(i) The bond formed between two amino acids by the elimination of a water molecule is called peptide linkage or bond
(ii) The amino group of one amino acid and a carboxyl group of other amino acid undergoes condensation to remove a water molecule and it results in the formation of bond.

(iii) The product formed by linking amino acid molecules through peptide linkages is called a peptide.
(iv) In peptide formation the two different amino acid molecules may react in one of the two ways.
\({ NH }_{ 2 }-{ CH }_{ 2 }-COOH+{ H }_{ 2 }N-\overset { \underset { | }{ { CH }_{ 3 } } }{ CH } -COOH\longrightarrow { H }_{ 2 }N-{ CH }_{ 2 }-CO-NH-\overset { \underset { | }{ { CH }_{ 3 } } }{ CH } -COOH\) 
\({ H }_{ 2 }N-\underset { Alanine }{ \overset { \underset { | }{ \overset { Glycine }{ { CH }_{ 3 } } } }{ CH } } -COOH+{ H }_{ 2 }N-\underset { Glycine }{ { CH }_{ 2 } } -COOH\longrightarrow \underset { Alanyl\quad glycine\quad (Dipeptide) }{ { H }_{ 2 }N-\overset { \underset { | }{ { CH }_{ 3 } } }{ CH } -CO-NH } -{ CH }_{ 2 }COOH\)

Importance of proteins:
Proteins are the functional units of living things : play vital role in all biological processes
(i) All biochemical reactions occur in the living systems are catalysed by the catalytic proteins called enzymes.
(ii) Proteins such as keratin, collagen acts as structural back bones.
(iii) Proteins are used for transporting molecules (Haemoglobin), organelles (Kinesins) in the cell and control the movement of molecules in and out of the cells (Transporters).
(iv) Antibodies help the body to fight various diseases
(v) Proteins are used as messengers to coordinate many functions. Insulin & glucagon controls the glucose level in the blood.
(vi) Proteins act as receptors that detect presence of certain signal molecules and activate the proper response.
(vii) Proteins are also used to store metals such as iron (Ferritin) etc.

(i) Primary structure of proteins: Proteins are polypeptide chains made up of amino acids connected through peptide bonds. The relative arrangement of the amino acids in the polypeptide chain is called the primary structure of the protein.
(ii) Secondary structure of proteins: The amino acids in the polypeptide chain forms highly regular shapes (substructures) through the hydrogen bond between the carbonyl oxygen (-C=O) and the neighbouring amine hydrogen (- NH) of the main chain.
(iii) β-Strands are extended peptide chain rather than coiled. The hydrogen bonds occur between main chain carbonyl group one such strand and the amino group of the adjacent strand resulting in the formation of a sheet like structure. This arrangement is called β-sheets.

Importance of carbohydrates:
(i) Carbohydrates, widely distributed in plants and animals, acts mainly as energy sources and structural polymers.
(ii) Carbohydrate is stored in the body as glycogen and in plant as starch.
(iii) Carbohydrates such as cellulose which is the primary components of plant cell wall, is used to make paper, furniture (wood) and cloths (cotton)
(iv) Simple sugar glucose serves as an instant source of energy.
(v) Ribose sugars are one of the components of nucleic acids.
(vi) Modified carbohydrates such as hyaluronate (glycosaminoglycans) act as shock absorber and lubricant.

(i) The amino acids are linked covalently by peptide bonds. The carboxyl group of the first amino acid react with the amino group of the second amino acid to give an amide linkage between these amino acids. This amide linkage is called peptide bond. The resulting compound is called a dipeptide. Addition an another amino acid to this dipeptide a second peptide bond results in tripeptide.

(ii) Thus we can generate tetra peptide, penta peptide etc... When you have more number of amino acids linked this way you get a polypeptide. If the number of amino acids are less it is called as a polypeptide, if it has large number of amino acids (and preferably has a function) then it is called a protein.
(iii) The amino end of the peptide is known as N - terminal or amino terminal while the carboxy end is called C-terminal or carboxy terminal. In general protein sequences are written from N-Terminal to C-Terminal.The atoms other than the side chains (R-groups) are called main chain or the back bone of the polypeptide.

