This article is on the Biomolecules Notes Class 12 of Chemistry. The notes on biomolecules of class 12 chemistry have been prepared with great care keeping in mind the effectiveness of it for the students. This article provides the revision notes of the chapter Biomolecules of Class 12 Chemistry for the students so that they can get a quick glance of the chapter.
The study of chemical make-up and structure of living matter and of the chemical changes that take place within them is called biochemistry.
The various activities of living organisms are regulated by complex organic molecules, such as carbohydrates, lipids, proteins and nucleic acids, called biomolecules.
Carbohydrates are products of plants and are a part of an extremely large group of naturally occurring organic compounds. Cane sugar, glucose, starch and so on are a few examples of carbohydrates. The general formula for carbohydrates is Cz(H2O)y. Carbohydrates are generally hydrates of carbon, which is where the name was derived. So, carbohydrates on hydrolysis produce polyhydroxy aldehydes or polyhydroxy ketones.
Classification of Carbohydrates
(a) On the basis of Physical Characteristics
(i) Sugar: Characteristics of sugars are crystalline substances, taste sweet and readily water soluble. Because of their fixed molecular weight, sugars have sharp melting points. A few examples of sugars are glucose, fructose, sucrose, lactose, etc.
(ii) Non-Sugars: Amorphous, Tasteless, water-insoluble substances with variable melting points e.g., Starch.
(b) On the basis of Hydrolysis
Monosaccharides: A carbohydrate that can be hydrolyzed only once to break down into simpler units of polyhydroxy aldehyde or ketone is called monosaccharide. These include glucose, mannose, etc.
Oligosaccharides: Sugars that on hydrolysis produce two or more molecules of monosaccharides are called oligosaccharides. These are further classified as di-, tri- or tetrasaccharides, etc.
- Disaccharides: These are sugars that produce two molecules of the same or different monosaccharides on hydrolysis. Examples are sucrose, maltose, and lactose. An example of disaccharides is sucrose: C12H22O11.
- Trisaccharides: Sugars that yield three molecules of the same or different monosaccharides on hydrolysis are called trisaccharides. An example of trisaccharides is Raffinose C18H32O16
- Polysaccharides: On hydrolysis, polysaccharides yield a large number of monosaccharides units. An example of a polysaccharide is starch cellulose.
(c) On basis of test with reagents (like Benedict’s solution, Tollen’s reagent and Fehling’s solution):
(i) Reducing Sugars:
- These have a free aldehyde (-CHO) or ketone group.
- These have the ability to reduce the cupric ions (Cu2+; blue) in Fehling’s or Benedict’s Solution to cuprous ions (Cu+; reddish) that separates out as cuprous oxide (Cu2O) from the solution.
- Examples include maltose, lactose, melibiose, gentiobiose, cellobiose, mannotriose.
(ii) Non-reducing sugars:
- A free aldehyde or ketonic group is absent.
- No cuprous oxide (Cu2O) producing chemical reaction takes place.
- Examples are sucrose, trehalose, raffinose, gentianose, melezitose.
(a) From sucrose (cane sugar): Boiling sucrose with diluted HCl or H2SO4 in alcoholic solution produces glucose and fructose in equal proportions.
(b) From starch: Industrially, glucose is manufactured by the hydrolysis of starch by boiling it with dil. H2SO4 at 393 K under pressure.
Structures of Glucose
Glucose was assigned open chained structure on the basis of the following evidence.
Glucose has one aldehyde group, one primary (—CH2OH) group and four secondary (—CHOH) hydroxyl groups, and gives the following reactions:
(a) Acetylation of glucose with acetic anhydride forms a pentaacetate, proving the presence of five hydroxyl groups in glucose.
(b) Glucose reacts with hydroxylamine to form monoxime and adds up a molecule of hydrogen cyanide to form a cyanohydrin.
The above reactions validate the occurrence of a carbonyl group in glucose.
(c) On prolonged heating with HI, glucose forms n-hexane, indicating that all the six carbon atoms in glucose are linked linearly.
