membrane transport- madhu

8/6/2019 Membrane Transport- Madhu

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Thus a complete cellmembrane is formed.Now let us see about theMembrane transport.

A SMALL ANIMATION

ON MEMBRANETRANSPORTt

Active transport is the energy ²

demanding transfer of a substance across acell membrane against its concentrationgradient,i.e., from lower concentration tohigher concentration.

Special proteins within the cell membraneact as specific proteins ¶carriers·.

The energy for active transport comesfrom ATP generated by respiration(inmitochondria).

Re-absorption of glucose, aminoacids and salts by the proximalconvoluted tubule of the nephron in

the kidney.

Sodium/potassium pump in cell

membranes(especially nerve cells)

The transport occurs against theelectrochemical gradient of thesubstance transported .

Energy utilized for the active transport isderived from the breakdown of ATP.Therefore, mechanisms that prevent thesupply of ATP hinder the active transportprocess is susceptible to metabolic

process.

Like any carrier mediated transport, thishas also a saturation point for limitation inthe rate of availability of carriers and thesupply of energy.

The energy is derived from ATP.The ion pumps hydrolyze ATP to ADP anduse energy in the third phosphate bondfor the transport process. As the ion

pumps hydrolyze ATP, these are alsocalled ATPases.

It is present in all eukaryoticcells.

This pumps K+ into the cell andNa+ out of the cell against their concentration gradient.

It is a heterodimeric proteinmade of two subunits:

subunit- MW 100,000subunit- MW55,000

(unknown function)

The subunit hasATPase activity.

It has the bindingsites for 3Na+, ATPmolecule andphosphate .

It has binding sites for 2K+ & ouabain .

The Na+-K+ ATPase pumps three Na+

ions out of the cell and two K+ into thecell.A carrier protein is phosphorylated

during the process. So it is called a P-type ATPase.

1. Cytosolic ion concentration2. Cell Volume3. Protein Synthesis4. Resting membrane potential5. Hormone actions

3 Na+ ions and 1 ATPmolecule bind to their respective sites on asubunit.

ATPase enzyme isactivated.

This catalysis thehydrolysis of ATP to ADP& liberate a high energyphosphatebond( Phosphorylation ).

Conformational change in the carrier protein occurs.

3 Na+ ions are transfered from inside to outside of the cell.

2K+ ions bind to the alpha subunit, and leads hydrolysis ofphosphate bond( Dephosphorylation ).

Reconformational change in the carrier protein occurs.2K+ ions are transferred from inside to outside of the cell.

CYCLESO DIUMPUMP

3NA+ ions arepumped out

2K+ ions arepumped in

1 ATP ishydrolyzed

Lets watch a small

animated video on sodiumpump

Ca++ pump is another example of Primary activetransport.It is present in all cellmembranes, membrane ofendoplasmic reticulum,and in sarcoplasmicreticulum in muscle cell.This is also a type of P-typeATPase.This pump actively

transports calcium out ofthe cell, and therefore,maintains a higher concentration of calciumin the ECF .

The pump present on the membrane ofsarcoplasmic reticulum in muscle cell andendoplasmic reticulum in other cellstransports calcium out of the cytoplasm into

these organelles, and therefore maintains alow Cytosolic concentration of calcium.

It helps storage of calcium in theseorganelles for ready availability of it at thetime of need like muscle contraction .

The H+ pump or protonpump is located on themembrane of lysosomeand endoplasmic

reticulum.It is a V-type ATPase,named for its discoveryin the vacuoles andvesicles of the cells.

This pumps H+ fromcytosol into theseorganelles. Thus theseorganelles becomemore acidic.

Proton pump is alsolocated in the inner mitochondrialmembrane. It is F-typeATPase in thisorganelle.Its main function is tosynthesize ATP inmitochondria byutilizing energy storedin the proton gradientcreated by respiratorychain.

There is no direct coupling of ATP.Electrochemical potential difference

created by pumping ions out of the cellis used.

Counter TransportCo-Transport

Uses the downhill movement of one solute species fromhigh to low concentration to move another moleculeuphill from low to high concentration.Eg: Glucose Symporter

Two species of ion or other solutes are pumped inopposite directions across a membrane.One of these species is allowed to flow from highconcentration to low concentration which yields the entropy energy to drive the transport of theother solute from a low to high concentration.Eg: Sodium- Calcium exchanger

Two common features of passive transport:-

(a)Transport is downhill(b)Transport does not require utilization of ATP

Simple Diffusion

Facilitated DiffusionOsmosisFiltration

Bulk FlowSolvent Drag

It is a carrier mediated diffusion as theprocess of diffusion is facilitated by acarrier protein in the membrane.

Features of Facilitated Diffusion:F aster rate of transportSaturation KineticsCompetitive inhibitionSpecificity

HERE IS AN AN IMATIO N O NFAC ILITATED D IFFUSIO N«

Selectively permeableTubular structures that connect the exterior with the interior of cells

Permeability depends on molecular size, shapeand charge.

Integral Proteins

Formed by severalpolypeptide units

An aqueous pore ispresent around whichthe polypeptide subunitsare arranged

Ions pass through theaqueous pore from oneside of the membrane tothe other.

A polypeptide subunit forms a gate at one endof the channel that opens in response to specificstimulus.

There is a specific filter that permits specific ionsto pass through the channel .

Sodium Channels

Aqueous pore-

0.5nmInner surface ofpore- (-ve)chargedSelective for passage of Na+

B locked byTetrodotoxin

PotassiumChannels

Aqueous pore-

nmB locked bytetrethylammonium or 4 aminopyridine

Calcium andChloride Channels

These allow the

passage ofCalcium ions andChlorine ions

Aquaporins form tetramers in the cellmembrane, and facilitate the transportof water and, in some cases, other small solutes, as glycerol, across themembrane.

The water pores are completelyimpermeable to charged species, suchas protons.

The aquaporins are predicted to havesix membrane-spanning segments.

Aquaporins exist in the plasmamembrane as homotetramers.

Each aquaporin monomer containstwo hemi-pores, which fold together toform a water channel

Uniport:-It is a carrier protein

It transports only one substanceEg. Na+ channels transport Na+Symport:-

It transports two or more substances from one sideof the membrane to the other in thesame direction.Eg. Transport of Na+ and glucose from the lumenon intestine into the epithelial cellsThese substances are said to beco-transported.

It transport substances inopposite directions(inwardand outward).

These substances are said to becounter-transported.

Eg:1 Sodium ² Potassium pump

Eg:2 Na+-H+ exchanger in the proximal tubular cells

CH EMICA L C O MP O UNDS ( M O B ILE IO N CARR IERS)

CHANN EL FO RMERS

Antibiotic Properties

Increases permeability of biological membranes

Growth enhancing feed additives

REFERENCE:Medical Physiology - Dr.G.K.PalGoogle ImagesAnimations from YOU TUBE

membrane transport- madhu

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