What is a Lipid, Anyway?

14 Pages·2012·1.19 MB·English

Know
Your
Inner

F


A



T


T


Y

:

All
about
Lipids


Have
you
ever
thought
understanding
lipids
was
a
simple
task…until
you
tried
to
tackle
the
exam
questions?


This
tutorial
will
mainly
serve
as
a
guided
outline
to
comprehending
and
completing
the
toughest
and
most

intensive
questions
about
lipids.



Part
A:

What
is
a
Lipid,
Anyway?

By
definition,
a
lipid
is
“an
[organic]
molecule
of
biological
origin
that
is
soluble
in
solvents
of
low
polarity
and

insoluble
in
solvents
of
high
polarity.”1

At
first
glance,
there
seem
to
be
many
complex
parts
to
this
definition,
so
let’s
break
it
down:

First,
a
lipid
is
an
organic
molecule,
meaning
the
molecule
must
contain
carbon.

Furthermore,
lipids
are

biomolecules,
meaning
that
they
derive
from
biological
origins
(ie.
are
synthesized
by
a
living
organism).

As

such,
at
its
most
base
level,
the
molecular
formula
for
lipids
will
contain
carbons,
hydrogens,
and
oxygens.

Secondly,
lipids
are
soluble
in
solvents
of
low
polarity,
which
simply
means
that
lipids
are
soluble
in
nonpolar

solvents.

This
solubility
property
is
a
result
of
the
large
nonpolar
regions
that
are
indicative
of
the
lipid

molecule.




HOW
CAN
YOU
DISTINGUISH
A
LARGE
NONPOLAR
REGION?

Long
hydrocarbon
chains
in
lipid
molecules
develop
the
largely
nonpolar
regional
property
of
lipids.

Due
to

carbon
and
hydrogen
forming
nonpolar
bonds
due
to
their
low
electronegative
difference
(as
opposed
to

Oxygen‐Hydrogen,
which
have
a
high
electronegative
difference
and
are
thus
polar),
a
large
hydrocarbon

chain
or
ring
creates
a
nonpolar
region.



For
example:
PHOSPHOLIPID






































NONPOLAR
REGION

Another
example:
STEROIDS
The
large
hydrocarbon
rings,
methyl
groups,
and
hydrocarbon
chains
all
consist
of
nonpolar
C‐H
bonds
due
to

its
low
electronegative
difference,
and
as
such
contributed
to
its
non‐polarity.



When
finals
day
approaches,
it
will
be
important
to
understand
what
areas
of
the
lipid
molecule
contribute
to

its
non‐polarity,
so
take
heed!

What’s
the
significance
of
the
non‐polarity/low
polarity?

The
most
simplistic
(and
not
entirely
chemically
true)
rule‐of‐thumb
is
that
“like
dissolves
like”,
which

indicates
that
a
solute
will
dissolve
in
a
solvent
of
identical
polar
property,
but
two
substances
of
opposing

polar
properties
will
not
interact.

For
example,
a
nonpolar
molecule
(ie.
a
fatty
acid)
will
not
dissolve
in
a

polar
substance
(ie.
water),
and
vice
versa.



As
such,
thirdly,
lipids
are
insoluble
in
water
(a
polar
molecule
due
to
the
high
electronegative
O‐H
bond

difference),
for
non‐polar
molecules
are
insoluble
in
polar
molecules.

Thus,
the
presence
of
a
nonpolar

molecule
(or
region
of
molecule)
means
that
the
region
or
molecule
is
hydrophobic
(Greek,
“water
fearing”),

and
produces
a
hydrophobic
effect,
“which
causes
the
polar
ends
to
be
oriented
outwards
towards
the
solvent

and
the
nonpolar
regions
oriented
inwards
away
from
the
polar
solvent.”2,
when
in
contact
with
a
polar

solvent
(ie.
solvent),
making
the
nonpolar
molecule
is
insoluble
in
water.

This
term
is
interchangeable
with

lipophilic
(Greek,
“fat
loving”),
indicated
that
nonpolar
lipids
are
soluble
in
other
nonpolar
solvents.



Thus,
polar
molecules
are
considered
to
be
hydrophilic
(“water
loving)
and
lipophobic
(“water
fearing”),
due

to
the
ability
of
highly
polar
bonds
being
soluble
in
polar
solvents,
and
insoluble
in
nonpolar
solvents
(ie.
fats,

lipids).



However,
there
are
categories
of
lipids
that
are
both
hydrophobic
and
hydrophilic.

These
lipids
are
called

amphiphiles,
molecules
that
contain
both
large
areas
of
polarity
and
nonpolarity,
and
are
thus
both

hydrophilic
and
lipophilic.
Phospholipids
have
both
a
long
hydrophobic
hydrocarbon
tail,
and
a
hydrophilic

head,
giving
phospholipids
integral
biological
properties,
which
will
be
later
discussed.



