Carbohydrates Overview and Classification

Study Notes

Definition of Carbohydrates

Carbohydrates are polyhydroxy aldehydes or ketones or compounds that yield these on hydrolysis.
They have a general molecular formula of $C_n(H_2O)_n$.
Glucose, with a molecular formula of $C_6H_{12}O_6$, is an example.

Classification of Carbohydrates

Monosaccharides are the simplest sugars, containing only one sugar unit.
- They cannot be further hydrolyzed into smaller units.
- Aldoses have aldehyde groups, ketoses have keto groups.
- They are classified by the number of carbon atoms: trioses (C3), tetroses (C4), pentoses (C5), hexoses (C6), heptose (C7), etc.
Disaccharides are formed when two monosaccharides combine with the elimination of a water molecule.
Oligosaccharides contain 3-10 monosaccharide units.
Polysaccharides are formed when more than 10 sugar units combine.

Importance of Glucose

Found in fruits, fruit juices, and is the principal sugar in the blood.
High glucose levels in the blood (hyperglycemia) and in urine (glucosuria) indicate diabetes mellitus.
It is a major source of energy for cells, especially neurons.
Glucose can be used to synthesize other carbohydrates like glycogen, galactose, and ribose.

Importance of Fructose

It is the main nutritional source of energy for spermatozoa and is found in the seminal fluid
Found in fruits and fruit juices, as well as honey.
Can be converted to glucose in the liver.

Pentoses

Ribose is a key component of nucleic acids (RNA and DNA), and biologically important nucleotides like ATP, GTP, AMP, etc.
Ribose also enters the structure of coenzymes NAD, NADP and FAD.

Hexoses

D-Glucose
D-Mannose
D-Galactose
D-Fructose

Importance of Galactose

It can be converted into glucose in the liver.
It is a component of lactose (milk sugar).
It is used in the structure of glycoproteins.

Importance of Mannose

It is a constituent of many glycoproteins.

Homopolysaccharides

Polysaccharides containing only one type of monosaccharide unit.

Heteropolysaccharides

Polysaccharides containing different types of monosaccharide units.

Derived Carbohydrates

Formed from carbohydrates through chemical reactions.

Sugar Acids

Formed by oxidizing sugars.
- Mild Oxidation (Hypobromous Acid, Br₂/H₂O) oxidizes the aldehyde group to a carboxyl group.
  - Glucose → Gluconic acid
  - Mannose → Mannonic acid
  - Galactose → Galactonic acid
- Moderate Oxidation oxidizes the last carbon into a carboxyl group, producing uronic acids.
  - Glucose → Glucuronic acid
  - Mannose → Mannuronic acid
  - Galactose → Galacturonic acid
- Strong Oxidation (Nitric Acid + Heat) oxidizes both the first and last carbons to dicarboxylic acids, known as saccharic acids.
  - Glucose → Glucosaccharic acid
  - Mannose → Mannaric acid
  - Galactose → Mucic acid

Sugar Alcohols

Formed by reducing the carbonyl group of a sugar.
Glucose is reduced to sorbitol, mannose and galactose form sorbitol and mannitol, respectively.
Sorbitol, mannitol, and dulcitol can be used to reduce intracellular levels of sorbitol.
Their accumulation can lead to abnormalities like cataracts and riboflavin (vitamin B2) deficiencies.

Glycosides

Formed when a hydroxyl group on the anomeric carbon of a monosaccharide (carbon 1) combines with another compound via a glycosidic bond.
Can be formed with another monosaccharide (e.g., lactose and sucrose) or with a non-carbohydrate compound (aglycone) (e.g., sugar nucleotides like purines and pyrimidines).

Description

This quiz covers the definition, classification, and importance of carbohydrates, focusing particularly on monosaccharides, disaccharides, oligosaccharides, and polysaccharides. It also highlights glucose as a key example of carbohydrates and its significance in nutrition. Test your knowledge on this essential biomolecule!