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  Membranes-General Structure and Function, Biology tutorial

Introduction:

Membranes are very significant to living organisms. The organism has at least one kind of membrane; plasma membrane is very common to all living organisms. In animal cells it makes contact with outside environment of cell; nothing goes in or come out except through plasma membrane. In plants, cell has a rigid cell wall covering plasma membrane. By its nature, cell wall is freely permeable to several substances.

General Structure of Membrane:

Membranes are lipo-protein complexes the molecular structure of that has been subject of several speculative models. The significant concepts of different models have been embodied in so-called "Fluid mosaic model" that states (i) that lipids and integral proteins are disposed in type of mosaic arrangement and (ii) that biological membranes are quasi -fluid structures in which both lipids and integral protein are able to carry out transitional movement within overall bilayer.

Existence of plasma membrane was discovered by such indirect way. When cells were in isotonic solutions they stay intact. When placed in hypotonic solutions they imbibe water until they inflate beyond the point of no return and then burst. In the hypertonic media cells shrink. Due to these findings it was suggested that cell is surrounded within semi-permeable membrane. Further elaboration with electron microscope has revealed more about plasma membrane. Categorization of cell into either as prokaryotes or eukaryote is based on abundance and placement of membranes inside cell. Prokaryotes, in general are generally unicellular cells that lack internal membrane system known as organelles. The membranes framework in eukaryotes comprise plasma membranes, the cytopasmic and vacuolar system (comprising SER, RER, microsomal fraction, golgi complex, lysosomes), mitochondria, chloroplasts and nuclear envelope. The most widely favored lamella mode considers membrane as consisting of double layer of lipids between 2 layers of proteins, in a sort of sandwich in which lipid forms meat and protein the bread. Particulate hypothesis, on the contrary, considers that membrane is composed of single layer f globular subunits like layer of marbles.

Hydrophilic portions of membrane lipids are supposed to face outwards towards surface of bimolecular layer of lipids, at these surfaces lipids are related with proteins. Protein layers on inside surface of the membrane (surface facing cytoplasm) perhaps differ, in particular molecular composition, from protein layer which faces external environment.

This model is attractive as it agrees with usual electron microscope image that sows trilarmellar membrane structure that looks like railroad tract with two dark parallel lines each approx 30Å thick, and inner lighter zone also approx 30Å thick. Most probably, central layer of unit membrane would correspond to hydrophobic portions of lipids whereas two dense lines would represent protein and hydrophilic portions of lipids. This structure is known as unit membrane.

Differences in thickness and asymmetry of layers have been seen in different membrane types. Thickness of unit membrane has been found to be greater in plasma membrane (10m) than in intracellular membranes of ER or Golgi complex (5 to 7mm). Significant concepts of different models of membranes have been embodied in so-called fluid mosaic model of membrane structure. This defines, (i) that lipids and integral proteins are disposed in type of mosaic arrangement and (ii) that biological membranes are quasi-fluid structures in which both lipids and integral proteins are capable to do translational movement inside overall bilayer. This model regards both lipid and integral protein to be amphipathic is show hydrophilic and hydrophobic group inside same molecules. Hydrophobic chains of lipids are surrounded in inner space of bilipid layer; protein too has their polar regions protruding from surface and non-polar regions embedded in hydrophobic interior of membranes. Fluidity of membranes is supported by experiments that have shown rapid lateral diffusion of lipids and fact that integral proteins can also suffer translational displacements inside bilayers.

The fluidity of membranes depends on state of unsaturation of fatty acid components of lipids. More unsaturation which exists inside the fatty acids the more fluid the membrane is. Membranes suffer a physical phase transition from the flexible fluid-like liquid crystalline state to solid gel structure as function of temperature. Temperatures at which phase transition occur are dependent on composition of amphipathic lipids.

The lipids with more unsaturated fatty acids have lower transitions than those with more saturated fatty acids; longer chain lengths have higher transition temperatures than shorter chain lengths; Cis fatty acids have a lower transition temperature than trans-unsaturated fatty acids. Membranes lipids include matrix which give form and structure to membranes in which membrane proteins are imbedded. All members have amphipathic lipids which comprise phospholipids and glycolipids. Phospholipids are made up of fatty acids related to glycerol backbone. Usually two fatty acids are joined to glycerol with third position available for one of particular compounds encountered in phospholipids, such as, choline or ethanolamine. Three main kinds of lipids found in nature are: fats, phospholipids and steroids. Fats comprises of fatty acids, the series of long hydcrocarbon, chains, related to glycerol backbone. Triglyceride results when all three carbons of glycerols are joined to fatty acid. Most phospholipids have structure like triglycerides, except that in place of one of fatty acids they have a more complex chain phosphate and nitrogen-containing groups. Steroids have a skeleton based on structure of cholesterol. Glycolipids are found in cellular membranes. Phospholipids and steroids are polar molecule that is amphipathic with hydrophobic and hydrophitic ends. Plasma membranes and other cellular membranes are rich in polar lipids.

Role of Membranes:

Membranes are involved in:

i) Pinocytosis - intake of small molecules-plasma membrane

ii) Phagocytosis - ingestion of large particles-plasma membrane

iii) Exocytosis - expulsion of particles from cell-Golgi, plasma membrane.

Bioenergetics:

iv) Photosynthesis in plant chloroplasts.

v) Oxidatve phosphorylation-production of ATP in mitochondria

vi) Storage - like lysosomes are stored I membrane (b) protection, were it not for membrane autolysis by lysozomal enzymes will occur.

vii) Cell recognition - contact inhibition where it fails cancer results.

viii) Anrtigen - antibody reaction (Ag - Ab) - plasma membrane

ix) Shape and size - plasma membrane.

x) Increase in surface area like microvilli, infoldings of mitochordria and chloroplast membranes, ER and Golgi apparatus.

xi) Transmision of nerve impulse-nerve cell membranes (ii) protection of nerves-myeling sheath.

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