Carbohydrates — Revision Notes

⚡ 30-Second Revision

Monosaccharides: — Glucose, Fructose, Galactose. Simple sugars. All are reducing.
Disaccharides: — Sucrose (Glucose + Fructose, $alpha-1,2$ bond, non-reducing), Lactose (Galactose + Glucose, $\beta-1,4$ bond, reducing), Maltose (Glucose + Glucose, $alpha-1,4$ bond, reducing).
Polysaccharides:

- Starch (Plants): Amylose ($alpha-1,4$ unbranched) + Amylopectin ($alpha-1,4$ & $alpha-1,6$ branched). Blue-black with Iodine. - Glycogen (Animals): Highly branched $alpha-1,4$ & $alpha-1,6$ linkages. Red-brown with Iodine. - Cellulose (Plants): Unbranched $\beta-1,4$ linkages. Indigestible by humans. No Iodine reaction.

Chitin: — Unbranched $\beta-1,4$ linkages of N-acetylglucosamine. Structural.
Glycosidic Bond: — Covalent link between sugar units, formed by dehydration.
Reducing Sugar: — Has a free anomeric carbon (e.g., all monosaccharides, maltose, lactose). Reduces Benedict's reagent.

2-Minute Revision

Carbohydrates are polyhydroxy aldehydes or ketones, serving as primary energy sources and structural components. They are classified into monosaccharides (single sugar units like glucose, fructose), disaccharides (two units like sucrose, lactose, maltose), and polysaccharides (many units like starch, glycogen, cellulose).

Monosaccharides exhibit isomerism, including D/L forms, epimers (differ at one chiral carbon, e.g., glucose and galactose at C-4), and anomers ($alpha$ and $\beta$ forms differing at the anomeric carbon).

Glycosidic bonds link sugar units; $alpha$-linkages are found in digestible starch and glycogen, while $\beta$-linkages in cellulose make it indigestible for humans. Reducing sugars have a free anomeric carbon (all monosaccharides, lactose, maltose), while non-reducing sugars (sucrose) do not.

Starch and glycogen are energy storage molecules, giving blue-black and red-brown colors with iodine, respectively. Cellulose and chitin are structural polysaccharides. Remember these key distinctions for NEET success.

5-Minute Revision

Carbohydrates are fundamental biomolecules, categorized by the number of sugar units. Monosaccharides are the simplest, like glucose (an aldohexose, primary energy source), fructose (a ketohexose), and galactose (an aldohexose, C-4 epimer of glucose).

They exist in cyclic forms and exhibit isomerism: D/L forms (based on the furthest chiral carbon), epimers (differ at one chiral center, e.g., glucose and galactose), and anomers ($alpha$ and $\beta$ forms at the anomeric carbon).

All monosaccharides are reducing sugars due to their free anomeric carbon.

Disaccharides are formed by two monosaccharides via a glycosidic bond (a dehydration reaction). Key examples include:

Sucrose: — Glucose + Fructose, linked by an $alpha-1,2$ glycosidic bond. It's a non-reducing sugar as both anomeric carbons are involved in the bond.
Lactose: — Galactose + Glucose, linked by a $\beta-1,4$ glycosidic bond. It's a reducing sugar because the glucose unit has a free anomeric carbon.
Maltose: — Glucose + Glucose, linked by an $alpha-1,4$ glycosidic bond. It's also a reducing sugar.

Polysaccharides are long chains of monosaccharides. Their properties depend on the monomer, linkage type, and branching:

Starch (plants): — Energy storage. Composed of amylose (unbranched, $alpha-1,4$ glucose units, blue with iodine) and amylopectin (branched, $alpha-1,4$ and $alpha-1,6$ glucose units, reddish-purple with iodine).
Glycogen (animals): — Energy storage. Highly branched, similar to amylopectin but more extensive. Gives a red-brown color with iodine.
Cellulose (plants): — Structural. Unbranched polymer of $\beta-1,4$ linked glucose units. Indigestible by humans due to the $\beta$-linkage and lack of cellulase enzyme. No color with iodine.
Chitin: — Structural. Unbranched polymer of N-acetylglucosamine units with $\beta-1,4$ linkages, found in fungal cell walls and insect exoskeletons.

Understanding these structures, their linkages, and their functional implications (energy, structure, recognition) is vital for NEET. Remember the specific tests: Benedict's for reducing sugars and Iodine for starch/glycogen.

Prelims Revision Notes

Carbohydrates are polyhydroxy aldehydes or ketones. General formula $C_x(H_2O)_y$.

I. Monosaccharides:

Simplest sugars, cannot be hydrolyzed.
Classified by carbon number (triose, tetrose, pentose, hexose) and functional group (aldose, ketose).
Aldoses: — Glucose, Ribose, Galactose.
Ketoses: — Fructose, Dihydroxyacetone.
Important Hexoses: — Glucose (aldohexose), Fructose (ketohexose), Galactose (aldohexose, C-4 epimer of glucose).
Important Pentoses: — Ribose (RNA), Deoxyribose (DNA).
Isomerism:

* D/L Isomers: Based on -OH at chiral carbon furthest from carbonyl. Most natural are D-forms. * Epimers: Differ at one chiral carbon (e.g., Glucose & Galactose are C-4 epimers). * Anomers: Differ at the anomeric carbon (C-1 for aldoses, C-2 for ketoses) in cyclic forms ($alpha$ vs. $\beta$).

All monosaccharides are reducing sugars (have a free hemiacetal/hemiketal group).

II. Disaccharides:

Two monosaccharides linked by a glycosidic bond (dehydration reaction).
Sucrose (Table Sugar): — Glucose + Fructose. $alpha-1,2$ glycosidic bond. Non-reducing sugar (both anomeric carbons involved).
Lactose (Milk Sugar): — Galactose + Glucose. $\beta-1,4$ glycosidic bond. Reducing sugar (glucose's anomeric carbon is free).
Maltose (Malt Sugar): — Glucose + Glucose. $alpha-1,4$ glycosidic bond. Reducing sugar.

III. Polysaccharides:

Many monosaccharide units linked by glycosidic bonds. Can be homopolysaccharides or heteropolysaccharides.
Storage Polysaccharides:

* Starch (Plants): Mixture of Amylose (unbranched, $alpha-1,4$ linkages, blue with Iodine) and Amylopectin (branched, $alpha-1,4$ and $alpha-1,6$ linkages, reddish-purple with Iodine). * Glycogen (Animals): Highly branched, $alpha-1,4$ and $alpha-1,6$ linkages. Red-brown with Iodine. Stored in liver and muscles.

Structural Polysaccharides:

* Cellulose (Plants): Unbranched polymer of $\beta-1,4$ linked glucose units. Indigestible by humans (lack cellulase). No Iodine reaction. Provides rigidity to plant cell walls. * Chitin (Fungi, Arthropods): Unbranched polymer of N-acetylglucosamine units, $\beta-1,4$ linkages. Forms exoskeletons and fungal cell walls.

IV. Chemical Tests:

Benedict's Test: — For reducing sugars. Blue $ightarrow$ Green/Yellow/Orange/Red precipitate upon heating.
Iodine Test: — For starch (blue-black) and glycogen (red-brown). No reaction with cellulose.

Vyyuha Quick Recall

For remembering the reducing/non-reducing nature of common disaccharides: 'S' for Sucrose, 'S' for Sad (Non-reducing). 'M' for Maltose, 'M' for Merry (Reducing). 'L' for Lactose, 'L' for Lively (Reducing).