Biology: Carbohydrates and Biomolecules

Carbohydrates

• Carbohydrates are the most abundant organic molecules in nature
• They are classified based on the number of carbon atoms, sugar subunits, or functional groups
• Carbohydrates exist as isomers, which have the same chemical formula but different structures
• Carbohydrates' functions include short-term energy generation, intermediate-term energy storage, and structural components of cells
• Carbohydrates can be attached to non-carbohydrate molecules by glycosidic bonds

Biomolecules

• Biomolecules are organic molecules formed by living organisms
• They are primarily composed of carbon, hydrogen, oxygen, and nitrogen
• Four major classes of biomolecules include carbohydrates, proteins, lipids, and nucleic acids

Monosaccharides

• Monosaccharides are simple sugars that cannot be hydrolyzed into simpler carbohydrates
• They are classified based on the number of carbon atoms (e.g., trioses, tetroses, pentoses, hexoses)
• Monosaccharides can be aldoses (aldehyde functional group) or ketoses (ketone functional group)
• Common monosaccharides include glucose, fructose, and galactose

Isomers

• Isomers have the same chemical formula but different structures
• Examples include glucose, galactose, and fructose, all with the formula C₆H₁₂O₆.
• Enantiomers are nonsuperimposable mirror images

Epimers

• Epimers are isomers that differ in the configuration around only one specific carbon atom (excluding the carbonyl carbon)
• Glucose and mannose are C-2 epimers
• Glucose and galactose are C-4 epimers

Cyclisation

• Monosaccharides often exist in ring formations
• Less than 1% of monosaccharides exist in their open-chain form
• Cyclization creates an anomeric carbon

Glycosidic bonds

• Glycosidic bonds are the bonds that link monosaccharides to form disaccharides, oligosaccharides, and polysaccharides
• They are formed through dehydration reactions (condensation)
• Glycosidic bonds are named based on the configuration of the anomeric hydroxyl group (α or β)

Common disaccharides

• Disaccharides are formed from two monosaccharides
• Examples include lactose (β-galactose + α-glucose), maltose (α-glucose + α-glucose), and sucrose (α-glucose + β-fructose)

Reducing sugars

• Reducing sugars can act as reducing agents because the anomeric carbon is not involved in a glycosidic bond
• All monosaccharides are reducing sugars

Glucose – biomedical importance

• Glucose is a crucial energy source in all cells
• It cannot be stored in this form because it would affect osmotic balance
• It is stored as glycogen in animals
• Ribose is present in nucleotides and nucleic acids.

Polymers of glucose

• Glycogen: Storage form of glucose in animals, highly branched
• Starch: Storage form of glucose in plants; composed of amylose (linear) and amylopectin (branched)
• Cellulose: Structural component of plant cell walls, composed of unbranched chains of β-glucose

Cellulose vs Starch

• The main difference between cellulose and starch is that cellulose uses β-glucose while starch uses α-glucose units in its structure

Complex carbohydrates

• Carbohydrates can be attached to non-carbohydrate molecules via glycosidic bonds
• Structures include glycoproteins, proteoglycans, and glycolipids
• The types of glycosidic bonds depend on the attached non-carbohydrate molecule (-NH₂ group = N-glycoside, -OH group = O-glycoside)