Questions and Answers
What happens to a beam of plane-polarised light when it passes through a solution exhibiting optical activity?
What is the term for a mixture of equal amounts of dextrorotatory and laevorotatory isomers?
What is the process of separating optically active isomers from a racemic mixture?
What is the term for the carbon atom that becomes asymmetric after cyclization in a monosaccharide?
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What is the term for a pair of cyclic compounds that are not mirror images of each other?
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What is the general formula of monosaccharides?
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What is the classification of carbohydrates based on the number of carbon atoms?
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What is the type of carbohydrate that cannot be hydrolyzed further into simpler forms?
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What is the general formula of disaccharides?
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What type of carbohydrates yields two molecules of the same or different molecules of monosaccharide on hydrolysis?
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Study Notes
Carbohydrates
- Consist of covalently linked monosaccharide units, often associated with proteins (glycoproteins) and lipids (glycolipids) with structural and regulatory functions.
Polysaccharides (Glycans)
- Composed of more than 10 molecules of monosaccharides on hydrolysis, with a general formula of (C6H10O5)n.
- Have molecular masses ranging into millions of daltons and serve indispensable structural functions in all types of organisms.
- Most conspicuous in plants, where cellulose comprises up to 80% of dry matter.
Classification of Polysaccharides
- Divided into two groups: homopolysaccharides (homoglycans) and heteropolysaccharides (heteroglycans).
- Homopolysaccharides: polymers of same monosaccharide units, e.g. starch, glycogen, inulin, cellulose, dextrins, and dextrans.
- Heteropolysaccharides: polymers of different monosaccharide units or their derivatives, e.g. mucopolysaccharides (glycosaminoglycans).
General Properties of Carbohydrates
- Isomerism: due to asymmetric carbon atoms, compounds can exhibit stereoisomerism, resulting in isomers with identical composition but differing only in spatial configuration.
- D-Series and L-Series: the orientation of H and OH groups around the carbon atom determines whether a sugar belongs to D-series (OH group on the right) or L-series (OH group on the left).
- Most monosaccharides in mammals are D-sugars, and enzymes responsible for their metabolism are specific for this configuration.
Chemistry of Carbohydrates
- Carbohydrates or saccharides (Greek: sakcharon, sugar) have a chemical composition of “carbon hydrate” (CH2O)n, where n ≥ 3.
- Essential and most abundant class of biological molecules, with the basic units being monosaccharides.
- Aldehyde or ketone derivatives of higher polyhydric alcohols or compounds that yield these derivatives on hydrolysis.
Classification of Carbohydrates
- Divided into four major groups: monosaccharides, disaccharides, oligosaccharides, and polysaccharides.
Monosaccharides
- Also known as simple sugars, cannot be hydrolyzed further into simpler forms.
- Aldehyde or ketone derivatives of straight-chain polyhydroxy alcohols containing at least three carbon atoms, with a general formula of CnH2nOn.
Sub-classification of Monosaccharides
- Based on the number of carbon atoms: trioses, tetroses, pentoses, hexoses, etc.
- Based on aldehyde (– CHO) or ketone (– CO) groups: aldoses and ketoses.
Disaccharides
- Hydrolysis produces two molecules of the same or different monosaccharides, with a general formula of Cn(H2O)n-1.
- Examples include maltose, lactose, sucrose, and lactulose.
Oligosaccharides
- Composed of 3 to 10 monosaccharide units on hydrolysis.
Optical Activity of Carbohydrates
- Presence of asymmetric carbon atoms confers optical activity on compounds.
- Dextrorotatory (D or + sign) and laevorotatory (L or – sign) compounds rotate plane-polarized light to the right or left, respectively.
- Racemic mixtures: equal amounts of dextrorotatory and laevorotatory isomers, resulting in no optical activity.
- Resolution: separation of optically active isomers from a racemic mixture.
Cyclic Structures of Carbohydrates
- Formation of cyclic structures from open-chain forms of monosaccharides, resulting in anomers.
- Anomeric carbon atom: the carbon atom that becomes asymmetric after cyclization, leading to the formation of different anomers.
Biomedical Importance of Carbohydrates
- Energy source
- Structural functions
- Carbon skeleton
Optical Activity and Isomerism
- Presence of asymmetric carbon atoms confers optical activity on a compound.
- When a beam of plane-polarized light is passed through a solution exhibiting optical activity, it will be rotated to the right (dextrorotatory, D or + sign) or left (laevorotatory, I or – sign).
- A racemic mixture contains equal amounts of dextrorotatory and laevorotatory isomers, resulting in no optical activity.
- Resolution is the separation of optically active isomers from a racemic mixture.
Cyclic Structures and Anomers
- The reaction between an aldehyde group and an alcoholic-OH group on the same molecule forms a cyclic structure.
- This reaction can form two different forms of glucose: α-D-Glucose (OH group extends to the right) and β-D-Glucose (OH group extends to the left).
- Carbon-1 becomes asymmetric after cyclization and is called an anomeric carbon atom.
- Compounds related in this way are called anomers.
Biomedical Importance and Energy Structure
- Carbohydrates have both structural and regulatory functions in biological systems.
- They are associated with proteins (glycoproteins) and lipids (glycolipids).
- Polysaccharides consist of many covalently linked monosaccharide units and have molecular masses ranging from millions of daltons.
Polysaccharides
- Polysaccharides are composed of more than 10 molecules of monosaccharides on hydrolysis.
- General formula: (C6H10O5)n.
- They have indispensable structural functions in all types of organisms.
- Polysaccharides are further divided into two groups: homopolysaccharides (homoglycans) and heteropolysaccharides (heteroglycans).
Homopolysaccharides and Heteropolysaccharides
- Homopolysaccharides are polymers of the same monosaccharide units, e.g., starch, glycogen, inulin, cellulose, dextrins, and dextrans.
- Heteropolysaccharides are polymers of different monosaccharide units or their derivatives, e.g., mucopolysaccharides (glycosaminoglycans).
General Properties of Carbohydrates
- Isomerism: the presence of asymmetric carbon atoms in a compound gives rise to the formation of isomers.
- Stereoisomerism: compounds that are identical in composition and differ only in spatial configuration.
- D-Series and L-Series: the orientation of the H and OH groups around the carbon atom determines the series.
Chemistry and Structure of Carbohydrates
- Carbohydrates are also known as saccharides (Greek: sakcharon, sugar).
- They have the general formula (CH2O)n, where n ≥ 3.
- The basic units of carbohydrates are monosaccharides.
- Carbohydrates are essential and the most abundant class of biological molecules.
Classification of Carbohydrates
- Carbohydrates are divided into four major groups: monosaccharides, disaccharides, oligosaccharides, and polysaccharides.
- Monosaccharides are also known as simple sugars and cannot be hydrolyzed further into simpler forms.
- They have the general formula CnH2nOn.
Sub-classification of Monosaccharides
- Based on the number of carbon atoms: trioses, tetroses, pentoses, hexoses, etc.
- Based on the aldehyde or ketone groups: aldoses and ketoses.
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Description
This quiz covers the structure and function of monosaccharides and polysaccharides, including their relationship with proteins and lipids. Test your knowledge of carbohydrate chemistry!