CLASSIFICATION OF CARBOHYDRATES: MONOSACCHARIDES, DISACCHARIDES, POLYSACCHARIDES AND OTHERS

High yield summary: Carbohydrate classification

Based on your lecture PDF.

Classification of carbohydrates-Monosaccharides-Disaccharides-Polysaccharides and others.pdf

1. Basic classification

Carbohydrates are polyhydroxy aldehydes or ketones, or substances that yield these compounds after hydrolysis. Many follow (CH₂O)ₙ, but this is not universal.

Class

Sugar units

Examples

Monosaccharides

1

Glucose, fructose, galactose

Disaccharides

2

Maltose, lactose, sucrose

Oligosaccharides

Short chains, commonly 3–10*

Sugars on glycoproteins and glycolipids

Polysaccharides

>10

Starch, glycogen, cellulose

Disaccharides can also be classified within the broader oligosaccharide group.

Main functions: energy, energy storage, structural support, and cell recognition.


2. Monosaccharides: classify by functional group + carbon number

  • Aldose: terminal aldehyde group; carbonyl usually at C1.

  • Ketose: internal ketone group; carbonyl commonly at C2.

Carbon number

Name

Important examples

3

Triose

Glyceraldehyde = aldotriose; dihydroxyacetone = ketotriose

4

Tetrose

Four-carbon sugars

5

Pentose

Ribose, deoxyribose

6

Hexose

Glucose, galactose, mannose, fructose

7

Heptose

Seven-carbon sugars

Memorize these identities:

Sugar

Classification

Key association

Glucose

Aldohexose

Blood sugar; major cellular fuel; also called dextrose

Galactose

Aldohexose

Component of lactose

Mannose

Aldohexose

C2 epimer of glucose

Fructose

Ketohexose

Fruits, honey, sucrose

Ribose

Aldopentose

RNA

2-Deoxyribose

Deoxy pentose

DNA

Monosaccharides are generally water soluble and insoluble in nonpolar solvents.


3. Stereochemistry: D/L, epimers, and anomers

Term

What it means

Example/rule

Enantiomers

Nonsuperimposable mirror images

D- and L-glyceraldehyde

D/L configuration

Determined by the chiral carbon farthest from the carbonyl

In a Fischer projection: OH right = D; OH left = L

Epimers

Differ at one chiral carbon

Glucose–mannose: C2; glucose–galactose: C4

Anomers

Differ specifically at the anomeric carbon

α- and β-glucose

Must know:

  • Most common biological hexoses belong to the D-series.

  • Dihydroxyacetone has no chiral carbon.

  • The open-chain carbonyl carbon is not chiral.

  • D/L does not tell you the direction of optical rotation.

  • D/L and α/β describe different structural features.

Memory hook:
Mannose = 2; Galactose = 4.


4. Ring formation and the anomeric carbon

The carbonyl group reacts with an –OH group within the same molecule, producing a cyclic sugar.

Starting sugar

Cyclic functional group

Anomeric carbon

Aldose

Hemiacetal

C1, e.g., glucose

Ketose

Hemiketal

C2, e.g., fructose

The anomeric carbon is the former carbonyl carbon.

Ring

Total ring atoms, including oxygen

Pyranose

6

Furanose

5

  • Glucose commonly occurs as glucopyranose.

  • Fructose can form furanose or pyranose rings.

  • In fructose, C2 reacting with the C5 hydroxyl → furanose; reacting with the C6 hydroxyl → pyranose. This corrects an imprecise statement in the slides.

α versus β in the usual Haworth drawing of D-glucose:

  • α: anomeric –OH is opposite the CH₂OH group → usually down.

  • β: anomeric –OH is on the same side → usually up.

Memorize the same-side/opposite-side rule; “up/down” depends on the sugar and drawing.


5. Glycosidic bonds and reducing sugars

An O-glycosidic bond links the anomeric carbon of one sugar to an oxygen of another group.

Example: α(1→4) means the first sugar’s C1 in α configuration is linked to the second sugar’s C4.

