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Introduction :
* The branch of science that deals with:
- The study of chemical composition and structure of living organisms.
- Various chemical changes taking place within them is called biochemistry.
* The complex organic molecules such as carbohydrates, proteins, lipids, nucleic acids, etc which form the basis of life and are also required for their growth and maintenance are called biomolecules.
Types of biomolecules :
* Carbohydrates
* Proteins
* Lipids
* Nucleic acid
* Enzymes
* Vitamins
What are carbohydrates ?
* Carbohydrates are organic biomolecules abundantly present in the nature.
* Found in the cells of plants and animals.
* The term "carbohydrates" was coined by "Karl Schmidt".
Carbohydrates Biosynthesis
* Carbohydrates are predominantly biosynthesized by plants through photosynthesis.
* Glucose is synthesized in plants from CO2, H2O and solar energy from the sun.
Sources
* Glucose is produced in plants through photosynthesis from CO2 and H2O.
* Glucose is converted in plants to other small sugars and polymers (cellulose, starch).
* Dietary carbohydrates provides the major source of energy required by organisms.
Definition :
* Most of them have a general formula, CX(H2O)Y, and were considered as hydrates of carbon from where the name carbohydrates was derived.
* For example, the molecular formula of glucose (C6H12O6) fits into this general formula, C6(H2O)6.
* But all the compounds which fit into this formula may not by classified as carbohydrates.
* Carbohydrates are also called saccharides (Greek: Sakcharon means sugar).
Old Definition of Carbohydrates
* Empirical formula/ General formula for simple carbohydrates : Cn(H2O)n
* Where n= number of carbon atom present in carbohydrates structure.
Old Definition -
Carbohydrates are "Hydrates of Carbon"
Monosaccharide Sub Classification
- Monosaccharide are sub classified on the basis of :
- Functional Group
- Number of Carbon atoms.
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Disaccharides
* Disaccharides has 2 monosaccharides units linked by glycosidic bond.
* Disaccharides may be reducing or non reducing.
Types of Disaccharides
Reducing Disaccharides -
* Lactose (Glu - Gal)
* Maltose (Glu - Glu)
Non reducing Disaccharides -
* Sucrose (Glu - Fru)
Oligosaccharides sub classification
* Oligosaccharides has 3 -10 monosaccharide units linked by glycosidic bonds.
* Oligosaccharide are sub classified on the basis of number of saccharide unit.
Polysaccharide sub classification
* Polysaccharide/Glycans contains more than 10, same/different monosaccharide units linked by glycosidic linkages.
Types of polysaccharides
* Homopolysaccharides/Homoglycans -
contains more than 10 same repeating units.
* Heteropolysaccharides/Heteroglycans -
contains more than 10, different repeating units.
Hetero Polysaccharides
(more than 10 different Repeating units)
Animal Heteropolysaccharides
* Mucopolysaccharide (MPS)
OR
* Glycosaminoglycans (GAGs)
Types of Examples of Mucopolysaccharides
* Acidic Non Sulfated MPS :
* Hyaluronic acid
* Acidic Sulfated MPS:
* Heparin
* Heparan sulfate
* Chondritin sulfate
* Dermatan sulfate
* Karatan sulfate
* Neutral MPS
* Blood Group substances
Plant Heteropolysaccharides
* Agar
* Pectin
* Lignin
* Gum
Sugar & Non-sugar
* Carbohydrates which are sweet on taste are collectively called sugars white those which are not sweet are called non-sugars. Monosaccharides and disaccharides are sugars but polysaccharides are non-sugars.
Reducing Sugars
* Any carbohydrates whose structure comprises of an aldehyde, or a hemiacetal in equilibrium with an aldehyde.
* This aldehyde group can be oxidized, with resultant reduction of the oxidizing agent.
* Aldehydes and Keto groups have reducing characters and reduce Tollens reagent and Fehling's (Benedict's) solution.
* Carbohydrates containing free aldehyde and keto functional group are thus reducing sugars.
Reducing sugar
* Sugar structure possessing free or potential (reactive) aldehyde or ketone group is termed as reducing sugar.
* Reducing sugar show reducing property efficiently in alkaline medium and reduces certain metallic ions as - Cu , Bi , Fe .
