INTRODUCTION

Membranes function to organize biological processes by compartmentalizing them. Indeed, the cell, the basic unit of life, is essentially defined by its enveloping plasma membrane. Moreover, in eukaryotes, many subcellular organelles, such as nuclei, mitochondria, chloroplasts, the endoplasmic reticulum, and the Golgi apparatus (Fig. 1-5), are likewise membrane bounded. Biological membranes are organized assemblies of lipids and proteins with small amounts of carbohydrate. Yet they are not impermeable barriers to the passage of materials. Rather, they regulate the composition of the intracellular medium by controlling the flow of nutrients, waste products, ions, etc., into and out of the cell.

The central architectural feature of biological membranes is a double layer of lipids, which acts as a barrier to the passage of polar molecules and ions. Membrane lipids are amphipathic: one end of the molecule is hydrophobic, the other hydrophilic. Their hydrophobic interactions with each other and their hydrophilic interactions with water direct their packing into sheets called membrane bilayers.

The lipids in cell membranes combine two very different properties in a single molecule: each lipid has a hydrophilic (“water-loving”) head and a hydrophobic (“water-fearing”) tail. The most abundant lipids in cell membranes are the phospholipids, which have a phosphate-containing, hydrophilic head linked to a pair of hydrophobic tails. Phosphatidylcholine, for example, has the small molecule choline attached to a phosphate group as its hydrophilic head.

Molecules with both hydrophilic and hydrophobic parts are termed amphipathic, a property shared by other types of membrane lipids, including the cholesterol, which is found in animal cell membranes and the glycolipids, which have sugars as part of their hydrophilic head. Having both hydrophilic and hydrophobic parts plays a crucial part in driving these lipid molecules to assemble into bilayers in an aqueous environment.

FIGURE DEPICTING Structure of membrane lipids
FIGURE DEPICTING :- Structure of membrane lipids

PHOSPHOLIPIDS

Phospholipids are made up of four components an alcohol [glycerol or sphingosine], fatty acids, phosphate and an alcohol attached to the phosphate. The fatty acid components are hydrophobic while remaining are hydrophilic molecules. There are two types of phospholipids first is glycerophospholipids and other is shingophospholipids.

GLYCEROPHOSPHOLIPIS

In all glycerophospholipids, the head group is joined to glycerol through a phosphodiester bond, in which the phosphate group bears a negative charge at neutral pH. The polar alcohol may be negatively charged (as in phosphatidylinositol 4,5-bisphosphate), neutral (phosphatidylserine), or positively charged (phosphatidylcholine, phosphatidylethanolamine). These charges contribute greatly to the surface properties of membranes. The fatty acids in glycerophospholipids can be any of a wide variety, so a given phospholipid (phosphatidylcholine, for example) may consist of several molecular species, each with its unique complement of fatty acids.

The distribution of molecular species is specific to the organism, to the particular tissue within the organism, and to the particular glycerophospholipids in the same cell or tissue. In general, glycerophospholipids contain a C16 or C18 saturated fatty acid at C-1 and a C18 or C20 unsaturated fatty acid at C-2. With few exceptions, the biological significance of the variation in fatty acids and head groups is not yet understood.

Some animal tissues and some unicellular organisms are rich in ether lipids, in which one of the two acyl chains is attached to glycerol in ether, rather than ester, linkage. The ether-linked chain may be saturated, as in the alkyl ether lipids, or may contain a double bond between C-1 and C-2, as in plasmalogens

FIGURE DEPICTING Common glycerophospholipids.
FIGURE DEPICTING :- Common glycerophospholipids

SPHINGOPHOSPHOLIPIDS

  • Carbons C-1, C-2, and C-3 of the sphingosine molecule are structurally analogous to the three carbons of glycerol in glycerophospholipids. When a fatty acid is attached in amide linkage to the —NH2 on C-2, the resulting compound is a ceramide, which is structurally similar to a diacylglycerol. Ceramides are the structural parents of all sphingolipids. There are three subclasses of sphingolipids, all derivatives of ceramide but differing in their head groups: sphingomyelins, neutral (uncharged) glycolipids, and gangliosides. Sphingomyelins contain phosphocholine or phosphoethanolamine as their polar head group and are therefore classified along with glycerophospholipids as phospholipids. Indeed, sphingomyelins resemble phosphatidylcholines in their general properties and three-dimensional structure, and in having no net charge on their head groups. Sphingomyelins are present in the plasma membranes of animal cells and are especially prominent in myelin, a membranous sheath that surrounds and insulates the axons of some neurons—thus the name “sphingomyelins.”
FIGURE DEPICTING Common sphingolipids
FIGURE DEPICTING :- Common sphingolipids

