CARBOHYDRATES
CARBOHYDRATES
A carbohydrate is a biological molecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually
with a hydrogen–oxygen atom ratio of 2:1 (as in water); in other words, with the empirical formula Cm(H2O)n(where m could be different from n).[1] Some exceptions
exist; for example, deoxyribose, a sugar
component ofDNA,[2] has the empirical formula C5H10O4.[3] Carbohydrates are
technically hydrates of carbon;[4] structurally it is
more accurate to view them as polyhydroxy aldehydes and ketones.[5]
The term is most common in biochemistry, where it
is a synonym of saccharide, a group that includes sugars,starch, and cellulose. The saccharides
are divided into four chemical groups: monosaccharides, disaccharides,oligosaccharides, and polysaccharides. In
general, the monosaccharides and disaccharides, which are smaller (lower molecular weight)
carbohydrates, are commonly referred to as sugars.[6] The word saccharide comes from the Greek word σάκχαρον (sákkharon),
meaning "sugar." While the scientific nomenclature of carbohydrates
is complex, the names of the monosaccharides and disaccharides very often end
in the suffix -ose. For example, grape sugar is the
monosaccharide glucose, cane sugar is the
disaccharide sucrose, and milk sugar
is the disaccharide lactose (see
illustration).
Carbohydrates perform numerous roles in
living organisms. Polysaccharides serve for the storage of energy (e.g. starch and glycogen) and as
structural components (e.g. cellulose in plants and chitin in arthropods).
The 5-carbon monosaccharide ribose is an important
component of coenzymes (e.g. ATP, FAD andNAD) and the backbone of the genetic molecule known as RNA. The related deoxyribose is a component of
DNA. Saccharides and their derivatives include many other important biomolecules that play key
roles in the immune system, fertilization,
preventing pathogenesis, blood clotting, and development.[7]
In food science and in many
informal contexts, the term carbohydrate often means any food that is
particularly rich in the complex carbohydrate starch (such as cereals,
bread and pasta) or simple carbohydrates, such as sugar (found in candy, jams, and desserts).
Often in lists of nutritional
information, such as the USDA National
Nutrient Database, the term "carbohydrate" (or
"carbohydrate by difference") is used for everything other than
water, protein, fat, ash, and ethanol.[8] This will include
chemical compounds such as acetic or lactic acid, which are
not normally considered carbohydrates. It also includes "dietary fiber"
which is a carbohydrate but which does not contribute much in the way of food energy (calories), even
though it is often included in the calculation of total food energy just as
though it were a sugar.[9]
STRUCTURE:
Formerly the name
"carbohydrate" was used in chemistry for any compound
with the formula Cm (H2O) n. Following this definition, some
chemists consideredformaldehyde (CH2O) to be the simplest carbohydrate,[10] while others claimed that title for glycolaldehyde.[11] Today, the term is
generally understood in the biochemistry sense, which excludes compounds with
only one or two carbons and includes many biological carbohydrates which
deviate from this formula. For example, while the above representative formulas
would seem to capture the commonly known carbohydrates, ubiquitous and abundant
carbohydrates often deviate from this. For example, carbohydrates often display
chemical groups such as: N-acetyl (e.g. chitin), sulphate (e.g.
glycosaminoglycans), carboxylic acid (e.g. sialic acid) and deoxy modifications
(e.g. fucose and sialic acid).
Natural
saccharides are generally built of simple carbohydrates called monosaccharides with general formula (CH2O)n where n is three or more. A typical monosaccharide has the
structure H–(CHOH)x(C=O)–(CHOH)y–H, that is, an aldehyde or ketone with many hydroxyl groups added,
usually one on each carbon atomthat is not part of the aldehyde or ketone functional group. Examples
of monosaccharides are glucose, fructose, and glyceraldehydes. However,
some biological substances commonly called "monosaccharides" do not
conform to this formula (e.g. uronic acids and deoxy-sugars
such as fucose) and there are
many chemicals that do conform to this formula but are not considered to be
monosaccharides (e.g. formaldehyde CH2O and inositol (CH2O)6).[12]
The open-chain form of a
monosaccharide often coexists with a closed ring form where the aldehyde/ketone carbonyl group carbon (C=O)
and hydroxyl group (–OH) react forming a hemiacetal with a new C–O–C
bridge.
Monosaccharides
can be linked together into what are called polysaccharides (or oligosaccharides) in a
large variety of ways. Many carbohydrates contain one or more modified
monosaccharide units that have had one or more groups replaced or removed. For
example, deoxyribose, a
component of DNA, is a modified version of ribose; chitin is composed of
repeating units of N-acetyl glucosamine, a nitrogen-containing form
of glucose.
DIVISION
Carbohydrates are polyhydroxy aldehydes,
ketones, alcohols, acids, their simple derivatives and their polymers having
linkages of the acetal type. They may be classified according to their degree
of polymerization and may be divided initially into three principal groups,
namely sugars, oligosaccharides and polysaccharides[13]
The major dietary
carbohydrates
|
||
Class (DP*)
|
Subgroup
|
Components
|
Malto-oligosaccharides
|
||
Other
oligosaccharides
|
||
Non-starch
polysaccharides
|
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DP * = Degree of polymerization
MONOSACCHARIDES
Monosaccharides are the simplest
carbohydrates in that they cannot be hydrolyzed to
smaller carbohydrates. They are aldehydes or ketones with two or more hydroxyl
groups. The general chemical formula of
an unmodified monosaccharide is (C•H2O) n,
literally a "carbon hydrate." Monosaccharides are important fuel
molecules as well as building blocks for nucleic acids. The smallest
monosaccharides, for which n=3, are dihydroxyacetone and D- and
L-glyceraldehydes.