Primary structure of Proteins:
Proteins are polypeptide chains, made up of amino acids are connected through peptide bonds. The relative arrangement of the amino acids in the polypeptide chain is called the primary structure of the protein. Knowledge of this is essential as even small changes have potential to alter the overall structure and function of a protein.
\(\mathrm{H}_{2} \mathrm{~N}-\mathrm{Gly}-\mathrm{Met}-\mathrm{Phe}-\mathrm{Cys}-\mathrm{Arg}-\mathrm{Asp}-\mathrm{COOH}\)
α - Helix:
In the α-helix sub-structure, the amino acids are arranged in a right handed helical (spiral) structure and are stabilised by the hydrogen bond between the carbonyl oxygen of one amino acid (n residue) with amino hydrogen of the fifth residue (n + 4^th residue). The side chains of the residues protrude outside of the helix. Each turn of an α-helix contains about 3.6 residues and is about 5.4 Å long. The amino acid proline produces a kink in the helical structure and often called as a helix breaker due to its rigid cyclic structure.

1. Linear sequence of aminoacids
2. Linear
3. Composed of peptide bonds formed between amino acids.
Secondary structure of Proteins:
The amino acids in the polypeptide chain forms highly regular shapes (sub-structures) through the hydrogen bond between the carbonyl oxygen (-C=O) and the neighbouring amine hydrogen (-NH) of the main chain. α-Helix and β-strands or sheets are two most common substructures formed by proteins.

β-Strand:
β-Strands are extended peptide chain rather than coiled. The hydrogen bonds occur between main chain carbonyl group one such strand and the amino group of the adjacent strand resulting in the formation of a sheet like structure. This arrangement is called β-sheets.

rRNA is mainly found in cytoplasm and in ribosomes, which contain 60% RNA and 40% protein. Ribosomes are the sites at which protein synthesis takes place.

Cellulose is used extensively in manufacturing paper, cellulose fibres and rayon explosive.

Humans cannot use cellulose as food because our digestive systems do not contain the necessary enzymes (glycosidases or cellulases) that can hydrolyse the cellulose.

Polysaccharides are called as non-sugars since they don't have an sweet taste.

Fructose is prepared commercially by hydrolysis of Inulin (a polysaccharide) in acidic medium.
\(\underset { Inulin }{ \left( { C }_{ 6 }{ H }_{ 12 }{ O }_{ 5 } \right) _{ 5 } } +n{ H }_{ 2 }O\overset { { H }^{ + } }{ \longrightarrow } \underset { Frutose }{ { nC }_{ 6 }{ H }_{ 12 }{ O }_{ 6 } } \)

Anomers are cyclic monosaccharide differing from each other in the configuration of C₁ if they are aldoses or in the configuration of C₂ if they are ketoses.
E.g. α and βD - glucose.

Lactose is a dissaccharide so on hydrolysis gives I two monosaccharides.
\(\underset { Lactose }{ { C }_{ 12 }{ H }_{ 22 }{ O }_{ 11 }+{ H }_{ 2 }O } \overset { + }{ \longrightarrow } \underset { D(+)G1ucoseD(+)Galactose }{ { C }_{ 6 }{ { H }_{ 12 }{ O }_{ 6 } }+{ C }_{ 6 }{ H }_{ 12 }{ O }_{ 6 } } \)

(a) C₄ carbon of the given sugar contains H and OH on the same configuration like C₄ carbon in L-Glyceraldehyde. Therefore the sugar is L-sugar.
(b) Because the H and OH on C₄ carbon are in the same configuration as the Hand OH on C₄ carbon in L-Glyceraldehyde

∴ Two dipeptides structures are possible from glycine and alanine. They are glycyl alanine and Alanyl glycine.

