Definition:
Carbohydrates are organic compounds made up of carbon (C), hydrogen (H), and oxygen (O), usually in the ratio of 1:2:1.
They are also called saccharides (from the Greek word “sakkharon” meaning sugar).
General formula: Cₙ(H₂O)ₙ
(this formula may vary in complex carbohydrates)
Functions: Energy source (glucose), energy storage (glycogen, starch), structural components (cellulose), and as components of DNA and RNA.
Classification of Carbohydrates:
Carbohydrates are classified into three main types:
1. Monosaccharides (Simple sugars)
- Cannot be broken down into smaller sugars.
- Sweet in taste, soluble in water.
Examples:
- Glucose (C₆H₁₂O₆) – main blood sugar
- Fructose – fruit sugar
- Galactose – found in milk
2. Disaccharides (Double sugars)
- Formed by two monosaccharide units linked by a glycosidic bond.
Examples:
| Disaccharide | Monosaccharides | Source |
| Sucrose | Glucose + Fructose | Cane sugar |
| Lactose | Glucose + Galactose | Milk |
| Maltose | Glucose + Glucose | Malt (grains) |
3. Polysaccharides (Complex carbohydrates)
- Long chains of monosaccharides linked together.
- Usually insoluble, tasteless.
Examples:
| Polysaccharide | Function/Location |
| Starch | Energy storage in plants |
| Glycogen | Energy storage in animals |
| Cellulose | Structural component in plant cell walls |
| Chitin | Found in fungal walls & exoskeletons of insects |
Chemical Properties of Carbohydrates
- Reducing Property
- Some sugars (like glucose, fructose) can reduce mild oxidizing agents like Benedict’s or Fehling’s solution → forms a red precipitate.
- Used in diabetes testing.
- Formation of Glycosidic Bonds
- Two monosaccharides link via condensation reaction (loss of water) to form disaccharides/polysaccharides.
- Hydrolysis
- Disaccharides and polysaccharides can be broken down into simpler sugars using acids or enzymes.
- Isomerism
- Many sugars have optical isomers (D- and L- forms, alpha and beta forms).
- Fermentation
- Some sugars like glucose can be fermented by yeast to produce alcohol and CO₂.
- Dehydration Reaction
- On strong heating with acids, carbohydrates lose water and form furfural derivatives, which is the basis of Molisch’s test.
Summary Table:
| Type | Example | Key Feature |
| Monosaccharide | Glucose | Simple sugar, basic unit |
| Disaccharide | Sucrose | Two sugars linked |
| Polysaccharide | Starch | Many sugars, storage/structural |
Monosaccharides
Definition:
Monosaccharides are the simplest form of carbohydrates, consisting of a single sugar unit. They are the building blocks of more complex carbohydrates like disaccharides and polysaccharides.
- General formula: CₙH₂ₙOₙ (commonly C₆H₁₂O₆ for hexoses)
- Types: Based on carbon atoms:
- Triose (3C), Tetrose (4C), Pentose (5C), Hexose (6C)
- Common Hexoses: Glucose, Fructose, Galactose
Structures of Glucose, Fructose, and Galactose
All three are hexoses (C₆H₁₂O₆) but differ in structure and function.
1. Glucose (Aldohexose)
- Structure Type: Has an aldehyde group (-CHO) on carbon 1 → Aldose
- Linear Formula (Fischer Projection):
CHO
|
H—C—OH
|
OH—C—H
|
H—C—OH
|
H—C—OH
|
CH2OH
- Cyclic Form: Forms a six-membered ring (pyranose) in solution
- Function: Main source of energy for cells
2. Fructose (Ketohexose)
- Structure Type: Has a ketone group (C=O) on carbon 2 → Ketose
- Linear Formula (Fischer Projection):
CH2OH
|
C=O
|
HO—C—H
|
H—C—OH
|
H—C—OH
|
CH2OH
- Cyclic Form: Forms a five-membered ring (furanose)
- Function: Found in fruits and honey; sweetest natural sugar
3. Galactose (Aldohexose)
- Structure Type: Similar to glucose (also an aldohexose), but differs at carbon 4 (epimer of glucose)
- Linear Formula (Fischer Projection):
CHO
|
H—C—OH
|
OH—C—H
|
H—C—OH
|
OH—C—H
|
CH2OH
- Function: Component of lactose (milk sugar), important in brain and nerve tissues
Key Differences Summary:
| Sugar | Type | Carbonyl Group | Ring Type | Found In |
| Glucose | Aldohexose | Aldehyde (C1) | Pyranose (6-ring) | Blood, fruits, starch |
| Fructose | Ketohexose | Ketone (C2) | Furanose (5-ring) | Fruits, honey, sucrose |
| Galactose | Aldohexose | Aldehyde (C1) | Pyranose (6-ring) | Milk (as part of lactose) |
Disaccharides
Definition:
Disaccharides are carbohydrates composed of two monosaccharide units linked together by a glycosidic bond, formed via a condensation reaction (removal of water).
