Cell membrane

“ In many instances this membrane is formed of very thin flattened cells, in other instances it is apparently a homogeneous membrane, and according to some observers is simply a modified part of the basal surface of the cell, while according to others it is a definite structure distinct from the epithelium.
In the secretory portion of the gland and in the smaller ducts the epithelial layer is one cell thick only. In the larger ducts there are two layers of cells, but even here the surface cell usually extends by a thinned-out stalk down to the basement membrane. ”

“ Now, it must have become evident from our previous chapters, that the further development of all other organisms out of such a cell presents no difficulty, for every animal and every plant, in the beginning of its individual life, is a simple cell. Man, as well as every other animal, is at first nothing but a simple egg-cell, a single lump of mucus, containing a kernel (p. 297, Fig. 5) .
In the same way as the kernel of the organic cell arose in the interior or central mass of the originally homogeneous lump of plasma, by separation, so, too, the first cell-membrane was formed on its surface. ”

“ Max Schultze showed that one and the same substance, protoplasm, occurred in unicellular forms and in the higher plants and animals; that in plants this substance, though usually enclosed within a cell membrane, was sometimes naked (e.g. swarm spores) , while in many animal tissues and in many of the unicellular forms the cell-membrane was always absent. He therefore concluded that in all cases the cell-membrane was unessential, and he redefined the “cell” of Schleiden and Schwann as “a small mass of protoplasm endowed with the attributes of life” (1861) . ”

“ But some cells, especially young ones, are entirely without the enveloping membrane, or stiff wall. Hence we now generally describe the cell as a living, viscous particle of protoplasm, enclosing a firmer nucleus in its albuminoid body. There may be an enclosing membrane, as there actually is in the case of most of the plants; but it may be wholly lacking, as is the case with most of the animals. There is no membrane at all in the first stage. The young cells are usually round, but they vary much in shape later on. ”

“ By the time the chick is ready to be hatched there are millions.
Usually, however, a cell is small—just as much protoplasm as its still more minute nucleus can keep going; though here, again, one must be guarded, for there may be several nuclei instead of only one. The protoplasm on the external surface and around the nucleus is specialized into a more or less definite membrane. To this outer envelope are attached fine fibrils, which join up to a small body within the cell, called the centrosome, and by the lengthening and shortening of these its shape can be altered. ”

“ In the first place the cell is a kind of inclosed sac, in which are found the elements of growth and life. Surrounding the cell is a thin wall known as the cell membrane. In plants this cell wall is composed of cellulose, a woody substance, which is thin and tender in green and growing plants, but hard and woody when the plant is mature. ”

“ The membranogenous substance, the chloride of calcium, diffuses uniformly on all sides from the solid nucleus, and forms an osmotic membrane where it comes into contact with the solution. This spherical membrane is extended by osmotic pressure, and grows gradually larger. Since the area of the surface of a sphere increases as the square of its radius, when the cell has grown to twice its original diameter, each square centimetre of the membrane will receive by diffusion but a quarter as much of the membranogenous substance. ”

“ Around each of these as a center the protoplasm of the mother cell is seen to have become differentiated in the form of a clear spherule, and we have thus corresponding to each of the new nuclei a young naked cell, which soon secretes over its surface a membrane of cellulose. The new cells, when once formed, multiply by division, press one on the other, and so combining into a cellular mass, constitute the completed endosperm. ”

“ A great deal of attention has been devoted of late years to the minute structure of cells, and the great improvements in microscopical powers and appliances have enabled investigators to ascertain a number of exceedingly interesting and important facts. The external surface of a cell is sometimes, but not always in the case of animals, bounded by a film or membrane. Within this membrane the substance of the cell is made up of a network of very delicate fibres (the plasmogen) , enclosing a more fluid material (the plasm) ; and this network seems to be the essential living substance. ”

“ Soon after, the cytoblasts are seen as granular coagulations from the granular mass; they grow considerably in this free condition, but as soon as they have reached their full size, a delicate transparent vesicle is formed upon them; this is the young cell, which at first presents the appearance of a very flat segment of a sphere, whose plane side is formed by the cytoblast, the convex by the young cell (the cell-membrane) , which rests upon the cytoblast as a watch-glass on a watch. ”

“ With regard to the former, we have seen that in the single cells, constituting unicellular organisms, there is always a bounding membrane of denser texture than the rest of the protoplasm. As the cell develops its capabilities, we have a shell or case of non-living matter secreted around it, with apertures for communication with the outside world, and increasingly effective protection is provided as protoplasm, whether in the single cell or the body, leaves the water, and has to face the inclemencies of terrestrial life. ”

“ The fact that a double plasma membrane delimiting the daughter cells is first formed in cell division, each cell having its own membrane, and that if a cellulose wall is formed at all it appears later between these membranes, indicates that the wall is of secondary importance, which is further supported by the fact that in many cell divisions, as in all the prothallial cells of Zainia and Ginkgo, no cellulose walls are formed, the cells being delimited only by the plasma membranes. ”

“ Response to stimulation thus depends on semipermeability, which implies polarizability of the membranes bounding the irritable cells or elements. This conclusion is one of far-reaching importance, because it localizes the primary change in electrical and hence in other forms of stimulation at the plasma membranes. Some membrane-process forms the first stage of the response to stimulation. The membrane is thus not to be regarded as a mere passive diffusion-preventing barrier between living substance and surroundings, but as the essentially sensitive and controlling portion of the cell. ”

