Question :
What are carbohydrates . Describe various types of carbohydrates and their Functions as well.
Answer:
CARBOHYDRATES:
Definition: The word carbohydrate literally means hydrated carbon. They are composed of carbon, hydrogen and oxygen and the ratio of hydrogen and oxygen is the same as in water. Their general formula is Cn (H2O)n, where n is the whole number from three to many thousands. Chemically, carbohydrates are defined as polyhydroxy aldehydes or ketones, or complex substances which on hydrolysis yield polyhydroxy aldehyde or ketone subunits. (Hydrolysis involves the break down of large molecules into smaller ones utilizing water molecules).
Sources of carbohydrates:
The sources of carbohydrates are green plants. These are the primary products of photosynthesis. Other compounds of plants are produced from carbohydrates by various chemical changes.
Occurrence:
Carbohydrates occur abundantly in living organisms. They are found in all organisms and in almost all parts of the cell. Cellulose or wood, cotton and paper, starches present in cereals, root tubers, cane sugar and milk sugar are all examples of carbohydrates.
Functions of Carbohydrates:
Carbohydrates play both structural and functionals roles.
1). Source of energy:
Simple carbohydrates e.g., glucose, are the main source of energy in cells.
2). Constituent of cell wall:
Some carbohydrates e.g., cellulose, are the main constituents of cell walls in plants and micro-organisms.
3). Conjugated molecules:
Carbohydrates in cell combine with proteins and lipids and the resultant compounds are called glycoproteins and glycolipids, respectively. 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.
Classifications of Carbohydrates:
Carbohydrates are also called saccharides (derived from Greek word sakcharon meaning sugar) and are classified into three groups:
1) Monosaccharides
2) Oligosaccharides
3) Polysaccharides
Monosaccharides:
a) Simple: These are simple sugars.
b) Sweet taste: They are sweet in taste.
c) Solubility: They are easily soluble in water, and cannot be hydrolysed into simple sugars.
d) Chemical nature: Chemically they are either polyhydroxy aldehydes or ketones. All carbon atoms in a monosaccharide 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 aldenyde group is called aldo-sugar and with the keto group as keto-sugar.
e) Classification of Monosaccharides:
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 (CH20).
Glyceraldehyde and dihydroxyacetorie, both trioses (C3H6O3) are, intermediates in respiration and photosynthesis. Tetroses are rare in nature and occur in some bacteria. Pentoses and hexoses are most common. From the biological point of view the most important hexose is glucose. It is an aldose sugar.
Ring Structure:
Most of the monosaccharides form a ring structure when in solution. For example ribose will form a five cornered ring known as ribofuranose, whereas glucose will form six cornered ring known as glucopyrariose.
Glucose:
In free state, glucose is present in all fruits, being abundant in grapes, figs, and date. Our blood 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. Glucose is naturally produced in green plants.
which take carbon dioxide from the air and water from the soil to synthesize glucose.
6CO2 + 12H20-----> C6H12O6 + 6O2+6H2O
Energy is consumed in this process which is provided by sunlight. This is why the process is called Photosynthesis. It is noteworthy that for the 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.
Importance of Monosaccharldes:
1. In free state, glucose is present in all fruits, being abundant in grapes, figs.
2. Our blood normally contains 0.08% glucose.
3. In combined form, it is found in many disaccharides and polysaccharides.
4. Starch, cellulose and glycogen yield glucose on complete hydrolysis.
5. Glucose is naturally produced in green plants, which take carbon dioxide from the air and water from the soil to synthesize glucose.
OLIGOSACCHARIDES:
1. These are comparatively less sweet in taste.
2. They are less soluble in water.
3. On hydrolysis oligosaccharides yield from two to ten monosaccharides. The ones yielding two monosaccharides are known as disaccharides, those yielding three are known as trisaccharides and so on.
4. The covalent bond between two monosaccharides is called glucosidic bond.
Important disaccharides:
are maltose, sucrose, and lactose. Most familiar disaccharide is sucrose (cane sugar), which on hydrolysis yields "o"H") glucose and fructose, both of which are reducing sugars. Its molecular formula is C12H22O11.
POLYSACCHARIDES:
Polysaccharides are the most complex and most abundant carbohydrates in nature.
1. They are usually branched.
2. They are tasteless.
3. They are formed by several monosaccharide units linked by glycosidic bonds.
4. Polysaccharides have high molecular weights.
5. They are only sparingly soluble in water.
Some biologically important polysaccharides are starch, glycogen, cellulose, dextrin, agar, pectin, and chitin.
Starch:
1. It is found in fruits, grains, seeds, and tubers.
2. Itis the main source of carbohydrates for animals.
3. On hydrolysis, it yields glucose molecules.
Starches are of two types:
Amylose starches:
Amylose starches have unbranched chains of glucose and are soluble in hot water.
Amylopectin starches:
Amylopectin starches have branched chains and are insoluble in hot or cold water. Starches give blue color with iodine.
GLYCOGEN:
1. It is also called animal starch.
2. It is the chief form of carbohydrate stored in animal body.
3. It is found abundantly in liver and muscles, though found in all animal cells. 4. It is insoluble in water, and gives red color with iodine.
5. It also yields glucose on hydrolysis.
Cellulose:
1. It is the most abundant carbohydrate in nature.
2. Cellulose gives no colour with iodine.
3. It is the main constituent of cell walls of plants.
4. It is highly insoluble in water.
5. On hydrolysis it also yields glucose molecules.
6. It is not digested in the human digestive tract.
7. In the herbivores, it is digested because of microorganisms (bacteria, yeasts, protozoa) in their digestive tract, which secrete an enzyme called cellulase for its digestion.
8. Cotton is the pure form of Cellulase.
FUNCTIONS OF CARBOHYDRATES:
Carbohydrates occur abundantly in living organisms. They are found in all organisms and in almost all parts of the cell. Cellulose of wood, cotton and paper, starches present in cereals, root tubers, cane sugar and milk sugar are all examples of carbohydrates. Carbohydrates play both structural and functional roles.
Structural role:
1. Some carbohydrates are the main constituents of cell walls in plants and microorganisms.
2. Carbohydrates in cell combine with proteins and lipids and the resultant compounds are called glycoproteins and glycolipids, respectively, which have structural role in the extracellular matrix of animals and bacterial cell wall. Both these conjugated moicoules are components of biological membranes.
Functional role:
1. Simple carbohydrates are the main source of energy in cells. This energy is used in various activities of cells/organisms.
2. Polysaccharides, such as starch and glycogen act as reserved food materials.
3. Carbohydrates are involved in many metabolic pathways.
4. Glyceraldehyde appears as intermediate compound during photosynthesis and respiration.
5. Ribose is an important component of RNA. 6. Deoxyribose is an important component of DNA.
7. They are needed to synthesize lubricants, e.g. mucus which consists of a carbohydrate and a protein.
8. Carbohydrates are needed to prepare the nectar in some flowers.
Pre-medical ist year biology notes
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