Mnosachharides

Monosaccharides are the simplest form of carbohydrates, consisting of single sugar molecules that cannot be hydrolysed into smaller units. They are the basic building blocks of more complex carbohydrates such as disaccharides and polysaccharides. Monosaccharides are essential biological molecules that serve as a primary source of energy, structural components, and metabolic intermediates in living organisms.
The most familiar monosaccharide is glucose, the key energy source for cells, but others such as fructose, galactose, and ribose also play vital biological roles.

Chemical Structure and Composition

Monosaccharides are polyhydroxy aldehydes or ketones, meaning they contain multiple hydroxyl (–OH) groups and either an aldehyde group (–CHO) or a ketone group (C=O). Their general chemical formula is CₙH₂ₙOₙ, where n typically ranges from 3 to 7.
According to the number of carbon atoms present, monosaccharides are classified as:

• Trioses (C₃H₆O₃): e.g., glyceraldehyde, dihydroxyacetone
• Tetroses (C₄H₈O₄): e.g., erythrose
• Pentoses (C₅H₁₀O₅): e.g., ribose, deoxyribose
• Hexoses (C₆H₁₂O₆): e.g., glucose, fructose, galactose
• Heptoses (C₇H₁₄O₇): e.g., sedoheptulose

Structurally, each monosaccharide molecule contains a carbon skeleton with hydroxyl groups attached to most of the carbons and one carbonyl group (aldehyde or ketone).

Classification of Monosaccharides

Monosaccharides are classified based on two main criteria:
1. By the Number of Carbon Atoms:

• Trioses (3C): e.g., Glyceraldehyde, Dihydroxyacetone
• Pentoses (5C): e.g., Ribose (in RNA), Deoxyribose (in DNA)
• Hexoses (6C): e.g., Glucose, Fructose, Galactose

2. By the Type of Carbonyl Group:

• Aldoses: Contain an aldehyde group (–CHO) at the terminal carbon. Example: Glucose.
• Ketoses: Contain a ketone group (C=O) within the carbon chain. Example: Fructose.

For example, glucose is an aldohexose, and fructose is a ketohexose.

Structural Isomerism

Monosaccharides exhibit several types of isomerism due to variations in the arrangement of atoms:

• Structural isomerism: Same molecular formula but different structural arrangements (e.g., glucose and fructose).
• Stereoisomerism: Occurs due to asymmetric carbon atoms leading to D- and L-forms (enantiomers). Naturally occurring sugars are mostly of the D-configuration.
• Anomerism: When a monosaccharide forms a ring structure, the carbonyl carbon becomes asymmetric, giving rise to two forms — α (alpha) and β (beta) anomers.
• Epimerism: Two monosaccharides that differ in configuration at a single carbon atom (e.g., glucose and galactose are C4 epimers).

Open-Chain and Ring Structures

In aqueous solutions, most monosaccharides do not exist in a linear form but instead form cyclic (ring) structures through an internal reaction between the carbonyl group and a hydroxyl group.

• Aldoses form hemiacetal rings.
• Ketoses form hemiketal rings.

Two common ring forms are:

• Furanose: Five-membered ring (e.g., fructose).
• Pyranose: Six-membered ring (e.g., glucose).

These rings can interconvert between α and β forms in solution through a process known as mutarotation.

Important Monosaccharides and Their Functions

1. Glucose (C₆H₁₂O₆):

• The most abundant monosaccharide in nature.
• Primary source of energy in cellular respiration.
• Found in blood as blood sugar.
• Synthesised by plants during photosynthesis and metabolised during glycolysis.

2. Fructose (C₆H₁₂O₆):

• A ketohexose found in fruits and honey, known as fruit sugar.
• The sweetest naturally occurring sugar.
• Metabolised in the liver to glucose or fat.

3. Galactose (C₆H₁₂O₆):

• A component of lactose (milk sugar).
• Less sweet than glucose and an essential part of glycolipids and glycoproteins.

4. Ribose (C₅H₁₀O₅) and Deoxyribose (C₅H₁₀O₄):

• Pentoses that form the backbone of nucleic acids — RNA (ribose) and DNA (deoxyribose).

5. Glyceraldehyde (C₃H₆O₃):

• Simplest aldotriose, important as an intermediate in glycolysis.

Biological Roles of Monosaccharides

Monosaccharides play vital roles in metabolism and structural biology:

1. Energy Source:
   • Glucose oxidation provides ATP through glycolysis, Krebs cycle, and oxidative phosphorylation.
2. Building Blocks:
   • Serve as monomers for the synthesis of disaccharides (e.g., sucrose, lactose) and polysaccharides (e.g., starch, cellulose, glycogen).
3. Metabolic Intermediates:
   • Intermediates in pathways such as photosynthesis, respiration, and nucleotide synthesis.
4. Structural Components:
   • Ribose and deoxyribose are essential in nucleic acid structure.
   • Modified monosaccharides like N-acetylglucosamine form part of bacterial cell walls and glycoproteins.
5. Cell Recognition and Communication:
   • Present on cell membranes as glycoproteins and glycolipids, helping in cell signalling and immune responses.

Chemical Reactions of Monosaccharides

Monosaccharides undergo several characteristic reactions:

• Oxidation: Produces acids; e.g., glucose oxidised to gluconic acid.
• Reduction: Converts carbonyl groups to alcohols, forming sugar alcohols (e.g., sorbitol, mannitol).
• Esterification: React with acids to form esters, such as phosphate esters in metabolism (e.g., glucose-6-phosphate).
• Fermentation: Certain monosaccharides, especially glucose, undergo fermentation to produce ethanol or lactic acid.

Detection and Tests

Several qualitative tests identify monosaccharides based on their reducing properties:

• Benedict’s test and Fehling’s test: Detect reducing sugars that can reduce copper(II) ions to copper(I) oxide.
• Tollen’s test: Identifies aldehyde-containing sugars by forming a silver mirror.
• Molisch’s test: A general test for carbohydrates using α-naphthol and sulphuric acid.

Importance in Health and Nutrition

Monosaccharides are critical for sustaining life, but imbalance or improper metabolism can lead to disorders:

• Hyperglycaemia and hypoglycaemia involve abnormal blood glucose levels.
• Diabetes mellitus results from impaired glucose regulation.
• Fructose intolerance and galactosaemia are metabolic disorders caused by enzyme deficiencies.

A balanced intake of monosaccharides is vital, as excess consumption — especially of refined sugars — may contribute to obesity and metabolic diseases.