i) Starch - Polysaccharide
ii) fructose - monosaccharide
iii) sucrose - oligosaccharides 
iv) lactose - oligosaccharides
iv) maltose - oligosaccharides

Hormone	Vitamin
Synthesized in animal bodies	Synthesised in plants
Produced in ductless (endocrine glands)	Have to be supplied in diet except (Vitamin D)
These are not stored in body but are continuously produced	These remain stored in the body to keep away diseases
Eg: Androgen, Estrogen, thyroxine etc.	Eg: Vitamin A (Retinol) Vitamin C (Ascorbic acid)

(i) Sucrose: D - glucose and D - fructose linked by α. - 1, 2 glycosidic bond.
(ii) Maltose: Two molecules of α. - D - glucose linked by α. - 1, 4 glycosidic bond.
(iii) Lactose: f3 - D - glucose and f3 - D galactose linked by β - 1, 4 glycosidic bond

At aqueous solution, the proton from carboxyl group can be transferred to the amino group of an amino acid leaving these groups with opposite charges. Despite having both positive and negative charges, this molecule is neutral and has amphoteric behaviour. These ions are called zwitter ions.

Zwitter ions behave as neutral molecules at pH isoelectric point.

Proteins are classified based on their structure (overall shape) into two major types. They are fibrous protein and globular proteins.
(i) Fibrous proteins: These proteins are linear molecules. These are generally insoluble in water and are held together by disulphide bridges and weak intermolecular hydrogen bonds. The Example: Keratin, Collagen etc
(ii) Globular proteins: These proteins have an overall spherical shape. The polypeptide chain is folded into a spherical shape. These proteins are usually soluble in water and have many functions including catalysis.

(i)

(ii) Glucose forms penta acetate with acetic anhydride suggesting the presence of five alcohol groups.

Glucose reacts with HCN to form cyanohydrin.

(i) Oxidation of fructose yields a mixture of glycollic acid and tartaric acid.
(ii) When fructose is treated with HCN, it forms an addition product which upon hydrolysis and subsequent reduction with hydroiodic acid and red phosphorous gives 2-methyl-hexanoic acid.
This indicates that the ketone group is

(i) Ribonucleic acids are similar to DNA. Cells contain up to eight times high quantity of RNA than DNA. RNA is found in large amount in the cytoplasm and a lesser amount in the nucleus. In the cytoplasm it is mainly found in ribosomes and in the nucleus, it is found in nucleolus.
(ii) RNA molecules are classified according to their structure and function into three major types
i. Ribosomal RNA (rRNA)
ii. Messenger RNA (mRNA)
iii. Transfer RNA (tRNA)
rRNA:
rRNA is mainly found in cytoplasm and in ribosomes, which contain 60% RNA and 40% protein. Ribosomes are the sites at which protein synthesis takes place.
tRNA:
tRNA molecules have lowest molecular weight of all nucleic acids. They consist of 73 - 94 nucleotides in a single chain. The function of tRNA is to carry amino acids to the sites of protein synthesis on ribosomes.
mRNA:
mRNA is present in small quantity and very short lived. They are single stranded, and their synthesis takes place on DNA. The synthesis of mRNA from DNA strand is called transcription. mRNA carries genetic information from DNA to the ribosomes for protein synthesis. This process is known as translation.

i) Reducing sugars:
1. Sugars which reduce Tollen's reagent or Fehling's solution or Benedict's solution are called reducing sugars.
2. These contain either α - hydroxyl ketone or cyclical hemi acetal or hemi ketal or structures in equilibrium with open chain forms having a free- CHO or C=O group.
3. E.g. a) All monosaccharide's like D - glucose, D - fructose (aldoses and ketoses)
b) Sugars like Lactose and maltose except sucrose.
ii) Non - reducing sugars:
1. Sugars which do not reduce either Tollen's reagent, Fehling's solution or Benedict's solution are called non-reducing sugars.
2. They contain a stable acetal or ketal structures which cannot be opened into a free carbonyl group.
E.g. Sucrose, starch, cellulose, glycogen, dextrin etc.