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Theory module: 05
INTRODUCTION TO CARBOHYDRATES AND LIPIDS
Introduction:
Carbohydrates are known as the key energy source for living things. The chemical
composition of carbohydrates is made of the elements carbon, hydrogen, and oxygen.
Carbohydrates are polyhydroxy aldehydes, polyhydroxy ketones, or compounds that
can be hydrolyzed to them. The term saccharide is derivative of the Latin word
“sacchararum" from the sweet taste of sugars. It also means “hydrate of carbon”. The
general formula of carbohydrates is Cx(H2O)y. The common names for carbohydrates
are sugars, starches, saccharides etc.
Carbohydrates are one of the three macronutrients, a group that also includes protein
and fat. These organic (carbon-containing) compounds are a vital part of both plant and
animal life, life as we know it could not exist without them.
Carbohydrates play a key role in a healthy, balanced diet. This nutrient category
includes sugars, starches, and fiber. A principal role of carbohydrate is to supply
energy in the form of glucose. Glucose is a simple sugar and is often called blood sugar,
since it is the main carbohydrate found in the blood of mammals. The health and
functioning of every cell relies on blood glucose. For example if our diet lacks
appropriate amount of carbohydrates then our body lack a key fuel source.
Carbohydrates, along with proteins and fats, comprise the major components of living
matter and are used for maintenance of cellular functional activities and as reserve and
structural materials for cells. The chemistry of carbohydrates is complex by the fact that
there is a functional group (alcohol) on almost every carbon. Carbohydrate may exist in
either a straight chain or a closed ring structure. Ring structures contain two additional
functional groups: the hemiacetal and acetal. Closed rings are the most common basic
structure of carbohydrates. Glucose is an example of a single closed ring carbohydrate
known as a monosaccharide.
How Carbohydrates Are Formed:
Carbohydrates are formed by green plants in the process of photosynthesis.
Photosynthesis is a complex series of reactions which utilize the energy from the sun,
carried out by algae, phytoplankton and the leaves in plants. The simplified version of
this chemical reaction is to utilize carbon dioxide molecules from the air and water
molecules and the energy from the sun to produce a simple sugar such as glucose and
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oxygen molecules as a byproduct. The simple sugars are then converted into other
molecules such as starch, fats, proteins, enzymes, and DNA/RNA i.e. all of the other
molecules in living plants. Humans acquire their carbohydrate needs most efficiently
from the plant world.
Carbohydrate Functions:
1. Carbohydrates are initially synthesized in plants from a complex series of
reactions involving photosynthesis.
2. Carbohydrate functions as Bio Fuel
3. Carbohydrate functions as Primary Source of Energy
4. Carbohydrate functions as storage food
5. Carbohydrate functions as framework in body
6. Carbohydrate functions as Anticoagulant
7. Carbohydrate functions as Antigen
8. Carbohydrate functions as Hormone
9. Carbohydrates provide raw material for industry
10. Carbohydrates are Essential for Fat Oxidation
11. Carbohydrates plays role in gastro-intestinal function
12. Carbohydrates add flavour to the diet
13. Carbohydrates play a important role in molecular recognition
Nomenclature and classification of carbohydrates:
Carbohydrates are classified according to the number of single simple carbohydrate
unit in each chemical structure. Carbohydrate compounds having one carbohydrate
unit are called monosaccharides; compounds with two carbohydrate unit are
called dissarcharides; compounds between two to ten carbohydrate unit are
called oligossarcharides and those compounds containing more than ten carbohydrate
unit are named polysaccharides. All carbohydrates can be broken down in the simplest
monosaccharides.
Carbohydrates in its simplest from are classified according to their number of C atoms
and functional groups. The suffix –ose indicates the sugar. For example hexose contains
six carbon sugars.
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Number of carbon atoms
Triose= 3 carbon atom
Tetrose= 4 carbon atom
Pentose= 5 carbon atom
Hexose=6 carbon atom
Heptose=7 carbon atom and so on
Monosaccahrides such as glucose can be in cyclic and acyclic form.
