Carbohydrates are composed of  carbon, hydrogen and oxygen. They are the cheapest sources of energy.Indian diets are characterized by excessive  amounts  of carbohydrates,  providing as much as 90 per cent of the required  calories.In balanced diets, carbohydrates provide not more than 50 to 60 per cent of total calories. Taken in excess, carbohydrates  are converted into body fat. 

1) Micromolecules in cells:

In  the following account, few micromolecules such as carbohydrates, lipids, amino acids, nucleotides,  energy carriers and coenzymes are described: 


Carbohydrates are polyhydroxy aldoses, ketoses, and their condensation products. Aldoses bear terminal aldehyde or _ CHO group ( e.g., xylose, ribose, glucos, galactose) while ketoses have an internal ketone or __ CO- group ( e.g., ribulose, xylulose, fructose, erythrulose). Carbohydrate is a hydrate of carbon or Cn( H2O)n in which n is an integer ranging from 3__ 7 .Some carbohydrates contain additional atoms of sulphur or nitrogen. Carbohydrates  are directly  produced  during photosynthesis of autotrophic plants, thus constituting major part of dry weight  of plant ( upon 80% ). They are called saccharides, because  their basic components  are sugars.

  Common Carbohydrates:

Sugars, glycogen ( animal starch), plant starch and cellulose are the common carbohydrates ( L.carbon = coal; G. hydor = water). Of these, sugars are categorized as  micromolecules and glycogen, starch and cellulose are grouped as macromolecules. 

Source of Carbohydrates:

The carbohydrates are formed by the green cells of algae, plants and many protists. The latter utilize water and carbon dioxide with the help of sun's light energy to form sugar and molecular oxygen. This process is called photosynthesis. Animals, fungi, most bacteria and many protists get carbohydrates from the plants. Thus, all organisms depends for nutrition on photosynthesis, which is the starting point in the production of food. The carbohydrates form about 80% of the dry weight of plants. They are the main components of our food.

There are three main sources  of  carbohydrates:

(1)  Starches : These are present in cereals  ( rice, wheat); roots and tubers ( potatoes).

(2) Sugars: (a) Monosaccharides: glucose, fructose, galactose. 

(b) Disaccharides: sucrose, lactose, maltose. 

(3) Cellulose: This is the  tough fibrous lining  found in vegetables, fruits, cereals, etc.It is hard to digest and has no nutritive  value. However,  cellulose  acts as " roughage" and prevents  constipation. 

Percentage and Composition: 

Percentage: Carbohydrates form about 1% of the cell contents.

Composition: Carbohydrates are compounds of carbon, hydrogen and oxygen. Hydrogen atoms and oxygen atoms generally occur in the ratio of 2 : 1 as in a molecule of water.This ratio gives carbohydrates their name__ hydrates of carbon¹. A general formula for the carbohydrates is CņH²nOņ. The carbohydrates are also called saccharides ( G.saccharan = sugar ) as sugars are their basic components. 

          Chemically the carbohydrates are defined as polyhydroxy aldehydes and ketones. This means they contain many hydroxyl or alcoholic ( __ OH) groups. A carbonyl group present at the end of a carbon chain is called an aldehyde and that located in the middle of a carbon chain is termed a ketone. Carbohydrates having aldehydes group or a ketone group are respectively known as aldoses or ketoses. The suffix__ ose denotes a carbohydrates. Glucose is an aldose and fructose is a ketose.Most names for sugars end in _ ose.

Notes: French for them is hydrate de carbone. 

Types: The carbohydrates are of three main types: monosaccharides, olgiosaccharides and polysaccharides. The latter two are together known as compound carbohydrates because they are made of a few to numerous monosaccharides units.

Explain how glycosidic bonds are formed? 


Monosaccharides ( simple Sugars):

Composition: The monosaccharides are the simplest carbohydrates ( G.monos= single, saccharon = sugar). They cannot be hydrolyzed into smaller units. Their geneal formula in most cases is CņH²nOn. The monosaccharides usually have 3 to 7 carbon atoms per molecule. They are named on the basis of the number of their carbon atoms as under__ 

(i) Trioses: These have 3 carbon atoms per molecule, C³H⁶O³. Examples: glyceraldehyde and dihyroxyacetone.

(ii) Tetroses: These have 4 carbon atoms per molecule, C⁴H⁸O⁴. Example : erythrose, threose.

(iii) Pentoses: These have 5 carbon atoms per molecule, C⁵H¹⁰O⁵.Examples: ribose, ribulose, xylose, arabinose and deoxyribose. Deoxyribose has a special chemical formula ( C⁵H¹⁰O⁴) as it contains one oxygen atom less.