Classification of monosaccharide
The α and β anomers of
glucose. Note the position of the hydroxyl group (red or green) on the anomeric
carbon relative to the CH2OH group bound to carbon 5: they either have
identical absolute configurations (R,R or S,S) (α), or opposite absolute
configurations (R,S or S,R) (β).[14]
Monosaccharides are classified according to
three different characteristics: the placement of its carbonylgroup, the number of carbon atoms it contains, and its chiral handedness.
If the carbonyl group is analdehyde, the monosaccharide is an aldose;
if the carbonyl group is a ketone,
the monosaccharide is aketose. Monosaccharides with three carbon atoms are
called trioses, those with four are called tetroses, five are called pentoses, six are hexoses, and so on.[15] These
two systems of classification are often combined. For example, glucose is
an aldohexose (a
six-carbon aldehyde), ribose is an aldopentose (a
five-carbon aldehyde), and fructose is
a ketohexose (a
six-carbon ketone).
Each carbon atom bearing a hydroxyl group (-OH),
with the exception of the first and last carbons, areasymmetric, making them stereo centers with
two possible configurations each (R or S). Because of this asymmetry, a number
of isomers may
exist for any given monosaccharide formula. Using Le Bel-van't Hoff
rule, the aldohexose
D-glucose, for example, has the formula (C·H2O) 6,
of which four of its six carbons atoms are stereogenic, making D-glucose one of
24=16 possible stereoisomers. In the case ofglyceraldehydes, an aldotriose, there is one pair of
possible stereoisomers, which are enantiomers andepimers. 1, 3-dihydroxyacetone, the ketose corresponding to the aldose
glyceraldehydes, is a symmetric molecule with no stereo centers. The assignment
of D or L is made according to the orientation of the asymmetric carbon
furthest from the carbonyl group: in a standard Fischer projection if the
hydroxyl group is on the right the molecule is a D sugar, otherwise it is an L
sugar. The "D-" and "L-" prefixes should not be confused
with "d-" or "l-", which indicate the direction that the
sugar rotates plane polarized light. This usage of "d-" and
"l-" is no longer followed in carbohydrate chemistry.[16]
The aldehyde or ketone group of a
straight-chain monosaccharide will react reversibly with a hydroxyl group on a
different carbon atom to form a hemiacetal or hemiketal, forming a heterocyclic ring
with an oxygen bridge between two carbon atoms. Rings with five and six atoms
are called furanose and pyranose forms,
respectively, and exist in equilibrium with the straight-chain form.[17]
During the conversion from straight-chain
form to the cyclic form, the carbon atom containing the carbonyl oxygen, called
theanomeric carbon, becomes a stereogenic center with two
possible configurations: The oxygen atom may take a position either above or
below the plane of the ring. The resulting possible pair of stereoisomers is
called anomers. In the α anomer, the -OH substituent on the anomeric carbon
rests on the opposite side (trans) of the ring from the CH2OH
side branch. The alternative form, in which the CH2OH
substituent and the anomeric hydroxyl are on the same side (cis) of the plane
of the ring, is called theβ anomer.
Monosaccharides are the major source of fuel
for metabolism, being used both as an energy source (glucose being
the most important in nature) and inbiosynthesis. When monosaccharides are not immediately
needed by many cells they are often converted to more space-efficient forms,
often polysaccharides. In many animals, including humans, this
storage form is glycogen, especially in liver and muscle cells. In
plants, starch is used for the same purpose. The most
abundant carbohydrate, cellulose, is a structural component of the cell wall of
plants and many forms of algae. Ribose is a component of RNA. Deoxyribose is
a component of DNA. Lyxose is a component of lyxoflavin found in the
human heart.[18] Ribulose and xylulose occur
in the pentose phosphate
pathway. Galactose, a component of milk sugar lactose, is found in galactolipids in plant cell membranes and
in glycoproteins in
many tissues. Mannose occurs
in human metabolism, especially in the glycosylation of
certain proteins. Fructose, or fruit sugar, is found in many plants and
in humans, it is metabolized in the liver, absorbed directly into the
intestines during digestion, and found in semen. Trehalose, a major sugar of insects, is rapidly
hydrolyzed into two glucose molecules to support continuous flight.
STRUCTURE
Monosaccharides (from Greek monos:
single, sacchar: sugar), also called simple sugars, are the most basic units
of carbohydrates.
