A living system evolves, reproduces itself, and sustains itself. The fact that a living system is made of nonliving atoms and molecules is the most incredible aspect of it. Biochemistry is the study of what happens within a living system chemically. Living systems are made up of carbohydrates, proteins, nucleic acids, lipids, and other complex biomolecules. Proteins and carbohydrates are essential nutrients in our diet. These biomolecules interact and form the molecular logic of biological processes.
Carbohydrates are polyhydroxy aldehydes or ketones that exist primarily in cyclic hemiacetal or acetal forms and are derived from their general formula Cx(H2O)y. Glucose’s molecular formula (C6H12O6) fits into this general formula, C6(H2O)6. However, not all of the compounds in this formula are carbohydrate-based. Acetic acid (CH3COOH), for example, has the general formula C2(H2O)2, but it is not a carbohydrate. Similarly, C6H12O5 (rhamnose)is a carbohydrate but not classified as such. Carbohydrates are also called sugars if they have a sweet taste. Sucrose is the most common type of sugar found in our homes, whereas lactose is found in milk. Carbohydrates are also known as saccharides.
Classification of carbohydrates
Carbohydrates are classified on the basis of their behaviour on hydrolysis.
- Monosaccharide: A carbohydrate whose structure cannot be hydrolyzed to a simpler one. Examples include fructose, glucose, and ribose.
- Disaccharide: A carbohydrate that can be hydrolyzed into two monosaccharide units, which do not have to be the same. For example: When sucrose is hydrolyzed, it yields one molecule of fructose and one molecule of glucose, whereas maltose yields two molecules of only glucose.
- Oligosaccharides: A carbohydrate that can be hydrolyzed into a small number of monosaccharide molecules (up to 10), sometimes of three or more types.
- Polysaccharide: A carbohydrate that can be hydrolyzed to produce a large number of monosaccharide molecules, frequently of three or more types.
Some common examples are starch, cellulose, glycogen, gums, etc. Carbohydrates can also be divided into two types: reducing sugars and nonreducing sugars. Reducing sugars are carbohydrates that reduce Fehling’s solution and Tollens’ reagent. All monosaccharides, aldose or ketose, are reducing sugars.
Starch is found in the roots, tubers, and seeds of plants. Important sources for human diets are potatoes, wheat and rice. Starch is a mixture of polyglucose structures, approximately 20% amylose and 80% amylopectin.
Amylose is composed of over 1000 D-glucopyranosyl units linked together by 𝝰-linkage between C1 of one unit and C4 of the next.
In water, amylose forms colloidal dispersions. When iodine (I2) is added to these solutions, it produces an intense blue-black colour. The nonpolar I2 migrates into the helix’s low polarity core, where it assembles with the surrounding groups on the outside, where it can make hydrogen bond with the surrounding water. This is a common method for detecting the presence of starch (or of I2). The hydrolysis of starch can be monitored by testing samples with a dilute iodine solution on a regular basis. As the starch is broken down into smaller fragments, the colour shifts from blue-black→ blue→ purple→ pink → colourless.
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Amylopectin has a structure similar to amylose, except that it has polyglucose side branches attached by 𝝰1-O-6 acetallinks at 20-25 glucose unit intervals. All other acetal links have the same formula as amylose, 𝝰1-O-4. Amylopectin is larger in size and contains between 6000- 35,000 glucose units.