Reducing sugar rule:

  • Free anomeric carbon → reducing sugar.

  • Both anomeric carbons locked in the bond → nonreducing disaccharide.

A free anomeric center allows the ring to open to a carbonyl-containing form.

Disaccharide

Components

Linkage

Reducing?

Maltose

Glucose + glucose

α(1→4)

Yes

Lactose

Galactose + glucose

β(1→4)

Yes

Sucrose

Glucose + fructose

α(1↔2)β

No

Why sucrose is nonreducing: glucose C1 and fructose C2 are both involved in the bond.


6. Polysaccharides: the most important comparison

  • Homopolysaccharide: one type of sugar monomer.

  • Heteropolysaccharide: two or more types.

Polymer

Role/location

Monomer

Main-chain linkage

Branch linkage

Amylose

Plant starch component

Glucose

α(1→4)

None

Amylopectin

Plant starch component

Glucose

α(1→4)

α(1→6)

Glycogen

Animal storage; liver and skeletal muscle

Glucose

α(1→4)

α(1→6)

Cellulose

Plant cell walls

Glucose

β(1→4)

None

Key distinctions:

  • Starch = amylose + amylopectin; amylopectin is the major component.

  • Amylopectin branches approximately every 24–30 glucose residues.

  • Glycogen is more extensively branched and more compact than amylopectin.

  • Amylose forms a coiled/helical chain.

  • Cellulose forms straight, extended chains with alternating glucose orientations.

  • Humans digest starch and glycogen, but cannot digest cellulose’s β(1→4) bonds.

Exam trap: Amylose and cellulose contain the same monomer, but their α versus β linkage produces different structures and biological roles.

Other examples:

  • Chitin: structural homopolysaccharide in exoskeletons.

  • Peptidoglycan: bacterial cell-wall material containing alternating sugar units.

  • Extracellular heteropolysaccharides provide support, protection, and tissue organization.


7. Sugar derivatives and cell recognition

Derivative

Structural change/key association

Glucosamine, galactosamine, mannosamine

C2 hydroxyl replaced by an amino group

N-acetylglucosamine

Acetylated amino sugar; structural polymers and bacterial cell-wall components

L-fucose

Deoxy sugar derived from L-galactose; glycoproteins and glycolipids

L-rhamnose

Deoxy sugar derived from L-mannose; plant polysaccharides

Oligosaccharides attached to proteins or lipids form glycoproteins or glycolipids.

High yield function: cell recognition, including ABO blood-group antigens.


8. Clinical relevance

Lactose intolerance

Low lactase activity → undigested lactose remains in the intestinal lumen → osmotic retention of water → watery diarrhea.

  • Lactase cleaves lactose’s β(1→4) bond.

  • Disaccharides require digestion before absorption.

  • Glucose, galactose, and fructose are absorbed as monosaccharides.

Benedict’s and Fehling’s tests

Both detect reducing sugars through copper reduction:

Blue Cu²⁺ → Cu₂O precipitate, which can be brick-red.

  • Benedict’s may show green → yellow → orange → red, depending on sugar amount.

  • Positive: glucose, galactose, maltose, lactose.

  • Sucrose is negative before hydrolysis.

  • These tests detect reducing substances and are not specific for glucose.


Memorize these first

  1. Glucose/galactose/mannose = aldohexoses; fructose = ketohexose.

  2. D = OH on the right at the farthest chiral carbon in Fischer projection.

  3. Glucose–mannose: C2; glucose–galactose: C4.

  4. Anomeric carbon: glucose C1; fructose C2.

  5. Pyranose = 6-membered ring; furanose = 5-membered ring.

  6. Maltose α1→4; lactose β1→4; sucrose α1↔2β.

  7. Maltose/lactose reduce; sucrose does not.

  8. Storage chains α1→4; branches α1→6; cellulose β1→4.

  9. Glycogen is more branched than amylopectin.

  10. Lactase deficiency → osmotic diarrhea.

Answers to the PDF’s sample questions: 1: D — fructose is a ketohexose; 2: D — polysaccharide; 3: C — D-series.