They are fundamental units of carbohydrates and cannot be further hydrolised to
simpler compounds. The general formula is C
nH
2nO
n. They are the simplest form of sugar and are usuallycolorless, water-soluble, and crystalline solids. Some monosaccharides have a sweet taste. Examples of monosaccharides include glucose (dextrose), fructose(levulose) and galactose. Monosaccharides are the building blocks of disaccharides (such as sucrose and lactose) and polysaccharides (such as cellulose andstarch). Further, each carbon atom that supports a hydroxyl group (so, all of the carbons except for the primary and terminal carbon) is chiral, giving rise to a number of isomeric forms, all with the same chemical formula. For instance, galactose and glucose are both aldohexoses, but have different physical structures and chemical properties.
nH
2nO
n. They are the simplest form of sugar and are usuallycolorless, water-soluble, and crystalline solids. Some monosaccharides have a sweet taste. Examples of monosaccharides include glucose (dextrose), fructose(levulose) and galactose. Monosaccharides are the building blocks of disaccharides (such as sucrose and lactose) and polysaccharides (such as cellulose andstarch). Further, each carbon atom that supports a hydroxyl group (so, all of the carbons except for the primary and terminal carbon) is chiral, giving rise to a number of isomeric forms, all with the same chemical formula. For instance, galactose and glucose are both aldohexoses, but have different physical structures and chemical properties.
Linear-chain monosaccharides
Simple monosaccharides have a linear and unbranched
carbon skeleton with one carbonyl (C=O) functional group, and one hydroxyl (OH)
group on each of the remaining carbon atoms.
Therefore, the molecular structure of a simple monosaccharide can be written as
H(CHOH)n(C=O)(CHOH)mH, where n + 1 + m = x; so that its elemental formula is CxH2xOx.
By convention, the carbon atoms are numbered from 1
to x along the
backbone, starting from the end that is closest to the C=O group.
If the carbonyl is at position 1 (that is, n or m is zero), the molecule begins
with a formyl group
H(C=O)− and is technically an aldehyde. In that
case, the compound is termed an aldose.
Otherwise, the molecule has a keto group, a
carbonyl −(C=O)− between two carbons; then it is formally a ketone, and is
termed a ketose. Ketoses
of biological interest usually have the carbonyl at position 2.
The various classifications above can be combined,
resulting in names such as "aldohexose" and "ketotriose".
A more general nomenclature for open-chain
monosaccharides combines a Greek prefix to indicate the number of carbons
(tri-, tetr-, pent-, hex-, etc.) with the suffixes "-ose" for aldoses
and "-ulose" for ketoses. In the latter case, if the carbonyl is not
at position 2, its position is then indicated by a numeric infix. So, for
example, H(C=O)(CHOH)4H is pentose, H(CHOH)(C=O)(CHOH)3H is
pentulose, and H(CHOH)2(C=O)(CHOH)2H is pent-3-ulose.
Open-chain stereoisomers
Two
monosaccharides with equivalent molecular
graphs (same chain length and same carbonyl
position) may still be distinct stereoisomers, whose molecules differ in the
three-dimensional arrangement of the bonds of
certain atoms. This happens only if the molecule contains a stereogenic center, specifically a carbon atom that ischiral (connected to four distinct molecular
sub-structures). Those four bonds can have any of two configurations in space
distinguished by their handedness. In a simple open-chain monosaccharide,
every carbon is chiral except the first and the last atoms of the chain, and
(in ketoses) the carbon with the keto group.
For example, the
triketose H(CHOH)(C=O)(CHOH)H (glycerone, dihydroxyacetone) has no stereogenic center, and therefore
exists as a single stereoisomer. The other triose, the aldose H(C=O)(CHOH)2H
(glyceraldehyde),
has one chiral carbon — the central one, number 2 — which is bonded
to groups −H, −OH, −C(OH)H2, and −(C=O)H. Therefore, it exists as two
stereoisomers whose molecules are mirror images of each other (like a left and
a right glove). Monosaccharides with four or more carbons may contain multiple
chiral carbons, so they typically have more than two stereoisomers. The number
of distinct stereoisomers with the same diagram is bounded by 2c, where c is
the total number of chiral carbons.
The Fischer projection is
a systematic way of drawing the skeletal
formula of an acyclic monosaccharide so that the
handedness of each chiral carbon is well specified. Each stereoisomer of a
simple open-chain monosaccharide can be identified by the positions (right or
left) in the Fischer diagram of the chiral hydroxyls (the hydroxyls attached to
the chiral carbons).
Most stereoisomers
are themselves chiral (distinct from their mirror images). In the Fischer
projection, two mirror-image isomers differ by having the positions of all
chiral hydroxyls reversed right-to-left. Mirror-image isomers are chemically
identical in non-chiral environments, but usually have very different
biochemical properties and occurrences in nature.
While most
stereoisomers can be arranged in pairs of mirror-image forms, there are some
non-chiral stereoisomers that are identical to their mirror images, in spite of
having chiral centers. This happens whenever the molecular graph is
symmetrical, as in the 3-ketopentoses H(CHOH)2(CO)(CHOH)2H,
and the two halves are mirror images of each other. In that case, mirroring is
equivalent to a half-turn rotation. For this reason, there are only three
distinct 3-ketopentose stereoisomers, even though the molecule has two chiral
carbons.
Distinct
stereoisomers that are not mirror-images of each other usually have different
chemical properties, even in non-chiral environments. Therefore, each mirror
pair and each non-chiral stereoisomer may be given a specific monosaccharide name. For example, there are 16 distinct
aldohexose stereoisomers, but the name "glucose" means a specific
pair of mirror-image aldohexoses. In the Fischer projection, one of the two
glucose isomers has the hydroxyl at left on C3, and at right on C4 and C5;
while the other isomer has the reversed pattern. These specific monosaccharide
names have conventional three-letter abbreviations, like "Glu" for
glucose and "Thr" for threose.
