Carbohydrates: A good energy source

What is Carbohydrates?

• Carbohydrates = also called saccharides (sugars).
• They are the most common biological molecules.
• Made of C, H, and O in ratio 1: 2: 1 → (CH₂O)_n. (where minimum n=3 or more)

Definition: They are aldehyde or ketone derivatives of polyhydroxy alcohols (means they contain many –OH groups along with aldehyde/ketone).

Classification of Carbohydrates

Quick Tip-

• Mono = single unit
• Di = double unit
• Oligo = few (3–10 units)
• Poly = many units

MONOSACCHARIDES-

• Definition: Monosaccharides are the simplest carbohydrates.
• They cannot be broken down further.
• They are aldehyde or ketone derivatives of straight-chain polyhydroxy alcohols (with 3 or more carbons).

Classification of Monosaccharides

1. Based on carbonyl group

1. Aldoses → contain aldehyde (–CHO) group

• Example: Glucose

2. Ketoses → contain ketone (–CO) group

• Example: Fructose

Ketose names are made by adding -ul before “-ose” (e.g. erythrose → erythulose).

2. Based on number of carbons

1. Trioses (3C)

• Smallest monosaccharides.
  • Examples:
    • Glyceraldehyde (aldotriose)
    • Dihydroxyacetone (ketotriose)

• Important in glycolysis as intermediates (Glyceraldehyde-3-phosphate, DHAP).

2. Tetroses (4C)

• Examples:
  • Erythrose (aldotetrose)
  • Erythrulose (ketotetrose)
• Important in HMP shunt pathway.

3. Pentoses (5C)

• Examples:
  • Ribose (aldopentose) → in RNA
  • Xylose (aldopentose)
  • Ribulose (ketopentose) → intermediate in HMP shunt.

4. Hexoses (6C)

• Most important sugars in human body.
• Examples:
  • Glucose (aldohexose) → main energy source (blood sugar).
  • Galactose (aldohexose) → part of lactose (milk sugar).
  • Fructose (ketohexose) → part of sucrose (table sugar).

Quick Revision-

• 3C = triose (glyceraldehyde, dihydroxyacetone)
• 4C = tetrose (erythrose, erythrulose)
• 5C = pentose (ribose, ribulose)
• 6C = hexose (glucose, galactose, fructose)

Derived Sugars/carbohydrates-

These are sugars or carbohydrates that are modified forms of monosaccharides:

1. Acid Sugars–

• When the aldehyde group (C1 of aldose) is oxidized → forms aldonic acid
  • Example: Gluconic acid
• When the alcohol group at C6 is oxidized – forms uronic acid
  • Example: Glucuronic acid

        CHO
         |
   H – C – OH   
         |
   OH – C – H   
         |
   H – C – OH   
         |
   H – C – OH   
         |
        COOH   
        

2. Sugar Alcohols–

• When sugars are reduced – they form polyhydroxy alcohols (alditols).
• Examples:
  • Ribitol – found in FMN & FAD (coenzymes)
  • Xylitol – used as sweetener (in gums, candies)

3. Amino Sugars–

• Formed when the –OH group at carbon-2 is replaced by –NH₂.
• Examples: Glucosamine, Galactosamine.
• Some exist as acetylated forms → N-acetyl derivatives.
• Found in:
  • Glycosaminoglycans (hyaluronic acid, chondroitin sulfate)
  • Antibiotics (erythromycin)
  • N-acetylneuraminic acid (sialic acid) → important in glycoproteins & glycolipids.

4. Glycosides–

• When carbon-1 of a sugar or carbohydrates reacts with an alcohol like methanol→ forms glycosidic bond (a type of bond that connect a carbohydrate to another group).
• Example: Methylglycoside (C₇H₁₄O₆).
• Found in glycosaminoglycans.

Disaccharides

Disaccharides have 2 monosaccharides joined by a glycosidic bond.

Examples: Maltose, Lactose, Sucrose (biologically important).

Maltose

• Made of: 2 α-D-glucose units.
• Linkage: α(1→4) glycosidic bond.
• Leaves one free –OH group at carbon-1 → reducing sugar.
• Formed by hydrolysis of starch & glycogen.
• Broken down by enzyme maltase → gives 2 glucose units.

Lactose

• Made of: β-D-galactose + β-D-glucose.
• Linkage: β(1→4) glycosidic bond.
• Found in milk.
• Reducing sugar (because glucose unit has free aldehyde or ketone group).

Lactose

• Also known as milk sugar.
• Structure: β-D-galactose + β-D-glucose
• Linkage: β(1→4) glycosidic bond
• Found in: milk (major sugar in mammalian milk)

• Properties:
  • Reducing sugar/ carbohydrates (because glucose part has free aldehyde or ketone group).
  • Hydrolyzed by enzyme lactase (β-D-galactosidase) → produces glucose + galactose.

Oligosaccharides

• Oligosaccharides made of 3–10 monosaccharide units linked by glycosidic bonds.
• Examples: Maltotriose, Raffinose, etc.
• Most are not digested in humans.
• Many are O-linked to proteins (glycoproteins, mucins).

• Functions:
  • Structural role in proteins.
  • Protein glycosylation (adding sugar chains to proteins) → affects stability, activity.

Polysaccharides

• Large polymers of more than 10 monosaccharides.
• Linked by glycosidic bonds.

• Properties:
  • Not sweet
  • Not reducing
  • Sparingly soluble in water

Types:

1. Homopolysaccharides → made of only one type of sugar

Examples: Starch, Glycogen, Cellulose (all polymers of glucose)

2. Heteropolysaccharides → made of different sugars (covered later, e.g. hyaluronic acid, heparin).