Biological Macromolecules: Structure, Function, and Classification

Biological Macromolecules

Biological macromolecules are large, complex molecules essential for life. The four major classes are carbohydrates, lipids, proteins, and nucleic acids. Each class has unique structures, functions, and elemental compositions.

Elemental Composition of Macromolecules

The table below summarizes the main elements found in each class of macromolecule:

Macromolecule	Carbon	Hydrogen	Oxygen	Nitrogen	Phosphorus	Sulfur
Proteins	✔️	✔️	✔️	✔️		✔️
Carbohydrates	✔️	✔️	✔️
Lipids	✔️	✔️	✔️
Nucleic Acids	✔️	✔️	✔️	✔️	✔️

Carbohydrates

Structure and Classification

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, typically with a hydrogen:oxygen ratio of 2:1. They serve as energy sources and structural components.

• Monosaccharides: Simple sugars (e.g., glucose, fructose) that cannot be hydrolyzed into smaller carbohydrates.

• Disaccharides: Composed of two monosaccharides joined by a glycosidic bond (e.g., sucrose, lactose).

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

Key Points:

• Monosaccharides are the building blocks of carbohydrates.

• Disaccharides are formed by dehydration synthesis, releasing a water molecule.

• Polysaccharides serve as energy storage (starch, glycogen) or structural support (cellulose in plants).

Example: Glucose ($C_6H_{12}O_6$) is a common monosaccharide; starch is a polysaccharide made of glucose units.

Lipids

Structure and Types

Lipids are hydrophobic molecules primarily composed of carbon, hydrogen, and oxygen. They include fats, oils, phospholipids, and steroids.

• Fatty Acids: Long hydrocarbon chains with a carboxyl group at one end. Can be saturated (no double bonds) or unsaturated (one or more double bonds).

• Triglycerides: Formed from one glycerol and three fatty acids. Serve as energy storage.

• Phospholipids: Contain a phosphate group and are key components of cell membranes.

• Steroids: Four fused carbon rings (e.g., cholesterol).

Key Points:

• Saturated fatty acids have no double bonds; unsaturated fatty acids have one or more double bonds, causing kinks in the chain.

• Phospholipids form bilayers in cell membranes due to their hydrophilic heads and hydrophobic tails.

• Steroids function as hormones and membrane components.

Example: Phospholipids arrange into bilayers, forming the structural basis of biological membranes.

Proteins

Structure and Function

Proteins are polymers of amino acids linked by peptide bonds. They perform a vast array of functions, including catalysis, transport, structure, and signaling.

• Amino Acids: Organic molecules with a central carbon, amino group, carboxyl group, hydrogen, and variable R group (side chain).

• Peptide Bonds: Covalent bonds formed between the amino group of one amino acid and the carboxyl group of another, releasing water ($\text{dehydration synthesis}$).

• Levels of Structure:

  • Primary: Sequence of amino acids.

  • Secondary: Local folding (α-helix, β-sheet) stabilized by hydrogen bonds.

  • Tertiary: 3D folding due to side chain interactions.

  • Quaternary: Association of multiple polypeptide chains.

Key Points:

• There are 20 standard amino acids, each with unique side chains.

• Protein function depends on its 3D structure.

• Enzymes are proteins that catalyze biochemical reactions.

Example: Hemoglobin is a protein that transports oxygen in the blood.

Nucleic Acids

Structure and Function

Nucleic acids store and transmit genetic information. The two main types are DNA and RNA.

• Nucleotides: The building blocks of nucleic acids, each consisting of a phosphate group, a five-carbon sugar (ribose or deoxyribose), and a nitrogenous base.

• DNA (Deoxyribonucleic Acid): Double-stranded helix containing genetic instructions.

• RNA (Ribonucleic Acid): Single-stranded, involved in protein synthesis and gene regulation.

• Nitrogenous Bases: Purines (adenine, guanine) and pyrimidines (cytosine, thymine in DNA; uracil in RNA).

Key Points:

• DNA stores genetic information; RNA translates and expresses it.

• Nucleotides are joined by phosphodiester bonds.

• ATP (adenosine triphosphate) is a nucleotide that serves as the cell's energy currency.

Example: The sequence of bases in DNA encodes the instructions for building proteins.

Summary Table: Macromolecule Comparison

Macromolecule	Monomer	Bond Type	Main Function
Carbohydrates	Monosaccharide	Glycosidic bond	Energy, structure
Lipids	Fatty acid, glycerol	Ester bond	Energy storage, membranes
Proteins	Amino acid	Peptide bond	Catalysis, structure, transport
Nucleic Acids	Nucleotide	Phosphodiester bond	Genetic information

Additional info:

• Some questions in the file prompt students to organize molecular structures and identify functional groups, which is a common activity in introductory biology labs.

• Images provided show molecular structures of various macromolecules, including glucose, fatty acids, amino acids, nucleotides, and a phospholipid bilayer.

• Students are asked to classify, compare, and analyze these molecules, reinforcing understanding of structure-function relationships in biology.