STEROLS

  • Sterols are structural lipids present in the membranes of most eukaryotic cells. The characteristic structure of this fifth group of membrane lipids is the steroid nucleus, consisting of four fused rings, three with six carbons and one with five. The steroid nucleus is almost planar and is relatively rigid; the fused rings do not allow rotation about C—C bonds. Cholesterol, the major sterol in animal tissues, is amphipathic, with a polar head group (the hydroxyl group at C-3) and a nonpolar hydrocarbon body (the steroid nucleus and the hydrocarbon side chain at C-17), about as long as a 16- carbon fatty acid in its extended form. Similar sterols are found in other eukaryotes: stigmasterol in plants and ergosterol in fungi, for example. Bacteria cannot synthesize sterols; a few bacterial species, however, can incorporate exogenous sterols into their membranes. The sterols of all eukaryotes are synthesized from simple five-carbon isoprene subunits, as are the fat-soluble vitamins, quinones, and dolichols.

    Cholestrol is the major sterol present in the plasma membrane. They are also major determinant of membrane fluidity it high temperature it interferes with  movement of phospholipid fatty acid chain making the membrane less fluid. At low temperature it interferes with interaction between fatty acid chain making the membrane and prevents the membrane from freezing.

FIGURE DEPICTING - Structure cholestrol
FIGURE DEPICTING - Structure cholestrol

GLYCOLIPIDS

  • Glycolipids are carbohydrates covalently attached to the lipid.these can derived from either with glycerol or sphingosine.  Glycosphingolipids, which occur largely in the outer face of plasma membranes, have head groups with one or more sugars connected directly to the —OH at C-1 of the ceramide moiety; they do not contain phosphate. Cerebrosides have a single sugar linked to ceramide; those with galactose are characteristically found in the plasma membranes of cells in neural tissue, and those with glucose in the plasma membranes of cells in nonneural tissues.

CONCLUSION

  • A living cell is a self-reproducing system of molecules held inside a container. That container is the plasma membrane—a protein-studded, fatty film so thin that it cannot be seen directly in the light microscope. Every cell on Earth uses such a membrane to separate and protect its chemical components from the outside environment. Without membranes, there would be no cells, and thus no life. The structure of the plasma membrane is simple: it consists of a two-ply sheet of lipid molecules about 5 nm—or 50 atoms—thick, into which proteins have been inserted. Its properties, however, are unlike those of any sheet of material we are familiar with in the everyday world. Although it serves as a barrier to prevent the contents of the cell from escaping and mixing with the surrounding medium, the plasma membrane does much more than that. If a cell is to survive and grow, nutrients must pass inward across the plasma membrane, and waste products must pass out. To facilitate this exchange, the membrane is penetrated by highly selective channels and transporters—proteins that allow specific, small molecules and ions to be imported and exported.
  • Regardless of their location, all cell membranes are composed of lipids and proteins and share a common general structure. The lipids are arranged in two closely apposed sheets, forming a lipid bilayer. This lipid bilayer serves as a permeability barrier to most water-soluble molecules. The proteins carry out the other functions of the membrane and give different membranes their individual characteristics.

REFERENCES

  • Lehninger  principles of biochemistry seventh edition By  David L. Nelson and Michael M. Cox
  • voets and voets biochemistry 4th edition
  • Life sciences  fundamental and practices sixth edition, pathfinder publication By Pranav Kumar and Usha Mina
  • Essential cell biology (fourth edition) by ALBERTS, BRAY, HOPKIN, JOHNSON, LEWIS, RAFF, ROBERTS, WALTER

:- Article Written By Zahra Madraswala

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