Excel A-Level Biology: Structure of cell membrane
Phospholipids are the major class of membrane lipids.
A phospholipid molecule is constructed from 4 components:
Phosphoglyceride consists of a glycerol backbone to which are attached 2 fatty acid chains and a phosphorylated alcohol.
The hydroxyl group of the glycerol backbone is esterified to phosphoric acid. When no further additions are made, the resulting compound is phosphatidate (diacylglycerol 3-phosphate) -- the simplest phosphoglyceride.
The major phosphoglycerides are derived from phosphatidate by the formation of an ester bond between the phosphate group of phosphatidate and the hydroxyl group of one of several alcohols. The common alcohol moieties of phosphoglycerides are the amino acid serine, ethanolamine, choline, glycerol, and inositol. Some common phosphoglycerides found in the membranes are phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol, and diphosphatidylglycerol (cardiolipin).
Sphingomyeline is not derived from glycerol, instead, the backbone is sphingomyelin (an amino alcohol that contains a long, unsaturated hydrocarbon chain). The amino group of the sphingosine backbone is linked to a fatty acid by an amide bond. In addition, primary hydroxyl group of sphingosine is esterified to phosphorylcholine.
The second major class of membrane lipids, glycoplipids.
The glycolipids in animal cells are derived from sphingosine. The amino group of the sphingosine backbone is acylated by a fatty acid, as in sphingomyelin. One/ more sugar (rather than phosphorylcholine) are attached to the primary hydroxyl group of the sphingosine backbone. The simplest glycolipid, cerebroside, contains a single sugar residue (glucose/galactose). More complex glycolipids, gangliosides, contain a branched chain of as many as 7 sugar residues.
Cholesterol is a lipid based on a steroid nucleus.
The 3rd major type of membrane lipid has a structure that is quite different from that of phospholipids. It is a steroid (4 linked hydrocarbon rings). A hydrocarbon tail is liked to the steroid at one end, and a hydroxyl group is attached at the other end. In membranes, the orientation of the molecule is parallel to the fatty acid chains of the phospholipids, and the hydroxyl group interacts with the nearby phospholipid head group. It constitutes almost 25% of the membrane lipids in certain nerve cells but is essentially absent from some intracellular membranes.
Phospholipids and Glycolipids readily form bimolecular sheets in aqueous media.
Membrane formation is a consequence of the amphipathic nature of phospholipids. Their polar head groups favour contact with water, whereas their hydrophobic hydrocarbon tails interact with one another in preference to water. One way is to form a globular structure called micelle. Alternatively, the strongly opposed preferences of the hydrophilic and hydrophobic moieties of the membrane lipids can be satisfied by forming a lipid bilayer/bimolecular sheet. The 2 opposing sheets are called leaflets. The favoured structure for most phosphalipids and glycolipds in aqueous media is a bimolecular sheet rather than a micelle because the 2 fatty acid chains of a phospholipid/a glycolipid are too bulky to fit into the interior of a micelle. This is of critical biological importance as a micelle is a limited structure but a bimolecular sheet can extend to macroscopic dimensions. Lipid bilayers form spontaneously by a self-assembly process. In other words, the structure of a bimolecular sheet is inherent in the structure of the constituent lipid molecules. The growth of lipid bilayers from phospholipids is rapid and spontaneous in water. Hydrophobic interactions are the major driving force for the formation of lipid bilayers. Water molecules are released from the hydrocarbon tails as these tails become sequestered in the nonpolar interior of the bilayer. Van der Waals attractive forces between the hydrocarbon tails favour close packing of the tails. Finally, the electrostatic and hydrogen-bonding attractions are formed between polar head groups and water molecules. Because lipid bilayers are held together by many reinforcing, noncovalent interactions, they are cooperative structures.
A phospholipid molecule is constructed from 4 components:
- one or more fatty acids, provide a hydrophic barrier
- a plaform to which the fatty acids are attached
- a phosphate
- an alcohol, attached to the phosphate
- hydrophilic properties that enable interaction with aqueous environment.
Phosphoglyceride consists of a glycerol backbone to which are attached 2 fatty acid chains and a phosphorylated alcohol.
The hydroxyl group of the glycerol backbone is esterified to phosphoric acid. When no further additions are made, the resulting compound is phosphatidate (diacylglycerol 3-phosphate) -- the simplest phosphoglyceride.
The major phosphoglycerides are derived from phosphatidate by the formation of an ester bond between the phosphate group of phosphatidate and the hydroxyl group of one of several alcohols. The common alcohol moieties of phosphoglycerides are the amino acid serine, ethanolamine, choline, glycerol, and inositol. Some common phosphoglycerides found in the membranes are phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol, and diphosphatidylglycerol (cardiolipin).
Sphingomyeline is not derived from glycerol, instead, the backbone is sphingomyelin (an amino alcohol that contains a long, unsaturated hydrocarbon chain). The amino group of the sphingosine backbone is linked to a fatty acid by an amide bond. In addition, primary hydroxyl group of sphingosine is esterified to phosphorylcholine.