Generally, a
monosaccharide with n asymmetrical
carbons has 2n stereoisomers. The number of open chain
stereoisomers for an aldose monosaccharide is larger by one than that of a
ketose monosaccharide of the same length. Every ketose will have 2(n−3) stereoisomers
where n >
2 is the number of carbons. Every aldose will have 2(n−2) stereoisomers
where n >
2 is the number of carbons. These are also referred to as epimers which have
the different arrangement of −OH and −H groups at the asymmetric or chiral
carbon atoms (this does not apply to those carbons having the carbonyl
functional group).
Chirality nomenclature
Like many chiral molecules, the two stereoisomers
of glyceraldehyde will gradually rotate the polarization
direction of linearly polarized light as it
passes through it, even in solution. The two stereoisomers are identified with
the prefixes D- and L-,
according to the sense of rotation: D-glyceraldehyde
is dextrorotatory (rotates the polarization axis
clockwise), while L-glyceraldehyde
is levorotatory (rotates it counterclockwise).
D- and L-glucose
The D-
and L-
prefixes are also used with other monosaccharides, to distinguish two particular
stereoisomers that are mirror-images of each other. For this purpose, one
considers the chiral carbon that is furthest removed from the C=O group. Its
four bonds must connect to −H, −OH, −C(OH)H, and the rest of the molecule. If
the molecule can be rotated in space so that the directions of those four
groups match those of the analog groups inD-glyceraldehyde's C2, then the isomer
receives the D-
prefix. Otherwise, it receives the L-
prefix.
In the Fischer projection, the D- and L-
prefixes specifies the configuration at the carbon atom that is second from
bottom: D-
if the hydroxyl is on the right side, and L-
if it is on the left side.
Note that the D- and L-
prefixes do not indicate the direction of rotation of polarized light, which is
a combined effect of the arrangement at all chiral centers. However, the two
enantiomers will always rotate the light in opposite directions, by the same
amount. See also d/l system.
Cyclic isomers
A monosaccharide
often switches from the acyclic (open-chain) form to a cyclic form,
through a nucleophilic addition reaction
between the carbonyl group and one of the hydroxyls of the same molecule. The
reaction creates a ring of carbon atoms closed by one bridgingoxygen atom.
The resulting molecule has an hemiacetal or hemiketal group,
depending on whether the linear form was an aldose or a ketose. The reaction is
easily reversed, yielding the original open-chain form.
In these cyclic
forms, the ring usually has 5 or 6 atoms. These forms are called furanoses and pyranoses, respectively — by analogy with furan and pyran, the simplest compounds with the same
carbon-oxygen ring (although they lack the double bonds of these two
molecules). For example, the aldohexose glucose may
form a hemiacetal linkage between the hydroxyl on carbon 1 and the oxygen on
carbon 4, yielding a molecule with a 5-membered ring, called glucofuranose. The same reaction can take place between
carbons 1 and 5 to form a molecule with a 6-membered ring, called glucopyranose. Cyclic forms with a 7-atom ring (the same
of oxepane), rarely encountered, are called heptoses.
Conversion between the
furanose, acyclic, and pyranose forms of D-glucose.
Pyranose forms of some
pentose sugars.
Pyranose forms of some hexose sugars.
For many
monosaccharides (including glucose), the cyclic forms predominate, in the solid
state and in solutions, and therefore the same name commonly is used for the
open- and closed-chain isomers. Thus, for example, the term "glucose"
may signify glucofuranose, glucopyranose, the open-chain form, or a mixture of
the three.
Cyclization
creates a new stereogenic center
at the carbonyl-bearing carbon. The −OH group that replaces the carbonyl's
oxygen may end up in two distinct positions relative to the ring's midplane.
Thus each open-chain monosaccharide yields two cyclic isomers (anomers), denoted by the prefixes α- and β-. The
molecule can change between these two forms by a process called mutarotation, that consists in a reversal of the
ring-forming reaction followed by another ring formation.[1]
The stereochemical
structure of a cyclic monosaccharide can be represented in a Haworth projection. In this diagram, the α-isomer has the -OH
of the anomeric
carbon below the plane of the carbon atoms, and the
β-isomer has the -OH of the anomeric carbon above the plane. Pyranoses
typically adopt a chair conformation, similar to that of cyclohexane. In this conformation, the α-isomer has the
-OH of the anomeric carbon in an axial position, whereas the β-isomer has the
OH- of the anomeric carbon in equatorial position.[2]
·
α-D-Glucopyranose
·
β-D-Glucopyranose
PROPERTIES:
Monosaccharides such as glucose and fructose are crystalline solids at
room temperature, but they are quite soluble in water, each molecule having
several OH groups that readily engage in hydrogen bonding. The chemical
behavior of these monosaccharides is likewise determined by their functional
groups.
An important reaction of monosaccharides is the oxidation of the
aldehyde group, one of the most easily oxidized organic functional groups.
Aldehyde oxidation can be accomplished with any mild oxidizing agent, such as Tollens’
reagent or Benedict’s reagent. With the latter, complexed
copper(II) ions are reduced to copper(I) ions that form a brick-red precipitate
[copper(I) oxide; Figure 16.5.1].
