Macromolecules: Structure and Function of Large Biological Molecules

Macromolecules: The Molecules of Life

Introduction to Macromolecules

All living organisms are composed of four major classes of large biological molecules, known as macromolecules. These molecules are essential for life and are built from thousands of covalently connected atoms.

• Carbohydrates

• Lipids

• Proteins

• Nucleic acids

Macromolecules are large molecules, often polymers, that play critical roles in cell structure and function.

Macromolecules of Polymers

Polymers and Monomers

A polymer is a long molecule consisting of many similar or identical building blocks linked by covalent bonds. The repeating units that serve as the building blocks of a polymer are called monomers.

• Three of the four classes of life's organic molecules are polymers: carbohydrates, proteins, and nucleic acids.

• Lipids are not true polymers.

The Synthesis and Breakdown of Polymers

• Dehydration reaction (synthesis): Two monomers are covalently bonded through the loss of a water molecule, forming a polymer.

• Hydrolysis (breakdown): Polymers are disassembled to monomers by the addition of a water molecule, breaking the covalent bond.

Equation for Dehydration Synthesis:

$\text{Monomer}_1 + \text{Monomer}_2 \xrightarrow{\text{Dehydration}} \text{Polymer} + H_2O$

Equation for Hydrolysis:

$\text{Polymer} + H_2O \xrightarrow{\text{Hydrolysis}} \text{Monomer}_1 + \text{Monomer}_2$

The Diversity of Polymers

• Each cell contains thousands of different macromolecules.

• Macromolecules vary among cells of an organism, more within a species, and even more between species.

• An immense variety of polymers can be built from a small set of monomers.

Carbohydrates

Overview

Carbohydrates include sugars and the polymers of sugars. They serve as fuel and building material for cells.

• Monosaccharides: Single sugar molecules (simple sugars).

• Polysaccharides: Macromolecules composed of many sugar building blocks.

Monosaccharides

• Have molecular formulas that are usually multiples of $CH_2O$ (e.g., glucose: $C_6H_{12}O_6$).

• Classified by:

  • The location of the carbonyl group (as aldose or ketose).

  • The number of carbons in the carbon skeleton (triose, pentose, hexose, etc.).

• Can exist as linear chains or ring forms in aqueous solutions.

Example: Glucose, galactose, and fructose are all hexoses (6-carbon sugars).

Isomerism in Sugars

• Structural isomers: Compounds with the same molecular formula but different structures (e.g., ribose and ribulose).

• Cis-trans isomers and enantiomers are other types of isomerism found in sugars.

Disaccharides

A disaccharide is formed when a dehydration reaction joins two monosaccharides. The covalent bond formed is called a glycosidic linkage.

• Maltose: Glucose + Glucose (1-4 glycosidic linkage)

• Sucrose: Glucose + Fructose (1-2 glycosidic linkage)

Polysaccharides

Polysaccharides are polymers of sugars and serve storage and structural roles.

Storage	Structural
Plant: Starch	Plant: Cellulose
Animal: Glycogen	Fungi/Arthropods: Chitin

Storage Polysaccharides

• Starch: Storage polysaccharide in plants, composed entirely of glucose monomers. Stored as granules in plastids (e.g., chloroplasts). The simplest form is amylose (unbranched), while amylopectin is branched.

• Glycogen: Storage polysaccharide in animals, highly branched, stored mainly in liver and muscle cells.

Structural Polysaccharides

• Cellulose: Major component of plant cell walls. Like starch, it is a polymer of glucose, but with different glycosidic linkages (β-1,4 linkages).

• Polymers with α-glucose are helical (starch), while those with β-glucose are straight (cellulose).

• Parallel cellulose molecules form microfibrils, providing structural support.

• Most animals cannot digest cellulose due to lack of enzymes for β-linkages; some herbivores rely on symbiotic microbes for digestion.

• Chitin: Structural polysaccharide in the exoskeleton of arthropods and cell walls of fungi.

Lipids

Overview

Lipids are a diverse group of hydrophobic molecules that do not form true polymers. They are characterized by their insolubility in water due to the presence of nonpolar covalent bonds.

• Major types: fats, phospholipids, and steroids.

Fats

• Constructed from glycerol (a three-carbon alcohol) and fatty acids (carboxyl group attached to a long hydrocarbon chain).

• Formed by ester linkages through dehydration reactions.

• Main function: energy storage (1 g of fat stores more than twice the energy of 1 g of polysaccharide).

• Stored in adipose tissue in animals, which also cushions organs and insulates the body.

Fatty Acids

• Saturated fatty acids: No double bonds, maximum number of hydrogen atoms, solid at room temperature (e.g., animal fats).

• Unsaturated fatty acids: One or more double bonds, liquid at room temperature (e.g., plant oils, fish oils). Cis double bonds cause bending.

Cis vs Trans Unsaturated Fats

• Cis fats: Naturally occurring, hydrogens on the same side of the double bond, cause kinks in the chain.

• Trans fats: Produced by hydrogenation, hydrogens on opposite sides, associated with increased cardiovascular risk.

Phospholipids

• Composed of two fatty acids and a phosphate group attached to glycerol.

• Amphipathic: hydrophilic (phosphate head) and hydrophobic (fatty acid tails) regions.

• Form bilayers in aqueous environments, making up the fundamental structure of cell membranes.

Steroids

• Lipids with a carbon skeleton consisting of four fused rings.

• Cholesterol: Essential component of animal cell membranes; precursor for steroid hormones. High levels can contribute to cardiovascular disease.

Summary Table: Major Classes of Biological Macromolecules

Class	Monomer	Polymer	Bond Type	Main Functions
Carbohydrates	Monosaccharide	Polysaccharide	Glycosidic linkage	Energy storage, structure
Proteins	Amino acid	Polypeptide	Peptide bond	Catalysis, structure, transport, signaling
Nucleic acids	Nucleotide	Polynucleotide	Phosphodiester bond	Genetic information storage and transfer
Lipids	Glycerol, fatty acids	Not true polymers	Ester linkage	Energy storage, membranes, signaling

Additional info: This summary covers the structure and function of carbohydrates and lipids, as well as the general principles of macromolecule synthesis and diversity. For proteins and nucleic acids, see subsequent sections or chapters.