In nucleoside a base is attached at $1\text{'}$ position of sugar moiety. Nucleotide is formed by linking of phosphoric acid unit to the sugar unit of nucleoside. At which position of sugar unit is the phosphoric acid linked in a nucleoside to give a nucleotide?
 

(i) Carbohydrates are optically active polyhydroxy aldehydes or ketones or molecules which produce such units on hydrolysis. They are broadly classified into three groups monosaccharides, disaccharides and polysaccharides.

(ii) Glucose, the most important source of energy for mammals, is obtained by the digestion of starch.

(iii) Preparation of glucose:
$\underset{\mathrm{Sucrose}}{{\mathrm{C}}_{12}{\mathrm{H}}_{22}{\mathrm{O}}_{11}}+{\mathrm{H}}_2\mathrm{O}\stackrel{{\mathrm{H}}^{+}}{⟶}\underset{\mathrm{Glucose}}{{\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_{6 }}+\underset{\mathrm{Fructose}}{{\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6}$

                               Or

$\underset{\mathrm{Starch} \mathrm{or} \mathrm{cellulose}}{{\left({\mathrm{C}}_6{\mathrm{H}}_{10}{\mathrm{O}}_5\right)}_{\mathrm{n}}+{\mathrm{nH}}_2\mathrm{O}}\underset{393 \mathrm{K};2-3\text{ atm }}{\overset{{\mathrm{H}}^{+}}{\to }}\mathrm{n}\underset{\mathrm{Glucose}}{{\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6}$

(iv) Monosaccharides are held together by glycosidic linkages to form disaccharides or polysaccharides.

2. Proteins: 

(i) Proteins are the polymers of about twenty different α-amino acids which are linked by peptide bonds.

(ii) Proteins which contain only α-amino acids are called simple proteins.

(iii) The secondary or tertiary structure of proteins when subjected to change of pH or temperature, will not be able to perform their functions. This is called denaturation of proteins.

3. Enzymes

Enzymes are biocatalysts which speed up the reactions in biosystems. They are very specific and selective in their action and chemically all enzymes are proteins.

4. Vitamins:

Vitamins are accessory food factors required in the diet. They are classified as fat soluble (A, D, E and K) and water soluble (Β group and C). Deficiency of vitamins leads to many diseases.

5. Nucleic acids :

(i) Nucleic acids are the polymers of nucleotides which in turn consist of a base, a pentose sugar and phosphate moiety. Nucleic acids are responsible for the transfer of characters from parents to offsprings.

(ii) There are two types of nucleic acids $—\mathrm{DNA}$ and $\mathrm{RNA}$.

(iii) $\mathrm{DNA}$ contains a five carbon sugar molecule called 2-deoxyribose whereas $\mathrm{RNA}$ contains ribose. Both $\mathrm{DNA}$ and $\mathrm{RNA}$ contain adenine, guanine and cytosine. The fourth base is thymine in $\mathrm{DNA}$ and uracil in $\mathrm{RNA}$.

(iv) The structure of $\mathrm{DNA}$ is a double strand whereas $\mathrm{RNA}$ is a single strand molecule. $\mathrm{DNA}$ is the chemical basis of heredity and have the coded message for proteins to be synthesized in the cell. There are three types of $\mathrm{RNA} — \mathrm{mRNA}, \mathrm{rRNA}$ and $\mathrm{tRNA}$ which actually carry out the protein synthesis in the cell.

6. Hormones:

Hormones Chemical substances which are produced in the ductless glands in the body are hormones. They act as intercellular messengers.