Structures and Functions of the Four Important Classes of Biological Molecules

Overview of Biological Macromolecules

Biological molecules are essential for life and can be classified into four major groups: carbohydrates, proteins, nucleic acids, and lipids. Three of these classes (carbohydrates, proteins, nucleic acids) are macromolecules composed of repeating units called monomers, which form polymers through covalent bonding. Lipids, the fourth class, are not true polymers but are crucial for cellular structure and function.

• Carbohydrates: Monomer is glucose; polymer is starch.

• Proteins: Monomer is amino acid; polymer is protein (e.g., alcohol dehydrogenase).

• Nucleic Acids: Monomer is nucleotide; polymer is DNA or RNA.

• Lipids: Not polymers; main types include fats, phospholipids, and steroids.

Nucleic Acids

Structure of Nucleic Acids

Nucleic acids are macromolecules that store and transmit hereditary information. The two main types are DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). Each nucleic acid is a polymer made of monomers called nucleotides.

• DNA: Double-stranded helix, composed of nucleotides with bases adenine (A), thymine (T), cytosine (C), and guanine (G).

• RNA: Usually single-stranded, with bases adenine (A), uracil (U), cytosine (C), and guanine (G).

• Chromosome: A DNA molecule packaged with proteins; the largest known biological polymer.

Example: The DNA in a single human cell, if stretched out, would be over 2 meters long. The total DNA in the human body could stretch to the sun and back many times.

Functions of Nucleic Acids

Nucleic acids are responsible for storing, transmitting, and expressing genetic information. DNA stores hereditary information, while RNA is involved in gene expression and, in some viruses, also stores genetic information.

• Storage of Hereditary Information: DNA contains the instructions for building and maintaining an organism.

• Transmission of Hereditary Information: DNA can copy itself through DNA replication.

• Gene Expression: DNA is transcribed into messenger RNA (mRNA), which is then translated into proteins. This process is called the central dogma of molecular biology.

• RNA Viruses: Some viruses (e.g., influenza, COVID-19, Ebola) use RNA to store and transmit genetic information.

Key Equations:

• DNA Replication:

• Transcription:

• Translation:

Example: The process of gene expression involves transcription of DNA into mRNA in the nucleus, followed by translation of mRNA into a polypeptide (protein) in the cytoplasm.

Summary Table: DNA vs. RNA

Lipids

Structure and Types of Lipids

Lipids are a diverse group of hydrophobic molecules that consist mostly of hydrocarbon regions. Unlike other macromolecules, lipids are not polymers. The main types of biologically important lipids are fats (triglycerides), phospholipids, and steroids.

• Fats (Triglycerides): Composed of glycerol and three fatty acids joined by ester linkages.

• Phospholipids: Composed of glycerol, two fatty acids, and a phosphate group; major component of cell membranes.

• Steroids: Characterized by a four-ring carbon skeleton; includes cholesterol and steroid hormones.

Example: Phospholipids form bilayers in water, which is the basis of biological membranes.

Functions of Lipids

Lipids serve several essential functions in living organisms, including energy storage, insulation, and forming cellular membranes.

• Energy Storage: Fats store energy more efficiently than carbohydrates; 1 kg of fat provides more than twice the energy of 1 kg of starch.

• Insulation and Protection: Adipose tissue cushions vital organs and insulates the body.

• Membrane Structure: Phospholipids form the structural basis of cell membranes.

• Hormones: Steroids such as cholesterol and estradiol function as hormones and signaling molecules.

Example: Seeds and plant products like avocados and nuts are rich in fats, providing high-density energy reserves for growth.

Saturated vs. Unsaturated Fats

Fats can be classified based on the presence of double bonds in their fatty acid chains.

• Saturated Fats: No double bonds; solid at room temperature; commonly found in animal fats.

• Unsaturated Fats: One or more double bonds; liquid at room temperature; commonly found in plant and fish fats.

Example: Butter is high in saturated fat, while olive oil is high in unsaturated fat.

Summary Table: Types of Lipids

Additional info:

• The central dogma of molecular biology describes the flow of genetic information: DNA is transcribed into RNA, which is then translated into protein.

• Some viruses use RNA instead of DNA to store genetic information, which is important in disease transmission and evolution.

• Phospholipids spontaneously form bilayers in aqueous environments, which is critical for the formation of biological membranes.