1. Transport and Transport Protein
in cell biology
Assistant Professor in Biotechnology
Tumkur University, Tumakuru
2. Transport and Transport Protein
• Transport – Diffusion, Osmosis and Concentrated Gradient
• Membrane proteins: involved in passive and active transport of
• Example: Plasmodesmata, Ion Channels, Voltage gated channels, gap
junctions and tight junction all these are the types of transport
system in plants and animals
• All these types involved in one of the transport like diffusion, osmosis
and concentration or density gradient
3. Diffusion, Osmosis and Concentration Gradient
• Diffusion – the movement of a substance from a high concentration
to a low concentration
• Osmosis – the movement of WATER from a high concentration to a
• Concentration Gradient – the difference in concentration between a
region of high concentration and a region of lower concentration
4. Passive or Active Transport:
• Passive Transport - does not require cell energy
• Examples: Diffusion, Facilitated diffusion and Osmosis
• Active Transport Requires cell energy (ATP)
• Examples: Carrier mediated active transport, Endocytosis and Exocytosis
• Diffusion is a process of migration of solute molecules from a region of higher concentration to a
region of lower concentration and is brought by random molecular motion.
• Movement from one side of membrane to another side.
• Diffusion is a time dependent process.
• Movement is based on kinetic energy(speed), charge, and mass of molecule
• Diffusion Gradient - The molecules are more densely packed on the left and so they tend to
diffuse into the space on the right. This is a diffusion gradient
A diffusion gradient
Definition of Diffusion:
It is defined as a process of mass transfer of individual
molecules of a substance brought about by random
molecular motion and associated with a driving force
such as a concentration gradient.
Diffusion is a PASSIVE process
which means no energy is used
to make the molecules move,
they have a natural KINETIC
10. Diffusion of Water Across A Membrane
• High water concentration Low water concentration
• Low solute concentration High solute concentration
• Diffusion of water across a
• Moves from HIGH water
concentration to LOW water
• Water is attracted to solutes (like salt) so
it will also travel to areas of low solute
concentration to high solute
• Osmosis: the diffusion of water through a selectively permeable
• Passive transport
• Water molecules move from a higher concentration OF WATER to a lower
concentration OF WATER.
• Water will move to where there is a greater amount of solute because there is
less water there
13. Isotonic Solution
• Isotonic solutions: the concentration of solute
inside and outside of the cell is the same.
• Water in = Water out
• No net movement of water.
• Molecules in equilibrium.
• Normal state for animal cells.
• Cell in homeostasis.
14. Hypotonic Solution
• Hypotonic solutions: the concentration of solute is lower outside the
cell than inside the cell.
• Have more water outside the cell so water moves into the cell
• Causes an increase in pressure inside the cell: called turgor pressure (plants)
or osmotic pressure (animals).
• Increase in pressure in animal cells causes them to swell or even burst; gives
plant cells shape and support.
16. Hypertonic Solution
• Hypertonic solutions: the concentration of solute is higher outside
the cell than inside the cell.
• Have more water inside the cell so water moves out of the cell
• Causes a drop in turgor or osmotic pressure: called plasmolysis.
• Plasmolysis causes animal cells to shrivel up and plants to wilt.
19. Cells in Solutions
• Isotonic solution hypotonic solution hypertonic solution
• No net movement
• of water. EQUAL CYTOLYSIS PLASMOLYSIS
• amounts leaving and
20. Cells in Solutions
• The destruction of a cell.
• Cells swell and burst
• The shrinking of a cell.
• Cells shrink and shrivel
24. Three Forms of Transport Across the Membrane
• Passive Transport Active Transport
25. Passive Transport: Simple Diffusion
• Simple Diffusion
• Doesn’t require energy
• Moves high to low concentration
• Example: Oxygen or water diffusing into
a cell and carbon dioxide diffusing out.
26. Passive Transport: Facilitated Diffusion
• Facilitated Diffusion
• Does not require energy
• Uses transport proteins to
move high to low concentration
• Examples: Glucose or amino acids
moving from blood into a cell.
