CHAPTER 17
LIPIDS
WHAT ARE LIPIDS?
Naturally occurring compounds that are soluble
in nonpolar solvents, but not in water
 Examples include fats, oils, waxes, steroids
(including cholesterol)
 Variety of functions in the body (vitamins,
hormones/chemical messengers, protection of
internal organs…)

A. LIPIDS
 Types


of Lipids
Lipids such as waxes, fats, oils, and
glycerophospholipids can be hydrolyzed, giving
free fatty acids and an alcohol.
Steroids, on the other hand, do not contain
fatty acids, and therefore cannot undergo a
hydrolysis reaction to liberate fatty acids.
Steroids are characterized by a structure of
four fused carbon rings.
CLASSES OF LIPIDS
B. FATTY ACIDS
Simplest type of lipid
 As we’ve seen, can be a component of a more
complex lipid
 Composition of a fatty acid: long carbon chain
with a carboxylic acid group at one end.
Example: lauric acid

FATTY ACIDS CONT.
Saturated fatty acids contain all single bonds in
their carbon chain.
 Monounsaturated fatty acids have only one
double bond in the carbon chain.
 Polyunsaturated fatty acids have two or more
double bonds in the carbon chain.
 Double bonds in fatty acids are capable of cistrans isomerism.

FATTY ACIDS CONT.

Properties of Fatty Acids

Saturated fatty acids can pack/stack together well,
allowing for strong attractions all along the carbon
chains.
This means -- it takes a LOT of energy to separate the
chains. The longer the chains, the stronger the
attraction.
Therefore -- saturated fatty acids are usually solids at
room temp, and the melting point increases as the chain
length increases.
FATTY ACIDS CONT.

Properties of Fatty Acids cont.

On the other hand, unsaturated fatty acids have lower
melting temperatures than saturated fatty acids.
Most naturally occurring fatty acids have cis double
bonds. This puts a kink into the carbon chain.
Therefore, unsaturated fatty acids don’t pack together
quite so well. Not so much energy required to separate
the unsaturated fatty acids = lower melting point.
Often liquid at room temperature.
SATURATED/UNSATURATED
EXAMPLE
 Consider
palmitoleic acid (structure on
board)
How many carbon atoms does it contain?
 Is it saturated or unsaturated?
 Is it more likely to be solid or liquid at room
temperature?

PROSTAGLANDINS
Hormone-like substances produced at low-levels
in the body, formed from arachidonic acid (an
unsaturated fatty acid)
 Many roles in the body -- involved in regulation
of blood pressure, uterine contractions during
birth, and stimulation of pain and inflammation
during tissue injury
 NSAIDs (such as aspirin or ibuprofen) block the
production of prostaglandins in order to reducce
pain and inflammation

C. WAXES, FATS, AND OILS

Waxes: often found as waterproof coating on the
outside of plants and animals

Structurally: ester of a saturated fatty acid and a longchain alcohol, 14-30 carbons each
FATS AND OILS

Fats and Oils: Triacylglycerols




Also known as triglycerides
Triesters of glycerol and fatty acids. General formula:
glycerol backbone esterified to three fatty acids
The three fatty acids may be identical, but are typically
different/mixed
Major form of energy storage for animals (this is what
bears use during hibernation!)
FATS AND OILS

Melting Points and Fats and Oils



Fat: solid at room temp (meat, butter, cheese)
Oil: liquid at room temp (olive oil, peanut oil)
Question: What kind of fatty acid (thinking of
structure) would you expect to find in fat? In oil?
OLIVE OIL LOOKS LIKE…
D. CHEMICAL PROPERTIES OF
TRIACYLGLYCEROLS

Hydrogenation

A hydrogenation reaction involves conversion of a
carbon-carbon double bond to a carbon-carbon single
bond through the addition of hydrogen.

This reaction can be applied to an entire triacylglycerol
molecule.
CHEMICAL PROPERTIES…

Hydrogenation cont.

As you continue to hydrogenate your molecule…
Melting point increases
 Fat becomes more solid at room temp


However, these “partially hydrogenated vegetable oil”
products still contain no cholesterol, unlike butter and
other animal products.
CHEMICAL PROPERTIES…
Oxidation of unsaturated fats can occur in the
presence of oxygen and microorganisms. This
results in rancid oils that have “disagreeable”
odors.
 Triacylglycerols can be hydrolyzed into glycerol
and free fatty acids in the presence of water and
a catalyst.
 And, a similar reaction to hydrolysis -saponification -- can occur when the fat is heated
in the presence of NaOH.

(SECTIONS TO SKIP!)

Don’t worry about…



Glycerophospholipids
Sphingolipids
It’s not that they aren’t important -- we already
have enough on our plate to memorize, eh?
E. STEROIDS: CHOLESTEROL, BILE
SALTS, AND STEROID HORMONES

Steroid: See p. 631 in the text. All steroids
contain the same 4 fused ring structure -- three
cyclohexane rings, one cyclopentane ring.

They do not hydrolyze to give fatty acids
CHOLESTEROL
Technically a steroid, since it contains the 4fused ring structure
 Classified as a sterol because it contains a
hydroxyl group
 A key component of cellular membranes, so we
must have some in our diets
 But… most Americans consume too much



Associated with the formation of gallstones
Also associated with the formation of plaques lining the
coronary arteries
BILE SALTS
Synthesized from cholesterol and stored in the
gallbladder
 When you eat fat, the bile salts act as soap and
help emulsify the large globules of fat.

LIPOPROTEINS

Since lipids are nonpolar and therefore insoluble
in the blood, how do they get to their target
tissues?


They are combined with a more polar protein, forming a
lipoprotein, which can be transported in the
bloodstream.
There are several types of lipoprotein, differing in
density, lipid composition, and function.
STEROID HORMONES

What is a hormone?


Chemical messengers -- allow one part of the body to
communicate with another part
Steroid hormones: include sex/adrenocortical
hormones (testosterone, estrogens, progesterone,
cortisone)
F. CELL MEMBRANES
Role: Separate inner contents of cell from outer
environment; regulate what comes into and out
of the cell
 Cell membranes are mainly made of
phospholipids




Phospholipid: a triglyceride where one of the fatty acids
has been replaced by a polar phosphate group.
The phospholipid has a polar head and a nonpolar “tail”
Cell membranes are arranged in a bilayer. Polar
outside so the cell is soluble in water, nonpolar
inside serves as a barrier.
VIEW OF A CELL MEMBRANE
TRANSPORT THROUGH CELL
MEMBRANES
Some molecules are small enough to diffuse
through the cell membrane on their own = simple
passive transport
 Other molecules are too big, or are charged,
needing a protein to help get across = facilitated
(passive) transport
 When a substance moves against its
concentration gradient (from lower to higher
concentration) = active transport, since this
requires energy

MEMBRANE TRANSPORT