Biomolecules: Structure and Function of Carbohydrates and Lipids

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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, serving as monomers for more complex sugars. 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)

Number of C atoms	General Name	Examples	Function
3	Triose	Glyceraldehyde	Intermediate in respiration/photosynthesis
5	Pentose	Ribose, Deoxyribose	Component of RNA, DNA, ATP
6	Hexose	Glucose, Fructose, Galactose	Main energy source, component of disaccharides

Structures of aldoses and ketoses

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.

Benedict's test color scale for reducing sugars

Structural Isomerism in Monosaccharides

Monosaccharides like glucose, fructose, and galactose share the same molecular formula (C6H12O6) but differ in structure and function. Glucose exists in equilibrium between open-chain and ring forms, with α- and β-anomers differing in the position of the hydroxyl group on carbon 1.

Open chain and ring forms of glucose Alpha and beta glucose isomers

Disaccharides

Disaccharides are formed by the condensation of two monosaccharides, creating a glycosidic bond and releasing water. The type of glycosidic bond determines the properties and digestibility of the disaccharide.

Maltose: Glucose + Glucose (α-1,4-glycosidic bond)
Sucrose: Glucose + Fructose (α-1,2-glycosidic bond)
Lactose: Glucose + Galactose (β-1,4-glycosidic bond)

Disaccharides can be hydrolyzed by acids or enzymes to yield their constituent monosaccharides.

Disaccharide	Components	Bond Type	Function
Maltose	Glucose + Glucose	α-1,4-glycosidic	Intermediate in starch digestion
Sucrose	Glucose + Fructose	α-1,2-glycosidic	Transport sugar in plants
Lactose	Glucose + Galactose	β-1,4-glycosidic	Milk sugar, energy for young mammals

Formation of glycosidic bond in maltose

Polysaccharides

Polysaccharides are large polymers of monosaccharides linked by glycosidic bonds. They are classified as storage (e.g., starch, glycogen) or structural (e.g., cellulose) polysaccharides.

Storage Polysaccharides

Starch: Main storage carbohydrate in plants, composed of amylose (unbranched, α-1,4-glycosidic bonds) and amylopectin (branched, α-1,4- and α-1,6-glycosidic bonds).
Glycogen: Main storage carbohydrate in animals, highly branched (α-1,4- and α-1,6-glycosidic bonds).

Amylose structure Amylopectin structure Amylose helical structure Amylopectin branched structure Alpha-amylose structure Helical structure of amylose Starch grains in chloroplast Glycogen granules in animal cells Amylopectin and glycogen branching Branching in glycogen Highly branched structure of glycogen

Structural Polysaccharide

Cellulose: Main component of plant cell walls, composed of β-glucose units linked by β-1,4-glycosidic bonds. Chains are straight and form microfibrils via hydrogen bonding, providing high tensile strength.

Comparison of starch and cellulose structure Cellulose straight chain structure Cellulose microfibril formation Cellulose in plant cell wall

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).

Saturated fatty acids: Only single C–C bonds, straight chains, solid at room temperature (fats)
Unsaturated fatty acids: One or more C=C double bonds, kinked chains, liquid at room temperature (oils)

Saturated and unsaturated fatty acids Triglyceride structure and packing Glycerol structure Esterification reaction forming triglyceride Triglyceride molecule

Properties and Functions of Triglycerides

High energy storage (more energy per gram than carbohydrates)
Insoluble in water, soluble in organic solvents
Provide insulation, protection, and buoyancy

Phospholipids

Phospholipids are major components of cell membranes. They consist of a glycerol backbone, two fatty acids, and a phosphate group. The molecule is amphipathic, with a hydrophilic head and hydrophobic tails.

Phospholipid structure Phosphatidylcholine structure

Form bilayers in aqueous environments, fundamental to membrane structure
Participate in cell signaling and recognition (as glycolipids)

Steroids and Sterols

Steroids are lipids with a characteristic four-ring structure. Cholesterol is a key sterol in animals, serving as a precursor for steroid hormones and an essential component of cell membranes.

Insoluble in water, transported in blood as lipoproteins
Regulate membrane fluidity and serve as hormone precursors

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