Plasma Membrane

No living cells on earth one finds without plasma membrane; it is one of the most important components of the cells, which finds in prime position of the cell, whether it is a plant cell or an animal or bacterial cell or Archaeal cells.

 

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Membranes are the most important structural components of the cell; it is the protecting layer of the cell bounding the protoplasm and provides the interface for interaction between the outer and inner components. All cells in all living systems have such membranes around the protoplasm. Various cell organelles too are bounded by membrane. Chemically, structurally cell protoplasm is covered by a membrane called plasma membrane. In fact the active cytoplasmic fluid is pervaded with membranes. Some of the organelle membranes are highly specialized to perform specific functions. Most of the intracellular organelles are bounded by membranes, which actually make such structures compartmentalization of the protoplasm.

http://education-portal.com/; Phospholipid layers interspersed with proteins form a mosaic structure, but fluid and dynamic, which has different functions.

Fig. 2.

Plasma membrane- Cell Wall Contacts; http://www.plantphysiol.org/

Cellulose synthase (CSA) forms a rosette complex in the membrane and secrete cellulose. Underlying and cellulose microfibrils have similar orientations. Ambinogabactus proteins (AGPs) are composed primarily of carbohydrates linked to smaller protein cores Some AGPs are anchored to membranes. Secreted AGPs and membrane bound forms are associated with a variety of cellular components perhaps provide adhesive or positional cues. Several glucanases are also bound to membranes. Several cell-wall associated kinases (WAK) having Ser/Thr kinase domains in the ECM (extracellular domains).

Cellulose is synthesized and secreted by CSA complex of cellulase synthase, between adjacent structures found in between adjacent cells. Pectin and hemicellulose are also synthesized and secreted through endomembranes and complexed with ECM as shown below. http://www.plantphysiol.org/

Fig. 1.

http://www.plantphysiol.org/

 

An image of phospholipids.

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Phospholipids are not the same in both layers, in plants they vary in their composition. Note all phospholipids have the same glycerol but the fatty acids attached can be different. Phosphoglycerol provide negative surface and fatty acids provide hydrophobic surface, hence fatty acid attracted to each other and form two layers.

Phospholipid layer may contain cholesterol; Lipid to protein molar ration is ~50:1 to 100:1; the kind of phospholipids vary from one plant tissue to thew other. In addition the composition of two layers cab vary

 

 

Lateral separation of lipids result in the formation f small domains called membrane rafts. These are rich in specific proteins, sterols and sphingolipids. Composition proteins are highly variable, some are fund at the external surface and some at internal surface and some show transmembrane traverse the entire cross section. Surface proteins on the outer side are modified as glycoproteins. Many proteins act as receptors, and transmembrane transporters (both passive and active- or facilitated). The also contain Aquaporins for transport of water, which is also transported through diffusion.

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Asymmetric distribution of proteins and phospholipids are discerned by the diagram; http://en.wikibooks.org/

 

 

 

 

 

A schematic diagram shows a cross section of four plasma membrane proteins performing different functions. The four proteins include a transporter, a receptor, an enzyme, and an anchor.

Role of transmembrane proteins; Transporters carry a molecule (such as glucose) from one side of the plasma membrane to the other. Receptors can bind an extracellular molecule (triangle), and this activates an intracellular process. Enzymes in the membrane can do the same thing they do in the cytoplasm of a cell: transform a molecule into another form. Anchor proteins can physically link intracellular structures with extracellular structures. http://www.nature.com/

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Schematic depiction of water movement through the narrow selectivity filter of the aquaporin channel.; The 2003 Nobel Prize in Chemistry was awarded jointly to Peter Agre for the serendipity discovery of aquaporins 1993 and Roderick MacKinnon for his work on the structure and mechanism of potassium channels.;Aquaporins selectively conduct water in and out of the cells, they form a kind of water channels. Often they are also called glycroaquaporins; However the first report of protein mediated water transport through membranes was by Gheorghe Benga in 1986; in the membrane they are found and act as transmembrane tetrameric proteins; In plants they have a symplastic pathway for the movement of water; http://en.wikipedia.org/;

A schematic diagram shows a piece of plasma membrane in three dimensions. The diagram includes the phospholipids, membrane proteins, cholesterol, glycosylphosphatidylinositol (GPI), and carbohydrates.

Like a mosaic, the cell membrane is a complex structure made up of many different parts, such as proteins, phospholipids, and cholesterol. The relative amounts of these components vary from membrane to membrane, and the types of lipids in membranes can also vary. http://www.nature.com/

Transportes

Plant cell receptors:

Recent studies revealed that higher plants also possess genes coding for putative receptor kinases (Receptor-like Kinases, RLK). For instance, a completely sequenced Arabidopsis genome contains over 600 genes encoding RLKs (Shiu and Becker, 2001), suggesting that higher plants, like animals, use receptor kinase signaling commonly and broadly in responding to vast arrays of stimuli to modulate gene expressions. RLKs act as superfamily of receptors

Receptor-like kinases in plants Figure.1

Plant RLKs are classified into subfamilies based on the structural feature of the extracellular domain, which is thought to act as a ligand-binding site. A common feature of these putative receptor kinases (RLKs), is that each has an N-terminal signal sequence, an extracellular domain that varies in structure, a single membrane-spanning region, and a cytoplasmic protein kinase catalytic domain. Unlike animals, where a majority of the receptor kinases possess tyrosine kinase activity, all of the plant RLKs thus far are shown to phosphorylate serine-and threonine residues, except one that displays dual specificity in vitro (Walker, 1994; Torii and Clark, 2000,; many other great reviews are available!). S-domain class, TM domain class, LRR-domain class, RCCI-like repeats, PR5-Likedomain class, TNFR-like repeats, Lectin-like domains, Serine/threonine protein kinase class, EGF-like repeat class, PR-class; http://faculty.washington.edu/

 

Chemical Composition:

Analysis of chemical components found in membranes show variation from membrane to membrane, from organelle to organelle. Generally membranes are made up of proteins and lipids in various proportions as such as 1:1 to 1:3. The analysis of proteins and lipids of various types of membranes show wide variety of structural components.

