Brief Notes On Carbohydrtaes

Carbohydrates                                              

              

Carbohydrate, class of naturally occurring compounds and derivatives formed from them.

 In the early part of the 19th century, substances such as wood, starch, and linen were found to be composed mainly of molecules containing atoms of carbon (C), hydrogen (H), and oxygen (O) and to have the general formula C₆H₁₂O₆

Other organic molecules with similar formulas were found to have a similar ratio of hydrogen to oxygen. The general formula C_x(H₂O)_y is commonly used to represent many carbohydrates, which means “watered carbon.”

Classification Of Carbohydrates

Slight changes in structural arrangements are detectable by living things and influence the biological significance of isomeric compounds.

It is known, for example, that the degree of sweetness of various sugars differs according to the arrangement of the hydroxyl groups (―OH) that compose part of the molecular structure. A direct correlation that may exist between taste and any specific structural arrangement, however, has not yet been established; that is, it is not yet possible to predict the taste of a sugar by knowing its specific structural arrangement.

Two molecules of a simple sugar that are linked to each other form a disaccharide, or double sugar. The disaccharide sucrose, or table sugar, consists of one molecule of glucose and one molecule of fructose; the most familiar sources of sucrose are sugar beets and cane sugar. Milk sugar, or lactose, and maltose are also disaccharides.

 Before the energy in disaccharides can be utilized by living things, the molecules must be broken down into their respective monosaccharides.

 Oligosaccharides, which consist of three to six monosaccharide units, are rather infrequently found in natural sources, although a few plant derivatives have been identified.

Polysaccharides (the term means many sugars) represent most of the structural and energy-reserve carbohydrates found in nature.

Several hundred distinct types have thus far been identified. Cellulose, the principal structural component of plants, is a complex polysaccharide comprising many glucose units linked together; it is the most common polysaccharide.

 The starch found in plants and the glycogen found in animals also are complex glucose polysaccharides.

Starch (from the Old English word stercan, meaning “to stiffen”) is found mostly in seeds, roots, and stems, where it is stored as an available energy source for plants.

Large molecules that may consist of as many as 10,000 monosaccharide units linked together, polysaccharides vary considerably in size, in structural complexity, and in sugar content; several hundred distinct types have thus far been identified. 

Cellulose, the principal structural component of plants, is a complex polysaccharide comprising many glucose units linked together;

 it is the most common polysaccharide.

The starch found in plants and the glycogen found in animals also are complex glucose polysaccharides.

Starch (from the Old English word stercan, meaning “to stiffen”) is found mostly in seeds, roots, and stems, where it is stored as an available energy source for plants. Plant starch may be processed into foods such as bread, or it may be consumed directly—as in potatoes, for instance.

Glycogen, which consists of branching chains of glucose molecules, is formed in the liver and muscles of higher animals and is stored as an energy source.

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The generic nomenclature ending for the monosaccharides is -ose; thus, the term pentose (pent = five) is used for monosaccharides containing five carbon atoms, and hexose (hex = six) is used for those containing six.

Biological significance

The importance of carbohydrates to living things can hardly be overemphasized. The energy stores of most animals and plants are both carbohydrate and lipid in nature; carbohydrates are generally available as an immediate energy source, whereas lipids act as a long-term energy resource and tend to be utilized at a slower rate.

 Glucose, the prevalent uncombined, or free, sugar circulating in the blood of higher animals, is essential to cell function. The proper regulation of glucose metabolism is of paramount importance to survival.

The ability of ruminants, such as cattle, sheep, and goats, to convert the polysaccharides present in grass and similar feeds into protein provides a major source of protein for humans.

 A number of medically important antibiotics, such as streptomycin, are carbohydrate derivatives. The cellulose in plants is used to manufacture paper, wood for construction, and fabrics.

During photosynthesis, an immediate phosphorous-containing product known as 3-phosphoglyceric acid is formed.

 This compound then is transformed into cell wall components such as cellulose, varying amounts of sucrose, and starch—depending on the plant type—and a wide variety of polysaccharides, other than cellulose and starch, that function as essential structural components. For a detailed discussion of the process of photosynthesis, see photosynthesis.

Role in plant and animal structure

Whereas starches and glycogen represent the major reserve polysaccharides of living things, most of the carbohydrate found in nature occurs as structural components in the cell walls of plants.

Carbohydrates in plant cell walls generally consist of several distinct layers, one of which contains a higher concentration of cellulose than the others.

 The physical and chemical properties of cellulose are strikingly different from those of the amylose component of starch.

In most plants, the cell wall is about 0.5 micrometre thick and contains a mixture of cellulose, pentose-containing polysaccharides (pentosans), and an inert (chemically unreactive) plastic-like material called lignin.

The amounts of cellulose and pentosan may vary; most plants contain between 40 and 60 percent cellulose, although higher amounts are present in the cotton fibre.

Polysaccharides also function as major structural components in animals. Chitin, which is similar to cellulose, is found in insects and other arthropods. Other complex polysaccharides predominate in the structural tissues of higher animals.

    Representative disaccharides and oligosaccharides

common name	component sugars	linkages	sources
*The linkage joins carbon atom 1 (in the β configuration) of one glucose molecule and carbon atom 4 of the second glucose molecule; the linkage may also be abbreviated β-1, 4.
**Note that raffinose and stachyose are galactosyl sucroses.
cellobiose	glucose, glucose	β1 → 4*	hydrolysis of cellulose
gentiobiose	glucose, glucose	β1 → 6	plant glycosides, amygdalin
isomaltose	glucose, glucose	α1 → 6	hydrolysis of glycogen, amylopectin
raffinose**	galactose, glucose, fructose	α1 → 6, α1 → 2	sugarcane, beets, seeds
stachyose**	galactose, galactose, glucose, fructose	α1 → 6, α1 → 6, α1 → 2	soybeans, jasmine, twigs, lentils