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• In addition to an external cell membrane (called the plasma membrane ), eukaryotic cells also contain internal
membranes that form the boundaries of organelles such as mitochondria, chloroplasts, peroxisomes, and
lysosomes .
• Phospholipids are the major class of membrane lipids
• A phospholipid molecule is constructed from four components :-
a) one or more fatty acids - a platform to which the fatty acids are attached
b) a phosphate
c) an alcohol - attached to the phosphate
▪ The fatty acid components provide a hydrophobic barrier, whereas the remainder of the molecule has
hydrophilic properties that enable interaction with the aqueous environment.
Cholesterol
Phosphatidic acid ( Phosphatidate )
• Glycerophospholipids or phosphoglycerides are phospholipids derived from glycerol.
▪ A phosphoglyceride consists of a glycerol backbone to which are attached two fatty acid chains and a
phosphorylated alcohol.
▪ Phosphatidic acid, in which glycerol is esterified to phosphoric acid and to two different carboxylic acids, is the
parent compound for glycerophospholipids .
▪ In phosphoglycerides, the hydroxyl groups at C-1 and C-2 of glycerol are esterified to the carboxyl groups of the
two fatty acid chains.
▪ The C-3 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 membrane
• Cholesterol is a lipid based on a steroid Nucleus and another major type of membrane lipids
• It is a steroid, built from four linked hydrocarbon rings.
▪ A hydrocarbon tail is linked 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 groups. Cholesterol is absent from prokaryotes and
essentially absent from some intracellular membranes.
Essential components of cell membranes
• Membrane formation is a consequence of the amphipathic nature of the molecules.
• Their polar head groups favor contact with water, whereas their hydrocarbon tails interact with one another in
preference to water.
• How can molecules with these preferences arrange themselves in aqueous solutions? One way is to form a globular
structure called a micelle .
• The polar head groups form the outside surface of the micelle, which is surrounded by water, and the hydrocarbon
tails are sequestered inside, interacting with one another
• Micelles are the preferred form of aggregation in water for detergents or soaps
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