Membrane transport mechanism is the movements of solutes into the cell are mediated and regulated through the action of specific transport proteins that are present on the cell membrane. The proteins are therefore required for movements of ions, such as Na⁺, K⁺, Ca²⁺, and Cl^–, as well as metabolites such as pyruvate, amino acids, sugars, and nucleotides, and even water. Transport proteins are also responsible for biological electrochemical phenomena such as neurotransmission.

Permeability of molecules across phospholipid bilayer

The lipid bilayer of the membranes is permeable to only hydrophobic molecules and small uncharged moelcules such as CO₂, N₂, O₂, NO, Urea, ethanol, anaesthetics and to some extent water by simple diffusion.

• The lipid bilayer is impermeable to most ions, polar molecules such as Na⁺, K⁺, Ca²⁺, Cl^-1, HPO₄^2–, HCO₃-, Glucose, other sugars, amino acids, ADP₃^–, ATP₄^– and other organic molecules of biological importance.
• These ions and hydrophilic organic molecules are required by all living cells to carry out cellular activities and meet their metabolic requirements.
• Conclusively it can be said that lipid bilayers are highly impermeable to charged molecules (ions) by considering its size also because the charge and high degree of hydration of such molecules prevents them from entering the hydrocarbon phase of the bilayer.
• Thus, these bilayers are 10⁹ times more permeable to water than to even such small ions as Na⁺ or K⁺.

Table of Contents

• Permeability of molecules across phospholipid bilayer
• Thermodynamics of Diffusion and Transport
  • Thermodynamics of Diffusion
  • Thermodynamics of Transport
• Types of membrane transport
  • Non-mediated transport
    • Simple Diffusion
  • Mediated transport
    • Passive-mediated transport
      • Facilitated diffusion
• Membrane Transport by Proteins
  • Channel proteins
    • Ion Channels (specifically inorganic ion)
      • Voltage gated
      • Ligand gated
      • Signal or secondary messenger gated channels
      • Mechanosensitive channels
      • Naturally occurring toxins often act ion channels
    • Porins
    • Aquaporins
  • Carrier proteins (Transporter or Permeases)
    • Passive
      • Glucose Transporters
      • Electrogenic transporter (Charge separation)
      • Electroneutral transporter
    • Active Transport
      • Direct Active Transporter / Primary Active Transporter
        • P Type ATPase  (Phosphorylation)
        • V Type ATPase (Vacuole)
        • F Type ATPase (Factor)
        • ABC Cassette ATPase
      • Indirect Active Transporter / Secondary Active Transporter
        • Uniporter
        • Antiporter
        • Symporter

Thermodynamics of Diffusion and Transport

Before moving to types of membrane transport lets understand some basics of thermodynamics of transport.

Thermodynamics of Diffusion

• The process of the net movement of solutes from a region of high concentration to a region of low concentration is known as diffusion.
• The differences of concentration between the two regions are termed as concentration gradient and the diffusion continues till the gradient has been vanished.
• Diffusion occurs down the concentration gradient.
• Simple diffusion depend random thermal motion of solute and is an exergonic process driven by increase entropy.
• ∆G = ∆H-T∆S (∆G = (-)  Spontaneous,   ∆S = (+) ve , ∆H (–) ve, hence the reaction is favorable & Spontaneous )

Transport of molecules across planar lipid bilayer sheets or membrane separating two compartments, A₁ and A₂ is studied by adding radioactively labelled compounds in one compartment, say A₁, and determining the radioactivity that appears in the second compartment as a function of time. 

If the concentration of the solute / compound in compartment A₁ is C₁ and its concentration in compartment A₂ is C₂, then according to Fick’s law, the movement of solute ‘C’ across a barrier is dependent upon the concentration gradient of the solute in the two compartments. If [C₂] > [C₁] the movement is from A₂ to A₁ till the gradient is abolished and an equilibrium is reached. The net rate of transport, J (in moles per square centimeter per second, moles/cm²/s) is given by:

J = – D (∆C/ ∆X)

J= Flux / Unit area, D diffusion coefficient (cm²/second) ∆C=  Difference in concentration ∆X distance (membrane thickness in case of membrane transport). The negative sign is for diffusion towards the lower concentration.

