Transportation Of Molecule In Different membranes

Submitted to Dr. J.J.DhruveAsst Research scientists,BACA,AAU,Anand. Submitted by

Joshi Prathmesh GovindM.Sc (Agri) Biochemistry,BACA,AAU, Anand.

Submission date:-

Welcome

Membranes:-Definition

Membranes separate the cell from the outside world and separate organelles

inside the cell to compartmentalize important processes and activities.

Introduction:- 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.

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.

Membrane Transport: Definition

Membrane transport is defined as the movement of molecules across

cell membranes.

The bilayer is permeable to:

Small hydrophobic molecules

Small uncharged polar molecules

The bilayer is impermeable to:

Ions

Large polar molecules

Therefore, need membrane proteins to transport most molecules and all ions across

biomembranes

Gases diffuse freely, no

proteins required.

Water diffuses fast enough that

proteins aren’t required for

transport.

Water diffuses fast enough that

proteins aren’t required for

transport.

Sugars diffuse very slowly so

proteins are involved in

transport.

Charged molecules are virtually

impermeable.

Charged molecules are virtually

impermeable.

Fig:The phospholipid bilayer is a barrier that controls the transport of molecules in and out of the cell.

Selective transport across the lipid membrane requires transport proteins.

Transport proteins are integral membrane proteins that move molecules

and

ions.

There are two classes of transport proteins: transporters (pumps) and

channels.

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.

3. Transporters

uniport

symport

antiport

Differences1. Transporters:

uniporters transport a single molecule down its gradient (passive).

co-transporters couple movement of a molecule down its gradient

with

moving a molecule up its gradient (active).

2. Pumps hydrolyze ATP to move small molecules/ions up a concentration gradient or electric potential (active).

3. Channels

transport water/ions/small molecules down their concentration

gradients or electric potentials (passive)

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

transport.

2. Active transport: metabolic energy is used to transport a solute

from the side of low concentration to the side of high concentration.

Types of Diffusion

Free Diffusion

A. Non-channel mediated

-lipids, gasses (O2, CO2), water

B. Channel mediated

-ions, charged molecules

Facilitated diffusion molecule moves down its electrochemical gradient.

-glucose, amino acids

ATP powered pump

1. P- class

2 , 2 subunit; can phosphorylationα β

i.e. Na+-K+ ATP ase, Ca+ATP ase, H+pump

2. F-class

locate on bacterial membrane , chloroplast and mitochondria

pump proton from exoplasmic space to cytosolic for ATP synthesis

3. V-class

maintain low pH in plant vacuole

similar to F-class 4. ABC (ATP-binding cassete) superfamily several hundred different transport protein

Transport process requires ATP hydrolysis in which the free energy is liberated by breakdown of ATP into ADP and phosphate.

Must phosphorylation

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

synthesis.

V-class H+ ATP ase pump protons across lysosomal and vacuolar membrane

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

activity.

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

of

small molecules including:

Amino acids, sugars, inorganic ions, peptides.

ABC transporters also catalyze the flipping of lipids between monolayers

in membranes.

All ABC transporters each contain 2 highly conserved ATP- binding

domains.

Structure of ABC Transporter

2 T ( transmembrane ) domain,

each has 6 - helix form pathways α

for transported substance

2A ( ATP- binding domain) 30-

40% homology for membranes

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.

Example:-The Acetylcholine Receptor Is a Ligand-Gated

Ion Channel

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+, Ca2+, and K+ to pass through with equal ease,

but other cations and all anions are unable to pass.

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 ions/s under

physiological conditions.

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

conduction.

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

Na+/K+ ATPase:-

Four major domains:

M - Membrane- bound domain, which is composed of 10

transmembrane segments

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

states

Na+K+ ATPase

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

plasma membrane.

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

gradients.

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.

Ion Channel (non-gate)Nongated ion channels and the resting membrane potentialGated 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

Ion channels are selective pores in the membrane Ion channels have ion selectivity - they only allow passage of specific

molecules

Ion channels are not open continuously, conformational changes open

and close

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

membranes.

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

Ca2+ that triggers muscle contraction.

Electrical measurements of ion-channel function.

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

membrane.

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.

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.

Voltage-gated K+ channels have four subunits each containing six

transmembrane helicesα

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.

Transporters

Cotransporter:-

A cotransporter is an integral membrane protein that is involved in secondary active

transport.

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"

and -porter.

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

another.

Overall, the movement of the two molecules still acts to increase entropy.

Proton-sucrose cotransporters are common in plant cell membranes.

Uniporter 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

molecules.

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

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,

sending

an impulse to the vestibulocochlear nerve,

Antiporter

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

gradient.

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+/Ca2+ exchanger, used by many cells to remove cytoplasmic calcium,

exchanges one calcium ion for three sodium ions.

Symporter

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.

Fig:- Transporter

Aquaporins

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

high

density of aquaporin in their plasma membranes (2 10⨉ 5 copies of AQP-1 per

cell).

In the nephron (the functional unit of the kidney), the plasma

membranes of proximal renal tubule cells have five different aquaporin

types.

Need of Membrane Transporters The lipid bilayer is an effective barrier to the movement of small hydrophilic

molecules.

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

physiology.

Changes in turgor pressure, for example, require rapid movement of water

across a membrane.

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