Animal cell membranes

Plant cell membranes are similar in overall structure and organization to animal cell membranes but differ in lipid and protein composition. 

Marine toxins modify the functions of many different types of ion channels in animal cell membranes. These channels may be important for maintaining the cell s resting potential, for generating electrical membrane signals, such as impulses, and for controlling hormonally triggered or metabolic responses. Thus toxins may depolarize membranes, leading to a (sometimes transient) increase in cellular activities, or they may... 

In order to provide quantitative information on the elfeets of pressure on eross-peak intensities, we earned out 2D-NOESY experiments on pure POPC (l-palmitoyl-2-oleoyl-x -glyeero-3-phosphatidylcholine (Ci6 o, Cig c )) and DMPC (l,2-dimyristoyl-OT-glyeero-3-phosphatidylcholine (di-Ci4 o)) lipid bilayers.As an example, we present data on POPC, a phospholipid which is also a very important eomponent of animal cell membranes. It has an... 

Natural biological membranes consist of lipid bilayers, which typically comprise a complex mixture of phospholipids and sterol, along with embedded or surface associated proteins. The sterol cholesterol is an important component of animal cell membranes, which may consist of up to 50 mol% cholesterol. As cholesterol can significantly modify the bilayer physical properties, such as acyl-chain orientational order, model membranes containing cholesterol have been studied extensively. Spectroscopic and diffraction experiments reveal that cholesterol in a lipid-crystalline bilayer increases the orientational order of the lipid acyl-chains without substantially restricting the mobility of the lipid molecules. Cholesterol thickens a liquid-crystalline bilayer and increases the packing density of lipid acyl-chains in the plane of the bilayer in a way that has been referred to as a condensing effect. 

Spontaneous fusion of cultured cells occurs only rarely. However, the rate at which it happens can be markedly increased by the addition of certain viruses or chemical fusogens to the culture. Sendai virus, as used in early somatic cell fusions, has a lipoprotein envelope similar in structure to the animal cell membrane. It has been suggested that a glycoprotein in the envelope promotes cell fusion by an as yet unexplained mechanism. 

Figure 3.1 Three-dimensional structure of the animal cell membrane. Proteins (A) are interspersed in the phospholipid bilayer (B).

CDP-diacylglycerol is also a precursor of phosphati-dylinositol (fig. 19.6), a lipid that is unique to eukaryotes. Phosphatidylinositol accounts for approximately 5% of the lipids present in animal cell membranes (see table 17.3). Also present, at much lower concentrations, are phosphatidylinositol-4-phosphate and phosphatidylino-... 

Stretch-activated proteins in animal cell membranes that are candidates for osmosensing activity include mechanosensitive ion channels and the membrane-localized enzyme phospholipase A2 (PLA2). The former proteins remain to be conclusively linked to osmosensing. Activity of PLA2 is sensitive to packing of the lipid bilayer of the cell and is responsive to osmotic changes, two attributes that mark it as a prime candidate for a stretch-activated sensor (Lehtonen and Kinnunen, 1995). 

Hazel, J.R. (1988). Homeoviscous adaptation in animal cell membranes. In Advances in Membrane Fluidity. Physiological Regulation of Membrane Fluidity, pp. 149-188, ed. R.C. Aloia, C.C. Curtain, and L.M. Gordon. New York Liss. 

BSA + glucose/silica containing nanocomposites demonstrated much higher bioactivity than BSA + fructose/silica nanocomposites. It is interesting to note that glucose was chosen by evolution as a terminal carbohydrate in oligosaccharide chains used in animal cell membranes receptors, while fructose was not. 

Another stable form of aggregation is a lipid bilayer, which forms animal cell membranes (Figure 25-7). In a lipid bilayer, the hydrophilic heads coat the two surfaces of a membrane, and the hydrophobic tails are protected within. Cell membranes contain phosphoglycerides oriented in a lipid bilayer, forming a barrier that restricts the flow of water and dissolved substances. 

An aggregation of phosphoglycerides with the hydrophilic heads forming the two surfaces of a planar structure and the hydrophobic tails protected within. A lipid bilayer forms part of the animal cell membrane, (p. 1210)... 

