Cellular transport components, lipids

The cellular membrane is a hydrophobic barrier that surrounds the cytoplasm of every cell and is involved in complex cellular processes, such as signaling and transport, which are essential to maintain the normal life cycle of a cell major components of this cellular membrane are lipids, proteins, peptides, and carbohydrates. Peptides that interact with cellular membranes are referred to as membrane-active peptides and can be broadly divided into three major classes antimicrobial, cell-penetrating, and fusogenic peptides. Any of these may have variable lengths, hydrophobicities, and secondary structures, but they often exhibit similar effects on membranes. For example, some antimicrobial peptides have cell-penetrating properties, and vice versa [23,24] these peptides usually cause some degree of membrane destabilization. 

MEMBRANE TRANSPORT Membrane transport mechanisms are vital to living organisms. Ions and molecules constantly move across cell plasma membranes and across the membranes of organelles. This flux must be carefully regulated to meet each cell s metabolic needs. For example, a cell s plasma membrane regulates the entrance of nutrient molecules and the exit of waste products. Additionally, it regulates intracellular ion concentrations. Because lipid bilayers are generally impenetrable to ions and polar substances, specific transport components must be inserted into cellular membranes. Several examples of these structures, referred to as transport proteins or permeases, are discussed. 

Cholesterol is an essential component of cellular membranes. In addition to dietary sources, we can also synthesize cholesterol. Cholesterol is transported in the blood as a lipoprotein, which is an aggregate of water-soluble proteins, cholesterol, and other lipids, including triglycerides. Proteins are denser than lipids,... 

The mechanism in hepatic cellular metabolism involves an electron transport system that functions for many drugs and chemical substances. These reactions include O-demethylation, N-demethyla-tion, hydroxylation, nitro reduction and other classical biotransformations. The electron transport system contains the heme protein, cytochrome P-450 that is reduced by NADPH via a flavoprotein, cytochrome P-450 reductase. For oxidative metabolic reactions, cytochrome P-450, in its reduced state (Fe 2), incorporates one atom of oxygen into the drug substrate and another into water. Many metabolic reductive reactions also utilize this system. In addition, there is a lipid component, phosphatidylcholine, which is associated with the electron transport and is an obligatory requirement for... 

Lipids have several important functions in animal cells, which include serving as structural components of membranes and as a stored source of metabolic fuel (Griner et al., 1993). Eukaryotic cell membranes are composed of a complex array of proteins, phospholipids, sphingolipids, and cholesterol. The relative proportions and fatty acid composition of these components dictate the physical properties of membranes, such as fluidity, surface potential, microdomain structure, and permeability. This in turn regulates the localization and activity of membrane-associated proteins. Assembly of membranes necessitates the coordinate synthesis and catabolism of phospholipids, sterols, and sphingolipids to create the unique properties of a given cellular membrane. This must be an extremely complex process that requires coordination of multiple biosynthetic and degradative enzymes and lipid transport activities. 

Blood serum, usually bovine-derived (calf or fetal bovine), contains amino acids, growth factors, vitamins, proteins, hormones, lipids, and minerals, among other components, as indicated in Table 5.3. Besides fetal bovine serum, serum from horse (equine), and even from humans (less common) can also be used. The main functions of serum are to stimulate growth and other cellular activities through hormones and growth factors, to increase cellular adhesion through specific proteins, and to supply proteins for the transport of hormones, minerals, and lipids (Freshney, 2005). Supplementation with bovine fetal serum is performed at concentrations from 2 to 20% in volume. 

Biological membranes define the very existence of cells. They provide compartments for the different components of the living system interact with, transport and are permeable to substrates. They are involved in lipid and protein syntheses, energy transduction, ion and group transport, information transmission and molecular and cellular recognition. These multitude of activities are accomplished by the unique morphology of the biological membrane and by its ability to affect the transport of species by different mechanisms. 

Surfactant Effects on Microbial Membranes and Proteins. Two major factors in the consideration of surfactant toxicity or inhibition of microbial processes are the disruption of cellular membranes b) interaction with lipid structural components and reaction of the surfactant with the enzymes and other proteins essential to the proper functioning of the bacterial cell (61). The basic structural unit of virtually all biological membranes is the phospholipid bilayer (62, 63). Phospholipids are amphiphilic and resemble the simpler nonbiological molecules of commercially available surfactants (i.e., they contain a strongly hydrophilic head group, whereas two hydrocarbon chains constitute their hydrophobic moieties). Phospholipid molecules form micellar double layers. Biological membranes also contain membrane-associated proteins that may be involved in transport mechanisms across cell membranes. 

The plasma membrane is a selectively permeable barrier between the cell and the extracellular environment. Its permeability properties ensure that essential molecules such as ions, glucose, amino acids, and lipids readily enter the cell, metabolic intermediates remain in the cell, and waste compounds leave the cell. In short, the selective permeability of the plasma membrane allows the cell to maintain a constant Internal environment. In Chapter 5, we learned about the components and structural organization of cell membranes. Movement of virtually all molecules and ions across cellular membranes is mediated by selective membrane transport proteins em bedded in the phospholipid bllayer. Because different cell types require different mixtures of low-molecular-weight compounds, the plasma membrane of each cell type contains a specific set of transport proteins that allow only certain ions and molecules to cross. Similarly, organelles within the cell often have a different internal environment from that of the surrounding cytosol, and organelle membranes contain specific transport proteins that maintain this difference. 

---