SUMMARY,
PLEASE!:
Lipids
have
large
nonpolar
regions
deriving
from
nonpolar
hydrocarbon
bonds
(which

have
low
electronegative
difference).

Thus,
lipids
are
largely
insoluble
in
polar
solvents
(water),
and
are

soluble
in
nonpolar
molecules.
As
such,
the
nonpolar
regions
are
hydrophobic/lipophilic,
and
the
polar

regions
are
hydrophilic/lipophobic.
Part
2:

What
are
the
Different
kinds
of
Lipids?

There
are
8
major
classifications
of
lipids
(with
7
of
them
being
important
for
the
purpose
of
Chem
14C)

1. Fatty
Acids


2. Waxes


3. Triacylglycerides


4. Phospholipids


5. Prostaglandins
6. Steroids


7. Lipophilic
Vitamins


8. Terpenes
(plant‐based
lipids)
Part
3:

How
in
the
world
do
I
tell
these
lipids
apart?

This
section
will
be
the
most
invaluable
to
you
for
the
exam.

Most
exam
questions
from
previous
years
center

on
how
to
distinguish
and
classify
types
of
lipids,
and
the
important
definitions
behind
said
molecules.



Fatty
Acids:

Fatty
acids
are
lipids
that
consist
of
(1)
Carboxylic
acid
(‐COOH)
with
a
(2)
long,
un‐branched
hydrocarbon

chain.

Example:
Arachidic
acid
(C H O )

20 40 2













































































































































CARBOXYLIC
ACID
(‐COOH)

LONG
UNBRANCHED
HYDROCARBON
CHAIN
(C‐H)

Fatty
acids
commonly
have
the
following
characteristics
(be
sure
to
look
for
these
properties
on
the
exam)

‐ Even
number
of
carbons
(ex.
arachidic
acid
has
20
carbons).

The
most
common
fatty
acids
consist
of

12‐20
carbons,
while
the
most
biologically
important
fatty
acids
have
18
carbons
(for
example,
stearic,

oleic,
and
linoleic
acids)

‐ 
Act
as
a
basic
“building
block”
for
more
complex
lipid
molecules
(act
as
a
precursor
to
other
lipids).


For
example,
the
long
hydrocarbon
chain
provides
an
essential
nonpolar
region
for
the
hydrophobic

tail
of
phospholipids;
waxes,
triacylglycerides,
and
phospholipids
have
at
least
one
fatty
acid
as
an

attachment.

‐ Low
polarity
due
to
the
low
electronegative
difference
in
the
C‐H
bonds
of
the
long
hydrocarbon
chain,

producing
a
hydrophobic
effect.




Furthermore,
fatty
acids
can
be
divided
into
two
subcategories:
saturated
and
unsaturated.

This
is
main

categorized
by
the
presence
of
C=C
bonds
in
the
hydrocarbon
chain.

Saturated
carbons
have
full
C‐H
bonds
in

the
hydrocarbon
chain,
and
as
such
have
non
C=C
bonds.

Unsaturated
carbons,
however,
have
at
least
one

C=C
bond
(monounsaturated,
more
than
one
is
polyunsaturated)
in
the
chain
to
replace
two
hydrogen
C‐H

bonds.



Within
unsaturated
fatty
acids,
cis‐C=C
bonds
(Carbon
bonds
are
on
the
same
side)
are
much
more
prevalent

than
trans‐C=C
bonds
(carbon
bonds
on
opposite
sides)
in
nature.

“Trans‐fats”
are
not
as
easily
metabolized

by
catalytic
enzymes
as
cis‐fats,
and
are
left
uncatalyzed
and
can
damage
health
(plaque
buildup).
Waxes:

Waxes
are
(1)
esters
that
have
attached
(2)
a
fatty
acid
and
(3)
a
long‐chain
alcohol.

The
synthesis
of
waxes

usually
derives
from
the
dehydration
synthesis
of
a
fatty
acid
and
a
long‐chain
alcohol
group.




ESTER

Fatty
acid


Long‐chain


 alcohol
(‐OH)

Myricyl cerotate


 Present in beeswax, carnauba
wax

Due
to
the
low
polarity
caused
by
the
two
long
hydrocarbon
chains
on
either
side
of
the
ester
linkage,
waxes

are
highly
insoluble
in
polar
solvents
and
are
hydrophobic;
extremely
repellent
toward
water.

As
a
result,
the

major
biological
function
waxes
are
to
act
as
a
water
barrier
(ie.

bird
feathers
are
coated
in
wax
lipids,
which

minimizes
wetting
of
the
feathers,
leaves
are
coated
in
wax
to
prevent
the
evaporation
of
water,
damaging

plant
nourishment.
Triacylglycerols:

Triacyglycerols,
or
triacylglycerides,
are
lipids
similar
to
that
of
waxes
(in
that
they
contain
ester
and
an

alcohol‐like
function
group),
but
consist
of
(1)
a
triester,
(2)
a
fatty
acid,
and
(3)
glycerol,
or
glycerin,
a
three

carbon
alcohol
structure:


Glycerol Fatty acids

Glycerol
and
the
fatty
acid
undergo
dehydration
synthesis,
which
in
turn
creates
the
triacylglycerol
molecule.




