Cell plasma membrane permeability barrier

Microinjection is a tool to overcome the plasma membrane permeability barrier to the introduction of charged molecules, polypeptides, or DNA plasmids into cells. In essence, microinjection treats the cell as the test tube and uses a microinjection capillary needle as the pipette to add small volumes of solution to the cytoplasm or nucleus. The approach has major advantages (i) the technique is highly synchronous with a few hundred cells being injected over 10-20 minutes, (ii) intracellular environment and cell morphology is preserved, (iii) the reaction vessel is small, that is, the size of a cell, and correspondingly reagent dilution is confined to the... 

An important harmful effect of metals at the cellular level is the alteration of the plasma membrane permeability, leading to leakage of ions like potassium and other solutes (Passow and Rothstein, 1960 Wainwright and Woolhouse, 1978 De Filippis, 1979 De Vos et al., 1988, 1991). After supply of copper ions Ohsumi et al. (1988) demonstrated for yeast cells and De Vos et al. (1989) for root cells of Silene cucubalus that the permeability barrier (controlled by means of potassium leakage) of the plasma membrane was almost immediately lost. Oshumi et al. (1988) also reported a quick release of amino acids, especially glutamate and aspartate. After McBrien and Hassall (1965) and Overnell (1975), who studied potassium release from algal cells, the increased permeability of the plasma membrane may be considered to constitute the primary toxic effect of copper. 

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. 

Figure 1 General pathways through which molecules can actively or passively cross a monolayer of cells. (A) Endocytosis of solutes and fusion of the membrane vesicle with the opposite plasma membrane in an active process called transcytosis. (B) Similar to A, but the solute associates with the membrane via specific (e.g., receptor) or nonspecific (e.g., charge) interactions. (C) Passive diffusion between the cells through the paracellular space. (C, C") Passive diffusion (C ) through the cell membranes and cytoplasm or (C") via partitioning into and lateral diffusion within the cell membrane. (D) Active or carrier-mediated transport of an otherwise poorly membrane permeable solute into and/or out of a cellular barrier.

Each cell is surrounded by a plasma membrane that separates the cytoplasmic contents of the cell, or the intracellular fluid, from the fluid outside the cell, the extracellular fluid. An important homeostatic function of this plasma membrane is to serve as a permeability barrier that insulates or protects the cytoplasm from immediate changes in the surrounding environment. Furthermore, it allows the cell to maintain a cytoplasmic composition very different from that of the extracellular fluid the functions of neurons and muscle cells depend on this difference. The plasma membrane also contains many enzymes and other components such as antigens and receptors that allow cells to interact with other cells, neurotransmitters, blood-borne substances such as hormones, and various other chemical substances, such as drugs. 

Major differences between a general (nonneural) and brain capillary. In the brain capillary, the Intercellular clefts are sealed shut by tight Junctions. There are also reduced pinocytosis and no fenestrae. Exchange of compounds between the circulation and the brain must take place in the cells of the capillary wall, the major barriers of which are the inner and outer plasma membranes of the capillary endothelial cells. (Reprinted with permission from Oldendorf WA. Permeability of the blood-brain barrier. In Tower DB [ed.]. The Nervous System. Vol. 1 New York Raven, 1975.)... 

The plasma membrane surrounds the cell, separating it from the external environment. The plasma membrane is a selectively permeable barrier due to the presence of specific transport proteins. It is also involved in receiving information when ligands bind to receptor proteins on its surface, and in the processes of exocytosis and endocytosis. 

The plasma membrane of epithelial cells, in common with other cell types, is selectively permeable, allowing the penetration of some substances but not others. The construction of the membrane from amphipathic lipid molecules forms a highly impermeable barrier to most polar and charged molecules, thereby preventing the loss of most water-soluble contents of the cell. This selective permeability presents a physical barrier to drag absorption, limiting absorption to specific routes and mechanisms, as described below (see Section 1.3.3). 

C. Assuming that the partition coefficient for CO2 is 100 times greater in the cell wall than in the plasma membrane, in which barrier is the permeability coefficient larger, and by how much ... 

The resistance to diffusion of a molecular species across a barrier equals the reciprocal of its permeability coefficient (Chapter 1, Section 1.4B). In this regard, we will let f COi be the permeability coefficient for CO2 diffusion across barrier j. To express the resistance of a particular mesophyll or chlo-roplast component on a leaf area basis, we must also incorporate Am sIA to allow for the actual area available for diffusion—the large internal leaf area acts like more pathways in parallel and thus reduces the effective resistance (Fig. 8-4). Because the area of the plasma membrane is about the same as that of the cell wall, and the chloroplasts generally occupy a single layer around the periphery of the cytosol (Figs. 1-1 and 8-11), the factor AmesIA applies to all of the diffusion steps of CO2 in mesophyll cells (all five individual resistances in Eq. 8.21). In other words, we are imagining for simplicity that the cell wall, the plasma membrane, the cytosol, and the chloroplasts are all in layers having essentially equal areas (Fig. 8-11). Thus, the resistance of any of the mesophyll or chloroplast components for CO2 diffusion,, is reduced from 1 /P co, by the reciprocal of the same factor, Ames/A ... 