Monosaccharides
Glucose
Galactose
Fructose
Ribose
Glyceraldehyde
Disaccharides
Sucrose
Maltose
Lactose
Oligosaccahrides
Sucrose
Maltose
Lactose
Polysaccahrides
Starch
Glycogen
Cellulose
Dextrin
Monosaccharide:
Monosaccharides are the simplest carbohydrates and are classified according to the
aldehyde or ketone derivatives, as well as the number of atoms contained in the
molecule. Hexoses contain six carbon atoms, and are found in foods, while pentoses,
ribose and deoxyribose contain five carbon atoms and are formed during the
metabolism of foodstuffs. These are the only sugars that can be absorbed and utilized
by the body. Disaccharides and polysaccharides must be finally broken down into
monosaccharides in the digestive process known as hydrolysis.
Glucose: From Greek word sweet wine: grape sugar
In nature glucose is the most abundant sugar which is converted to polysaccharides.
Excess glucose gets converted to starch in plants, glycogen in animals and fungi. Plants
cells are made up of cellulose which in turn is made of glucose. Glucose is the most
important carbohydrates in nutrition. It is one of the body fuels which directly supply
the energy needs.
Galactose: From Greek word for milk--"galact": found as a portion of lactose in milk.
Galactose differs from the other simple sugars like glucose and fructose, as it does not
occur free in nature. Digestion of lactose, a disaccharide leads to its production
Fructose: From Latin word for fruit--"fructus": found in fruits and honey; sweetest
sugar. Fructose is classified as monosaccharide’s and is very similar to galactose. These
sugars have the same chemical formula but differ in the arrangements of their chemical
groups. The older name of fructose is levulose because of tis levorotatory property of
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rotating plane polarized light to the left. Fructose is the sweetest of the sugars and is
commonly found together with glucose and sucrose furit juice and honey. Bees collect
nectar from flowers which contain sucrose.
Ribose:
Ribose and its related compound, deoxyribose, are the building blocks of the nucleic
acids, known as DNA and RNA. Ribose is used in RNA and deoxyribose is used
in DNA. The deoxy- ribose means it lack an alcohol,-OH group. Ribose and deoxyribose
are classified as monosaccharides, aldoses, pentoses, and are reducing sugars
Disaccharide
Disaccharides, on hydrolysis, gives two monosaccharide molecules. Disaccharides are
compounds that contain a bond between the first carbon of one sugar and a hydroxyl
group at any position on the other sugar. Disaccharides are sugars containing two
hexose units, such as sucrose in cane sugar, maltose in malt sugar and lactose in milk
sugar. These sugars are hydrolyzed into monosaccharides in the digestive tract by
specific enzymes and each of these specific sugars like sucrose, maltose and lactose have
a role to play in human nutrition.
Sucrose: From french word for sugar--"sucre": a disaccharide containing glucose and
fructose.Sucrose or table sugar is extracted from sugar cane or sugar beets in which it is
plentiful. It is also found in fruits and vegetables. Sucrose contains glucose and fructose
units. The glucose and fructose units are joined by an acetal oxygen bridge in the alpha
orientation. The structure contains the six member ring of glucose and the five member
ring of fructose. Refined sugars and brown sugars contain almost 100% sucrose. Any
food containing significant amount of refined sugar are high in sucrose
Maltose: From french word for "malt"; a disaccharide containing two units of glucose;
Maltose or malt sugar is the least common disaccharide in nature. Maltose occurs in the
body as an intermediate product of starch digestion. When maltose is hydrolyzed, it
gives two molecules of glucose. It is found in germinating grains (which is used to make
beer) and in small proportion in corn syrup. It is a reducing sugar.
Lactose: From Latin word for milk--"lact"; a disaccharide found in milk
containing glucose and galactose. Lactose or milk sugar occurs in the milk of mammals
- 4-6% in cow's milk and 5-8% in human milk. It is also one of the byproduct in the
manufacture of cheese. Lactose is made from galactose and glucose units. The galactose
and glucose units are joined by an acetal oxygen bridge in the beta orientation. The
enzyme lactase is needed to digest lactose, and this enzyme is not present in most, if
any, people over age three. This is one of the many reasons why milk is an objectionable
food for people over three years of age.
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Polysaccharides:
Polysaccharides contain many sugar units in long polymer chains of many repeating
units joined together with the elimination of water molecule by glycosidic linkage.
Polysaccharides are not water soluble as are the mono- and disaccharides. Though not
water soluble, starches can be dispersed in water heated to a certain temperature. The
granules of the starch swell and gelatinize and when cooled this gelatin sets to a paste.