(d) Reaction with Bromine water.
Cyclic Structures of Glucose
The open chain structure of glucose could not explain the following facts : –
Despite having the aldehyde group, it does not give the 2,4-DNP test, Schiff ‘s test and it does not form the hydrogen sulphite addition product with NaHSO3. The pentaacetate of it does not react with hydroxylamine indicating the absence of free — CHO group in glucose pentaacetate. Glucose exists in two different crystalline forms: α-form (obtained by crystallization from conc. solution of glucose at 303 K) and β-form (obtained by crystallization from hot and saturated aq.. solution at 371 K).
α and β glucose have different configuration at anomeric (C-1) carbon atom, hence are called anomers and the C-1 carbon atom is called anomeric carbon (glycosidic carbon). The six-membered cyclic structure of glucose is called the pyranose structure.
Structures of Fructose
It is a ketohexose as it contains six C- atoms and ketonic group.
On the basis of its reactions, it was found to contain a ketonic functional group at C-2 atom and six carbon atoms in the straight chain as in the case of glucose. It belongs to D-series and is a laevorotatory compound. It is appropriately written as D-(-)-fructose.
Fructose also exists in two cyclic forms which are obtained by the addition of -OH at C-5 to the (>C = 0) group. The ring, thus formed is a five-membered ring and is named as furanose.
The cyclic structures of two anomers of fructose are represented by Haworth strictures:
(a) Preparation: Fractional hydrolysis of starch by enzyme diastase.
(b) Units: Two units of α-D glucose.
(c) Reducing sugar
(d) Linkage: α glycosidic linkage between C1 and C4 carbon atoms.
(a) Preparation: Prepared from sugarcane and beetroot
(b) Units: α-D glucose and α-D fructose.
(c) Non-reducing sugar
(d) Linkage: α glycosidic linkage with reference to glucose and β glycosidic linkage to fructose, both linked at C1 carbon.
(a) Preparation: Found in milk.
(b) Units: β-D glucose and β-D galactose.
(c) Reducing sugar.
(d) Linkage: β glycosidic linkage.
The laevo rotation of fructose (–92.4°) is more than the rotation of glucose (+52.5º), so the mixture is laevorotatory. Sucrose hydrolysis brings about a change in the sign of rotation, from dextro (+) to laevo (–) and the product is called invert sugar.
(a) Starch: It is the main storage polysaccharide of plants having the general formula (C8H16O5)n. The main source is maize, wheat, barley, rice, and potatoes. It is a polymer of α-glucose and consists of two components – Amylose and Amylopectin. Amylose is made up of a long, unbranched chain of α-D-(+)-glucose linkage. Amylopectin is a branched chain polymer of α-D-glucose units, in which chain is formed by C1–C6 glycosidic linkage whereas branching occurs by C1–C6 glycosidic linkage.
(b) Cellulose It occurs exclusively in plants and is the main constituent of the cell wall of a plant cell. It is a linear polymer of β-D glucose in which glucose units are linked together by C1–C4 glycosidic linkage. It is a non-reducing sugar.
It is also known as animal starch because its structure is similar to amylopectin and is rather more highly branched. It is found in the liver, muscles, and brain. Glycogen breaks down to glucose by the action of enzymes when needed by the body. It is also found in yeast and fungi.
Proteins are the complex nitrogenous organic substance that occurs in all animals and plants. They are called the most vital chemical substance and are necessary for the normal growth and maintenance of life. Protein serves the following functions in our body:
- Proteins promote growth
- Proteins supply essential amino acids to blood.
- They help maintain body tissues.
- Proteins synthesize various enzymes.
- Proteins protect the body from infections.
α-Amino Acids: Carboxylic acids in which one α-hydrogen atom of an alkyl group is substituted by amino (–NH2) group are called α-Amino acids. The general formula is
Structure of α-amino acids: The amino acids containing one carboxylic group and one amino group behave as a neutral molecule. This is because in aqueous solutions the acidic carboxylic group and basic amino group neutralize each other intramolecularly to produce an internal salt structure known as a zwitterion or dipolar ions.