29. Types of Transport Proteins
• Channel proteins are
embedded in the cell
membrane & have a pore
for materials to cross
• Carrier proteins can
change shape to move
material from one side of
the membrane to the
30. Facilitated Diffusion
• Some carrier proteins do not
extend through the membrane.
• They bond and drag molecules
through the lipid bilayer and release
them on the opposite side.
31. Active Transport
• Active Transport
• Requires energy or ATP
• Moves materials from LOW to
• AGAINST concentration
32. Active Transport
• Examples: Pumping Na+ (sodium
ions) out and K+ (potassium ions)
• Called the Sodium-Potassium Pump.
34. Active Transport--Exocytosis
Type of active transport
Moving things OUT
Molecules are moved
out of the cell by vesicles
that fuse the with the
This is how many
hormones are secreted
and how nerve cells
communicate with each
46. Types of ion channels
Further diversity gained through alternative splicing, editing, phosphorylation,
mixing and matching of different subunit types
48. Types of Transport System:
• Ionic Channels – Gated Ion Channels : 1. Chemical, Mechanical and Voltage gated
Channels – involved in signalling
• Gap Junctions
• Tight Junctions
• Channel structure
• Ion channels have three basic functional properties
51. Specialized Functions of Ion Channels
• Mediate the generation, conduction and transmission of
electrical signals in the nervous system
• Control the release of neurotransmitters and hormones
• Initiate muscle contraction
• Transfer small molecules between cells (gap junctions)
• Mediate fluid transport in secretory cells
• Control motility of growing and migrating cells
• Provide selective permeability properties important for
various intracellular organelles
52. Voltage Gated Channels• Action potentials in neurons are mostly based on the voltage-gated Na+ channel, some neurons use both the
voltage-gated Na+ channel and a voltage-gated K+ channel, some neurons use only the voltage-gated Na+
channel and some neurons use the voltage-gated Ca+2 channel.
• Voltage gated Na+ channel:
The channel has three states, closed, open and inactive.
Closed to Open: Depolarization (depolarization is a change within a cell, during which the cell undergoes a
shift in electric charge distribution, resulting in less negative charge inside the cell) is necessary to open the
channel and therefore it acts to activate itself in a regenerative cycle. More Na+ influx depolarizes the
membrane which opens more channels which depolarizes the membrane more.
Open to Inactive: Depolarization is also necessary to inactive the channel. Once the channel is open it will
then also switch to the inactive state and can not be opened again
Inactive to closed: The channel will not switch back to the closed state until the membrane has repolarized
(i.e. gone back towards the original resting membrane potential. Once in the closed state it can then be
• Voltage-gated K+ channel (called the delayed rectifying K+ channel)
This channel has only two states, closed and open.
Closed to open: The channel is opened with a strong depolarization, the type you would normally get in an
action potential. This channel works to bring the membrane back towards the Nernst potential for K+ i.e.
hyperpolarize the membrane
Open to closed: The channel will close when the membrane becomes hyperpolarized or repolarized.
Therefore this channel works to shut itself down.
53. Lodish 5th edition. Depolarization
of the plasma membrane due to
opening of gated Na+ channels. (a)
Resting neurons non gated K+
channels are open, but the more
numerous gated Na+ channels are
closed. The movement of K+ ions
outward establishes the inside-
negative membrane potential
characteristic of most cells. (b)
Opening of gated Na+ channels
permits an influx of sufficient Na+
ions to cause a reversal of the
54. Lodish 4th Edition. Ion channels in neuronal plasma membranes.
Each type of channel protein has a specific function in the electrical activity of neurons. (a) Resting K+ channels
are responsible for generating the resting potential across the membrane. (b) Voltage- gated channels are
responsible for propagating action potentials along the axonal membrane. (c, d) Two types of ion channels in
dendrites and cell bodies are responsible for generating electric signals in postsynaptic cells. One type (c) has a
site for binding a specific extracellular neurotransmitter (blue circle). The other type (d) is coupled to a
neurotransmitter receptor via a G protein; it responds to intracellular signals (red circle) induced by binding of
neurotransmitter to a separate receptor protein (not shown). Signals activating different channels include
Ca2+, cyclic GMP, and the Ga subunits of trimeric G proteins
55. Lodish 4th edition OR Figure 7-33 Lodish 5th edition. Structure and function of the voltage-gated Na+ channel.
Like all voltage-gated channels, it contains four transmembrane domains, each of which contributes to the central pore through
which ions move. The critical components that control movement of Na+ ions are shown in the cutaway views. (a) In the closed,
resting state, the gate obstructs the channel, inhibiting Na+ movement, and the channel-inactivating segment is free in the cytosol.