Any carbohydrate capable of reducing either Tollens’ or Benedict’s reagents without first undergoing hydrolysis is said to be a reducing sugar. Because both the Tollens’ and Benedict’s reagents are basic solutions, ketoses (such as fructose) also give positive tests due to an equilibrium that exists between ketoses and aldoses in a reaction known as tautomerism.
Figure 16.5.1 Benedict’s Test. Benedict’s
test was performed on three carbohydrates, depicted from left to right:
fructose, glucose, and sucrose. The solution containing sucrose remains blue
because sucrose is a nonreducing sugar.
These reactions have been used as
simple and rapid diagnostic tests for the presence of glucose in blood or
urine. For example, Clinitest tablets, which are used to test for sugar in the
urine, contain copper(II) ions and are based on Benedict’s test. A green color
indicates very little sugar, whereas a brick-red color indicates sugar in
excess of 2 g/100 mL of urine.
DISACCHARIDES:
Two joined monosaccharides are called a disaccharide and
these are the simplest polysaccharides. Examples includesucrose and lactose. They are composed of two monosaccharide
units bound together by a covalent bond
known as aglycosidic linkage formed
via a dehydration reaction, resulting in the loss of a hydrogen atom
from one monosaccharide and a hydroxyl
group from the other. The formula of
unmodified disaccharides is C12H22O11. Although there are numerous kinds of
disaccharides, a handful of disaccharides are particularly notable.
Sucrose, pictured to the right, is the most abundant
disaccharide, and the main form in which carbohydrates are transported in
plants. It is composed of one D-glucose molecule
and one D-fructose molecule.
The systematic
name for sucrose, O-α-D-glucopyranosyl-(1→2)-D-fructofuranoside,
indicates four things:
·
Its
monosaccharides: glucose and fructose
·
How they are
linked together: the oxygen on carbon number 1 (C1) of α-D-glucose is linked to
the C2 of D-fructose.
·
The -oside suffix indicates that the anomeric
carbon of both monosaccharides participates in the
glycosidic bond.
Lactose, a disaccharide composed of one D-galactose molecule
and one D-glucose molecule,
occurs naturally in mammalian milk. The systematic
name for lactose is O-β-D-galactopyranosyl-(1→4)-D-glucopyranose.
Other notable disaccharides include maltose (two
D-glucoses linked α-1,4) and cellulobiose (two D-glucoses linked β-1,4).
Disaccharides can be classified into two types: reducing and non-reducing
disaccharides. If the functional group is present in bonding with another sugar
unit, it is called a reducing disaccharide or biose.
A disaccharide (also called a double
sugar or biose[1]) is the sugar formed when two monosaccharides (simple sugars) are joined. Like
monosaccharides, disaccharides are soluble in water. Three common examples are sucrose, lactose,[2] and maltose.
Disaccharides are one of the four chemical groupings of carbohydrates (monosaccharides,
disaccharides, oligosaccharides,
and polysaccharides).
The most common types of disaccharides—sucrose, lactose,
and maltose—have
twelve carbon atoms, with the general formula C12H22O11.
The differences in these disaccharides are due to atomic arrangements within
the molecule.[3]
The joining of simple sugars into a double sugar happens
by a condensation reaction,
which involves the elimination of a water molecule from the functional
groupsonly. Breaking apart a double sugar into its two simple
sugars is accomplished by hydrolysis with the help of a type of enzyme called a disaccharidase. As building the larger sugar
ejects a water molecule, breaking it down consumes a water molecule. These
reactions are vital in metabolism.
Each disaccharide is broken down with the help of a corresponding
disaccharidase (sucrase, lactase,
and maltase).
Classification[edit]
There are two functionally
different classes of disaccharides:
·
Reducing
disaccharides, in which one monosaccharide, the reducing
sugar of the
pair, still has a free hemiacetal unit that can perform as a reducing aldehydegroup; cellobiose and maltose are examples of reducing disaccharides,
each with one hemiacetal unit, the other occupied by the glycosidic bond, which prevents it from acting as a reducing
agent.
·
Non-reducing
disaccharides, in which the component monosaccharides bond through an acetal linkage between their anomeric centers. This results in neither
monosaccharide being left with a hemiacetal unit that is free to act as a reducing
agent. Sucrose and trehalose are examples of non-reducing disaccharides
because their glycosidic bond is between their respective hemiacetal carbon
atoms. The reduced chemical reactivity of the non-reducing sugars in comparison
to reducing sugars, may be of advantage where stability in storage is
important.
Formation[edit]
The formation of a disaccharide
molecule from two monosaccharide molecules proceeds by displacing a hydroxyl radical from
one molecule and a hydrogen
nucleus(a proton) from
the other, so that the now vacant bonds on the monosaccharides join the two monomers together. The vacant bonds on the hydroxyl
radical and the proton unite in their turn, forming a molecule of water, that then
goes free. Because of the removal of the water molecule from the product, the
term of convenience for such a process is "dehydration reaction" (also "condensation reaction" or "dehydration synthesis"). For example, milk sugar
(lactose) is a
disaccharide made by condensation of one molecule of each of the
monosaccharides glucose and galactose,
whereas the disaccharide sucrose in sugar cane and sugar beet, is a
condensation product of glucose and fructose. Maltose,
another common disaccharide, is condensed from two glucose molecules.[6]
The dehydration reaction that
bonds monosaccharides into disaccharides (and also bonds monosaccharides into
more complex polysaccharides) forms
what are called glycosidic
bonds.[7]
Properties[edit]
The glycosidic bond can be
formed between any hydroxyl group on the component monosaccharide. So, even if
both component sugars are the same (e.g.,glucose),
different bond combinations (regiochemistry) and stereochemistry (alpha- or beta-)
result in disaccharides that are diastereoisomers with different chemical and physical
properties.
