CARBOHYDRATES

CARBOHYDRATES


CARBOHYDRATES: CARBOHYDRATES are aldehyde or ketone alcohols containing carbon, hydrogen and oxygen in which the hydrogen and oxygen are generally in the same ratio as in water. Because carbohydrates are the first products formed in photosynthesis, they are a convenient starting point for any discussion or constituents of vegetable drugs. More ever carbohydrates are the products from which, by subsequent organic reaction the plant synthesizes a greater number of other constituents.

Carbohydrates may be classified into 2 broad groups: sugars and polysaccharides.
Carbohydrates have the general molecular formula CH2O, and thus were once thought to represent "hydrated carbon". However, the arrangement of atoms in carbohydrates has little to do with water molecules.
Starch and cellulose are two common carbohydrates. Both are macromolecules with molecular weights in the hundreds of thousands. Both are polymers (hence "polysaccharides"); that is, each is built from repeating units, monomers, much as a chain is built from its links.
The monomers of both starch and cellulose are the same: units of the sugar glucose.

Sugars
Monosaccharides
Three common sugars share the same molecular formula: C6H12O6. Because of their six carbon atoms, each is a hexose.
They are:
glucose, "blood sugar", the immediate source of energy for cellular respiration
galactose, a sugar in milk (and yogurt), and
fructose, a sugar found in honey.
Although all three share the same molecular formula (C6H12O6), the arrangement of atoms differs in each case. Substances such as these three, which have identical molecular formulas but different structural formulas, are known as structural isomers.

Glucose, galactose, and fructose are "single" sugars or monosaccharides. Two monosaccharides can be linked together to form a "double" sugar or disaccharide.

Disaccharides
Three common disaccharides:
sucrose — common table sugar = glucose + fructose
lactose — major sugar in milk = glucose + galactose
maltose — product of starch digestion = glucose + glucose

Although the process of linking the two monomers is rather complex, the end result in each case is the loss of a hydrogen atom (H) from one of the monosaccharides and a hydroxyl group (OH) from the other. The resulting linkage between the sugars is called a glycosidic bond. The molecular formula of each of these disaccharides is
C12H22O11 = 2 C6H12O6 − H2O

All sugars are very soluble in water because of their many hydroxyl groups. Although not as concentrated a fuel as fats, sugars are the most important source of energy for many cells.
Carbohydrates provide the bulk of the calories (4 kcal/gram) in most diets, and starches provide the bulk of that. Starches are polysaccharides.

Polysaccharides
Starches
Starches are polymers of glucose.

Two types are found:
(1) amylose consists of linear, unbranched chains of several hundred glucose residues (units). The glucose residues are linked by a glycosidic bond between their #1 and #4 carbon atoms.
(2) amylopectin differs from amylose in being highly branched. At approximately every thirtieth residue along the chain, a short side chain is attached by a glycosidic bond to the #6 carbon atom (the carbon above the ring). The total number of glucose residues in a molecule of amylopectin is several thousand.

Starches are insoluble in water and thus can serve as storage depots of glucose. Plants convert excess glucose into starch for storage. The image shows starch grains (lightly stained with iodine) in the cells of the white potato. Rice, wheat, and corn are also major sources of starch in the human diet.

They must be digested. The hydrolysis of starch is done by amylases. With the aid of an amylase (such as pancreatic amylase), water molecules enter at the 1 -> 4 linkages, breaking the chain and eventually producing a mixture of glucose and maltose. A different amylase is needed to break the 1 -> 6 bonds of amylopectin.

Glycogen
Animals store excess glucose by polymerizing it to form glycogen. The structure of glycogen is similar to that of amylopectin, although the branches in glycogen tend to be shorter and more frequent.
Glycogen is broken back down into glucose when energy is needed (a process called glycogenolysis).

In glycogenolysis,
phosphate groups — not water — break the 1 -> 4 linkages
The phosphate group must then be removed so that glucose can leave the cell.

The liver and skeletal muscle is major depots of glycogen.
There is some evidence that intense exercise and a high-carbohydrate diet ("carbo-loading") can increase the reserves of glycogen in the muscles and thus may help marathoners work their muscles somewhat longer and harder than otherwise. But for most of us, carob loading leads to increased deposits of fat.

Cellulose
Cellulose is probably the single most abundant organic molecule in the biosphere. It is the major structural material of which plants are made. Wood is largely cellulose while cotton and paper are almost pure cellulose.
Like starch, cellulose is a polysaccharide with glucose as its monomer. However, cellulose differs profoundly from starch in its properties.