“ The spindle fibers have bulged out on each side of the old mother-cell nucleus, reaching the cell wall on each side. They have contracted away from the daughter nuclei, and seem at their outer ends to be in the process of gradual transformation into the normal reticulum of the cytoplasm. No visible thickening occurs on the fibers where the new plasma membrane is to be formed, but a space free of granules and stainable matter clearly shows where the new plasma membrane delimiting the two cells will form. This is particularly well shown in a photomicrograph of the cell shown in figure 63. ”

“ Like the latter, it is made of fibres, which form a reticulum (Fig. 23) , and these fibres, like those of protoplasm, have microsomes in intimate relation with them and hold a clear liquid in their meshes. The meshes of the network are usually rather closer than in the outer cell substance, but their general character appears to be the same. But a more close study of the nucleus discloses vast differences. In the first place, the nucleus is usually separated from the cell substance by a membrane (Fig. 23, c) . ”

“ In none of these cells is a cellulose wall laid down, there being nothing but a plasma membrane or hautschicht formed. The first prothallial cell is shaped like a plano-convex lens and arches out into the second prothallial cell. In the mature pollen grains it can not be determined whether the plasma membrane formed in cutting off this cell is attached at the sides to the plasma membrane of the tube cell, as must be the case if only two prothallial cells are formed, or to that of a third resorbed prothallial cell. ”

“ In the simplest unicellular animals the fluid protoplasm is contained in a membrane, or denser bounding layer, to which are attached fine filaments springing from a minute body known as the centrosome. These centrosomes—for there are sometimes several in a cell—seem to control the mechanical department, just as the nucleus does the chemical. ”

“ In some cases the protoplasm at the circumference is so modified as to give the appearance of a limiting membrane called the cell wall. In brief, then, a cell is a mass of nucleated protoplasm; the nucleus may have a nucleolus, and the cell may be limited by a cell wall. Every tissue of the human body is formed through the agency of protoplasmic cells, although in most cases the changes they undergo are so great that little evidence remains of their existence. ”

“ The naked cells which have been alluded to live in water, and call therefore for no differentiation in connexion with this necessity; but those which are surrounded by a cell-wall always develop within themselves a vacuole or cavity which occupies the greater part of their interior, and the hydrostatic pressure of whose contents keeps the protoplasm in contact with the membrane, setting up a condition of turgidity. ”

“ The animal cell, in its most complete form, is comparable in most parts of its structure to that of the plant, but differs from it in the entire absence of the 'cellulose wall' or of anything that represents it, the cell-contents being enclosed in only a single limitary membrane, the chemical composition of which, being albuminous, indicates its correspondence with the primordial utricle. ”

“ The yeast-cell with its bud is a simple example of a principle which we shall find to be very important. Our whole treatment of cell-form in relation to surface-tension depends on the fact (which Errera was the first to point out, or to give clear expression to) that the incipient cell-wall retains with but little impairment the properties of a liquid film281, and that the growing cell, in spite of the membrane by which it has already begun to be surrounded, behaves very much like a fluid drop. ”

“ A soluble substance thus surrounded by an osmotic membrane represents what Traube has called an artificial cell. In such a cell the dissolved substances have a very high osmotic pressure, an expansive force like that of steam in a boiler; the molecules of the solute exerting pressure on the walls of the extensible cell, and distending it like the {124}gas in a balloon. This pressure increases the volume of the cell, and in consequence water rushes in through the permeable membrane and still further distends the cell. ”

“ Most cells are inclosed by a cell-wall, and contain a watery cell-sap, and a mucilaginous semi-fluid substance called protoplasm, composed of different organic constituents; among these nitrogenous or albuminous matter is always present, and in the largest proportion. In some plants the protoplasm is not at first inclosed by cell-walls, but it is sooner or later enclosed in a more or less elastic membrane. No cell can exist in a living state or grow unless it contains protoplasm, which is therefore the basis of all vegetable life. ”

“ The cells met with in morbid parts which are in a state of active Structure of Pathological Cells. vitality are built up of the same components as those found in normal tissues (Pl. I.) . [1] Thus they are provided with a nucleus which is the centre of cell activity; both of the reproductive and chemical (metabolic) processes which occur in the cell protoplasm. The executive centre varies in shape, but is usually round or oval, and is sharply defined by a nuclear membrane from the cytoplasm in which it lies. ”

“ When the elaboration of the substance to be secreted is more active, the mechanism of the secretion is modified. The basal membrane of the platform may then be raised at several places at once; instead of a single vesicle projecting into the intestinal cavity, each cell may present a great number more or less voluminous. If all remain small and rapidly detach themselves from the glandular cell, the filaments of the platform are simply separated from each other at different points of the free face, as in Fig. ”

“ Every form of plant-life is made up, not of a compact and uniform mass of matter with no occasional empty spaces, but, on the contrary, with the aid of a microscope it is seen that an infinite number of very minute cavities called cells are interspersed throughout the body of the plant. These cells may be regarded as extremely small closed sacs, sometimes round, sometimes oval, but more often with irregular and angular outlines by reason of the mutual pressure exerted by the cells. The cell-wall is composed of an excessively fine membrane. ”

“ If it has been at rest for some time the cell contains very many granules which swell it out and increase its size. The nucleus is then largely hidden by the granules. In the opposite condition, i.e. when the cell has been actively secreting, the protoplasm is much clearer, the nucleus obvious and the cell shrunken in size, all these changes being due to the extrusion of the granules.
Endothelium and Mesothelium.—Lining the blood vessels, lymph vessels and lymph spaces are found flattened cells apposed to one another by their edges to form an extremely thin membrane. ”