Depending on the monosaccharide constituents, disaccharides are sometimes
crystalline, sometimes water-soluble, and sometimes sweet-tasting and
sticky-feeling.
Disaccharides
Disaccharides Definition
Disaccharides [Greek di = two; sacchar = sugar] are sugars composed of 2 monosaccharides (Chart
1). Disaccharides,
along with monosaccharides, are called simple carbohydrates.
Three Common
Disaccharides
The three most
common disaccharides in foods are sucrose, lactose and maltose.
A Typical Disaccharide Structure
A disaccharide molecule is formed by 2 monosaccharides,
joined by a glycosidic bond (Picture
1). The type of a
glycosidic bond can determine the properties of certain disaccharides. For
example, sucrose, isomaltulose and trehalulose are all composed of glucose and
fructose, which are linked by different types of glycosidic bonds.
Picture 1. A disaccharide sucrose composed of
monosaccharides glucose and fructose
BIOLOGICAL
IMPORTANCE
disaccharides are formed of 2 monosaccharide
units.so, on hydrolysis they give rise to monosaccharides,which are the actual
energy providers.
egs:lactose-the milk sugar
matose-the malt sugar
sucrose-the cane sugar
the biological importance of carbohydrates in general:
principal source of energy
structural components of cells
stored as glycogen in liver of higher animals (reserve)
carbohydrates like cellulose are rouhage.
egs:lactose-the milk sugar
matose-the malt sugar
sucrose-the cane sugar
the biological importance of carbohydrates in general:
principal source of energy
structural components of cells
stored as glycogen in liver of higher animals (reserve)
carbohydrates like cellulose are rouhage.
Types
Three
disaccharides exist in the human diet. Lactose is naturally found in the milk
of mammals and contains glucose and galactose. Sucrose is found in beets and
sugar cane. Sucrose, also called table sugar, contains glucose and fructose.
Maltose is a disaccharide that contains two molecules of glucose. Maltose forms
during the breakdown of certain starches, such as barley, during food
manufacturing. Glucose is the simple sugar cell that your body uses for energy.
Your body normally converts galactose and fructose, also called fruit sugar,
into glucose.
Lactose
Lactose,
also called milk sugar, is the nutritional source of energy for infants during
nursing. Lactose makes milk tastes sweet and is an ingredient in many processed
foods that contain dairy. Manufacturers add whey, a byproduct of dairy
production that contains lactose, to certain food products, such as breads,
cookies, cakes, doughnuts, breakfast bars and ice cream. Lactose requires you
have an enzyme called lactase to digest the disaccharide. Many people are
lactose intolerant and do not produce sufficient amounts of lactase to digest
lactose, causing such symptoms as nausea, diarrhea, gas, abdominal cramps and
bloating. You can take a lactase supplement to help you digest lactose and
ameliorate the symptoms.
Sucrose
Sucrose is the disaccharide present most in the
human diet. The function of sucrose is to sweeten foods for more taste appeal.
Manufacturers add sucrose as a sweetener to candies, ice cream, cookies, cakes,
breads, sauces, ketchup and canned goods. An advantage of using sucrose as a
sweetener is that it is stable in both liquid and crystallized solid states.
Manufacturers replaced sucrose as a sweetener in many food and beverage products with high-fructose corn syrup, a sweetener that contains a mixture of glucose and high concentrations of fructose and is less costly than sucrose. However, high-fructose corn syrup increases abdominal fat and weight more than sucrose does, according to research by scientists at Princeton University that was published in "Pharmacology, Biochemistry and Behavior" in November 2010.
Manufacturers replaced sucrose as a sweetener in many food and beverage products with high-fructose corn syrup, a sweetener that contains a mixture of glucose and high concentrations of fructose and is less costly than sucrose. However, high-fructose corn syrup increases abdominal fat and weight more than sucrose does, according to research by scientists at Princeton University that was published in "Pharmacology, Biochemistry and Behavior" in November 2010.
Maltose
POLYSACCHARIDES
Polysaccharides are often quite
heterogeneous, containing slight modifications of the repeating unit. Depending
on the structure, these macromolecules can have distinct properties from their
monosaccharide building blocks. They may be amorphous or even insoluble in water.[1][2] When all the monosaccharides in a
polysaccharide are the same type, the polysaccharide is called ahomopolysaccharide or homoglycan, but
when more than one type of monosaccharide is present they are called heteropolysaccharides or heteroglycans.[3][4]
Natural saccharides are
generally of simple carbohydrates called monosaccharides with general formula (CH2O)n where n is three or more. Examples of
monosaccharides are glucose, fructose, andglyceraldehyde.[5] Polysaccharides, meanwhile, have a general
formula of Cx(H2O)y where x is usually a large number between 200 and
2500. Considering that the repeating units in the polymer backbone are often
six-carbon monosaccharides, the general formula can also be represented as (C6H10O5)n where 40≤n≤3000.