Triacylglycero
l
Where
 “R”
indicates
the
long,
un‐branched
hydrophobic
tail
of
hydrocarbons
that
are
representative
of
the

fatty
acid.



Triacylglycerols
have
the
following
important
characteristics

‐ Varying
melting
points.

If
solid
at
room
temperature,
then
the
lipid
is
considered
a
fat,
if
liquid
at
room

temperature,
oil.

‐ The
most
abundant
naturally
synthesized
lipid.

‐ Its
main
biological
function
is
for
energy
storage
(ie.
consumption
of
fats
provides
a
slow
burning
long‐
term
storage
of
energy
that
can
be
metabolized).

‐ With
the
addition
of
water
in
a
hydrolysis
reaction,
the
“breaking”
of
fats
derived
from
animals
yields

soaps,
which
are
hydrophilic
CO ‐
groups
attached
to
a
hydrophobic
hydrocarbon
chain.



2
HOW
DO
SOAPS
WORK?

1. The
polar,
hydrophilic
head
of
soap
(CO ‐)
is
attracted
to
the
positively
charged
ends
of
water,
while
the

2
hydrophobic
hydrocarbon
tail
can
avoid
the
water.

2. This
hydrophobic,
lipophilic
tail
then
can
attach
to
dirts,
fatty
acids,
and
other
lipids,
due
to
their
low

polarity.

3. These
tails
surround
the
“dirts”
in
spheres
called
micelles,
which
isolate
the
dirt
and
remove
them

from
interaction
with
the
water
molecules.

4. Finally,
when
the
water
is
removed,
the
micelles
encapsulating
the
dirt
are
carried
away,
leaving
the

area
free
of
the
dirts.
Phospholipids:

A
phospholipid
similarly
consists
of
(1)
glycerol,
three‐carbon
alcohol
structures
which
then
forms
an
(2)

triester
with
(3)
two
fatty
acids
and
(4)
one
phosphate
group

Generic
example:

Phosphate
group
ester

2
Fatty
acid
esters


Where
“R”
represents
the
long
un‐branched
hydrocarbon
chain
representative
of
a
fatty
acid.



Phospholipids
have
the
following
key
properties:

‐ Second
most
abundant
naturally
synthesized
lipid

‐ Its
major
biological
function
is
to
form
a
integral
phospholipid
bilayer
in
the
cell
membrane
of
living

organisms,
a
critical
component
that
allows
for
the
selective
diffusion
of
ions
and
molecules
through

the
barrier

HOW
DOES
THE
PHOSPHOLIPID
BILAYER
WORK?


Two
phospholipids
face
apart
from
each
other,
the
polar,
hydrophilic
head
of
the
phosphate
group
facing

outwards,
and
the
hydrophobic,
hydrocarbon
chain
on
the
fatty
acids
facing
inwards.

This
creates
the

hydrophobic
effect
of
the
tails,
avoiding
water.

This
bilayer
of
the
cell
membrane
allows
for
the
cell
to
freely

move
through
water,
yet
maintain
the
internal
structure
of
the
cell
Prostoglandins:

Prostoglandins
are
lipid
molecules
that
contain
a
(1)
prostanoic
acid
skeleton,
which
consist
of
a
(2)

cyclopentane
ring
attached
to
a
upper
chain
of
a
(3)
fatty
acid
with
seven
carbons,
and
a
lower
(4)
long,

unbranched
chain
of
hydrocarbons,
with
8
carbons.



Ex.
Prostaglandin
F 

2alpha

 Cyclopentane

ring



 Upper
chain
of
fatty

acids


Lower
chain
of


 hydrocarbons


Prostaglandin F (PGF )

2 2
α α

The
nomenclature
of
prostaglandins
is
dependent
on
the
stereochemistry
of
the
lipid,
based
on
the
number
of

OH,
C+C,
and
C=O
groups
in
the
molecule.

Major
characteristics
of
prostaglandins
include:

‐ Major
biological
function
to
act
as
a
regulator
and
signal
molecule
(regulation
of
hormones,

inflammation,
calcium
movement;
control
of
cellular
growth;
constriction
of
smooth
muscle
cells;

regulation
of
platelet
growth;
spinal
neuron
sensitization)

‐ Prostoglandins
often
synthesize
at
wound
sites
to
regulate
and
signal
an
inflammatory
response,

leading
to
inflammation.

‐ Short
half‐life
from
origination,
about
5
minutes
or
less.



‐ While
each
structure
has
the
same
prostanoic
acid
skeleton,
the
differing
stereochemistry
of
each

prostaglandin
derives
vastly
differing
biological
functions
for
each
lipid.