(iv) Hexoses: These have 6 carbon atoms per molecule. Examples: glucose, fructose, galactose and mannose. Their formula is C⁶H¹²O⁶.The atoms in their molecules differ in arrangement, and this gives them slightly different chemical properties. The compounds with identical molecular  formulae but different arrangements of their atoms are called isomers. 

(v) Heptoses: These have 7 carbon atoms per molecule, C⁷H¹⁴O⁷.Example : sedoheptulose, glucoheptose and galactoheptose.

 Pentoses and hexoses exist in both open chain as well as ring forms. Ring forms are of two types Pyranose  and furanose. Pyranose has hexagon structure  with 5 carbons and one oxygen.  While Furanose has a Pentagon structure with 4 carbon and one oxygen. 

  Furanose and Pyranose forms further are of two  types each viz_ æ and ß ( called anomers ). In æ form the hydroxyl  group near the oxygen  atom of ring i.e., carbon atom no.1 written below while  in ß from it is written  above.
Many monosaccharides have asymmetric carbons and are able to rotate polarized light to right side ( d or dextrorotatory +) or left side (1 or levorataory, ø ). Among trioses with aldehyde  group, carbon atom in second position  is asymmetric one, giving two optically  active forms viz., d_ glycerose and I_ glycerose. ( It is to be noted that Captital L, D and  small, I d do not have the same  meaning). Small I and d stand for optical rotation  which has to be determined experimentally. Capital L and D refer to relative configuration of OH group around the lowest chiral ( asymmetric) centre, the carbonyl group  ( aldehyde) being  at the top. It is termed as D if OH is on the right side and L if OH is on the left side. Glyceraldehyde molecule is taken as the standard to which configurations of the carbohydrates are  compared.In case of amino acids  position of NH2 ( Instead of OH) is observed . It has been  found that all the æ__ amino acids  forming proteins  have L_ configuration.  Amongst  sugars D__ sugars are more common.It should also be remembered that D family  may contain leavo or dextro forms of different  sugars/ amino acids  and same is true for L family  also). 

Biologically Important Monosaccharides: Most common and biologically important simple sugars are hexoses and pentoses.Glucose is the most widespread and most important monosaccharide in living systems. It is common sugar in fruits. It is commonly called fruit sugar. It also occurs in nectar and honey. It is the sweetest among the natural sugars. Galactose is called brain sugar. Among pentoses, ribose is a components of  RNA and ATP. Deoxyribose occures in DNA. 
      ● Properties: The more important properties of simple sugars are given here:

 1: Physical Properties:

The simple sugars are swedish to taste. They readily dissolve in water, forming a true solution. In solution, they occur in two forms in equilibrium with each other, the open chain, or straight, form and the ring, or cyclic, form. Ring forms are further of two types: pyranose and furanose. Pyranose ring form is hexagonal, with 5 carbon atoms and 1 oxygen atom. Furanose ring form is pentagonal with 4 carbon atoms and 1 oxygen atom. Pyranose as well as furanose have œ and ß types. In the œ type, the hydroxyl group nearer the oxygen atom of the ring is written on the lower side, whereas in the ß type, it is written on the upper side. The simple sugars easily diffuse through the cell membrane. They do not give simple sugar units on hydrolysis. They can occur in many isomeric forms. Many simple sugars rotate the polarised light to the right or left. These are respectively called dextrorotatory and laevorotatory. 

 2: Chemical Properties:

The simple sugars have 2 important chemical properties __ 

(i) The sugars with a free aldehyde or ketone group Cu++ to Cu+. Hence, they are known as reducing sugars. This property is the basis for Benedict's test and Fehlig's test for the detection of glucose in the urine. Benedict's solution is blue, having copper ( cupric) sulphate dissolved in it. On gentle heating, a reducing sugar changes it to insoluble reddish coprous oxide, which precipitates. 

(ii) The aldehyde or ketone group of a simple sugar can join an alcoholic group of another organic compound, forming a new compound.This linkage is called glycosidic bond C___ O___C .The process involves loss of water and is called condensation. In condensation, one compound loses a hydrogen atom ( __ H) and the other loses a hydroxyl group (__OH). These two join to form a water molecule ( H + OH➡️ H²O). Condensation is also called dehydration as it loses water. A condensation reaction between molecules A and B is shown below.The condensation reactions are readily reversible.