However, the neutral zwitterion (dipolar ions) changes to the cation in an acidic solution and exist as an anion in an alkaline medium. Thus amino acids exhibit amphoteric character.
Therefore, amino acid exists as zwitterion when the solution is neutral or pH-7. The pH at which the structure of an amino acid has no net charge is called its isoelectric point.
Classification of Amino Acids: Based on the relative number of –NH2 and –COOH group, α-amino acids are classified in three main groups
(a) Neutral Amino acids: Amino acids containing one –NH2 group and one –COOH group. For example, glycine, valine, alanine etc.
(b) Basic amino acids: These contain one –COOH group and two –NH2 groups, such as lysine and arginine.
(c) Acidic amino acids: Amino acids containing two –COOH groups and one –NH2 group are called acidic amino acids; for example, aspartic acid and glutamic acid, etc.
Classification of proteins
Proteins are classified on the basis of two different methods. The first mode of classification, proteins are of two types is based on their shape and functions:
(a) Fibrous proteins (b) Globular proteins
(a) Fibrous proteins: They are thread-like molecules that lie side by side to form fibers. They are held together by hydrogen bonds. These are insoluble in water but soluble in concentrated acids and alkalis. A few examples are keratin (present in hair, nails, wood, feather, and horns). Muscles have myosin, silk is composed of fibroin, bones and cartilages have collagen.
(b) Globular proteins: These proteins have molecules folded into compact units that often acquire spheroidal shape. Such proteins are soluble in water, diluted acids and alkalis, such as insulin, haemoglobin, albumin, etc.
Structure of Proteins:
The structure of proteins is quite complex. Study of its structure is carried out under the following headings.
(a) Primary structure of a protein:
The primary structure of a protein refers to its covalent structure, that is, the sequence in which various α-amino acid are arranged in protein or in the polypeptide structure of a protein.
The linkage (–CO–NH–) is known as peptide linkage.
The dipeptide still has free amino and carboxyl groups that can react with other molecules of amino acid resulting in polypeptide formation. In the polypeptide chain, the free amino end is termed as N–terminal and the free carboxyl end is called C–terminal end.
(b) Secondary structure of a protein:
This refers to the arrangement of polypeptide chains into a definite three-dimensional structure that protein assumes as a result of hydrogen bonding. Depending upon the size of the R-group of the amino acids in polypeptides, two different types of secondary structure are possible:
(i) α-helix structure (ii) β-Pleated structure
(i) α-Helix structure: This type of secondary structure is possible when the alkyl groups present in amino acids are large and involved in coiling of the polypeptide chains. The intramolecular hydrogen bond between the >C = O group of one amino acid and –NH group of the fourth amino acid stabilizes the helical pattern in a right-handed coil and the shape.
(ii) β-Pleated structure: Such secondary structure is acquired when the alkyl groups of amino acids are small. In this kind of structure, the linear polypeptide chains are arranged side by side and are held together by the intermolecular hydrogen bond between the (>C=O) and –NH group.
(c) Tertiary structure of a protein:
The tertiary structure of a protein is the most stable shape that a protein assumes under the normal temperature and pH conditions. Attractive forces between the amino acid chains are involved in acquiring tertiary structure. These attractive forces, like hydrogen bond, disulphide bonds, ionic, chemical and hydrophobic bonds, results in a complex and compact structure of the protein. The two important tertiary structures of proteins are fibrous structures and globular structure. Fibrous proteins have largely helical structure and are rigid molecules of rod-like shape. Globular proteins show a polypeptide chain that consists partly of helical sections and partly β-pleated structure and remaining in random coil form. These different segments of secondary structure then fold up to give protein a spherical shape.
