytes and are said
to be phagocytic. E.g., white blood
cells that engulf bacteria.
⚫ Pinocytosis (cell drinking) – material
taken up is in liquid form.
⚫ Vesicles (pinosomes) are extremely
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small.
Endocytosis and Exocytosis
Endocytosis
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Endocytosis and Exocytosis
Exocytosis
The reverse of endocytosis.
⚫ Waste materials, such as solid
undigested remains from
phagocytic vacuoles, are
removed from cells
⚫ Useful material may be secreted
⚫
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by the cell this way.
Exocytosis
⚫ Vacuoles with
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secretory cellular
products migrate
from the interior
cytoplasm to the cell
surface where they
fuse with the plasma
membrane and
discharge their
products to the