Application The Cell Membrane

The cell membrane is a beautifully complex example of how the principles of organic chemistry come into play in a biological system. 

The basic unit of living organisms is the cell. The cytoplasm is the aqueous medium inside the cell, separated from water outside the cell by the cell membrane. The cell membrane serves two apparently contradictory functions. It acts as a barrier to the passage of ions, water, and other molecules into and out of the cell, and it is also selectively permeable, letting nutrients in and waste out. 

A major component of the cell manbrane is a group of organic molecules called phospholipids. Like soap, they contain a hydrophilic ionic portion, and a hydrophobic hydrocarbon portion, in this case two long carbon chains composed of C-C and C-H bonds. Phospholipids thus contain a polar head and two nonpolar tails. 

Cell membranes are composed of these lipid bilayers. The charged heads of the phospholipids are oriented toward the aqueous interior and exterior of the cell. The nonpolar tails form the hydrophobic interior of the membrane, thus serving as an insoluble barrier that protects the cell from the outside. 

How does a polar molecule or ion in the water outside a cell pass through the nonpolar interior of the cell membrane and enter the cell Some nonpolar molecules like O2 are small enough to enter and exit the cell by diffusion. Polar molecules and ions, on the other hand, may be too large or too polar to diffuse efficiently. Some ions are transported across the membrane with the help of molecules called ionophores. 

When soap is dissolved in H2O, the molecules form micelles with the nonpolar tails in the interior and the polar heads on the surface. The polar heads are solvated by ion-dipole interactions with H2O molecules. 

Cross-section of a soap micelle with a grease particle dissolved in the interior 

Problem 3.11 Today, synthetic detergents like the compound drawn here, not soaps, are used to clean clothes. Explain how this detergent cleans away dirt. 

Chapter 3 Introduction to Organic Molecules and Functional Groups 

---

The relation between the architecture of the molecules and the spatial morphology into which they assemble has attracted longstanding interest because of their importance in daily life. Lipid molecules are important constituents of the cell membrane. Amphiphilic molecules are of major importance for teclmological applications (e.g., in detergents and the food industry). 

Ion channels that open in response to the application of drugs will produce a change in the conductance of the cell membrane the longer the individual ion channels are open, the greater will be the conductance of the membrane. This change in the conductance of the membrane, produced by anthelmintic drugs that open ion channels, has been detected in Ascaris body muscle (Martin, 1980) and in the Ascaris pharyngeal muscle (Martin, 1996). 

The test is based on an in vitro assay of the uptake of the dye, neutral red (NR), in Balb/c 3T3 fibroblasts. It was developed to detect the phototoxicity induced by the combined interaction of the test substance and light of the wavelength range from 315 to 400 nm, the so-called UVA. The cytotoxicity is evaluated in the presence (+UVA) or absence (-UVA) of UVA light exposure, after application of a nontoxic dose of the compound. The cytotoxicological impact is assessed via the inhibition of the fibroblasts to take up the vital dye NR (NR is a weak cationic dye, penetrating easily into the cell membrane by a nonionic diffusion and accumulates in the lysosomes) one day after the initial treatment. Normally, healthy cells may incorporate and bind NR. Alterations of the cell surface or the lysosomal membranes, however, lead to a decreased uptake and binding of the dye. 

ESR spectra indicated that ultrasound enhanced the penetration of 16-DS into the structurally stronger sites of the inner and outer cell membranes. The effect of ultrasound on the cell membranes was transient in that the initial membrane permeability was restored upon termination of the ultrasound treatment. These results suggested that the resistance of gram-negative bacteria to the action of hydrophobic antibiotics was caused by a low permeability of the outer cell membranes and that this resistance may be reduced by the simultaneous application of antibiotic and ultrasound. 

---