The following points highlight the three common types of carbohydrates in plants. The types are: 1. Monosaccharides 2. Oligosaccharides and 3. Polysaccharides.
Carbohydrates Type # 1. Monosaccharides:
Six-member ring structure (Ring is in between C No. 1 and 5)
D- Fructose may also exist as D-Fructopyranose i.e., six membered ring:-
Carbohydrates Type # 2. Oligosaccharides:
(xv) Maltose (α-D-Glucopyranosyl-(1 → 4)-D-Glucopyranoside):
In maltose one molecule of D-Glucopyranose is joined through the —OH group at carbon 1 by means of an α-glycosidic linkage to the — OH group at C-4 of a second molecule of D- Glycopyranose.
It is a reducing sugar because one of the glucose units has free – OH at C-1 (Reducing centre).
This free – OH is not shown in α or β form because of the mutarotation.
(xvi) Cellobiose (β-D-Glucopryanosyl-(1→4)-D-Glucopyranoside):
In cellobiose the two D-Glucopyranose units are united by β-(1: 4) glycosidic linkage. In writing its formula the right hand unit is usually twisted so that the two glucopyranose units come on the same level.
This is also a reducing sugar
(xvii) Sucrose (Cane Sugar):
It may be designated as a-D-Glucopyranosy-(1→2) β-D- Fructofuranoside, because it consists of one α-Glucopyranose and one β-D-Fructofuranose units joined together by glycosidic linkage between C-1 of glucopyranose and C-2 of fructofuranose. Because the glycosidic bond blocks reducing groups of both the monosaccharide units, the sucrose is a non-reducing sugar. In this formula the β-D-Fructofuranose unit has been turned over as well as upside down (with respect to H and OH groups) so that the (1: 2) linkage may be shown more simply.
(xviii) Raffinose (non-reducing):
It consists of 3 monosaccharide units i.e., α-D-Galactose, α-D-Glucose, and β-D-Fructose and may be designated as α-D-Galactopyranosyl-1: 6-α-D- Glucopyranosyl-1: 2-β-D-Fructofuranoside.
In this formula also, the β-D-Fructose unit has been turned over as well as upside down so that the (1: 2) linkage may be shown more simply. α-D-Galactose unit has been twisted.
It consists of Gentiobiose + Fructose or Sucrose + Glucose
Carbohydrates Type # 3. Polysaccharides:
Cellulose is most abundant structural cell wall polysaccharide of plants. It is a long straight-chain polysaccharide consisting of many β-D-Glucose units joined together by (1: 4) glycosidic linkages. Because it consists only of glucose units, it is known as a glucosan or glucan.
Cellulose molecules occur in cell walls of plants in the form of microfibrills which are several hundred angstroms long. Microfibrills are formed from numerous cellulose chains arranged parallel to each other.
Hemi-celluloses, along with pectic substances and lignin form the matrix in which cellulose micorfibrills are embedded. Hemicelluloses are regarded as “non-cellulosic, non-pectic cell wall polysaccharides” and are composed of three different groups of polysaccharides.
These consist of large number of β-D-xylopyranose units joined together by β (1 → 4) glycosidic linkages and form usually linear or slightly branched chains. Common side chains are e.g., L-arabofuranose, D-Gulucuronic acid or a disaccharide composed of one D-xylopyranose + one L-arabofuranose unit. These side chains are attached to C2 or C3 of xylopyranose units of the linear xylan backbone through their potential reducing group i.e., C1.
(b) Mannans and glucomannans:
These polysccharides are chiefly linear molecules composed of a mixture of β (1 → 4) linked mannopyranose units and β (1 → 4) linked glucopyranose units. Mannans contain up to 95% of mannopyranose residues, the rest being glucopyranose residues. Glucommannans have a relatively higher proportion of glucopyranose units in the mixture.
(c) Galactans and arabogalactans:
These polysaccharides are highly branched molecules consisting of probably (1 → 3) and (1 → 6) linked D-Galactopyranose units to which are attached arabinose containing side chains as disaccharide groupings. Those molecules which have very low proportion of arabinose units are called as galactans while those with relatively larger proportion of arabinose units are called as arabogalactans.
(xxii) Pectic substances:
Pectic substances are important constituents of middle lamellae and primary cell walls of higher plants. These are a mixture of 3 different polysaccharides which have different chemical composition but similar solubility properties.
This polysaccharide is a linear molecule consisting of about 120 β(1 —>4) linked D-Galactopyranose units.
This polysaccharide is low mol. wt. branched chain molecule composed of β(1 → 5) linked L-arabofuranose units. About 50% of these units have β (1 → 3) linked L- arabofuranose single unit side chains.
This polysaccharide is a straight chain polymer of α (1 → 4) linked D galacturonic acid units. Some of these units are esterified with methyl groups. In the cell walls, some un-esterified carboxylic groups of D-galacturonic acid units form salts with some metal ions such as Ca and Mg.
Galacturonans form the largest component of the pectic substances. Therefore, the properties of pectic substances as a whole are usually reflected by properties of galacturonan.
This mucopolysaccharide is chief component of fungal cell walls among plants. It resembles in structure to cellulose and performs similar functions. Among animals, chitin is found in abundance in Arthropods. Chitin is composed of a linear array of β(1→4) linked 2-acetamido-2-deoxy-D-glucopyranose units. Those polysccharides which contain amino sugars or their derivatives such as chitin are called as mucopolysaccharides.
(Food Storage Polysaccharides):
It consists of two components:
(a) Amylose and
Amylose consists of about 300 α-D-Glucopyranose units joined together in α-(1 : 4) positions and may be represented as follows-:-
Amylopectin is a branched chain structure in which a-D-Glucopyranose units are mainly joined by α (1:4) linkage as in amylose, but there are also side branches with α (1: 6) linkage:
It is rather short chain polysaccharide consisting of about 30 fructofuranose units joined together by β(2→1) glycosidic linkages. Because it consists only of fructose it is called as a fructosan.