Carbohydrates
Question 1. Classification of carbohydrates
Answer.
Definition Of Carbohydrates
Carbohydrates may be defined as polyhydroxy aldehydes or ketones or compounds which produce them on hydrolysis
Classification Of Carbohydrates
Carbohydrates are classified into 3 groups:
- Monosaccharides
- Oligosaccharides
- Diasaccharides
- Polysaccharides
Monosaccharides
- Monosaccharides are the simplest group of carbohydrates and are referred to as simple sugars
- They cannot be further hydrolysed
- The monosaccharides are divided into different categories depending on the functional group and the number of carbon atoms
- When the functional group in monosaccharides is an aldehyde, they are known as aldoses e.g. glyceraldehyde, glucose
- When the functional group is a keto group, they are known as ketoses e.g. dihydroxyacetone, fructose.
- Based on the number of carbon atoms, the monosaccharides are regarded as trioses (3C), tetroses (4C), pentoses (5C), hexoses (6C) and heptoses (7C).
Oligosaccharides
- Oligosaccharides contain 2 to 10 monosaccharide molecules which are liberated on hydrolysis
- Based on the number of monosaccharide units present the oligosaccharides are further subdivided as –
- Disaccharides (2) – e.g. Maltose
- Trisaccharides (3) – e.g. Raffinose
- Tetrasaccharides (4) – e.g. Stachyose
- Pentasaccharides (5) – e.g. Fondaparinux
Disaccharides
- Disaccharides are sugars which yield two molecules of the same or different molecules of monosaccharides on hydrolysis e.g. maltose, lactose and sucrose
- Maltose yields two molecules of glucose on hydrolysis
- Lactose yields one molecule of glucose and one molecule of galactose on hydrolysis
- Sucrose yields one molecule of glucose and one molecule of fructose on hydrolysis
Polysaccharides
- Polysaccharides are sugars which yield more than ten molecules of monosaccharides on hydrolysis
- They are of two types
- Homo-polysaccharides – they are polymers of the same monosaccharide units e.g. – starch, glycogen, inulin, dextrin, dextran and cellulose
- Hetero-polysaccharides – they are polymers of different monosaccharide units or their derivatives.
They are also known as mucopolysaccharides or glycosaminoglycans (GAGS) e.g. – keratan sulfate, chondroitin sulfate, heparin and hyaluronic acid
Question 2. Functions of carbohydrates.
Answer.
Functions Of Carbohydrates
- Carbohydrates are the most abundant dietary source of energy (4 C/gm)
- Carbohydrates are the precursors for many organic compounds such as fats and amino acids
- Carbohydrates participate in the structure of cell membrane
- Carbohydrates play a role in cellular functions such as cell growth, adhesion and fertilization
- Carbohydrates serve as the storage form of energy (glycogen) to meet the immediate energy demands of the body
- Carbohydrate derivatives are used as drugs e.g. cardiac glycosides and antibiotics
- Lactose is the principal sugar of milk in the lactating mammary gland
- Carbohydrates are constituents of compound lipids and conjugated proteins
- Heparin is an anti-coagulant
- Hetero-polysaccharides form the ground substance of tissues.
Question 3. sources of carbohydrates.
Answer.
Sources of Carbohydrates
Following are main sources of carbohydrates:
- Starches: These are present in cereals, roots and tubers e.g., Rice, wheat, ragi, pulses, potatoes, tapiaco, yam and colassia.
- Sugars
- Monosaccharides (simple sugars) glucose, fructose and galactose
- Disaccharides (Double sugars) sucrose, lactose, maltose.
- Polysaccharides (Complex carbohydrates) e.g., cellulose.
Question 4 . Digestion of carbohydrates.
Answer.
Digestion Of Carbohydrates
- Digestion In Mouth
- Digestion of carbohydrates starts in the mouth where they come in contact with saliva during mastication
- Saliva contains a carbohydrate splitting enzyme called salivary amylase (ptyalin)
- Salivary amylase requires chloride ion for its activity and optimum pH of 6.7
- It breaks down starch and glycogen into glucose and maltose
- Digestion In Stomach
- No carbohydrate splitting enzymes are present in the gastric juice
- Some dietary sucrose may be hydrolysed to glucose and fructose by HCL
- Digestion In Duodenum
- The food reaches duodenum from the stomach, where it meets pancreatic juice
- Pancreatic juice contains a carbohydrate splitting enzyme pancreatic amylase
- Pancreatic amylase acts at a pH of 7.1 and requires chloride ion for its activity
- It breaks down starch and glycogen into glucose and maltose
- Digestion In Small Intestine
Intestinal juice contains the following enzymes- Intestinal amylase
It acts at a pH of 5.5 to 6 and breaks down starch and glycogen to glucose and maltose - Lactase
It acts at a pH of 5.4 to 6 and breaks down lactose to glucose and galactose - Maltase
It acts at a pH of 5.8 to 6.2 and breaks down maltose into 2 units of glucose - Sucrase
It acts at a pH of 5 to 7 and breaks down sucrose into glucose and fructose
- Intestinal amylase
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Question 5. absorption of carbohydrates.
Answer.
Absorption Of Carbohydrates
- Carbohydrate digestion is complete when the food materials reach the small intestine and all complex carbohydrates like starch and glycogen and disaccharides are ultimately converted to monosaccharides
- All monosaccharides are completely absorbed from the small intestine
- No carbohydrates higher than monosaccharides can be absorbed directly into the blood and if administered intravenously are eliminated as foreign bodies
Mechanisms Of Absorption
There are 4 mechanisms for carbohydrate absorption:
- Simple diffusion
- Facilitated transport
- Active transport
- Glut2 transport
Question 6. storage of carbohydrates.
Answer.
- Carbohydrates are stored in animals body in the form of glycogen.
- Glycogen is a multibranched polysaccharide of glucose that serves as a form of energy storage in animals.
- It is also known as animal starch
- Glycogen primarily stored in the cells of the liver and muscles.
- During starvation, glycogen act as primary source of energy by providing ATPs.
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