1. Transportation Of Molecule In Different
Asst Research scientists,
Anand. Submitted by
Joshi Prathmesh Govind
M.Sc (Agri) Biochemistry,
Membranes separate the cell from the outside world and separate organelles
inside the cell to compartmentalize important processes and activities.
Cellular membranes have diverse functions depending on their location
within the organelles of a cell.
However, at the electron microscopic level, membranes share a common
structure following routine preparative steps.
The figure below shows a typical "Unit" membrane which resembles a
railroad track with two dense lines separated by a clear space.
4. This figure was obtained by cell fixation/sectioning and staining with osmium
tetroxide (an electron opaque agent that binds to a variety of organic compounds).
This figure actually shows two adjacent plasma membranes, both of which have
the "unit membrane" structure.
5. Membrane Transport: Definition
Membrane transport is defined as the movement of molecules across
The bilayer is permeable to:
Small hydrophobic molecules
Small uncharged polar molecules
The bilayer is impermeable to:
Large polar molecules
Therefore, need membrane proteins to transport most molecules and all ions across
6. Gases diffuse freely, no
Water diffuses fast enough that
proteins aren’t required for
Water diffuses fast enough that
proteins aren’t required for
Sugars diffuse very slowly so
proteins are involved in
Charged molecules are virtually
Charged molecules are virtually
The phospholipid bilayer is a
barrier that controls the
transport of molecules in and
out of the cell.
7. Selective transport across the lipid membrane requires transport proteins.
Transport proteins are integral membrane proteins that move molecules
There are two classes of transport proteins: transporters (pumps) and
8. Three main class of membrane protein
1. ATP- power pump( carrier, permease) couple with
energy source for active transport binding of specific solute to transporter
which undergo conformation change.
2. Channel protein (ion channel) formation of hydrophilic pore allow
passive movement of small inorganic molecule.
uniporters transport a single molecule down its gradient (passive).
co-transporters couple movement of a molecule down its gradient
moving a molecule up its gradient (active).
hydrolyze ATP to move small molecules/ions up a concentration
gradient or electric potential (active).
transport water/ions/small molecules down their concentration
gradients or electric potentials (passive)
10. Two types of transport.
1. Passive transport: no metabolic energy is needed because the
solute is moving down its concentration gradient.
• In the case of an uncharged solute, the concentration of the solute on
each side of the membrane dictates the direction of passive
2. Active transport: metabolic energy is used to transport a solute
from the side of low concentration to the side of high concentration.
12. Types of Diffusion
A. Non-channel mediated
-lipids, gasses (O2, CO2), water
B. Channel mediated
-ions, charged molecules
molecule moves down its electrochemical gradient.
-glucose, amino acids
13. ATP powered pump
1. P- class
2α, 2β subunit; can phosphorylation
i.e. Na+-K+ ATP ase, Ca+ATP ase, H+pump
locate on bacterial membrane , chloroplast and mitochondria
pump proton from exoplasmic space to cytosolic for ATP synthesis
maintain low pH in plant vacuole
similar to F-class
4. ABC (ATP-binding cassete) superfamily
several hundred different transport protein
14. Transport process requires ATP
hydrolysis in which the free energy is
liberated by breakdown of ATP into ADP
16. The Fo integral membrane protein complex subscript o denoting its
inhibition by the drug oligomycin provides a transmembrane pore for
protons, and the peripheral protein F1 is a molecular machine that uses
the energy of ATP to drive protons uphill.
The reaction catalyzed by F-type ATPases is reversible, so a proton
gradient can supply the energy to drive the reverse reaction, ATP
18. This class of proton-transporting ATPases structurally related to the F-type
ATPases, are responsible for acidifying intracellular compartments in many
organisms (thus V for vacuolar).
Proton pumps of this type maintain the vacuoles of fungi and higher plants
at a pH between 3 and 6, well below that of the surrounding cytosol (pH 7.5).
Vtype ATPases have a similar complex structure, with an integral
(transmembrane) domain (Vo) that serves as a proton channel and a
peripheral domain (V1) that contains the ATP-binding site and the ATPase
19. ABC Transporters
Largest family of membrane transport proteins 78 genes (5% of genome)
encode ABC transporters in E coli
Many more in animal cells Known as the ABC transporter superfamily
They use the energy derived from ATP hydrolysis to transport a variety
small molecules including:
Amino acids, sugars, inorganic ions, peptides.
ABC transporters also catalyze the flipping of lipids between monolayers
All ABC transporters each contain 2 highly conserved ATP- binding
20. Structure of ABC Transporter
2 T ( transmembrane ) domain,
each has 6 α- helix form pathways
for transported substance
2A ( ATP- binding domain) 30-
40% homology for membranes
21. The channels
The channels form membrane-spanning pores that allow molecules to diffuse down
the electrochemical gradient into or out of the cell.
Some channels are gated.
They are opened or closed by binding of a ligand or by altered membrane potential.