Reducing sugars answer following testes positive
* Benedict's Test
* Fehling's Test
* Nylander's Test
* Form Osazones
* Reducing shows Matarotation (change in optical activity).
Examples of Reducing sugars
* All monosaccharides are reducing sugars.
* Monosaccharides are strong reducing agents.
* Monosaccharides -
* Ribose, Glucose, galactose, Fructose.
* Disaccharides are weak reducing agents.
* Reducing Disaccharides.
* Lactose, Maltose.
Non-Reducing Sugars
* A non reducing sugar is a carbohydrate that is not oxidized by a weak oxidizing agent (an oxidizing agent that oxidizes aldehydes but not alcohols, such as the Tollen's reagent) in basic aqueous solution.
* The characteristic property of non reducing sugars is that is a basic aqueous medium, they do not generate any compounds containing an aldehyde group.
* If the groups are not free, then they do not reduce Tollens reagent and Fehling's solution and are, therefore classified as Non-reducing sugars.
* Example : Sucrose, Trehalose.
Non Reducing sugars
* Sugar structure not possessing free or potential aldehyde or ketone group in its structure is termed as non reducing sugar.
* Non reducing sugar does not show reducing property and do not reduce metallic ions.
Non reducing sugars give following reducing tests negative :
* Benedict's Test
* Fehling's test
* Nylander's test
* Do not form osazones
* Non reducing sugars do not exhibit Mutarotation (Change in optical activity)
Examples of Non reducing sugars
* Non reducing Disaccharides
* Sucrose (Biomedically Important)
* Trehalose (Glu-Glu linked with α (1-1) glycosidic bond).
* Polysaccharides/ Complex carbohydrates are Non reducing.
Monosaccharides
* Monosaccharides (mono = "one", sacchar = "sweet") are simple sugars, the most common of which is glucose.
* In Monosaccharides, the number of carbon usually ranges from three to seven.
* Most monosaccharide names nd with the suffix -ose.
* If the sugar has an aldehyde group (the functional group with structure R-CHO), it is known as an aldose and if it has a ketone group (the functional group with the structure PC(=O)R'), it is known as ketose.
* Depending on the number of carbons in the sugar, they also may be known as trioses (three carbons), pentoses (five carbons), and or hexoses (six carbons).
Classification of monosaccharides
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Examples |
Examples |
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Number of Carbons |
Category Name |
Aldose |
Ketose |
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3 |
Triose |
Glyceraldehyde |
Dihydroxyacetone |
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4 |
Tetrose |
Erythrose |
Erythrulose |
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5 |
Pentose |
Ribose Xylose |
Ribulose Xylulose |
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6 |
Hexose |
Glucose Galactose |
Fructose |
Triose
* A triose is a monosaccharide, or simple sugar, containing three carbon atoms.
* There are only three possible trioses (including dihydroxyacetone):
* L-glyceraldehyde and D-glyceraldehyde, which are aldotrioses and dihydroxyacetone, the only ketotriose.
Monosaccharides
*Trioses are important in cellular respiration.
* During glycolysis, fructose-1,6-bisphosphate is broken down into glyceraldehyde-3-phosphate and dihydroxyacetone phosphate.
* Lactic acid pyruvic acid are later derived from these molecules.
Pentose
* A pentose is a monosaccharide (simple sugar) with five carbon atoms.
* Pentoses are very important in biochemistry.
* Ribose is a constituent of RNA and the related molecule, deoxyribose, is a constituent of DNA.
* Phosphorylated pentoses are important produces of the pentose phosphate pathway, most importantly ribose 5-phosphate, which is used in the synthesis of nucleotides and nucleic acid and erythrosy 4-phosphate, which is used in the synthesis of Aromatic amino acids.
* The aldopentoses are a subclass of the pentoses which, in the linear form, have the carbonyl at carbon 1.
* E.g. xylose, Arabinose and Ribose.
* Xylose is sugar first isolated from wood. It is derived from hemicellulose, one of the main constituents of biomass.
* D-ribose - a component of RNA, ribonucleic acid, vitamins (riboflavin) and coenzymes. It is also important in the high-energy compounds, ATP and ADP. In its reduced form, deoxyribose, it is a component of DNA.