- General formula: C₁₂H₂₂O₁₁
- Formed from 2 monosaccharides
- Can be reducing or non-reducing sugars, depending on whether a free anomeric carbon is present
Structures of Common Disaccharides
1. Maltose (Glucose + Glucose)
- Bond: α (1→4) glycosidic bond
(Alpha bond from C1 of first glucose to C4 of second glucose) - Monosaccharides: 2 × Glucose
- Reducing Sugar? Yes (free anomeric carbon on second glucose)
- Occurs in: Malt, starch breakdown
Structure Overview:
Glucose (α) —(1→4)— Glucose
- First glucose is in α-form
- The bond is between C1 of first and C4 of second glucose
2. Lactose (Glucose + Galactose)
- Bond: β (1→4) glycosidic bond
(Beta bond from C1 of galactose to C4 of glucose) - Monosaccharides: Galactose + Glucose
- Reducing Sugar? Yes (free anomeric carbon on glucose)
- Occurs in: Milk (milk sugar)
Structure Overview:
Galactose (β) —(1→4)— Glucose
- Galactose is in β-form
- Bond is β(1→4)
3. Sucrose (Glucose + Fructose)
- Bond: α(1→2)β glycosidic bond
(Between C1 of glucose and C2 of fructose) - Monosaccharides: Glucose + Fructose
- Reducing Sugar? No (both anomeric carbons involved in bond)
- Occurs in: Table sugar (from sugar cane/beet)
Structure Overview:
Glucose (α1) —(→2β)— Fructose
- Bond involves C1 of glucose and C2 of fructose
- No free anomeric carbon → non-reducing sugar
Comparison Table
| Disaccharide | Monosaccharides | Bond Type | Reducing? | Found In |
| Maltose | Glucose + Glucose | α(1→4) | Yes | Germinating grains, starch |
| Lactose | Galactose + Glucose | β(1→4) | Yes | Milk |
| Sucrose | Glucose + Fructose | α(1→2)β | No | Sugarcane, fruits |
Polysaccharides
Definition:
Polysaccharides are complex carbohydrates formed by the polymerization of many monosaccharide units (usually glucose) linked by glycosidic bonds.
- General formula: (C₆H₁₀O₅)ₙ
- Insoluble in water, tasteless, and do not form crystals.
- Can be storage or structural polysaccharides.
1. Starch
Chemical Nature:
- Source: Found in plants (seeds, roots, tubers – e.g., rice, potatoes)
- Monomer Unit: α-D-glucose
- Bond Type: α(1→4) and α(1→6) glycosidic bonds
Structure:
Starch is a mixture of two components:
- Amylose (20–30%)
- Linear chain of α-D-glucose
- α(1→4) glycosidic bonds
- Forms a helical structure
- Amylopectin (70–80%)
- Branched polymer
- α(1→4) in chains and α(1→6) at branch points (every ~25–30 glucose units)
- Larger and more complex than amylose
Properties:
- Insoluble in cold water, forms a colloidal paste when heated (gelatinization)
- Digested by enzyme amylase to form maltose and glucose
- Gives blue color with iodine (due to amylose)
2. Glycogen
Chemical Nature:
- Source: Found in animals (especially liver and muscle cells)
- Known as “animal starch”
- Monomer Unit: α-D-glucose
- Bond Type: α(1→4) in chains and α(1→6) at branch points
Structure:
- Similar to amylopectin but more highly branched
- Branches occur every 8–12 glucose units
- Compact and suitable for quick energy release
Properties:
- Soluble in water (more than starch)
- Rapidly mobilized when energy is needed
- Does not react strongly with iodine (reddish-brown color, not blue)
- Broken down by glycogen phosphorylase during glycogenolysis
Starch vs Glycogen – Comparison
| Feature | Starch (Plant) | Glycogen (Animal) |
| Monomer Unit | α-D-glucose | α-D-glucose |
| Components | Amylose & Amylopectin | Only one type (like amylopectin, more branched) |
| Bond Types | α(1→4) and α(1→6) | α(1→4) and α(1→6) |
| Branching | Moderate (every 25–30 units) | Extensive (every 8–12 units) |
| Solubility | Less soluble | More soluble |
| Found In | Plants (seeds, roots) | Animals (liver, muscles) |
| Iodine Test | Blue-black (amylose) | Reddish-brown (less intense) |
Both starch and glycogen are storage polysaccharides of glucose, but their branching patterns and solubility reflect their roles:
- Starch stores energy in plants.