A __ H + HO__ B➡️
                     A__ B + H²O
             The new compound can be split into its original components by addition of water molecules into the bond that links them. This reaction is termed hydrolysis ( G. hydro = water + lysis = loosening). This reaction is shown below__ 

                 A__ B  + H²O ➡️A__ H + HO _ B

● Functions: The monosaccharides serve many functions __ 

(i) Intermediate in Photosynthesis: Trioses, such as glyceraldehyde, pentoses, such as ribulose, and heptoses, such as sedoheptuloe, are intermediates in the photosynthetic pathway for glucose synthesis. 

(ii) Fuel: Glucose is the main fuel in cells. It is oxidized to get energy. It is the blood sugar of many animals. The cells draw it from the blood.

( iii) Components of Macromolecules and Cell Organelles: Ribose is a component of 
(a) Ribonucleic acids ( RNAs), 

( b) Certain coenzymes, such as nicotinamide adenine dinucleotide ( NAD) and nicotinamide adenine dinucleotide phosphate ( NADP), and 

(c) Energy carriers, such as adenosine triphosphate ( ATP). Deoxyribose is a component of deoxyribonucleic acid ( DNA).  The RNA and DNA are, in turn, the components of ribosomes and chromosomes respectively. 

(iv) Building Blocks: The monosaccharides serve as the building blocks or monomers ( G. monos= singles, meros= part) for the formation of 
(a) Disaccharides, such as lactose and sucrose, and 

( b) Polysaccharides, such as cellulose, lignocellulose, glycogen and starch.

(v) Formation of other Compounts: Carbon skeleton of monosaccharides is also used in the formation of fatty acids, chitin and amino acids.

Structures  of other important  monosaccharides 

1): Glucose :

2): Fructose : 

Two sugars which differ from one another only  in the configuration around  one specific carbon are called epimers of each other, e.g., D_ glucose and D_ galactose are epimers with respect to C_ 4. Similarly D_ glucose and D_ mannose differ only in stereochemistry at C__ 2.

Sugars  with same structural formula but differing in spatial  configuration and known as isomers, e.g., L and D forms of glucose.  Presence of asymmetric carbon atoms ( carbon atoms attached to four different  atoms or groups) allows the formation  of isomers.

  Amongst sugars D__ isomers are more common.

Monosaccharides of physiologic importance:

1): D_ ribose:It is the structural  element of  nucleic acids and coenzymes e.g., ATP , NAD, NADP etc. It is formed as an intermediate in pentose phosphate pathways. 

2): Fructose : It is sweetest of all  naturally  occurring  sugars and in also called fruit sugar because  of its common  occurrence  in fruits ( except in grapes). Nectar, honey and seminal fluid  also contain fructose.  Honey is largely a hydrolytic product of sucrose and presence of fructose accounts of greater sweetness of honey. Fructose is also called laevolose because of its laevorotatory  nature ( however, it belongs to D_ family).

3): Glucose : It is " sugar" of the body, which is carried  by blood, and principle  one used by tissue. Also called ' blood sugar'. It is formed by hydrolysis of starch, cane sugar, maltose and lactose.It is also known as group sugar, come sugar and dextrose. It is the main respiratory substrate and hence forms immediate source of energy.

4): Galactose: It is formed by hydrolysis  of lactose. It can be changed to glucose  in the liver. It is synthesized in the mammary  glands to make lactose of milk.

(☆ Derived Monosaccharides:

Monosaccharides are modified  to form a number of different  substances . Various such derivatives are

( a) Deoxy sugars: They are formed by the deoxygenation where hydroxyl group is replaced by a hydrogen atom e.g., deoxygenation of ribose produce deoxyribose  which is a constituent of deoxyribotide found in DNA.

( b) Amino sugars: Here hydroxyl group of a sugar is replaced by an amino  or an acetylamino group e.g., Glucosamine,  a products of hydrolysis of chitin, and galactosamine found in chondroitin sulphate. 

(c) Sugar Acids: They are produced by oxidation  if aldehydic carbon, terminal hydroxyl carbon or both. Ascobic acid, glucuronic acid and galacturonic acid are sugar acids.

(d) Sugar  alchohol: Aldoses and ketoses may be reduced  at carbonyl  carbon to yield corresponding  sugar alchohols e.g., D__ glucose yields D__ sorbitol, D__ mannose yields D__ mannitol. Mannitol is storage alcohol in some fruits and brown algae.

● Molisch test is +ve with all sugars. 

● Osazones are crystalline derivatives of sugars. Osazones are not formed by sucrose.



The Oligosaccharides are formed by joining together of 2__ 6 monosaccharide molecules through condensation. The bond formed between two monosaccharide units in called a glycosidic bond. It is normally formed between carbon atoms 1 and 4 of the neighbouring units ( œ 1, 4 bond). The monosaccharide units, when linked up,are called residues. Maltose has two glucose residues. The disaccharides are hydrolysed into two their component monosaccharides by adding a molecule of water into the oxygen bridge every two monomers.

