Macromolecules in Biochemistry: Structure, Function, and Detection

Unit 2 | Biochemistry

Lesson 4 | Macromolecules

This lesson introduces the four major categories of biological macromolecules, their chemical properties, and their roles in living cells. It also covers laboratory tests used to identify these macromolecules.

Overview of Macromolecules

Major Elements in Living Cells

Living cells are primarily composed of a small number of elements, including carbon (C), hydrogen (H), oxygen (O), nitrogen (N), phosphorus (P), and sulfur (S). These elements form the backbone of biological macromolecules.

• Carbon: Forms the framework of organic molecules due to its ability to make four covalent bonds.

• Hydrogen and Oxygen: Found in water and most organic compounds.

• Nitrogen: Essential for amino acids and nucleic acids.

• Phosphorus: Key component of nucleic acids and phospholipids.

• Sulfur: Found in some amino acids (e.g., cysteine, methionine).

Categories of Macromolecules

There are four major categories of macromolecules in living organisms:

• Carbohydrates

• Lipids

• Proteins

• Nucleic Acids

Macromolecule Structure and Formation

Monomers and Polymers

Macromolecules are large molecules formed by joining smaller subunits called monomers. When monomers are linked together, they form polymers.

• Carbohydrates: Monomer = monosaccharide (e.g., glucose); Polymer = polysaccharide (e.g., starch, cellulose)

• Proteins: Monomer = amino acid; Polymer = polypeptide

• Nucleic Acids: Monomer = nucleotide; Polymer = DNA or RNA

• Lipids: Not true polymers; typically composed of glycerol and fatty acids

Polymerization occurs through dehydration synthesis (removal of water to form bonds) and can be reversed by hydrolysis (addition of water to break bonds).

Types of Bonds in Macromolecules

• Carbohydrates: Glycosidic linkage

• Proteins: Peptide bond

• Nucleic Acids: Phosphodiester bond

• Lipids: Ester bond

Functions and Examples of Macromolecules

Carbohydrates

Carbohydrates serve as energy sources and structural components.

• Monosaccharides: Simple sugars (e.g., glucose, fructose)

• Disaccharides: Two monosaccharides joined (e.g., sucrose, lactose)

• Polysaccharides: Long chains (e.g., starch, cellulose, glycogen)

• Function: Energy storage (starch in plants, glycogen in animals), structural support (cellulose in plants)

Lipids

Lipids are hydrophobic molecules important for energy storage, membrane structure, and signaling.

• Types: Fats (triglycerides), phospholipids, steroids

• Function: Long-term energy storage, cell membrane structure (phospholipids), hormones (steroids)

Proteins

Proteins perform a wide variety of functions due to their diverse structures.

• Monomer: Amino acid

• Polymer: Polypeptide

• Function: Enzymes, structural support, transport, signaling

• Example: Hemoglobin (oxygen transport), collagen (structural protein)

Nucleic Acids

Nucleic acids store and transmit genetic information.

• Monomer: Nucleotide

• Polymer: DNA, RNA

• Function: Genetic information storage (DNA), protein synthesis (RNA)

Laboratory Tests for Macromolecules

Common Tests and Their Results

Several chemical tests are used to identify the presence of specific macromolecules in a sample.

Test	Positive Result	Negative Result	Explanation
Benedict's Test (Reducing Sugars)	Red	Blue	Detects reducing sugars (e.g., glucose, maltose, lactose) by color change from blue to red due to reduction of copper ions. The color intensity indicates sugar concentration.
Lugol's Iodine Test (Starch)	Blue-black	Red	Identifies starch by forming a blue-black complex with iodine, indicating the presence of polysaccharides with a helical structure.
Biuret Test (Proteins)	Purple	Blue	Detects peptide bonds in proteins; copper ions bind to peptide bonds in an alkaline solution, producing a purple color.
Sudan Dye Test (Lipids)	Red, bubbly	Pink	Sudan dyes stain non-polar lipids, making fat droplets visible as red bubbles in an aqueous solution.

Macromolecule Summary Table

Macromolecule	Monomer	Polymer	Bond	Function	Example
Carbohydrate	Monosaccharide	Polysaccharide	Glycosidic linkage	Energy storage, structure	Starch, cellulose, glycogen
Lipid	Glycerol + Fatty acids	Triglyceride, phospholipid	Ester bond	Energy storage, membranes	Fats, oils, phospholipids
Protein	Amino acid	Polypeptide	Peptide bond	Enzymes, structure, transport	Hemoglobin, collagen
Nucleic Acid	Nucleotide	DNA, RNA	Phosphodiester bond	Genetic information	DNA, RNA

Formation and Breakdown of Macromolecules

Dehydration Synthesis and Hydrolysis

Macromolecules are formed by dehydration synthesis, where a water molecule is removed to join two monomers. Breakdown occurs by hydrolysis, where water is added to split polymers into monomers.

• Dehydration Synthesis Equation:

$\text{Monomer}_1 + \text{Monomer}_2 \xrightarrow{\text{Enzyme}} \text{Polymer} + \text{H}_2\text{O}$

• Hydrolysis Equation:

$\text{Polymer} + \text{H}_2\text{O} \xrightarrow{\text{Enzyme}} \text{Monomer}_1 + \text{Monomer}_2$

Additional info:

• Macromolecules are essential for cell structure and function, and their identification is crucial in biological investigations such as forensic analysis (e.g., "Murder and a Meal" activity).

• Water's chemical and physical properties (e.g., polarity, hydrogen bonding) are fundamental to macromolecule interactions and cellular processes.