The jelling characteristics of starches are due to amylose while amylopectin is
considered to be responsible for the gummy and cohesive properties of the paste.
Polysaccharides cannot be directly utilized by the body as disaccharides but must first
be broken down into monosaccharides, the only sugar form the body can use. There are
four polysaccharides that are important in the study of nutritional science: starch,
dextrin, glycogen and cellulose.
Starch: Starch is abundant in plants as a reserve material and helps the plants to sustain
their root and tubers during winter and nourish the growing embryo during
germination. It is found in the granular form in the plants cells and differ in its size and
shape from plant to plant. For example in wheat the granule are oval shaped where as
in corn it is rounded and angular shape. Starch granules are present in the storage
organ of a plant like seed, root, stem pith and tubers. Starch is a polymer of glucose
unit. It is mixture of two different molecular structure, amylose and amylopectin.
Amylose is a linear polymer of α-D-glucose. It contains about 200 glucose units which
are linked to one another through α-linkage involving C1 of one glucose unit with C4 of
the other whereas amylopectin, on the other hand, is a highly branched polymer. It
consists of a large number of branches of short chains each containing 20-25 glucose
units which are joined together through α-linkages involving C1 of one glucose unit
with C4 of the other. The C1 of terminal glucose unit in each chain is further linked to
C6 of the other glucose unit in the next chain through C1 – C6 α-linkage which gives
amylopectin a highly branched structure. The ratio of the two fractions varies according
to the species of plant. For example, potato starch and most cereal starches have nearly
15-30% amylose. But the waxy cereal grains, some varieties of corn plus rice and grain
sorghum, have more quantity of amylopectin.
Glycogen : Glycogen is the reserve food in the form of glucose in humans and animals.
It is to animals as starch in plants. It is synthesized and stored in the liver and muscles.
About two-thirds of total body glycogen is stored in the muscles and about one-third is
stored in the liver. It consists of long polymer chain of glucose unit joined by alpha
acetal linkages. Glycogen is similar to amylopectin, having a high molecular weight and
branched-chain structures made up of thousands of glucose molecules, however, it has
even more branching and more glucose units are present than in amylopectin which
results in more compact bushlike molecule with greater solubility and lower viscosity.
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In amylopectin the branches are separated by 12-20 glucose units, while in glycogen, the
branches occur at intervals of 8-10 glucose units.
Cellulose : Cellulose is the most abundant naturally-occurring organic substance. It
contains over 50% of the carbon in vegetation and is the structural constituent of the cell
walls of plants. It is the major component in the rigid cell walls in plants. Cellulose is a
linear polysaccharide polymer with many glucose monosaccharide units connected by
a beta acetal linkage which results in a major difference in digestibility from starch. It is
characterized by its insolubility, its chemical inertness and its physical rigidity. Humans
cannot digest cellulose because the appropriate enzymes to breakdown the beta acetal
linkages are lacking but can be digested by herbivorous such as cows, sheep’s, horses,
goats etc. In humans it is passed through the digestive tract unchanged but undigestible
cellulose is the fiber which aids in the smooth working of the intestinal tract. No
vertebrate can digest cellulose directly. Though we cannot digest cellulose, we find
many uses for it like Wood for building; paper products; cotton, linen, and rayon for
clothes; nitrocellulose for explosives; cellulose acetate for films.
Dextrin: Dextrins are most commonly consumed in cooked starch foods, as they are
acquired from starch by the action of heat. Dextrins are intermediary products of starch
digestion, also, and are formed by the action of amylases on starches. They extract the
disaccharide maltose on hydrolysis.
Nomenclature:
The major carbohydrates encountered in the body are structurally related to the
aldotriose glyceraldehyde and to the ketotriose dihydroxyacetone. Stereoisomerism is
an important character of monosaccharides. Stereoisomers are the compounds that
have the same structural formulae but different in the spatial configuration.
A carbon is said to be asymmetric when it is attached to four different atoms or groups.
The number of asymmetric carbon atoms (n) determines the possible isomers of a given
compound which is equal to 2n. For example glucose contains 4 asymmetric carbon and
thus has 16 isomers.
Glyceraldehyde is the simplest monosaccharides with 1 asymmetric carbon atom.