Proteins are polymers of amino acid residues. They can be isolated and separated in the individual components by the methods of SDS, polyacrylamide gel electrophoresis, column chromatography and ammonium sulfate precipitation and other methods. Some of the membrane proteins are structural ones and others are found to be enzymes, receptors, transporters or carriers. Many of them are located at the outer face of the membranes, extrinsic proteins or they may be found in the core as intrinsic or integral proteins. Most of peripheral proteins are globular and they are either hydrophilic or partially hydrophobic. Integral proteins are however hydrophobic because they contain greater amount of nonpolar amino acids like leucine, Valine, isoleucine, etc at their surface. Even such proteins contain some hydrophilic amino acid residues. The presence of variety of proteins and protein complexes provide structural and functional heterogeneity to membranes.

Lipids:

The only semi viscous to viscous part of the membranes contains a wide variety of lipids like phospholipids, sphingolipids, sulpholipids, phytosterols etc. Phospholipids are dipolar molecules with hydrophilic phosphate group at one and non polar hydrophobic fatty acid chains as tails at the other. Among them phosphotidyl serine, phosphotidyl ethanolamine phosphotidyl choline, phosphotidyl glycerol and cardiolipins are important.

Many of the lipids are associated with carbohydrates, such lipids are called glycolipids. Some of the membrane proteins are associated with carbohydrates and such proteins are called glycoproteins. They play important roles. Among sterols, cholesterols and phytosterols are common in plant membranes. Having polar heads and non polar tails, phospholipids play an important role in structural organization of the membrane.

 

Structure:

Starting from Sand witch model proposed by Danielli-Davson, the concept of membrane structure has undergone many modifications over the years. The unit membranes model of Robertson has been further improved by S.J. Singer and Garth L. Nicholson as Fluid Mosaic model. This model is the most accepted one today, for it explains most of the observed membrane structures and functions. Singer and Nicholson model is based on studies like Freeze fracture electron microscopy, Nuclear Magnetic Resonance, X-ray diffraction, Fluorescence spectroscopy and biochemical analytical techniques. This model has also taken into account of energy relations like translational movements, vibrational movements and hydrophobic, elctrostatic and hydrogen bond interactions. Moreover the dynamic feature of the membranes has been explained mostly on the basis of energy translations.

 

According to Fluid Mosaic Model, various phospholipids and other lipid components from a bilayered structure at the interface of water, because the hydrophobic tails of lipids get oriented towards each other in such a way the hydrophilic heads are exposed towards water. As wide variety of proteins of different dimension are integrated into lipid bilayers so as to form mosaic of lipids and proteins.

Many proteins are located at the interphase between water and hydrophilic phospholipid layers, some are held and buried in the hydrophobic core and other are integrated in the core of lipid bilayers so as to occupy the entire core section of the membrane. There is a dynamic interaction between lipids and proteins; they exhibit lateral movement including rotational flip flop turnovers. The position of proteins with respect to lipids in the membrane is never constant and always there is constant flux thus exhibit in quasi fluidity as well as quasi crystalline semi solid state. The association of microfilaments and microtubules at inner face of the membranes further adds up to its dynamicity to a greater extent. The above described structure holds good for all the membranes. However plasma membrane and other cytoplasmic membranes differ in their chemical composition particularly with respect to proteins and specific lipids. Even the thickness of the membrane varies from 60 100 . The plasma lemma in most of the cells being the surface membrane contains a wide variety of receptor and carrier proteins. It produces invagination to produce cytosolic endoplasmic reticulum.

Functions:

Membranes being sheet like structures, posses a large surface area for many biochemical reactions. However, that function depends upon the protein and lipid contents. Plasma membrane as present at the outer surface, it has manifold function. Though they allow water to diffuse through in both directions, it prevents the free diffusion of both inorganic and organic solutes. Water movement is greatly facilitated by the presence of aquaporins. Thus it exhibits semi permeable property. However plasma lemma performs selective uptake of ions because specific solute carrier proteins found within the membrane. The plasma membrane is the first cellular structure that receives a wide variety of external stimuli like light, heat, chemicals, hormones, etc. Such stimuli are then passed on to cytoplasm or to the genetic material through specific receptor proteins. The surface membranes are also involved in bringing about changes in permeability and electro potential. They also take part in Pinocytosis and phagocytosis thus they facilitate the transportation of various substances in bulk. On the country, they are also responsible for the secretion of undigested materials and enzymes to the exterior surface. The presence of desmotubules of 200 A thicknesses, which traverse across the pit channels from one cell to another, is one of the unique features of the plasma membranes. Similar to plasma membrane other membranes also show specific functions. Thus membranes exhibit dynamisity in its structure and function.

Biogenesis of Membranes:

Plasma membrane can be considered as an organ by itself; it exhibits a characteristic structure and functions. Plasma membranes of different cell types exhibit different functions either in receiving the stimulus or transportation of materials to and fro. In spite of its diversity and uniqueness the synthesis of plasma membrane mainly depends upon the activity of endoplasmic reticulum and Golgi complex. A large number of endoplasmic reticular membranes which are engaged in protein translocation and modifications contribute to the plasma membrane synthesis. Many of the fatty acids required are synthesized at the cytosolic side of the ER membranes for this region contains anchored enzymes. There is continuous flow of membrane materials from trans-golgi to plasma membranes and from plasma membrane inwards, there is dynamic equilibrium between these two components.