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114
PANEL 2–5: Fatty Acids and Other Lipids
COMMON FATTY
ACIDS
TRIACYLGLYCEROLS
O
These are carboxylic acids
with long hydrocarbon tails.
COOH
COOH
COOH
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH
CH2
CH2
CH
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH3
CH2
CH2
palmitic
acid
(C16)
CH2
CH3
stearic
acid (C18)
CH3
oleic
acid (C18)
H2C
O
HC
O
H2C
O
Fatty acids are stored as an energy reserve (fats and
oils) through an ester linkage to glycerol to form
triacylglycerols, also known as triglycerides.
C
O
H 2C
OH
C
HC
OH
O
H 2C
OH
C
glycerol
Hundreds of different kinds of fatty acids exist. Some have one or more double bonds in their
hydrocarbon tail and are said to be unsaturated. Fatty acids with no double bonds are saturated.
–O
–O
O
O
C
C
This double bond
is rigid and creates
a kink in the chain.
The rest of the chain
is free to rotate
about the other C–C
bonds.
oleic
acid
space-filling model
carbon skeleton
UNSATURATED
CARBOXYL GROUP
SATURATED
PHOSPHOLIPIDS
Phospholipids are the major constituents
of cell membranes.
If free, the carboxyl group of a
fatty acid will be ionized.
hydrophilic
group
O
choline
O
C
O
stearic
acid
_
O
_
O
P
O
CH2
CH
CH2
But more usually it is linked to
other groups to form either esters
O
C
O
C
hydrophobic
fatty acid tails
or amides.
space-filling model of
the phospholipid
phosphatidylcholine
O
C
N
H
general structure
of a phospholipid
In phospholipids two of the –OH groups in glycerol are
linked to fatty acids, while the third –OH group is linked
to phosphoric acid. The phosphate is further linked to
one of a variety of small polar groups (alcohols).
CHAPTER 2 PANELS
115
POLYISOPRENOIDS
LIPID AGGREGATES
long-chain polymers of isoprene
Fatty acids have a hydrophilic head
and a hydrophobic tail.
O–
micelle
O
In water they can form a surface film
or form small micelles.
P
O–
O
Their derivatives can form larger aggregates held together by hydrophobic forces:
Triglycerides can form large spherical
fat droplets in the cell cytoplasm.
Phospholipids and glycolipids form self-sealing lipid
bilayers that are the basis for all cell membranes.
200 nm
or more
4 nm
OTHER LIPIDS
STEROIDS
Lipids are defined as the water-insoluble
molecules in cells that are soluble in organic
solvents. Two other common types of lipids
are steroids and polyisoprenoids. Both are
made from isoprene units.
CH3
C
CH2
CH
CH2
isoprene
Steroids have a common multiple-ring structure.
OH
HO
O
cholesterol—found in many membranes
testosterone—male steroid hormone
GLYCOLIPIDS
Like phospholipids, these compounds are composed of a hydrophobic
region, containing two long hydrocarbon tails, and a polar region,
which, however, contains one or more sugars and no phosphate.
galactose
H
OH
C
C
C
H
H
H
C
C NH
hydrocarbon tails
O
O
CH2
sugar
a simple
glycolipid
dolichol phosphate—used
to carry activated sugars
in the membrane-associated
synthesis of glycoproteins
and some polysaccharides