Cell structure Plasma membrane

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. 

P. Bonfante, At the interface between mycorrhizal fungi and plants. The structural organization of cell wall, plasma membrane, and cytoskeleton Mycota, Vol. IX, Fungal Associations (B. Hock, ed.). Springer-Verlag. In press. 

Every cell has a plasma membrane. More than just a boundary, the plasma membrane allows molecules to pass into and out of the cell and provides sites where important chemical reactions occur. In animal cells, the plasma membrane is the outermost part of the cell, but the plasma membranes of plant cells are bounded by a rigid cell wall that protects the cells and gives them structure. 

The postsynaptic nerve ending, which is usually the tip of an axonal dendrite, has its own set of proteins, which varies to some extent with the nature of the neurotransmitter. In excitatory cells the plasma membrane of the postsynaptic neuron is thickened to — 30—40 ran to form the "postsynaptic density," a disc-like structure of clustered receptors of two types, which extends 30 ran into the cytosol.593 594 Only single receptor channels are indicated in Fig. 30-20, but many receptors are present in the clusters594 595 as are other specialized proteins. One of these, designated... 

The normal cell has an outer layer with a chemical structure similar to fat that has been esterihed. It is found in the form of a cell membrane that separates the fluid and tissue outside a cell from the organelles inside the cell. The cell or plasma membrane is a lipid bilayer that contains proteins on the inner and outer surfaces (Figure 1.3). The function of the cell membrane is to (1) maintain ionic and chemical concentration gradients (2) carry specific surface markers and receptors such the human leukocyte anti-... 

Be able to describe the structures and general functions of cell walls, plasma membranes, nucleus and nucleoid region, mitochondria, lysosomes, peroxisomes, endoplasmic reticulum, cytoskeleton, and extracellular matrix. 

The ectoderm ensures that the inner space of a body is sharply distinct from the outside world, and all forms and shapes that we find in that inner space are entirely due to endogenous processes of three-dimensional organisation. And the same is true for every single cell. The plasma membrane controls virtually everything that is passing through, and is for a cell what ectoderm is for an embryo the structure... 

A protective barrier called the plasma membrane (see p.24) is one of the most important regulatory locales in a cell, serving as the point of entry and exit for a bounty of large and small molecules that are carried to a cell via the bloodstream. To function properly, cells need a constant supply of nutrients, electrolytes, and structural materials. While some of these necessary components are manufactured in-house, many are obtained from outside the cell. The plasma membrane is also an important communications hub, filtering messages sent by other cells and the outside environment. Membranes are studded with proteins called channels and pores that thread their way from the outside to the inside of the cell, or the other way around. Scientists often study these proteins because they are a key target for drugs. 

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. 

In all cells, the plasma membrane acts as a permeability barrier that prevents the entry of unwanted materials from the extracellular milieu and the exit of needed metabolites. Specific membrane transport proteins In the plasma mem brane permit the passage of nutrients Into the cell and metabolic wastes out of It others function to maintain the proper Ionic composition and pH ( 7.2) of the c3d osol. The structure and function of proteins that make the plasma membrane selectively permeable to different molecules are discussed In Chapter 7. 

As we explain in Chapter 13, small increases in the concentration of free Ca ions in the cytosol trigger a variety of cellular responses. In order for Ca to function in intracellular signaling, the concentration of Ca ions free in the cytosol usually must be kept below 0.1 - 0.2 )xM. Animal, yeast, and probably plant cells express plasma-membrane Ca ATPases that transport Ca out of the cell against its electrochemical gradient. The catalytic a subunit of these P-class pumps is similar in structure and sequence to the a subpump. 

A biological membranes system is typically formed by the combination of lipids and proteins. In eukaryotic cells, the plasma membrane, also referred to as the cell membrane, is a protective barrier which regulates what enters and leaves the cell. The endomembrane system is composed of different kinds of membranes which divide the cell into structural and functional compartments within a eukaryotic cell, such as the endoplasmic reticulum, Golgi apparatus, mitochondria, endosome and lysosome. Covalent modification of proteins with lipid anchors (protein lipidation) facilitates association of the lipidated proteins with particular membranes in eukaryotic cells. Protein lipidation is one of the most important protein post-translational modifications (PTMs). Studying lipidated protein function in vitro or in vivo is of vital importance in biological research. 