Polysaccharides contain more
than ten monosaccharide units. Definitions of how large a carbohydrate must be
to fall into the categories polysaccharides or oligosaccharides vary according to personal opinion. Polysaccharides
are an important class of biological
polymers. Their function in
living organisms is usually either structure- or storage-related. Starch (a polymer of glucose) is used as a storage
polysaccharide in plants, being found in the form of both amylose and the branched amylopectin. In
animals, the structurally similar glucose polymer is the more densely branched glycogen,
sometimes called 'animal starch'. Glycogen's properties allow it to be
metabolized more quickly, which suits the active lives of moving animals.
Cellulose and chitin are examples of structural polysaccharides.
Cellulose is used in the cell
walls of
plants and other organisms, and is said to be the most abundantorganic
molecule on
Earth.[6] It has many uses such as a significant role
in the paper and textile industries, and is used as a feedstock for the
production of rayon (via the viscose process), cellulose acetate, celluloid, and
nitrocellulose. Chitin has a similar structure, but has nitrogen-containing
side branches, increasing its strength. It is found in arthropod exoskeletons and in the cell walls of some fungi. It also has multiple uses, including surgical
threads. Polysaccharides also includecallose or laminarin, chrysolaminarin, xylan, arabinoxylan, mannan, fucoidan and galactomannan.
Nutrition polysaccharides are
common sources of energy. Many organisms can easily break down starches into
glucose; however, most organisms cannot metabolize cellulose or other
polysaccharides like chitin and arabinoxylans. These
carbohydrate types can be metabolized by some bacteria and protists. Ruminants andtermites, for
example, use microorganisms to process cellulose.
Even though these complex to
carbohydrates are not very digestible, they provide important dietary elements
for humans. Called dietary
fiber, these carbohydrates enhance digestion among other benefits.
The main action of dietary fiber is to change the nature of the contents of the gastrointestinal tract, and to change how other
nutrients and chemicals are absorbed.[7][8] Soluble fiber binds to bile
acids in the
small intestine, making them less likely to enter the body; this in turn lowerscholesterol levels in the blood.[9] Soluble fiber also attenuates the
absorption of sugar, reduces sugar response after eating, normalizes blood
lipid levels and, once fermented in the colon, produces short-chain fatty acids as byproducts with wide-ranging
physiological activities (discussion below). Although insoluble fiber is
associated with reduced diabetes risk, the mechanism by which this occurs is
unknown.[10]
Not yet formally proposed as an
essential macronutrient (as of 2005), dietary fiber is nevertheless regarded as
important for the diet, with regulatory authorities in many developed countries
recommending increases in fiber intake.[7][8][11][12]
Storage
polysaccharides[edit]
Starches[edit]
Starch is glucose polymer in which glucopyranose units are bonded by alpha-linkages. It is made up
of a mixture of amylose (15–20%) and amylopectin (80–85%). Amylose consists of a linear
chain of several hundred glucose molecules and Amylopectin is a branched
molecule made of several thousand glucose units (every chain of 24–30 glucose
units is one unit of Amylopectin). Starches are insoluble in water. They
can be digested which can break the alpha-linkages
(glycosidic bonds). Both humans and animals have amylases, so they can digest
starches. Potato, rice, wheat, and maize are
major sources of starch in the human diet. The formations of starches are the
ways that plants store glucose. .
Glycogen[edit]
Glycogen serves as the
secondary long-term energy storage in animal and fungal cells,
with the primary energy stores being held in adipose
tissue. Glycogen is made primarily by the liver and the muscles, but
can also be made by glycogenesis within the brain and stomach.[13]
Glycogen is the analogue of starch, a
glucose polymer in plants, and
is sometimes referred to as animal
starch,[14] having a similar structure to amylopectin but more extensively branched and compact
than starch. Glycogen is a polymer of α(1→4) glycosidic bonds linked, with
α(1→6)-linked branches. Glycogen is found in the form of granules in the cytosol/cytoplasm in many cell types, and plays an important role in the glucose
cycle. Glycogen forms an energy reserve that can be quickly mobilized to
meet a sudden need for glucose, but one that is less compact and more
immediately available as an energy reserve than triglycerides (lipids).
In the liver hepatocytes, glycogen can compose up to eight
percent (100–120 g in an adult) of the fresh weight soon after a meal.[15] Only the glycogen stored in the liver can
be made accessible to other organs. In the muscles, glycogen
is found in a low concentration of one to two percent of the muscle mass.
The amount of glycogen stored in the body—especially within the muscles, liver, and red
blood cells[16][17][18]—varies
with physical activity, basal metabolic rate, and eating habits such as intermittent fasting. Small amounts of glycogen are
found in the kidneys, and
even smaller amounts in certain glial cells in the brain and white
blood cells. The uterus also stores glycogen during pregnancy, to nourish
the embryo.[15]
Glycogen is composed of a
branched chain of glucose residues. It is stored in liver and muscles.
·
It is an energy reserve for animals.
·
It is the chief form of carbohydrate stored in animal body.
·
It is insoluble in water. It turns brown-red when mixed with
iodine.
·
Schematic 2-D cross-sectional view of glycogen. A core protein
of glycogenin is surrounded by branches of glucose units. The entire globular granule may
contain approximately 30,000 glucose units.[19]
·
Structural
polysaccharides[edit]
Arabinoxylans[edit]
Arabinoxylans are found in both the primary and secondary
cell walls of plants and are the copolymers of two pentose sugars: arabinose and xylose.
