Let us make an in-depth study of the carbohydrates in plants. After reading this article you will learn about 1. Meaning of Carbohydrates 2. Classification of the Carbohydrates 3. Structural Features of Open Chain and Ring Forms of Monosaccharides.
Meaning of Carbohydrates:
Carbohydrates are a group of organic compounds consisting of C, H, O usually in the ratio of 1: 2: 1 and include such well known compounds as sugars, starch, cellulose etc.
Previously, the carbohydrates were regarded as hydrates of carbon and corresponded to general formula (C.H2O) n.
But the group name ‘carbohydrates’ was sometimes found misleading because:
(i) Some organic compounds e.g., formaldehyde (HCHO), acetic acid (CH3COOH), lactic acid (C3H6O1) inositol (C6H12O6) etc. correspond to the general formula but are not carbohydrates.
(ii) Some carbohydrates e.g., rhamnose (C6H12O5), rhamnohexose (C7H14O6), digitoxose (C6H12O4) do not correspond to the above general formula.
(iii) Besides containing C, H and O, some carbohydrates also contain nitrogen and sulphur.
Therefore, the carbohydrates are more appropriately referred to as polyhydroxyaldehydes or polyhydroxyketones and their derivatives or the substances which yield these on hydrolysis. But, the group name ‘carbohydrates’ is still retained traditionally.
The metabolism of carbohydrates is of utmost importance to organisms individually and collectively. Fundamentally, all organic foodstuffs are ultimately derived from the synthesis of carbohydrates through photosynthesis.
The catabolism of carbohydrates provides the major share of the energy requirement for maintenance of life and preformation of work. Moreover carbohydrates act as energy reservoirs and serve architectural functions and are important constituents of nucleic acids.
Classification of the Carbohydrates:
Depending upon their complexity and behaviour on hydrolysis, the carbohydrates are classified into following 3 categories (Fig. 13.1).
1. Monosaccharides:
i. These are simplest of carbohydrates and are known as sugars.
ii. These are the building units of complex carbohydrates.
iii. These cannot be hydrolysed.
iv. These are sweet-tasting, crystalline and soluble in water.
v. They have a potential aldehyde or keto group and hence, are reducing in nature.
vi. Aldehyde group or the reducing centre always lies at C No. 1 of the monosaccharide molecule. Such sugars are known as aldoses or aldose sugars.
vii. Monosaccharide’s having keto group are known as ketoses or ketose sugars. In such sugars the keto group or the reducing centre always lies at C No. 2.
Depending upon the number of the C atoms, the monosaccharide’s are further classified as follows:
(i) Triose Sugars, C3H6O3 (e.g., glyceraldehyde, dihydroxyaeetone)
(ii) Tetrose Sugars, C4H8O4 (e.g., erythrose)
(iii) Pentose Sugars, C5H10O5 (e.g., ribose, ribulose, xylose, xylulose, arabinose).
(iv) Hexose Sugars, C6H12O6 (e.g., glucose, fructose, galactose mannose).
(v) Heptose Sugars, C7H14O7 (e.g., sedoheptulose).
2. Oligosaccharides:
i. These consist of more than one but fewer number of monosaccharide molecules joined together by glycosidic bonds.
ii. On hydrolysis, they yield the monosaccharide units which may be similar or dissimilar.
iii. These are also sweet tasting, crystalline, soluble sugars.
iv. These may or may not have a free -OH group at the reducing centre and accordingly may or may not be reducing.
Depending upon the number of the monosaccharide molecules which constitute them, the oligosaccharides are grouped in following categories:
(i) Disaccharides. C12H22O11 (e.g., sucrose, maltose, lactose etc.)
(ii) Trisaccharides, C18H32O16 (e.g., raffinose, gentianose etc.)
3. Polysaccharides:
i. These consist of a large number of (often thousands) monosaccharide units to form branched or un-branched chains.
ii. These can be hydrolysed to yield monosaccharide units which are usually similar.
iii. These are usually amorphous, tasteless, non-sugars and insoluble in water.
Polysaccharides can be grouped into two categories:
(i) Structural Polysaccharides (e.g., cellulose, hemi-cellulose, pectic substances, chitin, gum, mucilage etc.)
(ii) Storage Polysaccharides (e.g., starch, inulin, glycogen etc.)
