Biomolecules: Structure and Function of Carbohydrates and Lipids

Biomolecules: Structure and Function of Carbohydrates and Lipids

Introduction to Biomolecules

Biomolecules are essential organic molecules that constitute living organisms and are fundamental to cellular structure and function. The four major classes are carbohydrates, lipids, proteins, and nucleic acids. Each class is built from specific monomers and performs distinct biological roles.

• Carbohydrates: Main energy source and storage

• Lipids: Main component of cell membranes and energy storage

• Proteins: Structural and enzymatic functions

• Nucleic acids: Store and transmit hereditary information

Carbohydrates

Monosaccharides

Monosaccharides are the simplest carbohydrates and serve as the monomers for disaccharides and polysaccharides. They are classified by the number of carbon atoms and the position of their carbonyl group.

• General formula: (CH2O)n, where n > 3

• Functional groups: Carbonyl (C=O) and multiple hydroxyl (-OH) groups

• Classification: Aldoses (aldehyde group) or ketoses (ketone group)

No. of C atoms	General Name	Examples	Functions
3	Triose	Glyceraldehyde	Intermediate in respiration/photosynthesis
5	Pentose	Ribose, Deoxyribose	RNA, DNA, ATP components
6	Hexose	Glucose, Fructose, Galactose	Main energy source

Properties of Monosaccharides

• Physical: Sweet, soluble, crystalline

• Chemical: All are reducing sugars (can reduce Cu2+ to Cu+ in Benedict's test)

The Benedict's test is used to detect reducing sugars. The color change from blue to green, yellow, orange, or red indicates increasing concentrations of reducing sugar.

Monosaccharide Isomerism

Glucose exists in multiple forms: open-chain, α-glucose, and β-glucose. The ring forms are more stable and predominant in solution.

• α-glucose: Hydroxyl group on C1 is below the ring

• β-glucose: Hydroxyl group on C1 is above the ring

Other hexose isomers include fructose and galactose, which differ in atomic arrangement and function.

Disaccharides

Disaccharides are formed by a condensation reaction between two monosaccharides, creating a glycosidic bond and releasing water. The bond can be α-1,4, α-1,2, or β-1,4 depending on the sugars involved.

• Maltose: Glucose + Glucose (α-1,4-glycosidic bond)

• Sucrose: Glucose + Fructose (α-1,2-glycosidic bond)

• Lactose: Glucose + Galactose (β-1,4-glycosidic bond)

Polysaccharides

Polysaccharides are long chains of monosaccharide units linked by glycosidic bonds. They can be storage or structural polysaccharides.

Storage Polysaccharides

• Starch: Main storage in plants; composed of amylose (unbranched, α-1,4) and amylopectin (branched, α-1,4 and α-1,6)

• Glycogen: Main storage in animals; highly branched (α-1,4 and α-1,6)

Structural Polysaccharides

• Cellulose: Main component of plant cell walls; composed of β-glucose units linked by β-1,4-glycosidic bonds, forming straight, unbranched chains that associate into microfibrils for high tensile strength.

Lipids

Triglycerides

Triglycerides are the most common lipids, consisting of three fatty acids esterified to one glycerol molecule. Fatty acids can be saturated (no double bonds) or unsaturated (one or more double bonds).

• Formation: Condensation reaction forms three ester bonds and releases three water molecules.

• Properties: Insoluble in water, soluble in organic solvents, lower density than water, high energy content.

Phospholipids

Phospholipids are major components of cell membranes, consisting of a glycerol backbone, two fatty acids, and a phosphate group. They are amphipathic, with hydrophilic heads and hydrophobic tails, allowing them to form bilayers in aqueous environments.

Steroids and Sterols

Steroids have a characteristic structure of four fused carbon rings. Cholesterol is a key sterol in animals, serving as a precursor for steroid hormones and a regulator of membrane fluidity.

Summary Table: Key Biomolecules

Biomolecule	Monomer	Bond Type	Example	Function
Carbohydrate	Monosaccharide	Glycosidic	Starch, Glycogen, Cellulose	Energy storage, structure
Lipid	Fatty acid, Glycerol	Ester	Triglyceride, Phospholipid	Energy storage, membranes
Protein	Amino acid	Peptide	Enzymes, Collagen, Hemoglobin	Catalysis, structure, transport
Nucleic Acid	Nucleotide	Phosphodiester	DNA, RNA	Genetic information

Additional info: The notes above are structured to cover the core concepts of biomolecules, focusing on carbohydrates and lipids, as outlined in the syllabus. Images included are directly relevant to the chemical structures, reactions, and biological roles discussed in each section.