Cellulose
The structural component of plants are
formed primarily from cellulose. Wood
is largely cellulose and lignin, while paper and cotton are nearly pure cellulose. Cellulose is a polymer made with repeated glucose units bonded
together by beta-linkages.
Humans and many animals lack an enzyme to break the beta-linkages, so they do not
digest cellulose. Certain animals such as termites can digest cellulose, because bacteria
possessing the enzyme are present in their gut. Cellulose is insoluble in
water. It does not change color when mixed with iodine. On hydrolysis, it
yields glucose. It is the most abundant carbohydrate in nature.
Chitin
Chitin is one of many naturally occurring polymers. It
forms a structural component of many animals, such as exoskeletons. Over
time it is bio-degradable in the natural environment. Its breakdown may be
catalyzed by enzymes called chitinases, secreted by microorganisms such as bacteria and fungi, and produced by some plants. Some of these microorganisms have receptors to simple sugars from the decomposition of chitin. If chitin
is detected, they then produce enzymes to digest it by cleaving the glycosidic
bonds in
order to convert it to simple sugars and ammonia.
Chemically, chitin is closely
related to chitosan (a more water-soluble derivative of
chitin). It is also closely related to cellulose in that it is a long unbranched chain ofglucose derivatives. Both materials contribute
structure and strength, protecting the organism.
Pectins
Pectins are a family of complex polysaccharides
that contain 1,4-linked α-D-galactosyluronic acid residues. They are present in
most primary cell walls and in the non-woody parts of terrestrial plants.
Acidic
polysaccharides
Acidic polysaccharides are
polysaccharides that contain carboxyl
groups, phosphate groups and/or sulfuric ester groups.
Bacterial
capsular polysaccharides
Pathogenic bacteria commonly
produce a thick, mucous-like, layer of polysaccharide. This "capsule"
cloaks antigenic proteins on the bacterial surface that would
otherwise provoke an immune response and thereby lead to the destruction of the
bacteria. Capsular polysaccharides are water-soluble, commonly acidic, and havemolecular
weights on the
order of 100-2000 kDa. They are linear and consist of regularly
repeating subunits of one to six monosaccharides. There
is enormous structural diversity; nearly two hundred different polysaccharides
are produced by E. coli alone. Mixtures of capsular
polysaccharides, either conjugated or native are used as vaccines.
Bacteria and many other
microbes, including fungi and algae, often secrete polysaccharides to help them adhere to surfaces
and to prevent them from drying out. Humans have developed some of these
polysaccharides into useful products, including xanthan gum, dextran, welan gum, gellan gum, diutan gum and pullulan.
Most of these polysaccharides
exhibit useful visco-elastic properties when dissolved in water at very
low levels.[20] This makes various liquids used in everyday
life, such as some foods, lotions, cleaners, and paints, viscous when
stationary, but much more free-flowing when even slight shear is applied by
stirring or shaking, pouring, wiping, or brushing. This property is named
pseudoplasticity or shear
thinning; the study of such matters is called rheology.
Aqueous solutions of the
polysaccharide alone have a curious behavior when stirred: after stirring
ceases, the solution initially continues to swirl due to momentum, then slows
to a standstill due to viscosity and reverses direction briefly before
stopping. This recoil is due to the elastic effect of the polysaccharide
chains, previously stretched in solution, returning to their relaxed state.
Cell-surface polysaccharides
play diverse roles in bacterial ecology and physiology. They
serve as a barrier between the cell
wall and the
environment, mediate host-pathogen interactions, and form structural components
of biofilms. These polysaccharides are
synthesized from nucleotide-activated
precursors (callednucleotide
sugars) and, in most cases, all the enzymes necessary for
biosynthesis, assembly and transport of the completed polymer are encoded by
genes organized in dedicated clusters within the genome of the organism. Lipopolysaccharide is one of the most important cell-surface
polysaccharides, as it plays a key structural role in outer membrane integrity,
as well as being an important mediator of host-pathogen interactions.
The enzymes that make the A-band (homopolymeric) and B-band (heteropolymeric) O-antigens have been identified
and the metabolic
pathways defined.[21] The exopolysaccharide alginate is a linear copolymer
of β-1,4-linked D-mannuronic acid and L-guluronic acid residues, and is
responsible for the mucoid phenotype of late-stage cystic fibrosis disease. The pel and psl loci are two recently discovered gene
clusters that also encode exopolysaccharides found to be important for biofilm
formation. Rhamnolipid is a biosurfactant whose production is
tightly regulated at the transcriptional level, but the precise role that it plays
in disease is not well understood at present. Protein glycosylation,
particularly of pilin and flagellin, became a focus of research by
several groups from about 2007, and has been shown to be important for adhesion
and invasion during bacterial infection.[22]
Chemical
identification tests for polysaccharides
Periodic
acid-Schiff stain (PAS)
Polysaccharides with unprotected vicinal diols or amino sugars (i.e. some OH groups replaced with
amine) give a positive Periodic acid-Schiff stain (PAS). The list of polysaccharides that stain with PAS
is long. Although mucins of epithelial origins stain with PAS, mucins of
connective tissue origin have so many acidic substitutions that they do not
have enough glycol or amino-alcohol groups left to react with PAS.
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