Steroids are a third class of lipids found
in the membranes of eukaryotes, and,
very rarely, in bacteria.
Steroids, along with lipid vitamins and
terpenes, are classified as isoprenoids
because their structures are related to
the five carbon molecule isoprene.
Steroids contain four fused rings: three
six-carbon rings designated A, B, and C
and a five-carbon D ring.
Cholesterol, is derived from the
Ancient Greek word Chole - bile
and stereos - solid
Followed by the chemical
suffix - ol for an alcohol, is an
It is a derived lipid molecule –
a sterol or modified steroid
Molecular formula C27H46O
Molar mass 386.65 g/mol
Appearance white crystalline powder
Density 1.052 g/cm3
Melting point 148–150 °C
Boiling point 360 °C (decomposes)
Solubility in water 0.095 mg/L (30 °C)
soluble in acetone, benzene,
chloroform, ethanol, ether, hexane,
isopropyl myristate, methanol
PRODUCTION OF CHOLESTEROL IN
In vertebrates the hepatic cells
typically produce greater
amounts of Cholesterol than
It is almost completely absent
among prokaryotes (bacteria and
archaea), although there are
some exceptions such as
Mycoplasma, which require
cholesterol for growth.
BODY CHOLESTEROL CONTENT
For a man of about 68 kg (150 lb), typical
total body-cholesterol synthesis is
approximately 1 g (1,000 mg) per day,
Total body content of Cholesterol is
approximately 35 g, primarily located
within the membranes of all the cells of
Typical daily dietary intake of additional
cholesterol, is 200–300 mg.
Most ingested cholesterol is esterified, and
esterified cholesterol is poorly absorbed
. The body also compensates for any
absorption of additional cholesterol by
reducing cholesterol synthesis.
For these reasons, seven to ten hours after
ingestion of cholesterol, blood levels will
show little if any effect on total body
cholesterol content or concentrations of
cholesterol in the blood.
However, during the first seven hours
after ingestion of cholesterol, the levels
Cholesterol is recycled.
The liver excretes it in a non-esterified
form (via bile) into the digestive tract
Typically about 50% of the excreted
cholesterol is reabsorbed by the small
bowel back into the bloodstream.
CHOLESTEROL IS REQUIRED
TO BUILD AND MAINTAIN
It modulates membrane
fluidity over the range of
Cholesterol serves as a precursor
for the biosynthesis of steroids
hormones, bile acids, and vitamin D
Cholesterol is the principal sterol
synthesized by animals. All kinds of
cells in animals can produce it.
Recent studies show that vitamin D
is a potent antioxidant, helping to
detox the body and protect arteries
CELL MEMBRAIN INTEGRITY
It is biosynthesized by all
animal cells because it is an
essential structural component
of animal cell membranes
It is required to maintain both
the structural integrity and
fluidity of the cell membrane
(a) To protect membrane
enables animal cells not to need a
cell wall like plants & bacteria
And thus animal cells are able to
(b) change shape and
(c) move about (unlike bacteria and
plant cells which are restricted by
their cell walls).
The hydroxyl group on cholesterol interacts
with the polar head groups of the membrane
phospholipids and sphingolipids
While the bulky steroid and the
hydrocarbon chain are embedded in the
membrane, alongside the nonpolar fatty-
acid chain of the other lipids.
Through the interaction with the
phospholipid fatty-acid chains, cholesterol
increases membrane packing, which
reduces membrane fluidity.
o The structure of the tetracyclic ring of
cholesterol contributes to the decreased
fluidity of the cell membrane as the
molecule is in a trans conformation
making all but the side chain of
cholesterol rigid and planar.
o In this structural role, cholesterol reduces
the permeability of the plasma membrane
to neutral solutes, hydrogen ions, and
Within the cell membrane, cholesterol also
functions in intracellular transport, cell
signalling and nerve conduction.
Cholesterol is essential for the structure and
function of invaginated caveolae and clathrin-
coated pits, including caveolae-dependent
and clathrin-dependent endocytosis
The role of cholesterol in such endocytosis
can be investigated by using methyl beta
cyclodextrin (MβCD) to remove cholesterol
from the plasma membrane.
Recently, cholesterol has also been
implicated in cell signalling processes,
assisting in the formation of lipid rafts in
the plasma membrane.
Lipid raft formation brings receptor
proteins in close proximity with high
concentrations of second messenger
In many neurons, a myelin sheath, rich in
cholesterol, since it is derived from
compacted layers of Schwann cell
membrane, provides insulation for more
efficient conduction of impulses.
