MONOSACCHARIDES

• Simple carbohydrate monomers, which cannot be hydrolysed further into simpler or smaller subunits.
• Monosaccharides are generally colourless, crystalline and mostly sweet to taste.
• The empirical formula is (CH₂O)_n where n = 3 to 7.
• A monosaccharide with aldehyde group is called aldose, generally having suffix as ose.
• A monosaccharide with keto group is ketose, generally having suffix as ulose.

SOME COMMON ALDOSES AND KETOSES

1.       TRIOSES

• The monosaccharides posses three carbons, e.g. Glyceraldehyde (an aldose) and Dihydroxy acetone (a ketose).
• They are formed in both respiration and photosynthesis.

2.      TETROSES

• The monosaccharides posses four carbons, e.g. Erythrose,Threose, Erythrulose.
• Tetroses are intermediates of photosynthetic and respiratory pathways as well as raw materials for many biochemicals.
• Erythrose is raw material for synthesis of anthocyanin & lignin.

3.       PENTOSES

• They are with 5-carbon monosaccharides, e.g. Arabinose, Deoxyribose, Ribose, Ribulose, Xylose, Xylulose.
• Deoxyribose is also a pentose sugar but has one oxygen atom less at 2nd C, formula is C₅H₁₀O₄.It is component of DNA.
• Ribose is raw material for synthesis of ribonucleotides, cAMP, ATP, NAD, NADP, FAD and RNA.
• Ribose and Deoxyribose sugars are involved in formation of nucleotides.
• Some pentose sugars are intermediates of photosynthetic and respiratory pathways.
• Arabinose and xylose produce wall materials.
• Arabinose present in gum of Accacia.
• Ribulose present in RuBP.

4.       HEXOSES

• They are six-carbon monosaccharides, e.g. Fructose, Galactose,Glucose, Mannose.
• Fructose is fruit sugar but absent in grapes. It is also the sweetest of all natural sugars with sweetness index of 170. Also known as laevulose.
• Glucose is blood sugar with a sweetness index of 70. It is raw material for formationof complex carbohydrates.
• Glucose is the main respiratory substrate that is oxidised by every cell in order to obtain energy.
• Glucose is reserve carbohydrates in grapes (Grape Sugar).
• Gluscose is also known as dextrose.
• Glucose present in the form of open chain or ring form.

• Galactose does not occur freely but is a component of lactose, agar-agar, glycolipids and glycoproteins.
• Galactose is milk sugar or brain sugar.
• Galactose is fastly circulated in blood.
• Mannose is found in cell wall and many prosthetic polysaccharides.
• Mannose is also found in wood with component of hemicellulose.
• Mannose is not found in free form.

5.       HEPTOSES

• Heptoses are seven - carbon monosaccharides.
• e.g., Glucoheptose, Galactoheptose, Sedoheptulose.
• Sedoheptulose is intermediate of both photosynthetic and respiratory pathways.