* L-arabinose - a component of hemicelluloses. It is also a component of pectin and can be a major component of gums (gum Arabic). Arabans are polymers of arabinose.
* This ketopentoses have instead the carbonyl at positions 2 to 3.
* This is not known to occur in nature and are difficult to synthesize.
* In the cell, pentoses have a higher metabolic stability than hexoses.
* A polymer composed of pentose sugar is called a pentosan.
Hexose
* A hexose is a monosaccharide (simple sugar) with six carbon atoms.
* Hexoses are extremely important in biochemistry, both as isolated molecules (such as glucose and fructose) and as building blocks of other compounds such as starch, cellulose and glycosides.
*Hexoses can form dihexose (like sucrose) by a condensation reaction that makes 1,6-glycosidic bond.
* Examples: Glucose, Galactose, Fructose etc.
* The aldohexose that is most important in biochemistry is D-glucose.
* D-glucose has various common names, including grape sugar, dextrose, corn sugar (made from cornstarch).
* The chemical formula for glucose is C6H12O6.
* In humans, glucose is an important source of energy.
* During cellular respiration, energy is released from glucose and that energy is used to help make adenosine triphosphate (ATP).
* Plants synthesize glucose using carbon dioxide and water and glucose in turn is used for energy requirement for the plant.
* Excess glucose is often stored as stored as starch that is catabolized (the breakdown of larger molecules by cells) by humans and other animals that feed on plants.
* Galactose (part of lactose, or milk sugar) and fructose (part of sucrose, or fruit sugar) are other common monosaccharides.
* Although glucose, galactose and fructose all have the same chemical formula (C6H12O6), they differ structurally and chemically (and are known as isomers) because of the different arrangement of functional groups around the asymmetric carbon.
* All of these monosaccharides have more than one asymmetric carbon.
* Monosaccharides can exist as a linear chain or as ring-shaped molecules; in aqueous solutions they are usually found in ring forms.
* Glucose in a ring form can have two different arrangements of the hydroxyl group (-OH) around the anomeric carbon (carbon 1 that becomes asymmetric in the process of ring formation).
* If the hydroxyl group is below carbon number 1 in the sugar, it is said to be in the alpha (α) position and if it is above the plane, it is said to be in the beta (β) position.
*C5 and C6 monosaccharides exist in equilibrium between linear and ring forms.
* When the ring forms, the side chain it closes on is locked into an α or β position.
* Fructose and ribose also form rings, although they form five-membered rings as opposed to the six-membered ring of glucose.
Disaccharides
* A disaccharide (also called a double sugar or biose) is the sugar formed when two monosaccharides are joined by glycosidic linkage.
* Like monosaccharides, disaccharides are simple sugars soluble in water.
* Three common examples are sucrose, lactose and maltose.
* The have 2 carbon atoms, with the general formula C12H22O11.
* The differences in these disaccharides are due to atomic arrangement within the molecule.
* The joining of monosaccharides into a double sugar happens by a condensation reaction, which involves the elimination of a water molecule from the functional groups only.
* Braking apart a double sugar into its two monosaccharides is accomplished by hydrolysis with the help of a type of enzyme called a disaccharides.
* As building the larger sugar ejects a water molecule. These reactions are vital in metabolism.
* Each disaccharide is broken down with the help of a corresponding disaccharides (sucrase, lactose and maltase).
Disaccharides
A disaccharide consists of two monosaccharides
Disaccharide Monosaccharides
- Maltose + H2 O → Glucose + Glucose
- Lactose + H2 O → Glucose + Galactose
- Sucrose + H2 O → Glucose + Fructose
- Other disaccharides include isomaltose, cellobiose and trehalose.
- The disaccharides can be classified into :
* Homodisaccharides
* Heterodisacchrides
Glycosidic bond
* The glycosidic bond can be formed between any hydroxy group on the component monosaccharide.
* So, even if bond component sugars are the same (e.g., glucose), different bone combinations (regiochemistry) and stereochemistry (alpha- or beta-) result in disaccharides that are diastereoisomers with different chemical and physical properties.
* Depending on the monosaccharide constituents, disaccharides are sometimes crystalline, sometimes water-soluble, and sometimes sweet-testing and sticky-feeling.