KINETICS AND MECHANISM OF TRANSPORT

Nonmediated Transport: Through simple diffusion

Mediated Transport: Requires specific carriers

· Passive Mediated Transport (or facilitated diffusion): Specific molecules flow from High to Low concentration. Net flow will cease when the concentration gradient is no longer in existence.

· Active Transport: From L to H concentration. Endergonic processes. Must be coupled to a sufficiently energetic (exergonic) process to make it possible.

· Classification (Fig. 10-19)

Stoichiometry

Uniport: movement of a single (type of) molecule at a time.

Symport: simultaneous movement of two different molecules in the same direction.

Antiport: simultaneous movement of two different molecules in opposite directions.

Charge

Electroneutral: (1) neutral molecule «; (2) (+ and -) ®; (3) (+) ® and ¬ (+);

(4) (-) ® and ¬ (-).

Electrogenic: results in charge separation.

THERMODYNAMICS OF TRANSPORT

I. For molecule A neutral in Charge:

Chemical Potential (Partial Molar Free Energy)

= + RT ln(A) where “-” means quantity per mole

is the chemical potential of A

is the chemical potential of A at its standard state

For a Chemical Reaction: aA + bB ⇌ cC + dD

DG = c + d -a - b

= DG^o + RT ln

Chemical Potential Across a Membrane: (A)_in ⇌ (A)_out

A difference in concentrations of the substance on two sides of a membrane generates a Chemical Potential Difference D :

D = - = RTln

· When (A)_out > (A)_in, D < 0 Spontaneous net flow of A from out to in.

· When (A)_out < (A)_in, D > 0 Spontaneous net flow of A from in to out.

Net flow of A from out to in must be coupled to an energy-providing system to make the D negative.

II. For Molecule A with Charge:

D = Electrochemical Potential

= - + Z_AF [y_(in) - y_(out)]

= RTln + Z_AFDy where:

· Dy = y_(in) - y_(out) = membrane potential; ~ -100 mV in living cells (inside more negative) is common.

· Z_A is ionic charge of A

· F is Faraday constant = 96,485 C mol^-1 (C for coulomb) or 96,485 J V^-1 mol^-1

NONMEDIATED TRANSPORT

· through simple diffusion

· driven by (electro)chemical potential gradient

· The substance diffuses in the direction that eliminates its concentration gradient.

· The rate of diffusion is:

(a) proportional to the magnitude of the (electro)chemical gradient,

[For many nonelectrolytes, their fluxes across erythrocyte membranes increase linearly with (A)_out -(A)_in.]

(b) dependent on the solubility of the substance in the membrane’s nonpolar core.

[Steroids and O₂ readily diffuse through biological membranes.]

· Water also rapidly equilibrates across membranes.

MEDIATED TRANSPORT

Through the action of specific protein or other molecules referred to as carriers, permeases, channels, and transporters.

A. PASSIVE – MEDIATED TRANSPORT

1. Ionophores:

· Small organic molecules.

· Many are antibiotics.

· Vastly increase the permeability of membranes to particular ions.

· Can be CARRIERS or CHANNEL FORMERS. (Fig. 10-1)

· The efficiency of a carrier can be drastically reduced at lower T but not so for the channel formers.

· Carriers:

o The carrier-ionic complexes are soluble in non-polar solvents.

o Carriers cam move from one side to the other side of a lipid-bilayer membrance.

o Valinomycin (Fig. 10-3) contains 6 oxygen atoms complexing with a K⁺.

· Channel Former: Gramicidin (Fig. 10-5) Two molecules together form a channel. Facilitates the passage of cations (e.g. Na⁺, K⁺, H⁺) but is blocked by Ca²⁺.

2. Bacterial Porins:

· Channel forming protein.

· X-ray structure known (E. coli OmpF porin, Fig. 9-24).

· Some are cation selective, others are anion selective.

3. Ion Channels:

· Many ion-specific channels for rapid passage of ions.

· K⁺ passively diffuses from cytoplasm to the extracellular space through transmembrane proteins “K⁺ Channels.” (Figs. 10-7, 8)

· High selectivity for K+ over Na⁺.

· Are “Gated.” The physiological functions of ion channels depend on their ion specificity, speed of transport, and the ability to gate (i.e. to open and close).

o Mechanosensitive channels: respond to physical stimuli (touching, sound, etc.), which cause local deformation s in membranes.

o Ligand-gated channels: respond to extracellular chemical stimuli such as neurotransmitter.

o Signal-gated channels: open upon intracellular binding of Ca²⁺ or other signal molecules.

o Voltage-gated channels: respond to changes in membrane potential, such as in the generation of nerve impulses.

4. Aquaporins:

· Water passes through biological membranes extremely fast.

· Aquaporins discovered in 1992 by Peter Agre.

· AQP1, a homotetrameric glycoprotein. (Figs. 10-15, 16)..

5. Transport Proteins (Function in Mediated Passive and Active Transport)

· Evidence for Transport Proteins (Box 10-2)

o Speed and Specificity: The observed permeability coefficient of D-mannitol is close to that calculated on the basis of oil-water partition coefficient (10^-9 cm^.s^-1). In contrast, the observed permeability coefficient of D-glucose in human erythrocyte (~10^-4 cm^.s^-1) is 10⁵ higher than the calculated value.

o Saturation kinetics.

o Susceptibility to competitive inhibition (e.g. 6-o-benzyl-D-galactose competitively inhibits D-glucose transport).

o Susceptibility to inactivation by protein modification agents such as HgCl₂, etc.

· Example for Mediated Passive Transport: Erythrocyte Glucose Transporter

o Mechanism: Gated Pore Mechanism (Fig. 10-17)

Ø Glucose binding on one face of the membrane

Ø Conformational change of the carrier between two states.

Ø Release glucose to the other side.

Ø Bidirectional but the net flow of glucose is from H to L.