Types of Oligosaccharides:

The Oligosaccharides are classified according to the number of their monosaccharide units or monomers. There are disaccharides, e.g., sucrose, trisaccharides, e.g., raffinose, tetrasaccharides, e.g., stachyose, pentasaccharides and bexasaccharides.

   ● Disaccharides: The disaccharides are the most abundant Oligosaccharides in the cells
 They, as already mentioned, have two monosaccharide units joined by glycosidic bond. Th e are called the double sugars. Biologically important disaccharides are maltose, sucrose and lactose.

( a) Maltose is a degradation product of a larger polysaccharide molecule. It  is formed by the action of amylases ( enzymes) on starch during digestion, for example in animals, or in germinating starch seeds and malt¹.It is called the malt sugar. It is a reducing sugar.It is used in making beer.It is composed of two glucose molecules bonded together by carbons 1and 4.

Note: Seed of barley or other grain, artificially germinated and dried, containing a few sugars and amylolytic enzymes. 

(b) Sucrose is the common cane or table sugar. It is common in higher plants,but the commercial sources are sugarcane ( Saccharum officinarum), sugarbeet ( Beta vulgaris), and certain species of sorghum. It is the storage product of photosynthesis in these plants. Plants generally transport carbohydrates from leaves to other organs in the form of sucrose.Sucrose results from the union of a glucose molecule with a fructose molecule between their 1 and 2 carbons. Sucrose is a nonreducing sugar, unlike its components. Its glucose unit is in pyranose from and fructose unit is in furanose state. 

  (c)  Lactose is called the milk sugar. It is found only in the mammalian milk. It is synthesized in  the mammary glands.It is composed of one glucose molecule and one galactose molecule joined by 1 and 4 carbons. Souring of milk occurs when bacteria,  Streptococcus lactis, convert lactose into lactic acid. It is a reducing sugar.


       All the above disaccharides have an identical chemical formula  C12H22O11 derived as under. 



(d) Trehalose: It is found in some fungi and harmolymph of some insects.It is a disaccharide made up of two glucose units. Both the glucose molecules are in æ _ form and the linkage is 1➡️1.

(e) Cellbiose: It is a disaccharide formed during hydrolysis  of cellulose. It has two glucose molecules,  which have ß _ 1 ➡️ 4 linkage.

(f) Isomaltose: It is a disaccharide formed during hydrolysis  of cellulose. It has two glucose molecules, which have ß__ 1➡️4 linkage.

Trisaccharide: Raffinose, a trisaccharide, is a reducing  sugar, which is formed of glucose, fructose and galactose. It is found in sugar beets .Melezitose formed of glucose + fructose + glucose is another trisaccharide found in sap of some coniferous trees.

   Sweetening index of sucrose, maltose  and lactose is  respectively  100, 32 and 16, 170 for fructose and 40,000 for saccharin.

Reducing  Sugars: These are the sugars, which can reduce Cu²+ ions to Cu+ state. This property  is seen in all saccharides with free aldose or ketose groups. All monosaccharides are reducing sugars.  Amongst disaccharides sucrose is non_ reducing  because the two. Trahalose is also a non__ reducing  sugar. Other disaccharides  e.g., maltose and lactose  possess reducing groups this called reducing  sugars.  This reaction is useful in detection  of glucose  in urine. Benedicts test is commonly used test for sugar detection  in urine. Benedict's reagent contains  cupric sulphate in a solution  of Na2 CO2 and Sodium citrate. Positive test is indicated by formation  of brick red  precipitates of Cuprous oxide.

      Anothe test used is Fehling's test. Fehling's  contains cupric sulphate in solution of NaOH and sodium potassium  tartarate. Positive  test is indicated  by formation of yellow precipitates of cuprous oxide.

A disaccharide called trehalose is  the major sugar of insect haemolymph. It is also found in fungi. 

● Properties: Like the monosaccharides, the disaccharides are also sweedish to taste and form a true solution in water. They, however,  diffuse very slowly through cell membrane. They hydrolyse into free monosaccharide units when treated with a dilute acid or with an enzyme.Maltose and lactose are reducing disaccharides but sucrose is not.

       Oligosaccharides may form unbranched or branched chains. Large branched or unbranched Oligosaccharide chains are found attached to cell membranes. 

  ● Functions: Common functions of Oligosaccharides are as under___ 

(i) Cellular Interactions: The Oligosaccharides attached to the cell membrane help in recognising cells of their own kind or species. 