The channel protein contains four voltage-sensing alpha helices (maroon), which have positively charged side chains every third
residue. The attraction of these charges for the negative interior of resting cells helps keep the channel closed. (b) When the
membrane becomes depolarized (outside negative), the voltage-sensing helices move toward the outer plasma membrane surface,
causing an immediate conformational change in the gate segment that opens the channel for influx of Na+ ions. (c) Within a
millisecond after opening, the voltage-sensing helices return to the resting position and the channel inactivating segment (purple)
moves into the open channel, preventing further ion movements. When the membrane potential is reversed so that the inside is
again negative, the gate moves back into the blocking position (not shown). After 1 2 ms the channel-inactivating segment is
displaced from the channel opening and the protein reverts to the closed, resting state (a) where it can be opened again by
56. Ionic Channels
• Na+/K+-ATPase: Na+/K+-ATPase (sodium-potassium adenosine triphosphatase, also known as the
Na+/K+pump or sodium–potassium pump) is an enzyme (an electrogenic transmembrane ATPase) found in
the plasma membrane of all animal cells. It performs several functions in cell physiology.
• The Na+
-ATPase enzyme is a solute pump that pumps sodium out of cells while pumping potassium into cells, both
against their concentration gradients
For every ATP molecule that the pump uses, three sodium ions are exported and two potassium ions are
imported; there is hence a net export of a single positive charge per pump cycle.
The sodium-potassium pump was discovered in 1957 by the Danish scientist Jens Christian Skou, who was
awarded a Nobel Prize for his work in 1997.
significance for excitable cells such as nerve cells, which depend on this pump to respond to stimuli and
• The pump, after binding ATP, binds 3 intracellular Na+
• ATP is hydrolyzed, leading to phosphorylation of the pump at a highly conserved
aspartate residue and subsequent release of ADP.
• A conformational change in the pump exposes the Na+
ions to the outside. The phosphorylated form of the pump has a low affinity for Na+
ions, so they are released.
• The pump binds 2 extracellular K+
ions. This causes the dephosphorylation of the pump, reverting it to its previous
conformational state, transporting the K+
ions into the cell.
• The unphosphorylated form of the pump has a higher affinity for Na+
ions than K+
ions, so the two bound K+
ions are released. ATP binds, and the process starts again.
58. The sodium-potassium pump is found in many cell (plasma) membranes.
Powered by ATP, the pump moves sodium and potassium ions in opposite
directions, each against its concentration gradient. In a single cycle of the
pump, three sodium ions are extruded from and two potassium ions are
imported into the cell.
59. Gap Junction:
• A gap junction may also be called a nexus or macula communicans. (Although most nerve tissues
don't have gap junctions, when found in neurons or nerves it may also be called an electrical
synapse, like nerve cells in dental pulp.) While an ephapse has some similarities to a gap junction,
intercellular space is
between 2 and 4 nm
protein has four
6 Connexins create one Connexon (hemichannel). When different connexins join together to form one connexon, it is
called a heteromeric connexon
60. • Gap junctions are a specialized intercellular connection between a
multitude of animal cell-types. They directly connect the cytoplasm of
two cells, which allows various molecules, ions and electrical impulses
to directly pass through a regulated gate between cells.
