CARBOHYDRATES
Introduction
Introduction
Carbohydrates occur abundantly in living organisms. They are found in all organisms and in almost all parts of the cell.
Meaning
- The word carbohydrate literally means hydrated carbon. They are composed of carbon, hydrogen and oxygen, the ratio of hydrogen and oxygen is the same as in water.
- Their general formula is Cx(H2O)y, where x and y are the whole number from three to many thousands whereas y may be the same or different whole number.
Definition
- Chemically carbohydrates are defined as polyhydroxy aldehydes or ketones or complex substances, which on hydrolysis yield polyhydroxy aldehyde or ketone subunits. (Hydrolysis involves the breakdown of large molecules into smaller ones utilizing water molecules).
Source
The source of carbohydrates are green plants. These are the primary products of photosynthesis. Other compounds of plants are produced from carbohydrates by various chemical changes.
Cellulose of wood, cotton, and paper, starches present in cereals, root tubers, cane sugar and milk sugar are all examples of carbohydrates.
Conjugated Molecules of Carbohydrates
Carbohydrates in cell combine with
- Protein forming glycoproteins
- Lipids forming glycolipids
Glycoproteins and glycolipids have structural role in the extracellular matrix of animals and bacterial cell wall. Both these conjugated molecules are components of biological membranes.
Classification of carbohydrates
Carbohydrates are also called ‘saccharides’ (derived from Greek word ‘sakcharon’ meaning sugar) and are classified into three groups;
- Monosaccharides
- Oligosaccharides
- Polysaccharides
(i) Monosaccharides
These are simple sugars.
Features
- They are sweet in taste
- They are easily soluble in water and they cannot be hydrolyzed into simple sugars.
Composition
- Chemically they are either polyhydroxy aldehydes or ketones.
- All carbon atoms in monosaccharides except one, have a hydroxyl group.
- The remaining carbon atom is either a part of an aldehyde group or a keto group. The sugar with aldehyde group is called aldo-sugar e.g. glyceraldehydes and with the keto group as keto-sugar e.g. dihydroxyacetone.
These are indicated in the case of two trioses sketched below.
Structure of glyceraldehydes, a 3C Sugar (C3H6O3). The aldehyde form is glyceraldehydes, whereas ketonic form is dihydroxyacetone.
Types
In nature monosaccharides with 3 to 7 carbon atoms are found. They are called trioses (3C), tetroses (4C), pentoses (5C), hexoses (6C) and heptoses (7C). They have general formula (CH2O)n. Where n is the whole number from three to seven.
Examples
- Two trioses mentioned above i.e. glyceraldehydes and dihydroxyacetone, are intermediates in respiration and photosynthesis.
- Tetroses are rare in nature and occur in some bacteria.
- Pentoses and hexoses are most common. Form the biological point of view most important hexose is glucose, which is an aldose sugar.
Structures of ribose and glucose is give below.
Structure of Ribose and Glucose
Ring formation
Most of the monosaccharides form a ring structure when in solution. For example ribosewill form a five-cornered ring known as ribofuranose, whereas glucose will form six-cornered ring known as glucopyranose.
Ribose and glucose form ring shaped structures.
Glucose a common example
- Glucose is naturally produced in green plants, which take carbon dioxide from the air and water from the soil to synthesize glucose.
- As indicated in the equation, energy is consumed in this process which is provided by sunlight. That is why the process is called photosynthesis.
- For synthesis of 10g of glucose 717.6 Kcal of solar energy is used. This energy is stored in the glucose molecules as chemical energy and becomes available in all organisms when it is oxidized in the body.
- In Free State, glucose is present in all fruits, being abundant in grapes, figs and dates.
- Our body normally contains 0.08% glucose.
- In combined form, it is found in many disaccharides and polysaccharides. Starch, cellulose and glycogen yield glucose on complete hydrolysis.
Carbohydrates occur abundantly in living organisms. They are found in all organisms and in almost all parts of the cell.
Meaning
- The word carbohydrate literally means hydrated carbon. They are composed of carbon, hydrogen and oxygen, the ratio of hydrogen and oxygen is the same as in water.
- Their general formula is Cx(H2O)y, where x and y are the whole number from three to many thousands whereas y may be the same or different whole number.
Definition
- Chemically carbohydrates are defined as polyhydroxy aldehydes or ketones or complex substances, which on hydrolysis yield polyhydroxy aldehyde or ketone subunits. (Hydrolysis involves the breakdown of large molecules into smaller ones utilizing water molecules).
Source
The source of carbohydrates are green plants. These are the primary products of photosynthesis. Other compounds of plants are produced from carbohydrates by various chemical changes.
Cellulose of wood, cotton, and paper, starches present in cereals, root tubers, cane sugar and milk sugar are all examples of carbohydrates.
Conjugated Molecules of Carbohydrates
Carbohydrates in cell combine with
- Protein forming glycoproteins
- Lipids forming glycolipids
Glycoproteins and glycolipids have structural role in the extracellular matrix of animals and bacterial cell wall. Both these conjugated molecules are components of biological membranes.
