Protoplasmic membrane

It is difficult to explain why toxic hydrocarbons can be made selective to carrots by the addition of a nontoxic oil but not by the addition of water. Green (7) found some correlation between the toxicity of oils and their ability to emulsify. It is commonly found that high aromatic oils are easier to emulsify than are oils with low aromatic content. It is possible that some action between the aromatic hydrocarbons and the emulsifying agent results in increased toxicity. There is some evidence that the permeability of the protoplasmic membrane is the key to carrot resistance. If this is true, the presence of the emul er or the physical properties of the emulsion might increase the cell penetration of the hydrocarbons. Work is being continued along these lines and on the fundamental reasons for differential plant resistance to oils. 

Although the enzyme sediments with intact cells, alkaline phosphatase appears in the supernate when broken cells are centrifuged. Malamy and Horecker (5) discovered that alkaline phosphatase is quantitatively released from the cell when E. coli are converted to spheroplasts by lysozyme and ethylenediaminetetraacetic acid (EDTA) in a sucrose medium. This evidence, supported by the observation that substrates such as glucose 6-phosphate are rapidly hydrolyzed by intact cells with release of most of the phosphate into the medium, led Malamy and Horecker (6) to suggest that alkaline phosphatase is localized in the periplasmic space, a region described by Mitchell (7) as lying between the protoplasmic membrane and the wall layer, and that it is not in association with the wall (8). 

Localization of the enzyme in the periplasmic space is also consistent with the selective release of alkaline phosphatase during growth of an E. coli mutant which is osmotically sensitive because of a defective cell wall (14) and with the fact that phosphate esters which do not penetrate the protoplasmic membrane can be hydrolyzed by intact cells 15). In these latter measurements the activities found with intact cells as compared with equivalent cell extracts varied over wide limits depending upon the substrate and its concentration. This difference was assumed to result from a difference in the ease of penetration of the wall barrier by different phosphate esters. 

H. Souzu (1967b). Location of polyphosphate and polyphosphatase in yeast cells and damage to the protoplasmic membrane of the cell by freeze-thawing. Arch. Biochem. Biophys., 120, 344-351. 

The function of the microbial cell wall is as a protective external support to the cell. Its rigid polymeric material functions as a continuous envelope to preserve the protoplast s integrity, something the delicate protoplasmic membrane cannot adequately do in the... 

The use of soaps to solubilize phenols in water, for use as disinfectants, depends on the formation of mixed micelles of the soap and the phenol. It has been found that variation in the proportion of soap to phenol leads to several zones, as shown in Fig. 14.2 (Berry, Cook and Wills, 1956). The first of these exists below 0.03 M potassium laurate and is poorly bactericidal the maximal bactericidal effect is obtained when the soap reaches 0.03 M, a figure which is identical with the critical micelle concentration for this soap. It was concluded that this bactericidal action is a combined attack of the phenol (mainly) and the soap on the protoplasmic membrane (see Section 14.3 for membrane damage). The next zone, up to 0.045 M soap, is one of greatly diminished bactericidal effect. The interpretation is that the phenol has entered the micelles, many more of which must have formed, and hence little of it is available for disinfection. When still more soap is introduced, a final zone (vigorous disinfection) appears, due to the toxicity of the soap itself All the phenols commonly used as disinfectants, including j -chloro-m-xylenol, form zones like these. 

Double films derived from the above, in which these elements are also included, are appropriate as models of the protoplasmic membrane. 

Nonenzymatic proteins. (Structural proteins of protoplasm, membranes, ribosomes regulatory proteins, like histones and receptor proteins)... 

The transport of specific ions is a common function of a biomembrane. It is observed in an active transport of ions through the cell wall, a proton transport in oxidative phosphorylation, a selective transport of K and Na through the protoplasmic membrane, etc. To develop synthetic membrane having such functions is an important objective of polymer chemistry. 

Scarth (72) has handled the coarser structure of protoplasm, or cell organization. He distinguishes five layers which, in general, occur in all cells. In most plant cells there is an additional layer, the outermost wall of cellulose. Within the cellulose wall there are, in order, the protoplasmic membrane or ectoplasm, the plasma gel or cortical endoplasm, the plasma-sol or liquid endoplasm, and the inner tonoplast or vacuolar membrane... 

The last barrier that our hypothetical molecule will have to cross is the plasma membrane or the inner membrane. The area between the outer and the protoplasmic membrane has been designated as periplasmic space, which appears to harbor a number of transport proteins and degra-dative enzymes. The inner membrane s composition and functions have been described extensively and can be summarized here as follows Structurally, the membrane consists of an organized and dynamic phospholipid-protein array in which the proteins (enzymes) are complexed with lipids to form units whose function is largely dependent on the state of the neighboring lipid environment. A schematic representation of the various surface structures of a gram-negative bacterium is shown in Fig. 1. 