Fick’s first law states that

• The flux is proportional to the negative concentration gradient.
• D, diffusion coefficient is the proportionality constant.
• Minus sign comes from the fact that matter flows down the concentration gradient.

Or, can be expressed as

J= KD₁ / l x {[C₂]-[C₁]}    (i)

where, l is the thickness of the membranes,

D₁ is the diffusion coefficient of the diffusing molecule, C.

K is the partition coefficient for the diffusing molecule between lipid and water (the ratio of solubilities of the molecule in lipid and water).

• For ions and other hydrophilic molecules, K is a very small number, so that these molecules diffuse across the membrane at an extremely slow rate or not at all.
• If C₁ and C₂ are expressed in mole/cm³ and l in cm, then D₁ has the unit cm²/s (D₁ is not the same as D (Diffusion coefficient) of the molecule in an aqueous solution). D₁ depends on the shape and size of the molecule as well as the viscosity of the membrane lipid.
• Since K, D₁ and the exact thickness of the membrane are not known; the rate of passive diffusion is given by Permeability coefficient, P. This can be measured experimentally:

J=P {[C₂]-[C₁]}                                           (ii),

where, P = KD₁/l with units of cm²/s

Thermodynamics of Transport

According to the thermodynamic principles, the transfer of a 1 mole of a substance from compartment one to the other separated by a membrane or a barrier is given by

ΔG=RTln(C₂/C₁) ……….(iii)

Where C₁ and C₂ are the concentrations of the free substance in Compartment 1 and Compartment 2 respectively and the substance is transferred from Compartment 1 to 2. If C₂ is less than C₁, ΔG is negative and the process is thermodynamically favorable. The substance can be transferred till the concentration in the two compartments is the same, i.e. if C₂ = C_1, ΔG equals to zero and the system is in equilibrium, the rate of transport in the two directions is the same and no net transport occurs. Thus, movement of a solute can occur spontaneously down a concentration gradient.

For a charged molecule with a charge Z, the free energy of transport across a membrane involves besides the concentration term given in (iii), an additional contribution due to the diffusion of the ion across a potential difference:

ΔG=RT ln (C₂/C₁) + ZFΔΨ……..(iv)

Where F is Faraday and is equal to 96.5 kJ mol^-1.Vol^-1 and ΔΨ is difference in membrane potential in volts in the two compartments, Z is the charge on the molecule. However, if C₂>C₁, then an energy input is required to transport the molecules uphill or against the concentration gradient, as this process will not occur spontaneously. This is then referred to as Active Transport and the general equation now becomes

ΔG= ΔG⁰ + RTln ([C₂]/[C₁])

If there is ten fold difference in concentration between two compartments, the cost of moving 1 mol of an uncharged solute at 25⁰C across a membrane separating the two compartments is therefore :

ΔG= (8.315 J/mol) (298)(ln 10/1)= 5.7 kJ/mol

If the molecule is charged, then the cost of moving it will be

ΔG= RTln ([C₂]/[C₁]) + ZFΔΨ,

an additional contribution due to the moving of the ions across a potential difference.

Types of membrane transport

Living cells have, thus, evolved systems that enable them to transport ions and molecules of interest into and out of the cell and intracellular organelles.

Two major types of transport processes occur across the membrane

Non-mediated transport

Spontaneous process, the transport of substance through a medium depend on it chemical potential gradient (high concentration to low concentration in time eradicating concentration difference between two sections).

Simple Diffusion

For Simple diffusion must have two conditions:-

1. Substances must be present high concentration on one side
2. Membrane must be permeable.
   • Rate of penetration of compound through membrane is its size. If molecule small, more rapidly penetration.
   • Diffusion directly proportional to the hydrophobicity of molecule.