(d) Quaternary structure of proteins:
The quaternary structure of proteins develops when the polypeptide chains, which may or may not be identical, are held together by hydrogen bonds. It results in the increase of molecular mass of protein greater than 50,000 amu. For example, haemoglobin contains four subunits, two identical α-chains containing 141 amino acids each and the other two identical β-chains containing 161 amino acids each.
Denaturation of proteins
Proteins when subjected to the action of heat, mineral acids or alkali, the water-soluble form of globular protein changes to water-insoluble fibrous protein resulting in the precipitation or coagulation of protein. This is called the denaturation of proteins.
These are the essential biological catalysts which are needed to catalyze biochemical reactions. Almost all the enzymes are globular proteins. Enzymes have specificity, i.e., specific for a particular substrate and reaction.
The enzymes which catalyze the oxidation of one substrate with simultaneous reduction of another substrate are named as oxidoreductase enzymes.
Certain organic substances required for regulating some of the body processes and preventing certain diseases are called vitamins. These compounds cannot be synthesized by an organism. On the basis of solubility, the vitamins are divided into two groups.
(1) Fat soluble; Vitamin A, D, E and K.
(2) Water soluble; Vitamin B and C.
Nucleic acids are vital biomolecules that present in the nuclei of all living cells as nucleoproteins. These are long chain polymers with a high molecular mass. Also called biopolymer, they have nucleotide as a repeating structural unit (monomer). These play an important role in the transmission of the heredity characteristics from one generation to the next and also in the biosynthesis of proteins. Therefore, the genetic information coded in nucleic acid governs the structure of protein during its biosynthesis and hence controls the metabolism in the living system.
Structure of nucleic acids: The nucleic acid is the prosthetic component of nucleoproteins. Nucleic acid on stepwise hydrolysis gives the following products as shown in the chart.
Difference between DNA and RNA
The main points of difference between the two types of nucleic acids are given in the table.
(a)The pentose sugar in RNA is ribose.
(b) Adenine and guanine represent the purine bases of RNA; the pyrimidine bases are uracil and cytosine.
(c) The thymine in DNA is replaced by uracil in RNA.
(d) RNA is single-stranded, but double-stranded RNA is present in Reovirus and wound tumour virus.
(e) There are three major classes of RNA, each with specific functions in protein synthesis.
1. Messenger RNA is produced by DNA; the process is called transcription.
2. Messenger RNA encodes the amino acid sequence of a protein in their nucleotide base sequence.
3. A triplet of nitrogenous bases specifying an amino acid in mRNA is called a codon.
1. tRNA is also known as soluble RNA (sRNA) as it is soluble in 1 molar solution of sodium chloride.
2. Molecules of tRNA are single-stranded and relatively very small.
3. tRNA identifies amino acids in the cytoplasm and transports them to the ribosome.
4. Anticodon is a three-base sequence in a tRNA molecule that forms complementary base pairs with a codon of mRNA.
5. All transfer RNA possess the sequence CCA at their three ends; the amino acid is attached to the terminal as residue.
1. Ribosomal RNA is found in ribosomes of a cell and is also called insoluble RNA.
2. The main function of rRNA is to provide a large surface for spreading of mRNA over ribosomes during the translocation process of protein synthesis.
3. The relationship between the sequence of amino acids in a polypeptide with the base sequence of DNA or mRNA is genetic code.
4. Genetic code determines the sequence of amino acids in a protein.
5. A triplet would code for a given amino acid as long as three bases are present in a particular sequence.
6. Later in a cell-free system. Marshall Nirenberg and Philip (1964) were able to show that GUU codes for the amino acid valine.
7. The spellings of further codons were discovered by R. Holley. H. Khorana and M. Nirenberg.
8. They have been awarded the Nobel Prize in 1968 for researches in the genetic code.
This article has tried to highlight the chemistry of biomolecules in the form of lecture notes for class 12 students in order to understand the basic concepts of the chapter. The notes on biomolecules have not only been prepared for class 12 but also for the different competitive exams such as iit jee, neet, etc.
Also Read:Polymers Class 12 Chemistry