The Acetylcholine Receptor Is a Ligand-Gated
Another very well-studied ion channel is the nicotinic acetylcholine receptor,
essential in the passage of an electrical signal from a motor neuron to a muscle fiber
at the neuromuscular junction (signaling the muscle to contract).
Nicotinic receptors were originally distinguished from muscarinic receptors by the
sensitivity of the former to nicotine, the latter to the mushroom alkaloid muscarine.
Acetylcholine released by the motor neuron diffuses a few micrometers to the
plasma membrane of a myocyte, where it binds to the acetylcholine receptor.
This forces a conformational change in the receptor, causing its ion channel to open.
The resulting inward movement of positive charges depolarizes the plasma
membrane, triggering contraction.
The acetylcholine receptor allows Na+
, and K+
to pass through with equal ease,
but other cations and all anions are unable to pass.
23. Movement of Na through an acetylcholine receptor ion channel is unsaturable (its rate
is linear with respect to extracellular [Na]) and very fast about 2 *107
This receptor channel is typical of many other ion channels that produce or
respond to electrical signals: it has a “gate” that opens in response to stimulation by
a signal molecule (in this case acetylcholine) and an intrinsic timing mechanism that
closes the gate after a split second.
Thus the acetylcholine signal is transient an essential feature of electrical signal
24. Na+/K+ ATPase maintain the intracellular Na+ and
K+ concentration in animal cell
ATP-powered ion pumps generate and maintain ionic gradients
across cellular membranes.
Na+ transport out
K+ transport in
By Na+/K+ ATPase
Four major domains:
M - Membrane- bound domain, which is composed of 10
25. N- Nucleotide-Nucleotide binding domain, where adenine moiety of ATP
and ADP binds
P – Phosphatase domain, which contains invariant Asp residue, which
became phosphorylated during the ATP hydrolysis
A domain – essential for conformational transitions between E1 and E2
26. In virtually every animal cell type, the concentration of Na is lower in the cell than in
the surrounding medium, and the concentration of K is higher.
This imbalance is maintained by a primary active transport system in the
The enzyme Na+K+ ATPase, discovered by Jens Skou in 1957, couples breakdown of
ATP to the simultaneous movement of both Na and K against their electrochemical
For each molecule of ATP converted to ADP and Pi, the transporter moves two K ions
inward and three Na ions outward across the plasma membrane.
The Na+K +ATPase is an integral protein with two subunits (Mr ~50,000 and ~110,000),
both of which span the membrane.
27. Ion Channel (non-gate)
Nongated ion channels and the resting membrane potential
Gated need ligand to activation. Non-gated do not need ligand.
Generation of electrochemical gradient across plasma membrane
i.e. Ca+ gradient.
regulation of signal transduction , muscle contraction and triggers
secretion of digestive enzyme in to exocrine pancreastic cells.
i.e. Na+ gradient
uptake of a.a , symport, antiport; formed membrane potential
i.e. K+ gradient formed membrane potential.
Voltage-gated proton channels open with depolarization, but in a strongly pH-
sensitive . The result is that these channels open only when the
electrochemical gradient is outward, such that their opening will only allow
protons to leave cells.
i.e. Voltage -gated proton channels
28. Ion channels are selective pores in the membrane
Ion channels have ion selectivity - they only allow passage of specific
Ion channels are not open continuously, conformational changes open
29. Ion-Selective Channels Allow Rapid
Movement of Ions across Membranes
Ion-selective channels—first recognized in neurons and now known to be present in
the plasma membranes of all cells, as well as in the intracellular membranes of
eukaryotes—provide another mechanism for moving inorganic ions across
Ion channels, together with ion pumps such as the Na+K+ ATPase, determine a plasma
membrane’s permeability to specific ions and regulate the cytosolic concentration of
ions and the membrane potential.
In neurons, very rapid changes in the activity of ion channels cause the changes in
membrane potential (the action potentials) that carry signals
from one end of a neuron to the other.
In myocytes, rapid opening of Ca2+
channels in the sarcoplasmic reticulum releases the
that triggers muscle contraction.
31. The “activity” of an ion channel is estimated by measuring the flow of ions
through it, using the patch-clamp technique.
A finely drawn-out pipette (micropipette) is pressed against the cell surface, and
negative pressure in the pipette forms a pressure seal between pipette and
As the pipette is pulled away from the cell, it pulls off a tiny patch of membrane (which
may contain one or a few ion channels).
After placing the pipette and attached patch in an aqueous solution, the researcher can
measure channel activity as the electric current that flows between the contents of the
pipette and the aqueous solution.
In practice, a circuit is set up that “clamps” the transmembrane potential at a given
value and measures the current that must flow to maintain this voltage.
With highly sensitive current detectors, researchers can measure the current flowing
through a single ion channel, typically a few picoamperes.
32. The trace showing the current as a function of time (in milliseconds) reveals how fast
the channel opens and closes, how frequently it opens, and how long it stays open.
