Carbohydrates occur abundantly in living organisms. They are found in all organisms and in almost all parts of the cell. Cellulose of wood, cotton and paper, starches present in cereals, root tubers, cane sugar and milk sugar are all examples of carbohydrates. Carbohydrates play both structural and functional roles. Simple carbohydrates are the main source of energy in cells. Some carbohydrates are the main constituents of cell walls in plants and micro-organisms.
The word carbohydrate literally means hydrated carbons. They are composed of carbon, hydrogen and oxygen and the ratio of hydrogen and oxygen is the same as in water. Their general formula is Cx (H2O)y where (x) is the whole number from three to many thousands whereas y many be the same or different whole number. Chemically, carbohydrates are defined as polyhydroxy aldehydes or kentones, or complex substances which on hydrolysis yield polyhydroxy aldehyde or ketone subunits. (Hydrolysis involves the break down of large molecules into smaller ones utilizing water molecules).
The sources of carbohydrates are green plants. These are the primary products of photosynthesis. Other compounds of plants are produced from carbohydrates by various chemical changes.
Carbohydrates in cell combine with proteins and lipids and the resultant compounds are called glycoproteins and glycolipids, respectively. Glycoproteins and glycolipids have structural role in the extracellular matrix of animals and bacterial cell wall. Both these conjugated molecules are components of biological membranes.
Classification of Carbohydrates
Carbohydrates are also called saccharids (derived from Greek word sakcharon meaning sugar) and are classified into three groups: (i) Monosaccharides (ii) Oligosaccharids, and (iii) Polysaccharides.
Monosaccharides:
These are simple sugars. They are sweet in taste, are easily soluble in water, and cannot be hydrolysed into simpler sugars. Chemically they are either polyhydroxy aldehydes or ketones. All carbon atoms in a monosaccharide except one, have a hydroxyl group. The remaining carbon atom is either a part of an aldehyde group or a keto group. The sugar with aldehyde group is called aldo-sugar; and with the keto group as keto-sugar. These are “indicated” in the case of two trioses sketched below (fig.2.2).
Fig. 2.2: structure of glyceraldehydes, a 3C sugar (C3H6O3). The aldehyde form is glyceraldehyde, whereas ketonic from is kigydroxyacetone.
In nature monosaccharides with 3 to 7 carbon atoms are found. They are called trioses (3C), tetroses (4C), pentoses (5C), hexoses (6C), and heptoses (7C). they have general formula (CH2O)n. where n is the whole number from three to many thousands.
Two trioses mentoned above are, intermediates in respiration and photo-synthesis. Tetroses are rare in nature and occur in some bacteria. Pentoses and hexoses are most common. From the biological point of view the most important hexose is glucose. It is an aldose sugar. Structure of ribose and glucose is given below (fig. 2.3).
Fig. 2.3:Structure of ribose and glucose.
Most of the monsaccharides form a ring structure when in solution. For example ribose will form a five cornered ring known as ribofuranose. Whereas glucose will form six cornered ring known as glucopyranose (fig.2.4).
Fig. 2.4 ribose and glucose form ring shaped structures.
Carbohydrates (F.Sc-Biology-Chapter-2.4): In Free State, glucose is present in all fruits, being abundant in grapes, figs, and dates. Our blood normally contains 0.08% glucose. In combined form, it is found in many disaccharides and polysaccharides. Starch, cellulose and glycogen yield glucose on complete hydrolysis. Glucose is naturally produced in green plants which take carbon dioxide from the air and water from the soil to synthesize glucose.
As indicated n the equation, energy is consumed in this process which is provided by sunlight. This is why the process is called photosynthesis. It is noteworthy that for the synthesis of 10g of glucose 717.6 kcal of solar energy is used. This energy is stored in the glucose molecules as chemical energy and becomes available in all organisms when it is oxidized in the body.
Oligosaccharides:
These are comparatively less sweet in taste, and less soluble in water. On hydrolysis oligosaccharides yield from two to ten monosaccharide’s. The ones yielding two monosaccharide’s are known as disaccharides, those yielding three are known as trisaccarides and so on. The covalent bond between two monosaccharide’s is called glycoside bond.
Physiologically important disaccharides are maltose, sucrose, and lactose (see fig. 2.5). most familiar disaccharide is sucrose (cane sugar) which on hydrolysis yields glucose and fructose, both of which are reducing sugars. Its molecular formula is C12H22 O11. Its structural formula is given in fig. 2.5.
Fig. 2.5 A disaccharide. Not carefully the glycoside linkage between the two monosaccharide’s.
Polysaccharides:
Polysaccharides are the most complex and the most abundant carbohydrates in nature. They are usually branched and tasteless. They are formed by several monosaccharide units linked by glycoside bonds (fig. 2.6) polysaccharides have high molecular weights and are only sparingly soluble in water. Some biologically important polysaccharides are starch, glycogen, cellulose, dextrin’s, agar, pectin, and chitin.
Starch:
It is found in fruits, grains, seeds, and tubers. It is the main source of carbohydrates for animals. On hydrolysis, it yields glucose molecules. Starches are of two types, amylase and amylopectin. Amylase starches have unbranched chains of glucose and are soluble in hot water. Amylopectin starches have branched chains and are insoluble in hot or cold water. Starches give blue colour with iodine.
Fig. 2.6 : polysaccharides are polymers of monosaccharide’s.
Glycogen:
It is also called animal starch. It is the chief from of carbohydrate stored in animal body it is found abundantly in liver and muscles, though found in all animal cells. It is insoluble in water, and gives red color with iodine. It also yields glucose on hydrolysis.
Cellulose:
It is the most abundant carbohydrate in nature. Cotton is the pure form of cellulose. It is the main constituent of cell walls of plants and is highly insoluble in water. On hydrolysis it also yields glucose molecules. It is not digested in the human digestive tract. In the herbivores, it is digested because of micro-organisms (bacteria, yeasts, protozoa) in their digestive tract. These micro-organisms secrete an enzyme called cellulose for its digestion. Cellulose gives no color with iodine.