Mediated transport

• Mediated transport requires specific proteins.
• Thus, the substance diffuses in the direction that eliminates its concentration gradient; at a rate proportional to the magnitude of this gradient and also depends on its solubility in the membrane’s non-polar core.
• Mediated transport is classified into two categories depending on the thermodynamics of the system.

Passive-mediated transport

• In this type of process a specific molecule flows from high concentration to low concentration.
• Substances that are too large or polar diffuse across the lipid bilayer on their own through membrane proteins called carriers, permeases, channels and transporters.
• Unlike active transport, this process does not involve chemical energy.
• So the passive mediated transport is totally dependent upon the permeability nature of cell membrane, which in turn, is function of organization and characteristics of membrane lipids and proteins.

Facilitated diffusion

Membrane transport via facilitated diffusion of biological membranes have certain common features:-

• The membrane transport of the solute / molecule can be in either direction depending on the concentration gradient of the solute, from a higher to a lower concentration, i.e. there is a down hill movement.
• Selective passive membrane transport occur along the concentration gradient entropy increases.
• Process is exergonic because solute diffuses down the concentration or electrochemical gradient.
• Process mediated by transport protein (channels & carriers).
• Permit polar and charged molecule such as, nucleosides, amino acid, carbohydrate and ion across membrane.
• The membrane transport is highly specific for a particular molecule as well as it also shows stereospecificity i.e. can differentiate between D– and L– Sugars or between L– and D– aminoacids etc.
• The membrane transport shows saturation kinetics i.e. the rate of transport reaches a maximum as the concentration of the solute is increased till a maximum transport rate is reached. This distinguishes biological transport from simple diffusion or unfacilitated transport where the rate is directly proportional to concentration gradient.

• The kinetics of facilitated passive transport are similar to that of enzyme kinetics exhibiting maximum transport rate viz. Vmax for enzymes and a high affinity binding site for the solute transported across biological membranes.

• The membrane transport is inhibited by known protein reagents that react with specific groups of proteins as in the case of enzymes e.g. p–chloromericuribenzoate or other molecules that react with –SH groups (cysteine residue); fluorodi nitrobenzene, FDNB, which reacts with –NH₂ group etc.

These characteristics what we have discussed so far showed the involvement of proteins with specific binding sites and that the transport is not by simple passive diffusion but is facilitated by proteins.

These proteins have been called as mediators, carriers, porters, transporters by different workers as they were studied and the transport system as Facilitated Passive Transport Figure below

Membrane Transport by Proteins

As represented in above flow chart membrane translocation by facilitated diffusion can be achieved by

• Channel proteins
• Carrier proteins
• Group Translocation

Channel proteins

• Hydrophilic channel are membrane proteins that allows polar solutes to pass through it.
• Channels show less stereospecificity than carrier & are usually non-saturable.
• Most of channel either open or close said to gated.
• The channel proteins can be of three types

1. Ion Channels
2. Porins
3. Aquaporins

Ion Channels (specifically inorganic ion)

• Ion channels are selective, therefore allow passage of only one kind of ion, so separate channels are needed for transporting such ions as Na⁺, K^+,  Ca²⁺, Cl^–, etc.
• This selectivity provide small differences in size and charge.
• Ion channels are bidirectional.
• Ion channels allow the passage of ions in either direction, depending on the electrochemical gradient.
• Do not usually stay open for long period. Most of ion channel are gated.
• Gated”, which means that they can be opened and close by conformational changes in the protein, thereby regulating the flow of ions through the channels.
• Ion channels may be following four types:-
  1. Voltage gated
  2. Ligand gated
  3. Signal or secondary messenger gated channels
  4. Mechano sensitive

Voltage gated

• Open and close in response to membrane potential changes.
• Examples
  • Na⁺–Channels and K⁺ Channels are present in brain and nervous tissues.
  • Ca²⁺ channels are present in muscle, heart and neuromuscular junction where they have a role in muscle contraction and release of acetylcholine from vesicles.