When a virus infects a cell, several events occur that are specific for the invader and hence offer opportunities for selective attack. First of all, there is the contact with the cell, then the penetration of the cell s plasma membrane and the (often simultaneous) rejection of the viral coating-protein. If the virus is of the RN A type, reverse transcriptase is soon in manufacture, but in any case the synthesis of nucleic acid polymerases dominates this early stage of invasion. Next follows the synthesis of viral nucleic acids, structural proteins, and yet more enzymes, followed by the assembly of these components to form the complete virus. Finally, some thousands of these virions are liberated from each cell. Apart from the possibilities for finding selective inhibitors for each of these stages, the patient could also be helped by other drugs to control the secondary (non-viral) symptoms, which are often of an inflammatory or anaphylactic character. 

Cationic lipids (Fig. 2.4a) are usually feirly small molecules, which mimic the structure of the cell s plasma membrane and hence facilitate the passage of DNA into the cell by increasing the solubility of the DNA in the plasma membrane. These molecules consist of a hydrophobic... 

The plasma membrane (Figure 9.2) encapsulates the cell and physically separates the cytoplasm from the external environment. All substances which enter or leave the cell must pass through the plasma membrane which plays an important role in the selective uptake of nutrients from the extracellular medium and the discharge of waste products of metabolism from the cell. The plasma membrane is the most extensively researched and best understood of all cell membranes and its properties have led to the development of models of membrane structure from the fundamental lipid bilayer composed of amphipathic phospholipids (Section 8.5) to the currently most widely accepted model called the fluid mosaic model. 

Because both oxidative and photophosphorylation occur at large sub-entities in organelles of heterotrophic and autotrophic cell, the plasma membrane, the mitochondria inner membrane or the chloroplast thylakoid system, and in a cooperative fashion - one thylakoid contains about 10 CF CF complexes -, in reconstitution experiments the danger of misinterpretation is always inherent The reappearence of catalytic activity of the membrane system may be due to "structural reconstitution", i.e. repair of H leakeness by the added soluble component (Schatz et al., 1967, McCarty, Racker, 1967). 

Prokaryotic cells have only a single membrane, the plasma membrane or cell membrane. Because they have no other membranes, prokaryotic cells contain no nucleus or organelles. Nevertheless, they possess a distinct nuclear area where a single circular chromosome is localized, and some have an internal membranous structure called a mesosome that is derived from and continuous with the cell membrane. Reactions of cellular respiration are localized on these membranes. In photosynthetic prokaryotes such as the cyanobacteria,... 

The vesicular monoamine transporters (VMATs) were identified in a screen for genes that confer resistance to the parkinsonian neurotoxin MPP+ [2]. The resistance apparently results from sequestration of the toxin inside vesicles, away from its primary site of action in mitochondria. In addition to recognizing MPP+, the transporter s mediate the uptake of dopamine, ser otonin, epinephrine, and norepinephrine by neurons and endocrine cells. Structurally, the VMATs show no relationship to plasma membrane monoamine transporters. 

Submembranous microtubules are often present in parallel bundles beneath the plasma membrane in the cells of higher plants, particularly during cell wall formation (Hardham and Gimning, 1978). Circular submembranous bundles of microtubules are a feature of bird erythrocytes and mammalian blood platelets, where they maintain the discoid shape of these structures (Dustin, 1980). 

As plant cells grow, they deposit new layers of cellulose external to the plasma membrane by exocytosis. The newest regions, which are laid down successively in three layers next to the plasma membrane, are termed the secondary cell wall. Because the latter varies in its chemical composition and structure at different locations around the cell, Golgi-derived vesicles must be guided by the cytoskeleton... 

More than 50 proteins have been discovered in the cytosol of nonmuscle cells that bind to actin and affect the assembly and disassembly of actin filaments or the cross-linking of actin filaments with each other, with other filamentous components of the cytoskeleton, or with the plasma membrane. Collectively, these are known as actin-binding proteins (ABPs). Their mechanisms of actions are complex and are subject to regulation by specific binding affinities to actin and other molecules, cooperation or competition with other ABPs, local changes in the concentrations of ions in the cytosol, and physical forces (Way and Weeds, 1990). Classifications of ABPs have been proposed that are based on their site of binding to actin and on their molecular structure and function (Pollard and Cooper, 1986 Herrmann, 1989 Pollard et al., 1994). These include the following ... 

---