Question

What is the relationship between carbohydrates and starch?

Answer 1

Carbohydrates"

• Carbohydrates are polyhydroxy aldehydes or polyhydroxy ketones, or compounds that can be hydrolyzed to them.
• The majority of carbohydrates are present in the same ratio as in water, hence, also called hydrates of carbon.
• Carbohydrates are the most abundant biomolecules on Earth.

Classification of Carbohydrates"

Carbohydrates are classified into the following classes depending on whether they undergo hydrolysis and if so on the number of products formed:

• Monosaccharides: These are the compounds that do not break into simpler compounds. These have the general formula ${\left({\mathrm{CH}}_2\mathrm{O}\right)}_{\mathrm{n}}$. Examples of monosaccharides are glucose, fructose, etc.
• Oligosaccharides: These compounds give two to ten monosaccharide molecules on acidic hydrolysis. These are known as di, tri, or tetrasaccharides, depending upon the actual number of the monosaccharide molecules formed during hydrolysis. Sucrose and maltose are examples of disaccharides because they give two monosaccharide molecules during hydrolysis.
• Polysaccharides: These compounds give a very large number of monosaccharide molecules on acidic hydrolysis. Cellulose and starch are two important examples of polysaccharides. Their general formula is ${\left({\mathrm{C}}_6{\mathrm{H}}_{10}{\mathrm{O}}_5\right)}_n$ where n is an integer having a very high value.

Starch

• It occurs in plants in the form of granules which are found mainly in the seeds, tubers, and roots of the plants.
• It is a mixture of two polysaccharides, i.e., amylose $(20\%)$ and amylopectin $(80\%)$.

Relationship between Carbohydrates and Starch

• Carbohydrates are polyhydroxy aldehydes or polyhydroxy ketones, or compounds that can be hydrolyzed to them.
• Starch is composed of two polysaccharides: Amylose and Amylopectin.
• Amylose is water-soluble while amylopectin is insoluble. Both amylose and amylopectin are polymers of glucose.
• Amylose is composed of $100-3000$ D-glucose units linked together to form unbranched chains, whereas, amylopectin is a branched-chain polysaccharide made up of $25-30$ D-glucose units joined by α-glycosidic linkages between $\mathrm{C}-1$ of one glucose unit and $\mathrm{C}-4$ of the next glucose unit as in amylose. However, these chains are connected to each other by $1,6$ linkages.