Ligand gated

• Ligand Gated channels are the neurotransmitter receptors.
• Ligand Gated channels open on binding of a specific ligand such as Acetylcholine, Gama-amino butyric acid, GABA.
• Ligand Gated channels are present in brain and nerve tissues and have an important role in conduction of nerve impulse and maintaining ion gradient in the nervous tissue;
• Acetylcholine receptor is a ligand gated cation channel whereas GABA receptor is an ligand gated anion channel.

Signal or secondary messenger gated channels

• Secondary messenger gated channels are sensitive to second messengers like c–GMP and c–AMP that open  ion channels.
• Secondary messenger gated channels are abundant in the sensing cells and have a role in visual and olfactory systems.
• Examples
  • These are found in Ca²⁺ channels that are IP₃ induced (present in sarcoplasmic reticulum).
  • Calcium release activated calcium (CRAC) channels present in Drosophila and mammals.

Mechanosensitive channels

• Mechanosensitive channels respond to mechanical forces that act on the membrane.
• For example pressure sensitive channel, stretch sensitive channel and heat sensitive channel.

Naturally occurring toxins often act ion channels

• Tetrodotoxin is a paralytic poison which occur mainly in ovaries, liver and intestine of the puffer fish, acts by specifically blocking the Na⁺ channel.
• Saxitoxin, which is a product of marine dinoflagellate and this also blocks Na⁺ channel. Both of these neurotoxins have a cationic guanidine group, and both are effective only when applied to the external surfaces of a neuron.
• The snake venom toxin, tubacurarines, bungarotoxin and cobratoxin, bind specifically and irreversibly to the acetylcholine receptors and are potent inhibitors and lethal poison.
• Batrachotoxin, steroidal alkaloid secreted by skin of Columbian arrow poison frog Phyllobates aurotaenia, is the most potent venom. This toxin also specifically bind to voltage gated sodium ion channel but it renders the axonemal membrane highly permeable to Na⁺.
• The venom of black mamba snake contains dendrotoxin, which interferes with voltage gated potassium ion channel.

Porins

• Porins are present in outer membranes of bacteria.
• Also, mitochondria and chloroplast outer membrane contains pores which are nonspecific and allow movement of certain small molecules to pass through.
• They are trimeric transmembrane proteins.
• Each identical subunit consists of 16–stranded anti–parallel β–barrel which forms a channel along its barrel axis through which solutes can pass.

Aquaporins

• Aquaporins also temed as major intrinsic membrane proteins (MIPs)are a family of integral membrane proteins which act as specific water channels.
• Homotetramer, every mononer having 6 membrane spanning α helical domain with amino and carboxy terminal oriented towards cytosol.
• Two hydrophobic loop contain highly conserve seq motif Asp-Pro-Ala (NPA).
• Fast water flow (3 billion mole/sec). More than 350 aquaporins are identified. 11 isoform AQP0- AQP11 designated in mammals.
• These water channels are bidirectional allowing influx or efflux of water molecule in response changing osmotic condition.
• AQP3, AQP7, AQP10, are glyceroaquaporins. Also allow entry of NH₃, H₂O₂ NO, Urea, Glycine.
• Aquaporins are prominent in cell such as in those of kidney tubules (AQP2). ADH regulate water retention by mobilizing AQP2 stored in vesicle membranes.
• When the AQP2 vesicle fuse PM, water permeability increases & more water reabsorbed from collecting duct and return to blood. When ADH drops, AQP2 is resequestered within vesicles, reducing water retention.

Carrier proteins (Transporter or Permeases)

• Carrier proteins non-covalently bind specific molecule to be transported one side of membrane thereby undergoes conformational changes that allow molecule to pass.
• Carrier mediated transport may be Passive or Active.

Passive

Transport of solute in either direction down the concentration gradient via protein.

Glucose Transporters

Transporting glucose in either direction down the concentration gradient. It thus helps to maintain glucose concentration (levels) in the blood.