Classification of carbohydrates
Carbohydrates are also called ‘saccharides’ (derived from Greek word ‘sakcharon’ meaning sugar) and are classified into three groups;
- Monosaccharides
- Oligosaccharides
- Polysaccharides
(i) Monosaccharides
These are simple sugars.
Features
- They are sweet in taste
- They are easily soluble in water and they cannot be hydrolyzed into simple sugars.
Composition
- Chemically they are either polyhydroxy aldehydes or ketones.
- All carbon atoms in monosaccharides except one, have a hydroxyl group.
- The remaining carbon atom is either a part of an aldehyde group or a keto group. The sugar with aldehyde group is called aldo-sugar e.g. glyceraldehydes and with the keto group as keto-sugar e.g. dihydroxyacetone.
These are indicated in the case of two trioses sketched below.
Structure of glyceraldehydes, a 3C Sugar (C3H6O3). The aldehyde form is glyceraldehydes, whereas ketonic form is dihydroxyacetone.
Types
In nature monosaccharides with 3 to 7 carbon atoms are found. They are called trioses (3C), tetroses (4C), pentoses (5C), hexoses (6C) and heptoses (7C). They have general formula (CH2O)n. Where n is the whole number from three to seven.
Examples
- Two trioses mentioned above i.e. glyceraldehydes and dihydroxyacetone, are intermediates in respiration and photosynthesis.
- Tetroses are rare in nature and occur in some bacteria.
- Pentoses and hexoses are most common. Form the biological point of view most important hexose is glucose, which is an aldose sugar.
Structures of ribose and glucose is give below.
Structure of Ribose and Glucose
Ring formation
Most of the monosaccharides form a ring structure when in solution. For example ribosewill form a five-cornered ring known as ribofuranose, whereas glucose will form six-cornered ring known as glucopyranose.
Ribose and glucose form ring shaped structures.
Glucose a common example
- Glucose is naturally produced in green plants, which take carbon dioxide from the air and water from the soil to synthesize glucose.
- As indicated in the equation, energy is consumed in this process which is provided by sunlight. That is why the process is called photosynthesis.
- For synthesis of 10g of glucose 717.6 Kcal of solar energy is used. This energy is stored in the glucose molecules as chemical energy and becomes available in all organisms when it is oxidized in the body.
- In Free State, glucose is present in all fruits, being abundant in grapes, figs and dates.
- Our body normally contains 0.08% glucose.
- In combined form, it is found in many disaccharides and polysaccharides. Starch, cellulose and glycogen yield glucose on complete hydrolysis.
(ii) Oligosaccharides
Features
- These are less sweet in taste.
- These are less soluble in water.
- On hydrolysis, they yield from two to ten monosaccharides.
- The covalent bond between two monosaccharides is called Glycosidic linkage.
Types
- Those yielding two monosaccharides on hydrolysis are called disaccharides.
- Those yielding three are known as trisaccharides and so on.
Examples
- Physiologically important disaccharides are maltose, sucrose and lactose.
Most familiar disaccharide is sucrose (cane sugar), which on hydrolysis yields glucose and fructose, both of which are reducing sugars. Its molecular formula is C12H22O11. Its structural formula is given below.
A disaccharide. Note carefully the glycosidic linkage between the two monosaccharides.
(iii) Polysaccharides
Polysaccharides are the most complex and most abundant carbohydrates in nature.
Features
- They are usually branched and tasteless.
- They are formed by several monosaccharide units linked by glycosidic bonds.
- They have high molecular weights.
- They are sparingly soluble in water.
Examples
Some biologically important polysaccharides are starch, glycogen, cellulose, dextrins, agar, pectin and chitin etc.
(i) Starch
- It is found in fruits, grains, seeds and tubers.
- It is the main source of carbohydrates for animals.
- On hydrolysis, it yields glucose molecules.
- It gives blue colour with iodine.
- It occurs in two types;
- Amylose starches have unbranched chains of glucose and are soluble in hot water.
- Amylopectin starches have branched chains and are insoluble in hot or cold water.
Polysaccharides are polymers of monosaccharides
(ii) Glycogen
- It is also called animal starch It is the chief form of carbohydrate stored in animal body.
- Found abundantly found in liver and muscles, though found in all animal cells.
- It is insoluble in water.
- It gives red colour with iodine solution.
- It also yields glucose on hydrolysis.
(iii) Cellulose
- It is the most abundant carbohydrate in nature.
- It is the main constituent of cell walls of plants and is highly insoluble in water. Cotton is the pure form of cellulose.
- On hydrolysis, it also yields glucose molecules.
- It gives no colour to iodine solution.
- It is not digested by human digestive tract. In the herbivores, it is digested because of microorganisms (bacteria, yeasts, protozoa) in their digestive tract. These microorganisms secrete an enzyme called cellulase for its digestion.
Comments