Biomolecules: Structure and Function of Carbohydrates and Lipids

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Biomolecules

Introduction

Biomolecules are essential components of cells, playing critical roles in structure, function, and energy storage. The four major classes are carbohydrates, lipids, proteins, and nucleic acids. This guide focuses on carbohydrates and lipids, their monomers, structures, and functions.

Carbohydrates

Monosaccharides

Monosaccharides are the simplest carbohydrates, serving as monomers for disaccharides and polysaccharides. They are classified by the number of carbon atoms: trioses (3C), pentoses (5C), hexoses (6C), etc. Their general formula is $(CH_2O)_n$.

Structure: Monosaccharides contain a carbonyl group (C=O) and multiple hydroxyl groups (-OH). They are either aldoses (aldehyde group) or ketoses (ketone group).
Examples: Glucose, fructose, galactose, ribose, deoxyribose.
Function: Main energy source for cellular respiration; building blocks for larger molecules.

Type	Formula	Example	Function
Triose	C3H6O3	Glyceraldehyde	Intermediate in metabolism
Pentose	C5H10O5	Ribose, Deoxyribose	RNA, DNA component
Hexose	C6H12O6	Glucose, Fructose, Galactose	Energy source

Structures of aldoses and ketoses

Properties of Monosaccharides

Physical: Sweet, soluble, crystalline.
Chemical: Reducing sugars; can reduce Cu2+ to Cu+ in Benedict's test.

Benedict's test color scale for reducing sugars

Monosaccharides exist in equilibrium between open-chain and ring forms. For glucose, the ring form predominates in solution.

Oxidation of glucose in Benedict's test

Isomers of Glucose

Glucose exists as α-glucose and β-glucose, differing in the position of the hydroxyl group on carbon-1. These isomers have the same formula but different structures, affecting their biological roles.

Open chain and ring forms of glucose α-glucose and β-glucose ring structures

Disaccharides

Disaccharides are formed by condensation reactions between two monosaccharides, creating a glycosidic bond and releasing water. Common examples include maltose, sucrose, and lactose.

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 monosaccharides.

Formation of α-1,4-glycosidic bond in maltose Formation of α-1,2-glycosidic bond in sucrose Formation of β-1,4-glycosidic bond in lactose

Polysaccharides

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

Starch

Starch is a plant storage polysaccharide composed of α-glucose monomers. It consists of two components:

Amylose: Unbranched, α-1,4-glycosidic bonds, helical structure.
Amylopectin: Branched, α-1,4 and α-1,6-glycosidic bonds.

Amylose structure Amylopectin structure Amylose and amylopectin comparison Amylose and amylopectin glycosidic bonds α-1,4-glycosidic bond in amylose Helical structure of amylose

Glycogen

Glycogen is the main storage polysaccharide in animals, structurally similar to amylopectin but with more extensive branching, making it highly compact and easily hydrolyzed.

Starch grains in chloroplast Glycogen granules in liver/muscle cells Amylopectin and glycogen comparison Branching in glycogen Highly branched structure of glycogen

Cellulose

Cellulose is a structural polysaccharide in plants, composed of β-glucose monomers linked by β-1,4-glycosidic bonds. Its straight, unbranched chains form microfibrils, providing high tensile strength to cell walls.

Comparison of starch and cellulose structures β-1,4-glycosidic bond in cellulose Cellulose chain structure Cellulose microfibrils in plant cell wall

Lipids

Triglycerides

Triglycerides are the most common lipids, consisting of three fatty acids and one glycerol molecule. They are formed by condensation reactions, creating ester bonds and releasing water.

Fatty acids: Long hydrocarbon chains with a carboxyl group. Can be saturated (no double bonds) or unsaturated (one or more double bonds).
Glycerol: A three-carbon alcohol with hydroxyl groups.

Saturated and unsaturated fatty acids Triglyceride structure and packing Glycerol structure Formation of ester bonds in triglycerides Triglyceride molecule

Properties and Functions of Triglycerides

Physical: Fats (solid at room temperature) have saturated fatty acids; oils (liquid) have unsaturated fatty acids.
Solubility: Insoluble in water, soluble in organic solvents.
Function: Energy storage, heat insulation, protection, buoyancy.

Phospholipids

Phospholipids are major components of cell membranes, consisting of one glycerol, two fatty acids, and one phosphate group. They are amphipathic, with a hydrophilic head and hydrophobic tails.

Phospholipid structure Phosphatidylcholine structure

Functions of Phospholipids

Form cell membrane bilayers (Fluid Mosaic Model).
Participate in cell-cell recognition and adhesion.
Assist in fat transport and neurotransmitter formation.

Summary Table: Carbohydrate and Lipid Monomers

Biomolecule	Monomer
Carbohydrates	Sugars (saccharides)
Lipids	Fatty acids and glycerol
Proteins	Amino acids
Nucleic Acids	Nucleotides

Additional info: The notes above expand on the original content by providing detailed explanations, definitions, and examples for each biomolecule, as well as relevant chemical equations and structural comparisons. All included images directly reinforce the molecular structures and reactions discussed in the text.