• Examples
  • Movement of glucose by uniport such as GLUTs. All members of GLUT (GLUT family carrier, ̴ 500 amino acid 12 transmembrane) transport sugar, similar structure. 14 different type, ubiquitous and sodium and ATP independent.
  • GLUTs differ in their kinetic property like Km value, expression in different cell type and substrate specificity and regulation are imp for sugar metabolism in blood.
  • GLUT–1 is present in most tissues.
  • GLUT–2 mostly in pancreatic B cells.
  • GLUT–4 in muscle and fat cells which are responsive to insulin. (Number of GLUT–4 increases in the plasma membrane, when insulin binds to its receptor by exocytosis or movement of vesicles containing GLUT–4 from the cytoplasm to the plasma membrane on receiving the appropriate signals conveyed by insulin binding to insulin receptor.)

Electrogenic transporter (Charge separation)

A transport via symport and antiport in which charge is also move simultaneously is termed as electrogenic Na⁺/ K⁺ pump.

Electroneutral transporter

No net movement of charge H⁺/ K⁺ pump of animal gastric mucosa.

Anion Channel or Exchanger or Cl–HCO₃– Exchange Protein, is a dimeric protein which traverses the membrane 12 times.

Active Transport

• In this type of membrane transport a specific molecule is transported from low concentration to high concentration, that is, against its concentration gradient.
• Such an endergonic process must be coupled to a sufficiently exergonic process to make it favorable (ΔG <0).
• This type of transport is also carried out by transporters and is referred to as PUMPS.
• Active transport shows similarity to Passive Transport in
  1. Saturation kinetics.
  2. Specificity
  3. Stereospecificity
  4. Inhibition by specific protein reagents

• Active transport differs to Passive Transport in
  • In the movement of the solute across the biological membranes which is
    • Unidirectional and in a specific direction and
    • Against the concentration gradient or uphill.
  • Require an energy input i.e. are accompanied by hydrolysis of ATP or any other source of energy for the uphill movement of the solute and hence named as Pumps.
  • Are inhibited by cyanide and such reagents that inhibit energy production.
• There are two types of active transport
  • Direct Active Transporter / Primary Active Transporter
  • Indirect Active Transporter / Secondary Active Transporter

Direct Active Transporter / Primary Active Transporter

• Primary active transport, also called direct active transport, directly uses energy to transport molecules across a membrane.
• Generally, ATP dependent proton pumps are divided into 4 classes:
  • P Type ATPase  (Phosphorylation)
  • V Type ATPase (Vacuole)
  • F Type ATPase (Factor)
  • ABC Cassette ATPase

P Type ATPase (Phosphorylation)

