Maarg Manthan · Topic 7.5

Biomolecules: Carbohydrates, Proteins, Lipids, Nucleic Acids

Science & Technology › Chemistry in Daily Life and Biomolecules · Topic 7.5

This topic looks at the chemistry of the four classes of biomolecules. Carbohydrates are polyhydroxy aldehydes or ketones and their polymers, with the general idea of Cx(H2O)y. Proteins are polymers of amino acids joined by peptide bonds. Lipids are fats, oils and related compounds that are insoluble in water, and triglycerides are esters of glycerol with three fatty acids. Nucleic acids are polymers of nucleotides, each made of a sugar, a phosphate group and a nitrogen base. Chemical tests identify them: Benedict's and Fehling's tests for reducing sugars, the iodine test for starch, the biuret test for proteins and the grease spot test for fats. Frederick Sanger worked out the amino acid sequence of insulin in 1955 and received the Nobel Prize in 1958.

5 min read · UPSC Prelims · Science & Technology · Chemistry in Daily Life and Biomolecules

At a Glance

Glucose and fructose Monosaccharides
Sucrose Glucose + fructose
Peptide bond Links amino acids
Triglyceride Glycerol + 3 fatty acids
Insulin sequence Sanger
Exam link UPSC Prelims, Science & Technology: Chemistry in Daily Life and Biomolecules

Where Does It Fit?

This is the fifth topic of the macro on chemistry in daily life and biomolecules. It revisits the biomolecules of the first macro from a chemical point of view. Previous topic: Hydrocarbons and Their Derivatives. Next topic: Vitamins and Enzymes.

What Are the Classes of Carbohydrates and Their Tests?

Class	Examples and features
Monosaccharides	Glucose, fructose and galactose, with six carbons, and ribose and deoxyribose with five. Glucose is found in grapes and blood, and fructose in fruit and honey
Disaccharides	Sucrose (glucose plus fructose) in cane sugar, lactose (glucose plus galactose) in milk, and maltose (two glucose) in malt
Polysaccharides	Starch, with amylose and amylopectin, glycogen in animals, cellulose in plants and chitin in fungi and insects
Reducing sugars	Glucose, fructose, lactose and maltose reduce Benedict’s and Fehling’s solutions to a red precipitate. Sucrose is non-reducing
Starch test	Iodine turns starch blue-black
Optical activity	Sugars rotate polarised light, and natural glucose is the dextro form, so it is also called dextrose

How Are Proteins, Lipids and Nucleic Acids Built?

Amino acids: have an amino group and a carboxyl group on the same carbon. In water they exist as zwitterions with both charges. Twenty occur in proteins, and nine are essential in the diet of adults.
Peptide bond: joins the carboxyl group of one amino acid to the amino group of the next, with loss of water.
Protein structure: primary is the sequence, secondary is the alpha helix and beta sheet, tertiary is the folded shape and quaternary is the arrangement of several chains, as in haemoglobin.
Fibrous and globular proteins: keratin and collagen are fibrous and insoluble, while albumin and enzymes are globular and soluble.
Denaturation: heat, acids or heavy metals change the shape of proteins, as when an egg white turns solid on boiling, and function is lost.
Lipids: triglycerides in fats and oils, phospholipids in membranes, steroids such as cholesterol, and waxes. Saponification of fats with alkali gives soap, as in Chemistry of Pharmaceuticals.
Nucleic acids: a nucleoside is a sugar plus a base, and a nucleotide adds a phosphate. Nucleotides link through phosphodiester bonds. Friedrich Miescher found nucleic acid in 1869.

What Else Should You Remember?

Saturated and unsaturated fats: saturated fats have no double bonds and are solid, while unsaturated ones have double bonds and are liquid. Hydrogenation converts oils into vanaspati, and trans fats formed in the process raise the risk of heart disease.
Essential fatty acids: omega-3 and omega-6 fatty acids must come from the diet.
Energy values: fats give 9 kilocalories per gram, and carbohydrates and proteins give 4, as in Chemical Building Blocks of Life (Biomolecules).
Fibre: cellulose cannot be digested by humans but helps in bowel function.
Protein sequencing: insulin was the first protein to be sequenced, and later made by recombinant DNA technology, as in Recombinant DNA Technology.
Nucleic acids and genes: structure and function are in DNA and RNA: Structure and Function.
Hormones and enzymes: many hormones are proteins or steroids, and almost all enzymes are proteins, as in Vitamins and Enzymes.
Dietary sources: cereals give carbohydrates, pulses and milk give protein, and oils and ghee give fats.
Denaturation of proteins: denaturation of proteins is irreversible in many cases, and it is the reason why cooking changes the texture of meat and eggs.
Glycosidic bond: a glycosidic bond joins two sugar units with loss of water, and it is broken by acids and by enzymes such as amylase and sucrase.
Sweetness: fructose is the sweetest natural sugar, and sucrose is the common table sugar made from sugarcane or beet.

