CARBOHYDRATE 

Carbohydrates are organic biomolecules composed of carbon (C), hydrogen (H), and oxygen (O), usually in the ratio Cₙ(H₂O)ₙ. 
They are also called saccharides (Greek: sakkharon = sugar).
The name “carbohydrate” literally means hydrates of carbon.
They are the most abundant biomolecules on Earth.
Carbohydrates are one of the four major biomolecules, along with proteins, lipids, and nucleic acids.

General Characteristics:

Taste: Many are sweet (mono- and disaccharides).

1. Basic Nature: Polyhydroxy aldehydes or ketones, or compounds that yield these on hydrolysis.

• Aldoses → contain an aldehyde group (–CHO).
• Ketoses → contain a ketone group (–C=O).

2. Taste 

• Many simple carbohydrates (like glucose, sucrose, fructose) are sweet in taste.
• Complex carbohydrates (like starch, cellulose) are usually tasteless.

3. Solubility

• Simple carbohydrates (monosaccharides, some disaccharides) are soluble in water.
• Polysaccharides (starch, cellulose, glycogen) are mostly insoluble or partially soluble.

4. Optical Activity

Most carbohydrates are optically active, meaning they can rotate the plane of polarized light (due to chiral carbon atoms).

5. Energy Source

• Primary energy-providing biomolecules (4 kcal per gram).
• Glucose is the main immediate energy source in cells.

6. Reducing / Non-reducing Property

• Reducing sugars (e.g., glucose, fructose, maltose, lactose) can reduce mild oxidizing agents like Fehling’s solution or Tollen’s reagent.
• Non-reducing sugars (e.g., sucrose) do not show this property.

7. Formation of Glycosidic Bonds

• Carbohydrates can join via glycosidic linkages to form complex polysaccharides.

8. Role in Structure

Serve as structural components in organisms:

• Cellulose in plant cell walls.
• Chitin in the exoskeleton of arthropods.

9. Chemical Reactivity

Undergo oxidation, reduction, esterification, fermentation, and hydrolysis reactions.

Classification-

Fig.01

 Based on Number of Sugar Units: Carbohydrates are classified into three main groups:

A. Monosaccharides: 

• Definition: Simplest form of carbohydrates; cannot be hydrolyzed into smaller units.
• General formula: CₙH₂ₙOₙ
• Examples: Glucose, Fructose, Galactose.

Further classification by number of carbon atoms:

• Trioses (C₃): Glyceraldehyde, Dihydroxyacetone
• Tetroses (C₄): Erythrose, Erythrulose
• Pentoses (C₅): Ribose, Xylose, Arabinose
• Hexoses (C₆): Glucose, Fructose, Galactose, Mannose
• Heptoses (C₇): Sedoheptulose

B. Disaccharides:

Definition: Two monosaccharide units linked by a glycosidic bond.

Examples:

• Sucrose = Glucose + Fructose
• Lactose = Glucose + Galactose
• Maltose = Glucose + Glucose

C. Oligosaccharides

Definition: 2–10 monosaccharide units linked by glycosidic bonds.
Examples: 

• Raffinose (Galactose + Glucose + Fructose), Stachyose.
• Disaccharides – Sucrose (glucose + fructose), Lactose (glucose + galactose), Maltose (glucose + glucose).

D. Polysaccharides

Definition: Large molecules made up of hundreds to thousands of monosaccharide units.

Types:

1. Storage polysaccharides:

• Starch (plants) → Amylose + Amylopectin
• Glycogen (animals)

2. Structural polysaccharides: 

• C

  ellulose (plant cell wall)
• Chitin (exoskeleton of arthropods)

2. Based on Functional Group

• Aldoses: Contain an aldehyde group (e.g., Glucose, Galactose).
• Ketoses: Contain a ketone group (e.g., Fructose, Ribulose).

3. Based on Reducing Property

• Reducing sugars: Can reduce Fehling’s/Benedict’s solution due to free aldehyde or ketone group (e.g., Glucose, Maltose, Lactose).

• Non-reducing sugars: Do not have a free carbonyl group (e.g., Sucrose).

NOTE: 

Mnemonic for Carbohydrate Classification

We can arrange by number of sugar units:
Monosaccharides → Disaccharides → Oligosaccharides → Polysaccharides

🔹 Mnemonic:
"My Dear Old Parrot"

M → Monosaccharides
D → Disaccharides
O → Oligosaccharides
P → Polysaccharides

For Monosaccharide Carbon Numbers (Trioses, Tetroses, Pentoses, Hexoses, Heptoses)

🔹 Mnemonic:
"The Tiny Penguins Hugs Happily"

T → Triose (C₃)
T → Tetrose (C₄)
P → Pentose (C₅)

H→ Hexose (C₆)

H → Heptose (C₇)

Functions of Carbohydrates

1. Primary Energy Source

• Main fuel for the body (4 kcal/g).
• Glucose is the most important immediate energy source for cells.
• Stored as glycogen in animals and starch in plants.