• P Type ATPaseThese are multipass transmembrane proteins having two identical catalytic α-subunits that contain an ATP binding site.
• Some have two smaller β-subunits that usually have regulatory functions.
• During the transport process or pumping cycle at least one of the α-subunit must be phosphorylated and the H⁺ ions are thought to move through the phosphorylated subunit.
• This class includes many ion pumps that are responsible for setting up and maintaining gradients of Na⁺, K⁺, H⁺ and Ca²⁺ across the cell membrane.
  • H⁺/K⁺ ATPase
    • The common P-type pump is mostly found in parietal cells of the mammalian stomach which transport protons (H⁺ ions) out of cell and K+ ions into the cell and is mainly responsible for the acidification of the stomach contents.
    • The H⁺/K⁺ ATPase transports one H⁺ from the cytoplasm of the parietal cell in exchange for one K⁺ retrieved from the gastric lumen.
    • As an ion pump the H⁺/K⁺ ATPase is able to transport ions against a concentration gradient using energy derived from the hydrolysis of ATP.
    • Like all P-type ATPases, a phosphate group is transferred from ATP to the H⁺/K⁺ ATPase during the transport cycle.
  • Na⁺ – K⁺ ATPase or Sodium Pump of Plasma Membranes
    • It is (αβ) dimer, each consisting of α and β subunits.
    • The 110 KD α–submit contains the binding sites for the ions and the site phosphorylated by ATP.
    • The 55 KD β–subunit is glycoprotein whose function is not known
    • The α–subunit has around 8 α-helices forming the transmembrane domain and two large cytoplasmic domains.
    • An aspartyl group in α-subunit is phosphorylated by ATP to form a reactive aspartyl phosphate intermediate which has been isolated by reduction with borohydride to homoserine.
    • The β-subunit has a large extra cellular domain with carbohydrate moieties on the exterior side and a single Transmembrane helix.
    • Its known inhibitors cardiotonic steroids and ouabain (an arrow poison used in East Africa) binds on the external side, so also the K⁺, while the Na⁺ and ATP binding sits are on the cytoplasmic side.
    • It is called sodium pump as it pumps Na⁺ out of and K⁺ into the cell against a concentration gradient.
    • It is, thus, an electrogenic antiport as 3Na⁺ are pumped out and 2K⁺ pumped in.
    • The molecular model given for this pump involves two conformational states E₁ and E₂ where,
      1. E₁ faces cytoplasmic side and has a high affinity Na⁺ binding site.
      2. Na+ binding to E₁– [E₁ 3Na⁺] promotes phosphorylation of aspartyl residue by ATP [E₁ – P. 3Na⁺]
      3. Phosphorphorylation of E₁ results in a change of conformation to [E₂ – P. 3Na⁺].
      4. E₂ site faces outside and has a low afinity for Na+ which is released to form [E₂–P] where the site has a high affinity for K⁺.
      5. Binding of K⁺ forms [E₂ – P. 2K⁺] which results in dephosphorylation of and release of Pi into the cytoplasm and reversal of the conformation [E₁ – 2K⁺].
      6. E₁ having a low affinity for K⁺, releases K⁺ in the cytoplasm.
  • Ca²⁺ – ATPase or Calcium Pumps
    • Calcium Pumps are present in the plasma membrane and Endoplasmic reticulum and sacroplasmic reticulum of the muscle cells.
    • Calcium pumps regulates the Ca²⁺concentration in the cells which is crucial for a number of cellular responses and activity e.g. muscle contraction, release of neurotransmitters, and as a second messenger.

V Type ATPase (Vacuole)

• V type  structurally & functionally differ from P type.
• Transport of H⁺ against concentration gradient coupled with ATP hydrolysis.
• Generally found on vacuolar membrane (V type due to vacuole) or organelles as vesicles, vacuole, lysosome (maintain pH 3-6) endosome, golgi complex thus responsible for acidification.
• Insensitive to vandate but inhibited by bafilomycin.
• Priloses (heart burning drug) inhibited V type ATPases (cause acidity).
• These V-class proton pumps contain two domains: a cytosolic hydrophilic domain (V1) and a transmembrane domain (V₀) with multiple subunits in each domain. Binding and hydrolysis of ATP by the B subunits in V₁ provide the energy for pumping of H+ ions through the proton-conducting channel formed by the c and a subunits in V0. These V-class proton pumps are not phosphorylated and dephosphorylated during proton transport.

F Type ATPase (Factor)

• Found in bacteria mitochondria & chloroplast, Conserve energy of substrate oxidation as ATP.
• They generally behave as reverse proton pump by synthesizing ATP from ADP and Pi by movement of protons from the exoplasmic to the cytosolic face of the membrane down the proton electrochemical gradient.
• Therefore, these pumps are also known as ATP synthases or F₀F₁ complex.
• Similar to V type both transport H⁺ (not phosphrylate in catalysis).
• F class ion pumps contain different transmembrane and cytosolic subunits.
• They are known for only transport of protons, in a process that does not involve phosphoprotein intermediate.

ABC Cassette ATPase

• ATP-powered pumps is a large family of multiple membranes.
• Found in all organisms ranging from bacteria to mammals.
• Mostly found in PM but also on ER, mitochondria and lysosome.
• It has two Transmembrane Domains 2 cytosolic NBDs (ATP binding cassette).
• Transporter translocates a  variety of substances including ions, heavy metals sugar, drugs, amino acid, peptide & protein.
• Mostly are pumps but some are act as ion channels (use ATP).
• Some ABC transporter pump antibiotic & other drugs out of the cell, thereby making the cell resistant to the drug eg multidrug resistance transport protein (MDR) in human, uses energy of ATP hydrolysis to pump hydrophobic drugs (MDR in tumor cell uses ATP to export drug) out of cells (effectiveness as therapeutic agents).