Thus far we have considered only the plasma membrane of cells. Many bacteria such as E. coli have two membranes separated by a cell wall (made of proteins, peptides, and carbohydrates) lying in between (Figure 12 35). The inner membrane acts as the permeability barrier, and the outer membrane and the cell wall provide additional protection. The outer membrane is quite permeable to small molecules owing to the presence of porins. The region between the two membranes containing the cell wall is called the periplasm. Other bacteria and archaea have only a single membrane surrounded by a cell wall. 

The transfer of the information described in the preceding sections of this chapter to the in vivo situation is a matter where opinions are sharply divided, even if more than 20 years have elapsed since the discovery by Vasington and Murphy [4]. One key problem, naturally, is the impossibility of reproducing the composition and the conditions of the cytosol in in vitro experiments. The above mentioned effect of Mg on the rate of Ca influx into mitochondria is but one striking example of the difficulties inherent to the extrapolation to the in situ conditions. Of interest in this respect are recent experiments [124,125] in which methods have been devised to estimate simultaneously the membrane potential across the plasma membrane and the mitochondria of nerve endings in situ. The conclusion of this work has been that the concentration of free Ca in the cytosol correlates directly to the membrane potential across the mitochondrial membrane, and is maintained at a steady-state level below 1 jaM. Simulation of the in situ conditions has also been the aim of studies [126] in which isolated liver endoplasmic reticulum has been added to media in which isolated liver mitochondria were made to take up Ca, or in which liver cells have been treated with digitonin to abolish the permeability barrier of the plasma membrane. It was found that respiring mitochondria lower the external Ca " concentration to about 0.5 /iM. The addition of endoplasmic reticulum vesicles produces a further decrease of the external Ca " to about 0.2 jaM. Thus, mitochondria... 

When cells are placed in external apphed electric fields, they experience an electric force. Electroporation involves the use of short, high voltage pulses to overcome barrier of the cell membrane. When a cell is submitted to an external electric field of high intensity and short duration (kV/cm, p,s), transient and dramatic increase in the permeability of the plasma membrane occurs beyond a point. This phenomenon is popularly called electroporation or electropermeabilization, which allows entry of otherwise impermeable exogenous molecules into the cell interior. This phenomenon has been an active area of research in biology and bioelectrochemistry for more than three decades [3,4] and has found many apphcations in cell biology. 

The cornified cell envelope is the outermost layer of a corneocyte, and mainly consists of tightly bundled keratin filaments aligned parallel to the main face of the corneocyte. The envelope consists of both protein and lipid components in that the lipid is attached covalently to the protein envelope. The envelope lies adjacent to the interior surface of the plasma membrane. " The corneocyte protein envelope appears to play an important role in the structural assembly of the intercellular lipid lamellae of the stratum corneum. The corneocyte possesses a chemically bound lipid envelope comprised of A-co-hydroxyceramides, which are ester linked to the numerous glutamate side chains provided possibly by both the ot-helical conformation and p-sheet conformation of involucrin in the envelope protein matrix. In the absence of A-oo-hydro-xyceramides, the stratum corneum intercellular lipid lamellae were abnormal and permeability barrier function was disrupted. 

The cell membrane serves as a protective barrier in renal cells. It is the initial site which p-lactams encounter in their journey to the cellular environment from the blood or tubular fluid, p-lactams may disrupt the functional organization of the membrane through peroxidation of membrane lipids, which, in turn, leads to the inability of membrane to serve as an osmotic barrier and causes the cytosol contents to leak. As a result of the cephalosporins disruptive effect on cell membrane, increased leakage of the cytosolic enzyme lactate dehydrogenase (LDH) occurs. The increased LDH concentration was from the cytosol of the renal cortex [49,71] or from isolated proximal and distal tubular cells [39] or in the urine of experimental animals [39]. The results of these studies indicate that plasma membrane became permeable to large molecules such as LDH. After cephalosporin treatment, cephaloridine caused the greatest decrease of LDH concentration in cytosol [49]. Whereas, cephaloridine induced a greater release of LDH from proximal tubular cells than cepha-lothin and cephalexin, distal cells were not affected by any of these cephalosporins [38,39]. 

I have already shown several syntheses of sphingolipids as microbial metabolites or marine natural products. Sphingolipids are building blocks of the plasma membrane of eukaryotic cells. Their function is to anchor lipid-bound carbohydrates to cell surfaces, and to construct the epidermal water permeability barrier. The chemistry of sphingolipids is therefore closely related to dermatology or the science of skin. This section first treats sphingolipid in human epidermis. 

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