Cell The smallest structural unit of living matter capable of functioning independently it is a microscopic mass of protoplasm surrounded by a semipermeable membrane, including one or more nuclei and various non-living substances that are capable, either alone or with other cells, of performing all the fundamental functions of life. 

The drying protoplast will be subjected to tension as the result of volume contraction and its adherence to the cell wall. Early observations (Steinbrick, 1900) on desiccation tolerant species showed that the protoplasm does not separate from the wall, but rather that it folds and cavities develop in the wall. Where there are thick-walled cells, localised separation of the plasmalemma from the wall may occur. It seems unlikely, however, that rupture of the plasmalemma normally occurs during desiccation. A more subtle form of membrane damage may arise from dehydration-induced conformational changes. Certainly it is relatively easy to demonstrate that dehydrated membranes exhibit a loss of functional integrity... 

Membranes are highly viscous, plastic structures. Plasma membranes form closed compartments around cellular protoplasm to separate one cell from another and thus permit cellular individuality. The plasma membrane has selective permeabilities and acts as a barrier, thereby maintaining differences in composition between the inside and outside of the cell. The selective permeabilities are provided mainly by channels and pumps for ions and substrates. The plasma membrane also exchanges material with the extracellular environment by exocytosis and endocytosis, and there are special areas of membrane strucmre—the gap junctions— through which adjacent cells exchange material. In addition, the plasma membrane plays key roles in cellcell interactions and in transmembrane signaling. 

As early as 1848, it had been suggested that sensory receptors transduce only one sensation, independent of the manner of stimulation. Behavioral experiments tend to support this theory. In 1919, Renqvist proposed that the initial reaction of taste stimulation takes place on the surface of the taste-cell membrane. The taste surfaces were regarded as colloidal dispersions in which the protoplasmic, sensory particles and their components were suspended in the liquor or solution to be tested. The taste sensation would then be due to adsorption of the substances in the solution, and equal degrees of sensation would correspond to adsorption of equal amounts. Therefore, the rate of adsorption of taste stimulants would be proportional to the total substances adsorbed. The phenomenon of taste differences between isomers was partly explained by the assumption that the mechanism of taste involves a three-dimensional arrangement for example, a layer of fatty acid floating on water would have its carboxylic groups anchored in the water whereas the long, hydrocarbon ends would project upwards. 

Each cell consists primarily of a membrane, which separates it from the environment, preserves its structural integrity, and keeps it apart from other cells or from the surrounding environment. Plant cells, unlike animal cells, also have, in addition to a cell membrane, a cell wall, composed of cellulose and lignin. The cell wall provides structural strength not only to the vegetable cell itself but to all plant tissues as well. Inside the membrane, the interior of the cell, known as the protoplasm, includes two main... 

The term protoplasm is used to indicate the thick viscous semifluid or almost jelly-like colorless, transparent material which makes up the essential substance of both the cell body and the nucleus, including the cytoplasmic membrane but not the cell wall. It contains a high percentage of water and holds fine granules in suspension. 

When the cell wall is damaged, the protoplasm usually disintegrates. However, methods are available for removing the cell membranes without destroying the vital nature of protoplasm. The term protoplast is used to indicate living protoplasm exclusive of the cell membranes. 

Most of the early work on membranes was based on experiments with erythrocytes. These cells were first described by Swammerdam in 1658 with a more detailed account being given by van Leeuwenhoek (1673). The existence of a cell (plasma) membrane with properties distinct from those of protoplasm followed from the work of Hamburger (1898) who showed that when placed in an isotonic solution of sodium chloride, erythrocytes behaved as osmometers with a semipermeable membrane. Hemolysis became a convenient indication of the penetration of solutes and water into the cell. From 1900 until the early 1960s studies on cell membranes fell into two main categories increasingly sophisticated kinetic analyses of solute translocation, and rather less satisfactory examinations of membrane composition and organization. 

The cytoplasm is the semi-fluid polyphasic colloid that comprises the bulk of the cell s interior between the cell membrane and the nucleus it contains enzymes responsible for catalyzing the biosynthetic machinery of the cell and organelles responsible for specific tasks within the cell. Cytoplasm must be differentiated from protoplasm protoplasm is the whole material contained within the cell membrane and is further differentiated into the material found within the nucleus (nucleoplasm) and material external to the nucleus (cytoplasm). Organelles are important functional structures within the cytoplasm. Various structures visible by light microscopy are classified as organelles, including mitochondria, endoplasmic reticulum, and lysosomes. 

CENTRAL Capsule In the Radiolaria, a characteristic perforated membranous sac (a chitenoid internal skeleton) which lies embedded in the protoplasm dividing the protoplasm into intracapsular and extra-capsular regions. 

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