Glyceraldehyde exists as two stereoisomers
H-C=O
|
H-C-OH
|
CH2OH
D- Glyceraldehyde
H-C=O
|
OH-C-H
|
CH2OH
L- Glyceraldehyde
The D and L isomers are the mirror image of each other. The spatial arrangement of –H
and –OH groups on the carbon atom that is adjacent to the terminal primary alcohol
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carbon determines whether the sugar is D or L isomers. If the –OH is on the right side
the sugar is D type and if on the left side it is considered as L type. In naturally
occurring monosaccharides in the mammalian tissues are mostly D-configuration.
Optical isomerism
Carbohydrates contain asymmetric carbons, because of this carbohydrates are optically
active, and rotating the plane polarized light. If the plane polarized light is rotated to
the right (clockwise) the compound is dextrorotatory (indicated by d or +). If the plane
polarized light is rotated to the left (counter clockwise) the compound is levororotatory
( indicated by l or -).
Epimers
If two monosaccharides differ from each other in their configuration around a single
specific carbon (other than anomeric) atom they are considered as epimers to each
other. For example glucose and galactose are the epimers of each other with regard to
carbon 4. The interconversion of epimers is known as epimerization
andepimerases catalyzes this reaction
Enantiomers
Enantiomers are a special type of stereoisomers that are mirror images of each other.
Study of the chemical and physical properties of many sugars has shown the cyclic
forms predominate over chain structures, both in solution and in solid state. The
aldehyde and ketone moieties of the carbohydrates with five and six carbon will
spontaneously react with the alcohol groups present in neighboring carbons to produce
intramolecular hemiacetals and hemiketals respectively.
In glucose the –OH on carbon 5(C-5) can react intramolecularly with the carbonyl group
on (C-1) to from a stable, cyclic hemiacetal. The cyclic form of glucose is a six membered
ring such sugars are called pyranoses as they resemble pyran. Fructose forms a five
membered ring called a furanose. Hemiacetals of glucose namely a and β forms is
depicted in the figure. The configuration of glucose is conveniently represented by
Fischer formulae and or Haworth projection formulae. The spatial relationships of the
atoms of the furanose and pyranoses ring structures are more properly described by the
two conformations identified as the chair form and boat form
Mutarotation
Mutarotation is defined as the change in the specific optical rotation representing the
interconversion of a and β forms of D-glucose to an equilibrium mixture. The specific
optical rotation of a freshly prepared glucose ( a anomer) solution in water is +112.2°
which gradually changes and attains an equilibrium with a constant value of +52.7°.
The optical rotation of glucose is +18.7°
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LIPIDS
Introduction to lipids
Definition
Lipids are intracellular compounds which are insoluble, or only poorly soluble in water.
They are readily soluble in nonpolar solvents such as ether, chloroform or benzene. The
hydrophobic nature of lipids is due to the predominance of hydrocarbon chains (-CH2CH2-CH2-) in their structures.
Where do lipids come from?
Lipids are molecules that can be extracted from plants and animals using nonpolar
solvents such as ether, chloroform and acetone. Fats (and the fatty acids from which
they are made) belong to this group as do other steroids, phospholipids forming cell
membrane components etc.
Lipids functions
1.
2.
3.
4.
They serve as a storage form of metabolic fuel
They serve as a transport form of metabolic fuel
They provide structural components of membrane
They have a protective functions in bacteria, plants, insects and vertebrates,
serving as a part of the outer coating between the body of organism and the
environment
Classification
There are many different ways to classify lipids; in one of the more common, five class
are recognized
1.
2.
3.
4.
Fatty acids and their immediate derivatives, e.g., prostaglandins, leukotrienes
Glycerol esters, e.g., acylglycerols, phosphoglycerides
Sphingolipids, e.g., sphingomyelin, glycosphingolipids
Cholesterol and its derivatives, e.g., cholesterol esters, bile acids, steroid
hormones, vitamin
5. Isoprene derivatives, e.g., dolichols, vitamin A, vitamin E, vitamin K.
6. Fatty acids
The building blocks of neutral fat molecules are the fatty acid chains (three are attached
to a glycerol molecule). Fatty acids chain differs in the length of chain have an even
number and odd number of carbons. Saturated fatty acids have single bonds between
the carbon atoms. They formed from animal sources and are solid at room temperature.