Cholesterol inserts itself into the bilayer and keeps the
chains of the phospholipids from becoming entangled.
They also work with the proteins to allow for movement
around the cell surface.
On the top you can see how the cholesterol
(yellow) breaks up the monotony of the
The space makes it easy for proteins and lipids
to flow – hence the fluid mosaic model.
The Figure shows a lipid raft, made with
phospholipids with longer tails, and similar
proteins to do similar functions.
These rafts can move around to where they are
needed on the surface, break up and reform
later, all because the elements of them are
Thank you cholesterol.
CHOLESTEROL SERVES AS A
Vitamin ‘ D’
PRECURSOR OF STEROID
Cholesterol is the precursor to all steroid
hormones, which account for numerous
physiological functions your body needs to maintain
a normal, healthy state
Cholesterol is similar to a band aid used to repair
wounds and irritations on the arteries.
Pregnenolone is also a precursor to all other
Cholesterol is an important precursor
molecule for the synthesis of
Including the adrenal gland hormones
cortisol and aldosterone,
As well as the sex hormones
progesterone, estrogens, and
testosterone, and their derivatives.
Some research indicates cholesterol may
act as an antioxidant
Glucocorticoids are necessary for blood sugar
regulation, being extremely important for your power
houses, the mitochondria.
Mineralocorticoids are the key to balancing your
minerals, maintaining a perfect and very sensitive
homeostasis. This in turn adjust blood pressure quickly
and counteracts a loss of minerals e.g. through sweating.
Sex Hormones -All your sex hormones are made from
cholesterol. If you are male, testosterone (an androgen)
Ladies fight with estrogens and progesterone on a
Within cells, cholesterol is the precursor
molecule in several biochemical pathways.
In the liver, cholesterol is converted to bile,
which is then stored in the gallbladder.
Bile contains bile salts, which solubilize
fats in the digestive tract and aid in the
intestinal absorption of fat molecules as
well as the fat-soluble vitamins, A, D, E,
WHAT ARE BILE SALTS ?
The bile salts are conjugates of cholic
acid and chenodeoxycholic acid with
glycine and taurine. Normally the ratio
of glycine : taurine conjugates is about
3 : 1.
They are synthesised in the liver by
hydroxylation and side-chain oxidation
of cholesterol, followed by conjugation
with glycine or taurine.
conjugated bilirubin 5.1
lecithin and other
bile salts 145.0
What are the main constituents of bile?
Apart from electrolytes, the main constituents of gall bladder bile
are as follows (mmol /L )
WHAT IS THE FUNCTION OF THE BILE SALTS ?
The function of the bile
salts in the small intestine is
to emulsify dietary lipids into
mixed micelles that are
small enough to be
absorbed across the
The bile salts are amphiphilic
molecules, with a hydrophobic
planar hydrocarbon ring and a
hydrophilic region provided by
the hydroxyl groups and the
glycine or taurine conjugated
to the carboxylic acid group
above the plane of the ring.
In an aqueous medium the bile
salts will form micelles on their
In bile they also emulsify the
cholesterol that is secreted in the
bile, so keeping it in solution.
The phospholipids in bile also
help to emulsify the cholesterol
REABSORPTION OF BILE SALTS
It seems likely that bile salts are reabsorbed
from the small intestine and recycled.
Interestingly, they are not absorbed from the
micelles that contain monoacylglycerol, non-
esterified fatty acids, phospholipids and
Instead, as the other lipids are absorbed into
intestinal epithelial cells from the micelles, the
remnants for micelles that consist more or less
completely of bile salt conjugates.
These are absorbed in the terminal ileum.
Analysis of bile shows the presence of four
bile acids, conjugated with glycine and
Chenodeoxycholic and Cholic acids, which
are synthesised from cholesterol by isolated
And Deoxycholic acid, as well as a small
amount of Lithocholic acid, neither of which
is synthesised by isolated hepatocytes.
When the glycine and taurine
conjugates of chenodeoxycholic
acid and cholic acid were
incubated with a mixed culture of
Free chenodeoxycholic acid and
cholic acid were found in the
incubation medium, together with
two new compounds: lithocholic
acid and deoxycholic acid.
Intestinal bacteria seem to be able to
deconjugate the conjugated bile acids, and
metabolise them further, to lithocholic and
Although some 75 mmol of bile salts are
secreted each day, the total body pool is only
about 7.5 - 12.5 mmol; each molecule is
secreted and reabsorbed some 6 - 10 time daily.
Chenodeoxycholic and cholic acid are generally
referred to as primary bile salts, because they
are synthesised in the liver; lithocholic and
deoxycholic acids are referred to as secondary