If as above we simply represent alicyclic rings sharing two Gs by a vertical line, then we can represent the basic tetracyclic structure of lanosterol as G61G61 G6 C5 (noting that there are two double bonds and various alkyl substituents and also a 3-hydroxyl on the first of the alicyclic rings). Many subsequent reactions yield cholesterol, a major triterpene membrane component that modifies the fluidity of animal cell membranes and is a precursor for synthesis of animal bile acids (fat solubilizing amphipathic detergents) plant triterpenes and steroid hormones such as the corticosteroids cortisol and cortisone, the mineralocorticoid aldosterone and the sex hormones testosterone and 17-(3-oestradiol. The structure and bioactivity of the plant terpenes is sketched below. 

Cholesterol is an extremely important biological molecule that modulates the fluidity of animal cell membranes and is the precursor of steroid hormones (such as progesterone, testosterone, oestradiol and cortisol) and bile acids. Cholesterol is either derived from the diet or synthesised de novo. Regardless of the source, cholesterol is transported through the circulation in lipoprotein particles, as are cholesterol esters, the cellular storage form of cholesterol. The amount of cholesterol synthesised daily in the liver of a normal person is usually double that obtained from dietary sources. Other sites of cholesterol synthesis include the intestine, and the degree of production is highly responsive to cellular levels of cholesterol. Over 1.2 g of cholesterol is lost in the faeces daily in the form of free sterol or as bile acids. 

Sphin oUpids are the second major group of pbospliolipidai These compounds, which have sphingosine or a related dihydroxyamine ns their backbone, are coonstituents of plant and animal cell membranes. They are... 

We now turn our attention to the synthesis of the fundamental lipid cholesterol. This steroid modulates the fluidity of animal cell membranes (Section 12.6.2) and is the precursor of steroid hormones such as progesterone, testosterone, estradiol, and cortisol. All 27 carbon atoms of cholesterol are derivedfrom acetyl CoA in a three-stage synthetic process (Figure 26.6). 

The role of glycolipids is still unclear. Certain glycolipid molecules may bind bacterial toxins, as well as bacterial cells, to animal cell membranes. For example, the toxins that cause cholera, tetanus, and botulism bind to glycolipid cell membrane receptors. Bacteria that have been shown to bind to glycolipid receptors include E. coli, Streptococcus pneumoniae, and Neisseria gonorrhoeae, the causative agents of urinary tract infections, pneumonia, and gonorrhea, respectively. 

FIGURE 3.1 The three-dimensional structure of the animal cell membrane. 

The influence of plant sterols on the phase properties of phospholipid bilayers has been studied by differential scanning calorimetry and X-ray diffraction [206]. It is interesting that the phase transition of dipalmitoylglycerophosphocholine was eliminated by plant sterols at a concentration of about 33 mole%, as found for cholesterol in animal cell membranes. However, less effective modulation of lipid bilayer permeability by plant sterols as compared with cholesterol has been reported. The molecular evolution of biomembranes has received some consideration [207-209]. In his speculation on the evolution of sterols, Bloch [207] has suggested that in the prebiotic atmosphere chemical evolution of the sterol pathway if it did indeed occur, must have stopped at the stage of squalene because of lack of molecular oxygen, an obligatory electron acceptor in the biosynthetic pathway of sterols . Thus, cholesterol is absent from anaerobic bacteria (procaryotes). 

Biophysical studies on membrane lipids coupled with biochemical and genetic manipulation of membrane lipid composition have established that the L state of the membrane bilayer is essential for cell viability. However, membranes are made up of a vast array of lipids that have different physical properties, can assume individually different physical arrangements, and contribute collectively to the final physical properties of the membrane. Animal cell membranes are exposed to a rather constant temperature, pressure, and solvent environment and therefore do not change their lipid makeup dramatically. The complex membrane lipid composition that includes cholesterol stabilizes mammalian cell membranes in the L phase over the variation in conditions they encounter. Microorganisms are... 

Phosphatidylcholines (PC), (l,2-diacyl-s -glycerol-3-phosphoryl-cholinc) are predominant in animal cell membranes [7]. They are often called lecithins (Greek name "lekilhos the egg yolk) and their content in the biomembranes or animal tissues is very high. They are the main lipid compounds of plant membranes but their proportion in bacterial... 

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