Why Does It Matter for the Exam?

Structure questions: bond types and building blocks of each class.
Test questions: which test detects which biomolecule.
Nutrition link: energy values, essential amino acids and fatty acids.
Technology link: protein engineering, insulin and enzyme products.
Common errors: calling sucrose a reducing sugar or confusing the peptide bond with the glycosidic bond.

As an analytical point, the structure of a biomolecule decides its function. A change in one amino acid, as in sickle cell haemoglobin, can change the whole behaviour of a protein.

Why Is This Topic Important?

Facts Building blocks asked
Tests Benedict, iodine, biuret
Link Nutrition and insulin

What Came Next?

The next topic covers vitamins and enzymes. Next topic: Vitamins and Enzymes.

Key People and Terms

Glycosidic bond Links sugar units
Peptide bond Links amino acids
Zwitterion Ion with both charges
Denaturation Loss of protein shape
Triglyceride Glycerol with three fatty acids
Nucleotide Sugar, phosphate and base

Exam Corner

Points to Remember

Sucrose = glucose + fructose.
Lactose in milk.
Iodine turns starch blue.
Peptide bond joins amino acids.
Sanger: insulin sequence.

Do Not Confuse With

Sucrose and glucose Non-reducing disaccharide versus reducing monosaccharide
Nucleoside and nucleotide No phosphate versus with phosphate
Saturated and unsaturated fat No double bond versus double bonds

Memory Hook

Sugar glycosidic, protein peptide, fat ester, nucleic phosphodiester; iodine blue for starch, Benedict red for reducing sugar.

Mains Angle

Questions link molecular structure with function. These points add depth.

Carbohydrates Classes and tests
Proteins Levels of structure
Lipids and nucleic acids Types

A Question You May Face

An original practice question, not a past paper question.

“Describe the structure and classification of carbohydrates, proteins, lipids and nucleic acids with the tests used to identify them.”

How to Answer

Carbohydrates Classes.
Proteins Peptide bond.
Lipids Fats.
Nucleic acids Nucleotides.

GS Relevance

Prelims: classification, structure, chemical tests, examples. GS Paper 3: science and technology.

Frequently Asked Questions

What are reducing sugars?

Reducing sugars are sugars that can reduce mild oxidising agents such as Benedict's and Fehling's solutions because they have a free aldehyde or ketone group. Glucose, fructose, lactose and maltose are reducing sugars, while sucrose is non-reducing, and the test gives a red precipitate of copper oxide.

What is a peptide bond?

A peptide bond is the covalent bond formed between the carboxyl group of one amino acid and the amino group of another, with loss of a molecule of water. Many amino acids joined by peptide bonds form a polypeptide chain, which folds to form a protein.

What is denaturation of proteins?

Denaturation is the loss of the three-dimensional shape of a protein, and so its function, caused by heat, strong acids or bases, heavy metals or alcohol. The peptide bonds stay intact, but the folding is disrupted, as when egg white turns solid on boiling or milk curdles.

What is a triglyceride?

A triglyceride is the main form of fat and oil, made by joining one molecule of glycerol with three fatty acid molecules through ester bonds. Fats are mostly saturated and solid, while oils are mostly unsaturated and liquid, and triglycerides store energy in the body and in seeds.

What is the difference between a nucleoside and a nucleotide?

A nucleoside is made of a five-carbon sugar and a nitrogen base, while a nucleotide has a phosphate group attached to the nucleoside as well. Nucleotides are the building blocks of DNA and RNA, and they link through phosphodiester bonds, and ATP is a nucleotide that carries energy.

PYQ Practice — Statement Analysis

1 Sucrose is a reducing sugar.

False

It is non-reducing.

2 Iodine solution turns starch blue-black.

True

A test for starch.

3 Proteins are polymers of amino acids joined by peptide bonds.