Carbohydrates

Full Notes

Carbohydrates, also known as saccharides, are essential for energy storage and structural support in living systems.

Classification of Carbohydrates

Carbohydrates are grouped based on their ability to hydrolyse into simpler sugars.

• Monosaccharides – Cannot be hydrolysed further.
• Disaccharides – Yield two monosaccharides on hydrolysis.
• Oligosaccharides – Yield 3–10 monosaccharide units.
• Polysaccharides – Yield many monosaccharides.
• Reducing sugars contain free aldehyde or ketone groups.
• Non-reducing sugars do not (e.g., sucrose).

Glucose

Glucose is the most abundant monosaccharide in nature and plays a central role in respiration, being used to release energy in the form of ATP.

Preparation

From sucrose (hydrolysis in dilute acids)

From starch (acidic hydrolysis under pressure)

Structure of Glucose

Molecular Formula: C₆H₁₂O₆

There are several reactions that can be used to confirm the structure of glucose.

Evidence of Straight Chain

On prolonged heating with HI, forms n-hexane.

Evidence of Carbonyl Group

• Reaction with hydroxylamine forms oxime.
• Reaction with HCN forms cyanohydrin.

Evidence of Aldehyde Group

Mild oxidation with bromine water gives gluconic acid.

Evidence of Five –OH Groups

Acetylation with acetic anhydride forms glucose pentaacetate.

Presence of Primary Alcohol: Oxidation with nitric acid → gives saccharic acid (dicarboxylic acid). Indicates glucose has a primary –OH group.

Configuration of Glucose

Fischer projections can be used to show the correct configuration of glucose, based on orientation of –OH group at the lowest asymmetric carbon. The carbon backbone is represented as a straight line, with bonds shown to each carbon.

D- and L- Notation:
There are two ways the the –OH group at lowest asymmetric carbon can be orientated, D- and L- notation is used to identify which configuraiton is present.

(+) D-isomer = –OH group on the right in Fischer projection
(−) L-isomer = –OH group on the left.

For glucose, we can determine the correct configuration via oxidation products and physical properties.

D-(+)-glucose is the correct form. ‘D’ refers to relative configuration compared to D-(+)-glyceraldehyde. ‘+’ indicates dextrorotatory nature.

Cyclic Structure of Glucose

Although glucose was initially proposed to have a linear structure, certain experimental observations couldn't be explained by the open chain model.

Absence of Free Aldehyde Behaviour

• Despite having an aldehyde group, glucose does not give a positive Schiff's test.
• Its pentaacetate does not react with hydroxylamine.

These suggest the aldehyde group is not free and is likely involved in a cyclic structure.

Existence in Two Crystalline Forms (α and β)

• Glucose crystallizes (forms a solid crystal) in two forms:
  • α-form from cold solution (m.p. 419 K)
  • β-form from hot solution (m.p. 423 K)
• This implies glucose exists in two isomeric cyclic forms.

Formation of Hemiacetal Ring (Cyclic Structure)

One of the –OH groups (specifically on C-5) reacts with the aldehyde (C-1) to form a six-membered ring (pyranose). This internal reaction produces two anomers: α and β, which differ in the position of the OH at C-1 (anomeric carbon).

Anomers and Haworth Representation

We can also use Haworth structures to represent the cyclic nature of glucose and its anomers.

• Anomers are isomers differing at C-1 (anomeric carbon) configuration: α-D-(+)-glucose and β-D-(+)-glucose.

Fructose

Fructose is a ketohexose, found in fruits and honey, and is isomeric with glucose.

• Open-chain form: CH₂OH–CO–(CHOH)₃–CH₂OH.
• Cyclic structure (furanose) formed by reaction between C-2 and C-5 hydroxyl

Disaccharides

Disaccharides are carbohydrates composed of two monosaccharide units joined via a glycosidic bond. They break apart into their two monosaccharide units during hydrolysis.

Example Sucrose hydrolyses to D-(+)-glucose and D-(−)-fructose

These units are linked by a glycosidic bond formed by the elimination of water.

Reducing vs. Non-reducing Sugars:

• If the aldehyde or ketone group is free, the sugar is reducing (e.g., maltose, lactose).
• If it is involved in bonding (as in glycosidic linkage), it is non-reducing (e.g., sucrose).

Sucrose

• Made from α-D-glucose and β-D-fructose.
• Joined via a glycosidic linkage between C1 of glucose and C2 of fructose.
• Properties:
  • Non-reducing sugar (both reducing groups involved in linkage).
  • Hydrolysis yields equimolar D-(+)-glucose and D-(−)-fructose.
  • The optical rotation changes from dextro to laevo due to higher laevorotation of fructose leading to invert sugar.

Maltose

• Composed of two α-D-glucose units.
• Linked between C1 of one glucose and C4 of the second.
• Properties:
  • Reducing sugar (free aldehyde group at C1 of second glucose).
  • Found as an intermediate in starch digestion.

Lactose

• Made from β-D-galactose and β-D-glucose.
• Linkage is between C1 of galactose and C4 of glucose.
• Properties:
  • Reducing sugar (free aldehyde group at C1 of glucose).
  • Found in milk.

Polysaccharides

Polysaccharides are long-chain carbohydrates formed by linking many monosaccharide units via glycosidic bonds.

Starch

Function: Main storage carbohydrate in plants. Major dietary source of glucose for humans.

Polymer of α-D-glucose made up of two components - amylose and amylopectin.

Amylose: Linear polymer, 200–1000 glucose units, C1–C4 linkages.

Amylopectin: Branched polymer, C1–C4 and C1–C6 linkages.

Solubility:

• Amylose: Soluble in water.
• Amylopectin: Insoluble in water, forms majority (80–85%) of starch.

Cellulose

• Unbranched polymer of β-D-glucose.
• Units joined by β(1→4) glycosidic linkages.
• Function:
  • Provides structural strength to plants.
  • Not digestible by humans due to absence of necessary enzymes.
  • Found in plant cell walls.
  • Most abundant organic compound in nature.

Glycogen

• Function: Storage form of carbohydrate in animals. Also called animal starch.
• Occurrence: Found in liver, muscles, brain. Also present in fungi and yeast.
• Structure: Similar to amylopectin but more branched. Provides quick source of glucose during fasting or activity.

Importance of Carbohydrates

Carbohydrates play a vital role in living organisms, acting as both energy providers and structural components. They are essential for metabolism, storage, and cellular functions.

• Glucose: Serves as a rapid and readily available source of energy, especially for brain and muscle function.
• Starch (in plants) and Glycogen (in animals): Function as storage forms of glucose, providing a reserve of energy that can be mobilised when needed.
• Cellulose: Provides structural support in the cell walls of plants, contributing to rigidity and strength.
• Biological functions: Involved in biosynthetic pathways and play crucial roles in cell signalling, immune response, and cellular recognition processes (e.g., glycoproteins and glycolipids on cell membranes).

Summary

• Carbohydrates include mono, di, oligo and polysaccharides.
• Glucose shows open-chain and cyclic forms with α and β anomers.
• Disaccharides are linked by glycosidic bonds and may be reducing or non-reducing.
• Polysaccharides such as starch, cellulose and glycogen have distinct structures and functions.
• Carbohydrates provide energy and structural support and are key to cellular processes.