Structure, Classification and functions of Carbohydrates
Carbohydrates are biological molecules made of carbon, hydrogen, and oxygen in a ratio of
roughly one carbon atom (C) to one water molecule (H2O). Their general structure is thus
(CH2O)n. Carbohydrate chains come in different lengths, they belong to three categories:
•Monosaccharides
•Disaccharides
•Polysaccharides
monosachharides
These are simple sugars, the most common of which is glucose. Monosaccharides have a
formula of (CH2O)n and they typically contain three to seven carbon atoms.
Most of the oxygen atoms in monosaccharides are found in hydroxyl (OH) groups, but one of
them is part of a carbonyl (C=O) group. The position of the carbonyl (C=O) group can be used
to categorize the sugars:
If the sugar has an aldehyde group, meaning that the carbonyl C is the last one in the chain,
such sugars are known as an aldose.
If the carbonyl C is internal to the chain, so that there are other carbons on both sides of it, it
forms a ketone group and the sugar is called a ketose.
Sugars are also named according to their number of carbons: some of the most common types
are trioses (three carbons), pentoses (five carbons), and hexoses (six carbons).
Monosaccharide include
•Glucose - A six-carbon sugar with the formula C6H12O6.
•Galactose -which forms part of lactose, the sugar found in milk.
•Fructose asix-carbon sugar found in fruits.
•Glucose, galactose,and fructose have the same chemical formula
C6H12O6,but they differ in the organization of their atoms, making
them isomers of one another.Fructose is astructural isomer of
glucose and galactose, meaning that its atoms are actually bonded
together in adifferent order.
Glucose and galactose are
stereoisomers (have atoms
bonded together in the same
order, but differently arranged in
space).They differ in their
stereochemistry at carbon 4.
Fructose is astructural isomer of
glucose and galactose
Rings of sugars
Many five-and six-carbon sugars can exist either as a linear chain or
in one or more ring-shaped forms. These forms exist in equilibrium
with each other, but equilibrium strongly favors the ring forms
(particularly in aqueous, solution).
In solution, glucose’s main configuration is a six-membered ring.
Over 97% of glucose is typically found in this form.
1
2
3
4
5
6
1
2
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6
5
Glucose structure Fructose structure
In this six-membered ring, it can occur in two different forms with different
properties. During ring formation, the Ofrom the carbonyl, which is converted to a
hydroxyl group, will be trapped either “above” or below the ring. When the
hydroxyl is down, glucose is said to be in its alpha (α) form, and when it’s up,
glucose is said to be in its beta (β) form.
α and β forms of Monosacharides
The linear form can convert into either the alpha or the beta ring form, with the
two forms differing in the position of the hydroxyl group derived from the
carbonyl of the linear form. If the hydroxyl is up (on the same side as the CH2OH
group), then the molecule is beta glucose, while if it is down, then the molecule is
alpha glucose.
Other common monosaccharides in plants
include:
•D-Galactose
•D-Mannose
•D-Xylose
•D-arabinose
•Ribose
Activity
Identify the functions of the monosacharides above in plants
Disaccharides
Disaccharides form when two
monosaccharides join together via
adehydration reaction, also known
as acondensation reaction.In this
process, the hydroxyl group of one
monosaccharide combines with the
hydrogen of another, releasing a
molecule of water and forming a
covalent bond known as a
glycosidic linkage.
Eg when glucose and fructose
monomers combining via a
dehydration reaction to form
sucrose or table sugar.
1-2 glycosidic linkage
Other common disaccharides include:
•Lactose -A disaccharide consisting of glucose
and galactose and is found naturally in milk
•Maltose -is a disaccharide made up of two
glucose molecules.
•Sucrose –This is the most common
disaccharide (table sugar), it is made of
glucose and fructose
Polysaccharides
This is along chain of monosaccharides linked by glycosidic bonds.The chain
may be branched or unbranched and may contain different types of
monosaccharides.The molecular weight of apolysaccharide can be quite high.
In plants, polysaccharides play two major roles;
Act as storage material and act as structural molecules roles
Storage Polysaccharides
Starch is the stored form of sugars in plants and is made up of amixture of two
polysaccharides, amylose and amylopectin (both are polymers of glucose).Plants are able to
synthesize glucose using light energy gathered in photosynthesis, and the excess glucose,
beyond the plant’s immediate energy needs, is stored as starch in different plant parts,
including roots and seeds.
The starch in the seeds provides food for the embryo as it germinates and can also serve as a
food source for humans and animals;it is broken to glucose monomers using digestive
enzymes before absorption into their bodies.
In starch, the glucose monomers are in the αform, and they are connected primarily by 1-
4glycosidic linkages.
Amylose and Amylopectin
•Amylose consists entirely of
unbranched chains of glucose
monomers connected by 1-
4linkages.
•Amylopectin is abranched
polysaccharide.Although
most of its monomers are
connected by 1-4 linkages,
additional 1-6 linkages occur
periodically and result in
branch points.
•Because of the way the
subunits are joined, the
glucose chains in amylose and
amylopectin typically have a
helical structure.
Structural polysaccharides
Although energy storage is one important role for polysaccharides, they
are also crucial for providing structure. Structural polyschaccharides
include cellulose, callose and pectin
Cellulose
Cellulose is a major component of plant cell walls, which are rigid
structures. Wood and paper are mostly made of cellulose. In plants the
functions of cellulose include
•Connecting cells to form tissues
•Signaling cells to grow and divide
•Controlling the shape of plant cells
•Allowing cells to withstand the turgor pressure of the fluids inside
them
•Protecting delicate organs and tissues as seeds
•Protecting roots stems from attack by parasites
Cellulose structure
Cellulose is made up of unbranched chains of glucose monomers
linked by 1-4 glycosidic bonds.cellulose is made of glucose
monomers in their βform, and this gives it very different properties
when compared to cellulose which has α 1-4 glycosidic linkages.
These chains cluster together to form parallel bundles that are held
together by hydrogen bonds between hydroxyl groups. This gives
cellulose its rigidity and high tensile strength, which are important to
plant cells.
Humans cannot break the β glycosidic linkages in cellulose because they
do not have enzymes that break them. However, some herbivores, such as
cows, have specialized microbes that help them process cellulose.
Callose
This is a β1-3-glucan with a long unbranched helical chain. Callose
forms very compact structures and functions as a universal isolating
material in the plant.
when cells are wounded, large amounts of callose is synthesized very
rapidly at the at the plasma membrane.
Pectin
This is apolysaccharide found in the non-woody cell
walls of plants where it binds cells together and helps
the plant take in water.
In fruits, it is broken down during the ripening
process, and this is why ripe fruits are soft.The fruits
that contain the most pectin are apples, plums,
grapefruits, and oranges.
Pectin is commonly used as afood additive,
especially as athickener for jams and marmalades,
where it provides the jellylike consistency
Pectin is amixture of polymers from sugar acids,
such as D-galacturonic acid, which are connected by
(α1-4) glycosidic linkages
Fuctions of Pectin in plants
•It is a structural component of the cell wall
•Pectins form a hydrated gel phase in which the
cellulose–hemicellulose network is embedded.
•They act as hydrophilic filler, to prevent aggregation
and collapse the cellulose network.
•They also determine the porosity of the cell wall to
macromolecules.
•It functions in cell adhesion and wall hydration
•Pectin crosslinking influences wall porosity and plant
morphogenesis.
•It helps plant to withstand abiotic stresses such as high
salinity