Their general formula is
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CH3-(CH2)n-COOH. Other fatty acids do not bind their maximum number of
hydrogen’s due to the double bonding between carbon atoms in the chain.
Formula
CH3(CH2)10CO2H
CH3(CH2)12CO2H
CH3(CH2)14CO2H
CH3(CH2)16CO2H
CH3(CH2)18CO2H
Saturated fatty acids
Common name
lauric acid
myristic acid
palmitic acid
stearic acid
arachidic acid
Melting point
45 ºC
55 ºC
63 ºC
69 ºC
76 ºC
Fatty acids with one double bond are monosaturated while those with two or more
double bonds are polysaturated fatty acids. Unsaturated fats originate from plants are
liquid at room temperature
Unsaturated fatty acids
Common
name
palmitoleic
CH3(CH2)5CH=CH(CH2)7CO2H
acid
CH3(CH2)7CH=CH(CH2)7CO2H
oleic acid
CH3(CH2)4CH=CHCH2CH=CH(CH2)7CO2H
linoleic acid
CH3CH2CH=CHCH2CH=CHCH2CH=CH(CH2)7CO2H linolenic acid
arachidonic
CH3(CH2)4(CH=CHCH2)4(CH2)2CO2H
acid
Formula
Melting point
0 ºC
13 ºC
-5 ºC
-11ºC
-49 ºC
The higher the melting points of the saturated fatty acids reveal the uniform rod-like
shape of their molecules. The cis-double bond(s) in the unsaturated fatty acids leads a
twist in their shape, which makes it more difficult to pack their molecules together in a
stable repeating array or crystalline lattice.
Two polyunsaturated fatty acids, linoleic and linolenic, are considered as essential
because their absence in the human diet leads with health problems, such as scaley skin,
stunted growth and increased dehydration. These acids are also precursors to the
prostaglandins, a family of physiologically effective lipids present in minute amounts in
most body tissues.
Fats and oil
The triesters of fatty acids with glycerol (a type of alcohol with a hydroxyl group on
each of its three carbons) make up the class of lipids known as fats and oils.
Thesetriglycerides (or triacylglycerols) are found in both plants and animals. Bread”
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and pastries from a “bread factory” often contain mono- and diglycerides as “dough
conditioners.” Triglycerides that are solid or semisolid at room temperature are
classified as fats, and those triglycerides that are liquid are called oils. Fats occur
predominantly in animals and oil originates chiefly in plants. Properties of fats show
the predominance of saturated fatty acids and oils composes of unsaturated fatty acids.
The difference between oils and fats lies in their melting temperatures rather than in
any fundamental structural difference.
The constituent compounds include:
tetradecanoic acid CH3(CH2)12COOH;
hexadecanoic acid, CH3(CH2)14COOH;
octadeca-9,12-dienoic acid (linoleic acid),
CH3(CH2)4(CH=CHCH2)2(CH2)6COOH;
octadeca-9-enoic acid (oleic acid), CH3(CH2)7CH=CH(CH2)7COOH;
propan-1,2,3-triol (glycerol), HOCH2CH(OH)CH2OH.
Waxes
Waxes are esters of fatty acids with long chain monohydric alcohols (one hydroxyl
group). Natural waxes are often combinations of such esters, and may also contain
hydrocarbons. Waxes are widely distributed in nature. Waxes from a protective
covering over plants and fruits like dehydration and small predators and the feathers of
birds and the fur of some animals have similar coatings which serve as a water
repellent. Carnuba wax is valued for its toughness and water resistance.
Phospholipids
Phospholipids are complex fat derivatives in which one fatty acid has been replaced by
a phosphate group and one of several nitrogen-containing molecules. Phospholipids
found the largest class of lipids in all biological membranes. All phospholipid molecules
are amphipathic in nature i.e. they have two distinct ends, one polar or water loving
(hydrophilic) head and two non-polar or water hating (hydrophobic) tails.
This property makes two layers of phospholipid molecules in all biological membranes,
which spread as sheets. The polar heads face towards the outer and cytoplasmic side,
while the non-polar tails are present in the inner side forming the hydrophobic core of
the membrane. Because of this property of biological membranes does not allow polar
molecules and ions to pass through them with relative ease.