(ii)  Storage  Products: Sucrose is a reserve food in sugarcane and sugarbeet. 

(iii) Fuel: Disaccharides also serve as fuel to provide energy. For this, they first break down to glucose. 

( iv) Lactose of milk provides nourishment. 

Functions  of small carbohydrates:

1): Trioses, glyceraldehyde and dihydroxy acetone, are important  intermediates of both respiratory and photosynthetic pathways. 

2): Erythrose, a tetrose monosaccharide is an intermediate of respiratory and photosynthetic pathways. It also acts as a raw material  for synthesis  of lignin, anthocyanines  and  some amino acids e.g. tryrosine, phenylalanine. 

3): Pentose  sugars, xylose  and arabinose form polymers which are constituents  of hemicellulose.

4): Ribulose  1, 5 biphosphate is the acceptor of carbon dioxide  in photosynthesis. 

5): Sedoheptulose,  a heptose sugar, is an intermediate  of respiratory  and photosynthetic  pathways. 

6):Ribose is found in FAD, NAD, NADP, and ATP.

7): Oligosaccharides attached to cell membranes  take part in recognition,  attachment and antigen specificity.

8): Glucose is polymerized to produce storage  carbohydrates, starch in plants and glycogen in animals.

Carbohydrate bonds:

The subunits  of disaccharide and polysaccharide are linked by means of glycosidic  bonds in which  carbon of  one sugar unit is bound to the oxygen atom of hydroxyl  of an adjacent sugar unit .Maltose is formed  by a glycosidi bond between  carbon__ 1 of one glucose to carbon __ 4 of second glucose  molecule. Lactose  is formed  by bond between  C1, of galactose  and C4 of glucose  while sucrose forms by bond between  C1 of glucose  and C2 of fructose. 

In glycoside bond formation , one carbon gives up its OH group and other loses hydrogen  from its OH group thus forming an H2O molecule. This reaction is known as dehydration synthesis. 

Described the role of Carbohydrates in the cells? 

Carbohydrates play several important roles in cells. Here are some of the key functions of carbohydrates:

1. Energy production: Carbohydrates are the preferred source of energy for cells. When carbohydrates are digested and broken down, they are converted into glucose, the simplest form of carbohydrate. Glucose can then be used by cells to produce ATP (adenosine triphosphate), the energy currency of cells.

2. Energy storage: Excess glucose that is not immediately needed for energy production can be stored in the form of glycogen. Glycogen is a highly branched carbohydrate polymer that is stored mainly in the liver and muscles. When energy demands increase, glycogen is broken down into glucose molecules, providing a readily available source of energy.

3. Structural support: Some carbohydrates, such as cellulose, are important components of plant cell walls. Cellulose provides strength and rigidity to plant cells, allowing them to maintain their structure and withstand external pressures.

4. Cell recognition and signaling: Carbohydrates play a crucial role in cell-cell recognition and communication. Carbohydrate molecules on the surface of cells act as receptors or ligands, allowing cells to identify and respond to specific molecules or signals. This is particularly important in immune responses and cell adhesion processes.

5. Nucleotide synthesis: Carbohydrates are involved in the synthesis of nucleotides, the building blocks of DNA and RNA. Ribose, a pentose sugar derived from glucose, is a key component of RNA molecules.

6. Protein modification: Some carbohydrates can be attached to proteins in a process called glycosylation. This post-translational modification plays a critical role in protein folding, stability, and function. Glycosylation can also affect protein-protein interactions and cell-cell adhesion.

Overall, carbohydrates play diverse and essential roles in cells, providing energy, storage, structural support, recognition, and signaling functions.

Main Difference Between Oligosaccharides and Polysaccharides? 


1:  Formed of fewer ( 2_9) monosaccharide units.

2: Are sweedish to taste.

3: Are soluble in water.

4: Are small, simple molecules with low molecular weights. 

5: Only some form storage products or structural components. 

Examples: maltose, sucrose, lactose.


1: Formed of numerous mononsaccharide units.

2: Are not sweet.

3: Are not soluble in water.

4: Are large, complex molecules with high molecular weights. 

5: Nearly all acts as storage products or structural components. 

  Example: starch, cellulose, glycogen. 

Differences Between Reducing and Nonreducing Sugars? 

Reducing Sugars 

1: Have a free aldehyde ( __ CHO) or a ketone (__ CO) group.

2: Reduce cupric ions ( Cu++) to coprous ions ( Cu+). 

Nonreducing Sugars 

1: Lack a free aldehyde ( __ CHO) or ketone (__CO) group.

2: Do not reduce cupric ions ( Cu++) to coprous ions ( Cu+). 


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