Ions and Molecules Across Membranes

Movement of Ions and Molecules Across Membranes Transport Across Membranes The Nernst Equation... 

The cell plasma membrane consists of a variety of proteins associated with the lipid bilayer and they perform multitasks in cell function. The control of transport of ions and molecules across membrane is accomplished through specialized function of membrane proteins. These proteins are distributed in membrane on the outer surface, some on the inner surface, and some others are transmembrane proteins with external and cytoplasmic domains. The majority of the transmembrane proteins are the ion channels or signaling proteins. Generally, hpid to protein ratio is 60 40 but this ratio is found variable in different cells and types of membranes. Membrane proteins impart the dynamic structure and selectivity to membrane function. Both proteins and hpids show motional and diffusion properties within the bUayer structure. 

Two Families of Membrane Proteins Use ATP Hydrolysis to Pump Ions and Molecules Across Membranes... 

The plasma membrane is semipermeable because it is not permeable to all solute particles present. As a result, it maintains a concentration difference for many ions and molecules across itself, although water crosses the membrane freely in either direction. The movement of water in and out of the... 

We must remember too that all the gradients of ions (or molecules) across membranes, represented sometimes by electrical potentials, are bulk sources of energy, not of specific chemical use but are of general value in uptake/rejection and signalling (see Chapter 9). 

Membrane proteins (which make up approximately one-third of the total number of known proteins) are responsible for many of the important properties and functions of biological systems. They transport ions and molecules across the membrane they act as receptors and they have roles in the assembly, fusion, and structure of cells and viruses. Presently, investigating membrane proteins is one of the most difficult challenges in the area of structural biology and biophysical chemistry. Our knowledge of membrane proteins is limited, primarily because it is very difficult to crystallize these protein systems due to the extreme hydrophobic interactions between the proteins and the membrane. New methods are needed and current techniques need to be extended to study the structural properties of membrane proteins. 

Proteins and peptides have diverse functions in the cell. They form the structural components of muscle, connective tissue, hair, and nails. They catalyze reactions and transport ions and molecules across cell membranes. Met-enkephalin, for example, a peptide with four amide bonds found predominately in nerve tissue cells, relieves pain and acts as an opiate by producing morphine-like effects. 

The rate of transmembrane diffusion of ions and molecules across a membrane is usually described in terms of a permeability constant (P), defined so that the unitary flux of molecules per unit time [J) across the membrane is 7 = P(co - f,), where co and Ci are the concentrations of the permeant species on opposite sides of membrane correspondingly, P has units of cm s. Two theoretical models have been proposed to account for solute permeation of bilayer membranes. The most generally accepted description for polar nonelectrolytes is the solubility-diffusion model [24]. This model treats the membrane as a thin slab of hydrophobic matter embedded in an aqueous environment. To cross the membrane, the permeating particle dissolves in the hydrophobic region of the membrane, diffuses to the opposite interface, and leaves the membrane by redissolving in the second aqueous phase. If the membrane thickness and the diffusion and partition coefficients of the permeating species are known, the permeability coefficient can be calculated. In some cases, the permeabilities of small molecules (water, urea) and ions (proton, potassium ion) calculated from the solubility-diffusion model are much smaller than experimentally observed values. This has led to an alternative model wherein permeation occurs through transient hydrophilic defects, or pores , formed by thermal fluctuations of surfactant monomers in the membrane [25]. 

The complex structure of cells requires a high degree of internal order. This is accomplished by four primary means (1) synthesis of biomolecules, (2) transport of ions and molecules across cell membranes, (3) production of movement, and (4) removal of metabolic waste products and other toxic substances. 

The energy produced on hydrolysis of ATP is utilised in many biological processes. These include muscle contraction, the transport of ions and molecules across cell membranes and the synthesis of various biomolecules. The biosynthesis of ATP is mainly by oxidative phosphorylation, photosynthetic phosphorylation and substrate-level phosphorylation (Chapter 11.5). In 1941, Lipmann introduced the concept of high-energy phosphate bonds and indicated that ATP was the universal energy carrier in bio systems. 

TRANSFER OF IONS AND MOLECULES ACROSS CELLULAR MEMBRANES... 

Lipids are components of the barrier opposing the diffusion of ions and molecules across the membrane. Interestingly, the hydrocarbon phase of this barrier is a relatively weak obstacle. Polar ends are much more active in opposing entry to polar and particularly, to charged solutes. Specific transport systems help some solutes pass through membranes (Section IV, B, b, and c). 

B. Tieke, A. Toutianoush, W. Jin, Selective transport of ions and molecules across layer-by-layer assembled membranes of polyelectrolytes, p-sulfonato-calix[n]arenes and Prussian Blue-type complex salts. Advances in Colloid and Interface Science, 116 (2005) 121-131. 

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