- Glycogen stores energy in animals and is mobilized quickly during fasting or exercise.
Qualitative Tests for Carbohydrates
These are chemical tests used to identify the presence and type of carbohydrates (monosaccharides, disaccharides, or polysaccharides).
1. Molisch’s Test (General Test for All Carbohydrates)
- Reagent: Molisch reagent (α-naphthol in alcohol) + concentrated sulfuric acid
- Positive Result: Violet ring at the interface
- Principle: Carbohydrates are dehydrated by acid to form furfural, which reacts with α-naphthol.
2. Benedict’s Test (Reducing Sugars)
- Reagent: Benedict’s solution (copper sulfate + sodium carbonate + sodium citrate)
- Positive Result: Green, yellow, orange, or red precipitate depending on sugar concentration
- Principle: Reducing sugars reduce Cu²⁺ to Cu⁺, forming colored precipitates
Detects: Glucose, fructose, lactose, maltose
Does not detect: Sucrose (non-reducing)
3. Fehling’s Test (Reducing Sugars)
- Reagent: Fehling’s A (CuSO₄) + Fehling’s B (alkaline tartrate)
- Positive Result: Brick-red precipitate of Cu₂O
- Principle: Similar to Benedict’s, but less stable
4. Barfoed’s Test (Monosaccharides)
- Reagent: Barfoed’s reagent (copper acetate in acetic acid)
- Positive Result: Red precipitate forms within 2–3 minutes
- Principle: Differentiates monosaccharides from disaccharides (only monosaccharides reduce in acidic medium)
5. Iodine Test (Starch and Polysaccharides)
- Reagent: Iodine solution
- Positive Result: Blue-black color for starch
- Principle: Iodine fits into the helical structure of amylose forming a colored complex
6. Seliwanoff’s Test (Ketoses like Fructose)
- Reagent: Seliwanoff reagent (resorcinol + HCl)
- Positive Result: Cherry-red color
- Principle: Ketoses dehydrate faster than aldoses and react with resorcinol to give red color
Summary Table
| Test | Detects | Positive Result |
| Molisch’s | All carbohydrates | Violet ring |
| Benedict’s | Reducing sugars | Green/yellow/red precipitate |
| Fehling’s | Reducing sugars | Brick-red precipitate |
| Barfoed’s | Monosaccharides | Red precipitate quickly |
| Iodine | Starch (polysaccharide) | Blue-black color |
| Seliwanoff’s | Ketoses (e.g., fructose) | Cherry-red color |
Biological Role of Carbohydrates
Carbohydrates serve several essential biological functions in both plants and animals:
1. Energy Source
- Glucose is the primary fuel for cells.
- ATP is generated through glycolysis and cellular respiration.
2. Energy Storage
- Starch in plants and glycogen in animals serve as energy reserves.
- Easily mobilized when energy is needed.
3. Structural Components
- Cellulose provides strength to plant cell walls.
- Chitin forms the exoskeleton in arthropods and cell walls in fungi.
4. Component of Biomolecules
- Carbohydrates are part of DNA and RNA (ribose and deoxyribose).
- Found in glycoproteins and glycolipids — important in cell recognition and signaling.
5. Metabolic Intermediates
- Intermediates of carbohydrate metabolism are precursors for:
- Amino acids
- Fatty acids
- Nucleotides
6. Brain and Red Blood Cells Function
- Glucose is the only energy source for the brain and red blood cells under normal conditions.
7. Role in Immunity and Cell Communication
- Glycoproteins on cell surfaces help in immune response and cell recognition.
- Bacterial cell walls contain carbohydrate components like peptidoglycan.