The second major class of membrane lipids, glycoplipids.
The glycolipids in animal cells are derived from sphingosine. The amino group of the sphingosine backbone is acylated by a fatty acid, as in sphingomyelin. One/ more sugar (rather than phosphorylcholine) are attached to the primary hydroxyl group of the sphingosine backbone. The simplest glycolipid, cerebroside, contains a single sugar residue (glucose/galactose). More complex glycolipids, gangliosides, contain a branched chain of as many as 7 sugar residues.
Cholesterol is a lipid based on a steroid nucleus.
The 3rd major type of membrane lipid has a structure that is quite different from that of phospholipids. It is a steroid (4 linked hydrocarbon rings). A hydrocarbon tail is liked to the steroid at one end, and a hydroxyl group is attached at the other end. In membranes, the orientation of the molecule is parallel to the fatty acid chains of the phospholipids, and the hydroxyl group interacts with the nearby phospholipid head group. It constitutes almost 25% of the membrane lipids in certain nerve cells but is essentially absent from some intracellular membranes.
Phospholipids and Glycolipids readily form bimolecular sheets in aqueous media.
Membrane formation is a consequence of the amphipathic nature of phospholipids. Their polar head groups favour contact with water, whereas their hydrophobic hydrocarbon tails interact with one another in preference to water. One way is to form a globular structure called micelle. Alternatively, the strongly opposed preferences of the hydrophilic and hydrophobic moieties of the membrane lipids can be satisfied by forming a lipid bilayer/bimolecular sheet. The 2 opposing sheets are called leaflets. The favoured structure for most phosphalipids and glycolipds in aqueous media is a bimolecular sheet rather than a micelle because the 2 fatty acid chains of a phospholipid/a glycolipid are too bulky to fit into the interior of a micelle. This is of critical biological importance as a micelle is a limited structure but a bimolecular sheet can extend to macroscopic dimensions. Lipid bilayers form spontaneously by a self-assembly process. In other words, the structure of a bimolecular sheet is inherent in the structure of the constituent lipid molecules. The growth of lipid bilayers from phospholipids is rapid and spontaneous in water. Hydrophobic interactions are the major driving force for the formation of lipid bilayers. Water molecules are released from the hydrocarbon tails as these tails become sequestered in the nonpolar interior of the bilayer. Van der Waals attractive forces between the hydrocarbon tails favour close packing of the tails. Finally, the electrostatic and hydrogen-bonding attractions are formed between polar head groups and water molecules. Because lipid bilayers are held together by many reinforcing, noncovalent interactions, they are cooperative structures.
These hydrophobic interactions have 3 significant biological consequences:
Liposomes are aqueous compartments enclosed by a lipid bilayer. They are formed by suspending a suitable lipid (phosphatidylcholine) in an aqueous medium, and then sonicating to give a dispersion of closed vesicles that are quite uniform in size and nearly spherical. Ions or molecules can be trapped in the aqueous compartments of lipid vesicles by forming the vesicles in the presence of these substances. Liposomes containing drugs/DNA for gene therapy experiments can be injected into patients. Drug delivery with liposomes often lessens its toxicity. Less of the drug is distributed to the normal tissues because long-circulating liposomes concentrate in regions of increased blood circulation (solid tumours and sites of inflammation). Moreover, the selective fusion of lipid vesicles with particular kinds of cells is a promising means of controlling the delivery of drugs to target cells.
Lipids = esters with at least one ester linkage
Classification based on chemical structure:
- Lipid bilayers have an inherent tendenct to be extensive.
- Lipid bilayers will tend to close on themselves so that there are no edges with exposed hydrocarbon chains, and so they form compartments.
- Lipid bilayers are self-sealing because a hole in a bilayer is energetically unfavourable.
Liposomes are aqueous compartments enclosed by a lipid bilayer. They are formed by suspending a suitable lipid (phosphatidylcholine) in an aqueous medium, and then sonicating to give a dispersion of closed vesicles that are quite uniform in size and nearly spherical. Ions or molecules can be trapped in the aqueous compartments of lipid vesicles by forming the vesicles in the presence of these substances. Liposomes containing drugs/DNA for gene therapy experiments can be injected into patients. Drug delivery with liposomes often lessens its toxicity. Less of the drug is distributed to the normal tissues because long-circulating liposomes concentrate in regions of increased blood circulation (solid tumours and sites of inflammation). Moreover, the selective fusion of lipid vesicles with particular kinds of cells is a promising means of controlling the delivery of drugs to target cells.
Lipids = esters with at least one ester linkage
Classification based on chemical structure:
- simple - only ester bonds
- compound - ester bonds + other bonds (phosphodiester bonds, amide bonds, ether bonds)
- derived - hydrolysed ester bonds
- complexed - noncovalent bonds
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