Some phospholipids from unicellular organisms and vertebrate heart tissue contain
ether linkages instead of ester linkages e.g. plasmalogen. Platelet activating factor is
another such phospholipids in vertebrates. It is released from basophils, which
stimulates platelet aggregation and release of serotonin.
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There are two classes of phospholipids
1. Glycerophospholipids (phosphoglycerides) that contain glycerol as the alcohol.
2. Sphingophospholipids (sphingomyelins) that contain sphingosine as the alcohol.
Glycerophospholipids are the main lipids that occur in biological membranes. It consist
of glycerol-3-phosphate esterified at its C1 and C2 with fatty acid. Generally C1 contains
saturated fatty acids whereas C2 contains unsaturated fatty acids
1. Phosphatidic acid: is the simplest phospholipid. It is intermediate in the
synthesis of triacylglycerols and phospholipids It is present in low concentration
in tissues.
2. Phosphatidylcholine( Lecithins): are the most abundant group in phospholipid in
cell membrane. Chemically it is a phosphatidic acid with choline as the base. It
represents the storage form of bodys choline
3. Phosphoethanolamine (Cephalins): is the nitrogenous base present in the
cephalins
4. Phosphatidyllinositol: the stereoisomer myo-inositol is attached to phosphatidic
acid to give Phosphatidyllinositol
5. Phosphatidylserine: named as amino acid serine is present in this group of
glycerophospholipid
6. Plasmalogens: When a fatty acids is attached by an ether linkage at C1 of glycerol
in the Glycerophospholipids.
7. Cardiolipin: named as it was first isolated from heart muscle. Chemically
it consists of two molecules of phosphatidic acid held by an additional glycerol
through phosphate groups
Sphingolipids:
It is the second largest class of membrane lipids. Sphingolipids like phospholipids, are
amphipathic having a polar head and two non-polar tails.They do not have glycerol in
their backbone.
All sphingolipids are derived from a compound called sphingosine or 4-sphingenine. A
molecule of fatty acid is attached to the amino group in an amide linkage at the C2
resulting in ceramide.
Ceramide is the parent compound for all sphingolipids. A polar head group is attached
to the first carbon atom by a phosphodiester or glycosidic bond. This results in an
amphipathic molecule.
There are three groups of sphingolipids: sphingomyelin, neutral glycospihngolipids
and gangliosides
1. Sphingomyelins: In sphingomyelins, the polar head group is phosphocholine
or phosphoethanolamine. It is present in the plasma membrane of all animal cells and
myelin.
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2. Glycosphingolipids: These are present on the outer face of plasma membranes.
The polar head groups are one or more monosaccharides attached to the hydroxyl
group at C, of ceramide. Galactose is the head group in the plasma membrane of neural
tissue, while glucose in the non-neural tissue. Glycosphingolipids act as cell surface
recognition molecules (antigens), especially as blood group substances
3. Gangliosides are complex sphingolipids with oligosaccharides as head groups
having, sialic acid at the termini. Sialic acid confers a negative charge to the
sphingolipid at pH 7.0.
Steroids:
Steroids are lipids that contain four carbon rings joined to form steroid nucleus,
cyclopentanoperhydrophenanthrene. Steroids are oxidized derivatives of sterols.
All steroid hormones are derived from the sterol, cholesterol, a 27 carbon compound.
An 8 carbon side chain is attached to the steroid nucleus which result in a cholesterol
molecule. Cholic acids (glycocholic acid and taurocholic acid) are the metabolic
products of cholesterol formed in the liver. These are released to the gall bladder, where
they form bile salts. Vitamin D a fat soluble vitamin is derived from the sterol,
ergosterol.
Lipid Soluble Vitamins
Vitamins are organic compounds that human tissues cannot synthesize but that are
required for normal growth and development. Vitamins are classified as water soluble
or fat soluble. Water soluble vitamins, such as vitamin C, are rapidly eliminated from
the body and their dietary levels need to be relatively high. The lipid soluble vitamins
are not as easily eliminated and may accumulate to toxic levels if consumed in large
quantity. Fat soluble vitamins are vitamin A, vitamin D, vitamin E and vitamin K, Fat
soluble vitamins can all be regarded as being built up from isoprene units {CH2C(CH3)-CH-CH2}. Human tissues can manufacture vitamin D, but not the other fat
soluble vitamins, although they can make and handle isoprene units in the synthesis of
cholesterol and ubiquinone.
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