• Most ABC transporters act as pumps but some members act as ionchannels that open and close by ATP hydrolysis e.g. CFTR / cystic Fibrosis Transmembrane Conductor Regulator) is a Cl^– channel.
  • Defect in CFTR protein responsible for Cystic fibrosis
  • Cystic fibrosis (Channel damaged) affects intestine, pancreases sweat gland, lungs, reproductive tract. CFTR channel necessary for producing thin freely flowing mucus. Mucus lubricate & protect lining of the airways and regulate function of Na⁺ channel.
  • Cystic fibrosis transmembrane conductance regulator (CFTR) is an ABC transporter (glycoprotein family, 170 KDa MDR).
  • The  CFTR channel requires both ATP hydrolysis and cAMP dependent phosphorylation.
  • It has 2 TMDs 2 cytosolic NBDs (ATP binding cassette). 
  • Phosphorylation of R domain by the cAMP dependent PKA & ATP binding with NBDs  (ATP binding cassette) is requirement  for channel opening. ATP hydrolysis is need for channel closing.
  • Defect in channel epithelial cell as mucous become thickened sticky that is very hard to propel out airways. Affected individual typically suffer from chronic lung infection inflammation.

Indirect Active Transporter / Secondary Active Transporter

• Secondary active transport or co-transport, also uses energy to transport molecules across a membrane. However, in contrast to primary active transport, there is no direct coupling of ATP; instead, the electrochemical potential difference created by pumping ions out of the cell is instrumental.
• The three main forms of active transport are
  • Uniporter
  • Antiporter
  • Symporter

Uniporter

• Type of passive transport
• Transport one molecule Amino acid, Nucleosides, Sugars
• Movement of single solute –> rate determined by their Vmax and Km

Antiporter

• In antiport two species of ion or solutes are pumped in opposite directions across a membrane.
• One of these species is allowed to flow from high to low concentration which yields the entropic energy to drive the transport of the other solute from a low concentration region to a high one.
• Example: the sodium-calcium exchanger or antiporter, which allows three sodium ions into the cell to transport one calcium out.

Symporter

• Symport uses the downhill movement of one solute species from high to low concentration to move another molecule uphill from low concentration to high concentration (against its electrochemical gradient).

• On the basis of movement of ions, cotransporters can also be categorized into:
  • Cation cotransporter: Example of cation transporter is Na⁺/H⁺antiporter, which exports H⁺ from cells coupled to the energetically favorable import of Na⁺.
  • Anion cotransporter: Example of anion transporter is exchange of Cl- and HCO₃– across the plasma membrane.
• Example

Na+–Glucose Symport (SGLT1)

Glucose symporter SGLT1, which co-transports one glucose (or galactose) molecule into the cell for every two sodium ions it imports into the cell. Na⁺ glucose and Na⁺ amino acids cotransport system are present in the specialized plasma membranes of intestinal and kidney epithelial cells. The extracellular high concentration of Na⁺ drives the symport of glucose and amino acids by specific symporters in the apical region of the cells. The Na⁺ concentration is maintained by the pumping of Na⁺ into blood by Na⁺–K⁺ ATPase present in the basal region of these cells.

Phlorizin inhibits Na⁺–glucose symporters whereas cytochalasin B inhibits glucose carriers or transporters and thus the two types of glucose transports can be differentiated.

Lactose – H⁺ Symporter

Lactose–H⁺ symporter in E. coli has been studied in considerable detail. Proton gradient established by proton pump drives the uphill movement of Lactose. When the energy yielding oxidation reductions are inhibited by cyanide (CN^–), Lactose permease transport lactose passively down a concentration gradient till equilibrium is attained.