2. Energy Storage

• Glycogen in liver & muscles (animals).
• Starch in seeds & tubers (plants).
• Stored energy can be mobilized when needed.

3. Structural Role 

• Cellulose forms plant cell walls.
• Chitin forms the exoskeleton in arthropods.
• Glycoproteins and glycolipids help maintain cell membrane structure.

4. Protein Sparing Action

• Adequate carbohydrate intake prevents excessive use of proteins for energy, preserving them for growth and repair.

5. Fat Metabolism Regulation

• Help prevent ketosis by ensuring fats are completely oxidized.
• Without carbs, fats break down incompletely, producing ketone bodies.

6. Biological Recognition & Signaling

• Glycoproteins and glycolipids on cell membranes act as “markers” for cell recognition and immune response.

7. Source of Carbon Skeletons

• Provide raw materials for synthesis of amino acids, nucleotides, and fatty acids.

8. Regulation of Blood Sugar

• Complex carbs help maintain stable blood glucose levels, preventing spikes and crashes.

9. Dietary Fiber Role

• Non-digestible carbohydrates (cellulose, hemicellulose, pectin) aid digestion, prevent constipation, and promote gut health.

Carbohydrates – Characteristics & Functions

Characteristics	Functions

Fig.02

                  

Important Examples of carbohydrate:

• Glucose: Blood sugar, main energy source
• Fructose: Fruit sugar, sweetest natural sugar
• Sucrose: Table sugar (Glucose + Fructose)
• Lactose: Milk sugar (Glucose + Galactose)
• Starch: Plant storage carbohydrate
• Glycogen: Animal storage carbohydrate
• Cellulose: Main plant structural component

Qualitative Tests for Carbohydrates-

Test Name	Purpose	Principle	Procedure	Observation	Inference
Molisch’s Test	General test for all carbohydrates	Conc. H₂SO₄ dehydrates carbohydrates → furfural/hydroxymethylfurfural, which reacts with α-naphthol to give violet ring	1. Take 2 ml sample 2. Add 2–3 drops α-naphthol (ethanolic) 3. Layer conc. H₂SO₄ down the side	Violet/purple ring at interface	Carbohydrate present (mono-, oligo-, poly-)
Benedict’s Test	Detect reducing sugars	Reducing sugars reduce Cu²⁺ (alkaline) → Cu⁺ (brick-red cuprous oxide)	1. Take 2 ml Benedict’s reagent 2. Add 0.5 ml sample 3. Heat in boiling water bath (2–5 min)	Blue → green → yellow → orange → brick-red ppt (depends on sugar conc.)	Positive for glucose, fructose, lactose, maltose (not sucrose unless hydrolyzed)
Barfoed’s Test	Differentiate monosaccharides & disaccharides	In acidic medium, monosaccharides reduce Cu²⁺ faster than disaccharides	1. Take 2 ml Barfoed’s reagent 2. Add 0.5 ml sample 3. Boil 2 min, then cool	Red ppt in < 2 min → monosaccharide; later → disaccharide	Identify sugar type
Seliwanoff’s Test	Distinguish ketoses from aldoses	Ketoses dehydrate faster than aldoses in acid to hydroxymethylfurfural, which reacts with resorcinol → red complex	1. Take 2 ml Seliwanoff’s reagent (resorcinol + conc. HCl) 2. Add 1 ml sample 3. Heat 2–3 min	Cherry-red color appears quickly in ketoses; slow/faint in aldoses	Ketose present (e.g., fructose)
Iodine Test	Detect starch	Iodine fits into amylose helix → blue-black complex	1. Take 2 ml starch solution 2. Add 2–3 drops iodine solution	Blue-black color (disappears on heating, reappears on cooling)	Starch present
Bial’s Test	Detect pentoses	Pentoses dehydrate to furfural, which reacts with orcinol + Fe³⁺ → green color	1. Take 2 ml Bial’s reagent 2. Add 1 ml sample 3. Heat 2–3 min	Green color	Pentose sugar present (e.g., ribose)
Osazone Test	Identify specific sugars	Reducing sugars react with phenylhydrazine to form crystalline osazones (shape depends on sugar)	1. Mix sugar soln with phenylhydrazine HCl + sodium acetate + water 2. Heat in boiling water bath	Characteristic crystals under microscope (e.g., glucose/fructose – needle-shaped)	Sugar identity based on crystal form

Fig.03

Tips for remembering in exams

• Molisch’s → Most sugars give violet ring
• Benedict’s → Brick-red Benedict = reducing sugar
• Barfoed’s → Barfoed = Boil for 2 minutes (fast = mono, slow = di)
• Seliwanoff’s → S for Seliwanoff = S for Speed (ketose reacts faster)
• Iodine → I for Iodine = Indigo-blue starch
• Bial’s → Bial = Blue/Green pentose
• Osazone → Observe under microscope