• two connexons (or hemichannels) -=> homo- or hetero-hexamers of
connexin proteins – vertebrates, invertebrates - proteins from the
innexin family - Innexins have no significant sequence homology with
connexins = One gap junction channel
• Gap junctions are analogous to the plasmodesmata that join plant
• Gap junctions occur in virtually all tissues of the body, with the
exception of adult fully developed skeletal muscle and mobile cell
types such as sperm or erythrocytes. Gap junctions, however, are not
found in simpler organisms such as sponges and slime molds.
61. • When two identical connexons come together to form a Gap junction
channel, it is called a homotypic GJ channel. When one homomeric
connexon and one heteromeric connexon come together, it is called a
heterotypic gap junction channel. When two heteromeric connexons
join, it is also called a heterotypic Gap Junction channel.
• Several gap junction channels (hundreds) assemble within a
macromolecular complex called a gap junction plaque.
62. • A connexon channel pair:
• Allows for direct electrical communication between cells, although different connexin
subunits can impart different single channel conductances.
• Allows for chemical communication between cells, through the transmission of small
second messengers, such as inositol triphosphate (IP3) and calcium (Ca2+ ), although
different connexin subunits can impart different selectivity for particular small molecules.
• In general, allows transmembrane movement of molecules smaller than 485 Daltons
(1,100 Daltons through invertebrate gap junctions), although different connexin subunits
may impart different pore sizes and different charge selectivity. Large biomolecules, for
example, nucleic acid and protein, are precluded from cytoplasmic transfer between cells
through gap junction connexin channels.
• Ensures that molecules and current passing through the gap junction do not leak into the
• Gap Junctions have been observed in various animal organs and tissues where cells
contact each other
64. Tight Junctions
• Epithelia are sheets of cells that provide the interface between
masses of cells and a cavity or space (a lumen). The portion of the cell
exposed to the lumen is called its apical surface.
• The rest of the cell (i.e., its sides and base) make up the basolateral
• Tight junctions seal adjacent epithelial cells in a narrow band just
beneath their apical surface. They consist of a network of claudins
and other proteins.
Tight junctions, also known as occluding junctions or zonulae occludentes (singular, zonula
occludens) are multiprotein junctional complex whose general function is to prevent leakage
of transported solutes and water and seals the paracellular pathway. Tight junctions may
also serve as leaky pathways by forming selective channels for small cations, anions, or
water. Tight junctions are present only in vertebrates.
65. Tight junctions perform two vital functions:
• They limit the passage of molecules and ions through the space
between cells. So most materials must actually enter the cells (by
diffusion or active transport) in order to pass through the tissue. This
pathway provides tighter control over what substances are allowed
• They block the movement of integral membrane proteins (red and
green ovals) between the apical and basolateral surfaces of the cell.
Thus the special functions of each surface, for example
• receptor-mediated endocytosis at the apical surface
• exocytosis at the basolateral surface
66. Adherens Junctions
• Adherens junctions provide strong mechanical attachments
between adjacent cells. They hold cardiac muscle cells tightly
together as the heart expands and contracts.
• They hold epithelial cells together.
• They seem to be responsible for contact inhibition.
• Some adherens junctions are present in narrow bands
connecting adjacent cells.
• Others are present in discrete patches holding the cells
• Adherens junctions are built from:
• cadherins — transmembrane proteins (shown in red) whose
• extracellular segments bind to each other and
• whose intracellular segments bind to
• catenins (yellow). Catenins are connected to actin filaments
Desmosomes are localized patches that hold two cells tightly together. They are common in epithelia
(e.g., the skin). Desmosomes are attached to intermediate filaments of keratin in the cytoplasm.
Pemphigus is an autoimmune disease in which the patient has developed antibodies against proteins
(cadherins) in desmosomes. The loosening of the adhesion between adjacent epithelial cells causes
Carcinomas are cancers of epithelia. However, the cells of carcinomas no longer have desmosomes.
This may partially account for their ability to metastasize.
These are similar to desmosomes but attach epithelial cells to the basal lamina ("basement membrane"
– View) instead of to each other.
Pemphigoid is an autoimmune disease in which the patient develops antibodies against proteins
(integrins) in hemidesmosomes. This, too, causes severe blistering of epithelia.
•Four kinds of junctions occur in vertebrates: