Membranes are thin sheet-like boundaries composed of lipids, proteins, and carbohydrates. Lipids form a bilayer that acts as a barrier to polar molecules. There are four main classes of lipids: triacylglycerols used for energy storage; glycerophospholipids that make up cell membranes; sphingolipids found in myelin sheaths and brain tissue; and cholesterol found in plasma membranes. Lipids aggregate to form structures like liposomes due to their amphiphilic nature, with hydrophobic fatty acid tails and hydrophilic heads. Lipoproteins transport lipids through the bloodstream as a complex of hydrophobic lipids surrounded by a shell of phospholipids and proteins.

1. LIPIDS AND MEMBRANES
Membranes are: sheet like, few molecules thick, closed
boundaries, asymmetric.
Lipids
Proteins
Carbohydrates
Lipids : bilayer, barrier for polar molecules.
Proteins : function as pumps, gates,receptors,enzymes, etc.
Carbohydrates: signal, receptor (cell-cell recognition), etc.
LIPIDS
Water insoluble, highly soluble in organic solvents.Role :
-Fuel molecules & energy stores
-Membranes
-Protection
-Precursors for hormones, prostaglandins, etc.
CLASSES
1.Triacylglycerol.(Triglycerides, TG)(neutral fat).
Fats and oils are TG . TG is a glycerol with 3 -OH groups
esterified with fatty acids.
H
FA OCH2
H
FA OCH
H
FA O CH2
Fatty acids (FA): is a long HC chain carboxylic acid. 12 – 24 C
(common 16 – 18 C ).
Saturated and Unsaturated.
Nomenclature of FA : from #C , or common name.
Sat.unsat.
18C octadec an oic acid 9-octa dec en oic acid
(stearic acid) -9 (oleic acid)
9,12-octa dec di en oic acid
( linoleic acid )

2. TG sat. FAfat ( m.p.) more vanderwaal
unsat. FA oil (m.p.) less vanderwaal
TG is the storage form of lipid in adipocytes (adipose tissues).
2.Glycerophospholipid. (Phophoglycerides)
Glycerol backbone in which two –OH groups are esterified to
FAs, and one –OH is esterified to phosphoric acid to give the
smallest (parent) phosphoglyceride called phosphotidic acid.
Phosphate could be esterified (i.e. –H is
substituted) by one of several alcohols :-
e.g.
ethanolamine ………… phosphotidyl ethanolamine
Choline ………….. phosphotidyl choline
3. Sphingolipid
Instead of glycerol the backbone is " ceramide" , which is
sphingosine long unsat. HC chain amine alcohol in which
amine is condensed with FA.

3. 1° -OH group is esterified to give Three different subclasses :
( l ) Shingomyelin Found in myelin surrounding nerves
isolating them.
( ll ) Cerebroside Found in neuronal cell membrane of brain.
(lll )Gangliosid brain lipid (constitutes receptor for
pituitary gland hormone).
4. Cholesterol
Found in plasma membrane of eukaryotic cells.
Lipoproteins
TG ,cholesterol and other lipids are transported as lipoprotein
(non-covalent binding). Lipoprotein particle consists of : a
core of hydrophobic lipids surrounded by a shell of polar
lipids(e.g. phospholipid).
Essential and Non-essential fatty acids
Essential FAs are important for body function but not
synthesized in our body. They have double bonds at less than
C9 from methyl end (ω9).
PROPERTIES OF LIPID AGGREGATION
Phospholipids, sphingolipids, and cholesterol are amphiphilic
molecules:
hydrophilic part(charged or polar) + hydrophobic part ( HC )
can form closed bilayer sphere or lipid vesicle "liposome" as
in cell membrane, impermeable to ions & most polar
molecules.

4. Lipoproteins
TG ,cholesterol and other lipids are transported as lipoprotein
(non-covalent binding). Lipoprotein particle consists of : a
core of hydrophobic lipids surrounded by a shell of polar
lipids(e.g. phospholipid) and apoprotein. Essential and Non-
essential fatty acids Essential
FAs are important for body function but not synthesized in
our body. They have double bonds at less than C9 from
methyl end (ω9).