Structural Features:
Open Chain and Ring Forms of Monosaccharides:
Many monosaccharide’s e.g ribose, glucose, fructose etc. exist both in open straight chain and ring form. If the ring is 5-membered it is called as furanose sugar. Sugar with a 6-membered ring is called as pyranose sugar e.g.,
(The names pyranose and furanose are derived from six and five membered cyclic ethers called pyran and furan respectively with which these sugars bear a formal resemblance).
If the highest numbered asymmetric carbon atom of the sugar molecules contains—OH group on right-hand side in open chain structure, the sugar is known as D-Sugar and if on left side it is known as L-Sugar. For example, in open chain structure of glucose carbon number 2, 3, 4 and 5 are asymmetric. The highest numbered asymmetric C-atom is therefore, 5. As shown below, it bears -OH group on right-hand side and hence, glucose is called a D-sugar and written as D-Glucose or D-Glucopyranose (in case of 6-membered ring structure).
In arabinose (a pentose sugar), the highest numbered asymmetric carbon atom is 4 which bears—OH group on left-hand side, therefore, this is a L-Sugar and called as L-Arabinose.
(The carbon atom whose 4 valancies are satisfied by four different groups or atoms is called as asymmetric carbon atom).
i. Almost all the sugars in plants are D-Sugars.
ii. Symbols D and L should not be confused with the optical activity. If the compound is optically active and rotates the plane polarised light to right, it is called as dextrorotary and is denoted by small italic letter d or + sign. But, if it rotates the plane to left side it is called as laevorotatory and is denoted by I or – sign. For example, D-Glucose is dextrorotatory and D- Fructose leavorotatory and are written as:-
D (+) Glucose
or
D (d) Glucose
D (-) Fructose
or
iii. The ring form of sugars is in fact a cyclic hemi-acetal or hemiketal structure formed by combination of the carbonyl group (i.e., aldehyde group at C1 in aldose sugars and keto group at C2 in ketose sugars) and one of the hydroxyl groups (usually at the highest numbered asymmetric C atom) of the sugar molecule in open chain structure.
The hemi acetal or hemiketal bond formation creates a new asymmetric centre at C1 in aldose sugars and C2 in ketose sugars which is now called as anomeric carbon atom. Taking for example a-D-Glucopyranose, the ring form of the sugars is usually depicted by following two structural formulas:
Although structure I indicates the basic features of a-D-Glucopyranose, it provides little insight as to the actual shape of the molecule and the spatial relationship of the various functional groups to one another. The latter features are clearly represented by structure II which is also called as Haworth’s formula.
iv. In Haworth’s perspective formula, the plane of the pyranose ring is considered to be perpendicular to that of the page on which it is written with the substituents (-H, -OH and -CH2OH groups) either above or below the plane of the ring. The thin bonds of the ring are behind the plane of the paper while the thick bonds in front of it.
(As a rule, the substituents written to the right side of the C chain in structure I are shown below the plane of the ring in Haworth’s representation while those on left side above the plane of ring. However, opposite rule applies to that C atom whose—OH group is involved in the formation of cyclic hemiacetal. Thus, the CH2OH group is written in an above position and the H atom on the same carbon written below despite the fact that it is on the left side in structure I.
In practice, however, the carbon atoms of the ring and the H atoms are omitted and often the thick bonds of the ring are not shown. The formula may also be inverted or twisted in oligo and polysaccharides to show the linkages between various monosaccharide molecules more clearly.)
If in the Haworth representation of the sugars the—OH group at the reducing centre (i.e., C No. 1 in aldose sugars and C No. 2 in ketose sugars) is present below the plane of the ring, the sugar is said to be in a-form. And if it is present above the plane of the ring, the sugar is said to be β-form. For example,
When α and β-forms change into one another the phenomenon is called as mutarotation.
There is an equilibrium between these two forms of ring compound. For example, the equilibrium in the mutarotation of a synthetic sample of D-Glucopyranose corresponds to a mixture of 36% of the a-form and 64% of the β-form.
α and β-forms of a given sugar are anomers of each other.
(The proportion of the open chain and ring forms of sugars in solution differs with different sugars. For most of the aldohexoses and aldopentoses the ring form is predominant one. Fructose (a ketose sugar) and most of the aldopentoses exist predominantly in the furnose form while glucose and other aldohexoses exist mainly in pyranose form. Ketopentoses such as ribulose usually prefer open chain form).