Clamping the Vm at different values permits determination of the effect
of membrane potential on these parameters of channel function.
34. In voltage gated ion channels, a change in transmembrane electrical
potential (Vm) causes a charged protein domain to move relative to the
membrane, opening or closing the ion channel.
Both types of gating can be very fast.
A channel typically opens in a fraction of a millisecond and may remain
open for only milliseconds, making these molecular devices effective
for very fast signal transmission in the nervous system.
A cotransporter is an integral membrane protein that is involved in secondary active
It works by binding to two molecules or ions at a time and using the gradient of
one solute's concentration to force the other molecule or ion against its gradient.
It is sometimes equated with symporter, but the term "cotransporter" refers both to
symporters and antiporters .
The word "symporter" is a conjunction of the Greek syn- or sym- for "together, with"
In order for any protein to do work, it must harness energy from some source. In
particular, symporters do not require the splitting of ATP because they derive the
necessary energy for the movement of one molecule from the movement of the
Overall, the movement of the two molecules still acts to increase entropy.
Proton-sucrose cotransporters are common in plant cell membranes.
A uniporter is an integral membrane protein that is involved in facilitated diffusion.
They can be either a channel or a carrier protein.
Uniporter carrier proteins work by binding to one molecule of solute at a time and
transporting it with the solute gradient.
Uniporter channels open in response to a stimulus and allow the free flow of specific
Uniporters may not utilize energy other than the solute gradient.
Thus they may only transport molecules with the solute gradient, and not against it.
There are several ways in which the opening of uniporter channels may be regulated:
1.Voltage - Regulated by the difference in voltage across the membrane
2.Stress - Regulated by physical pressure on the transporter.
3.Ligand - Regulated by the binding of a ligand to either the intracellular or extracellular
side of the cell
37. Uniporters are involved in many biological processes, including impulse
transmission in neurons.
Voltage-gated sodium channels are involved in the propagation of a nerve
impulse across the neuron.
During transmission of the signal from one neuron to the next, calcium is transported
into the presynaptic neuron by voltage-gated calcium channels.
Calcium released from the presynaptic neuron binds to a ligand-gated calcium channel
in the postsynaptic neuron to stimulate an impulse in that neuron.
Potassium leak channels, also regulated by voltage, then help to restore the
resting membrane potential after impulse transmission.
In the ear, sound waves cause the stress-regulated channels in the ear to open,
an impulse to the vestibulocochlear nerve,
An antiporter is an integral membrane protein involved in secondary active
transport of two or more different molecules or ions i.e., solutes across a phospholipid
membrane such as the plasma membrane in opposite directions.
In secondary active transport, one species of solute moves along its electrochemical
gradient, allowing a different species to move against its own electrochemical
This movement is in contrast to primary active transport, in which all solutes are
moved against their concentration gradients, fueled by ATP.
Transport may involve one or more of each type of solute.
Example, the Na+
exchanger, used by many cells to remove cytoplasmic calcium,
exchanges one calcium ion for three sodium ions.
A symporter is an integral membrane protein that is involved in movement of two or
more different molecules or ions across a phospholipid membrane such as the plasma
membrane in the same direction, and is therefore a type of cotransporter.
Typically, the ion(s) will move down the electrochemical gradient, allowing the other
molecule(s) to move against the concentration gradient.
The movement of the ion(s) across the membrane is facilitated diffusion, and is coupled
with the active transport of the molecule(s).
Although two or more types of molecule are transported, there may be several molecules
transported of each type.
Examples:- In the roots of plants, after pumping out H+, they use H+/K+ symporters to
create a chemiosmotic potential inside the cell. This allows the root hairs to take up
water, which moves by osmosis into the xylem so that way the root hair may stay in
a hypotonic environment.
A family of integral proteins discovered by Peter Agre, the aquaporins
(AQPs), provide channels for rapid movement of water molecules
across all plasma membranes Ten aquaporins are known in humans,
each with its specialized role.
Erythrocytes, which swell or shrink rapidly in response to abrupt changes in
extracellular osmolarity as blood travels through the renal medulla, have a
density of aquaporin in their plasma membranes (2 ⨉ 105
copies of AQP-1 per
In the nephron (the functional unit of the kidney), the plasma
membranes of proximal renal tubule cells have five different aquaporin
42. Need of Membrane Transporters
The lipid bilayer is an effective barrier to the movement of small hydrophilic
Two factors govern the rate at which molecules can diffuse across the lipid
bilayer. These are:
(1)the membrane solubility of the specific molecules in question and the size of
the molecule that diffuses across the cell membrane.
(2) These cells reabsorb water during urine formation, a process for which
water movement across membranes is essential.
The plant Arabidopsis thaliana has 38 genes that encode various types of
aquaporins, reflecting the critical roles of water movement in plant
Changes in turgor pressure, for example, require rapid movement of water
across a membrane.