Biological Molecules: Structure, Function, and Diversity

Biological Molecules

Introduction to Biological Molecules

Biological molecules, also known as biomolecules, are essential compounds that form the structure and function of all living organisms. They include carbohydrates, lipids, proteins, and nucleic acids, each with unique properties and roles in cellular processes.

• Key Elements: Most biological molecules are composed of carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur.

• Structure-Function Relationship: The molecular structure of biomolecules determines their function in biological systems.

• Examples: Glucose (carbohydrate), triglycerides (lipid), hemoglobin (protein), DNA (nucleic acid).

Water and Its Importance in Biology

Properties of Water

Water is vital for life due to its unique chemical and physical properties, which make it an excellent solvent and participant in biological reactions.

• Cohesion: Water molecules stick to each other due to hydrogen bonding.

• Adhesion: Water molecules can also stick to other polar substances.

• High Specific Heat: Water can absorb a lot of heat before changing temperature, helping to stabilize environments.

• Solvent Properties: Water dissolves many ionic and polar substances, facilitating biochemical reactions.

Example: Water's cohesive and adhesive properties are essential for the transport of water in plants (capillary action).

Carbohydrates

Types and Functions of Carbohydrates

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen. They serve as energy sources and structural components in cells.

• Monosaccharides: Simple sugars such as glucose, galactose, and fructose.

• Disaccharides: Formed by joining two monosaccharides (e.g., maltose, lactose, sucrose).

• Polysaccharides: Long chains of monosaccharides; examples include starch, glycogen, and cellulose.

Example: Starch is a storage polysaccharide in plants, while glycogen serves a similar function in animals.

Lipids

Types and Functions of Lipids

Lipids are hydrophobic molecules that include fats, oils, phospholipids, and steroids. They are important for energy storage, membrane structure, and signaling.

• Triglycerides: Composed of glycerol and three fatty acids; main form of stored energy in animals.

• Saturated vs. Unsaturated Fatty Acids:

  • Saturated: No double bonds between carbon atoms; solid at room temperature.

  • Unsaturated: One or more double bonds; liquid at room temperature.

• Phospholipids: Major component of cell membranes, forming a bilayer with hydrophilic heads and hydrophobic tails.

Example: The phospholipid bilayer forms the structural basis of all biological membranes.

Proteins

Structure and Function of Proteins

Proteins are polymers of amino acids that perform a vast array of functions, including catalysis, transport, structure, and signaling.

• Primary Structure: Sequence of amino acids in a polypeptide chain.

• Secondary Structure: Local folding into alpha helices and beta sheets, stabilized by hydrogen bonds.

• Tertiary Structure: Three-dimensional folding due to interactions among R groups (side chains).

• Quaternary Structure: Association of multiple polypeptide chains.

Example: Hemoglobin is a quaternary protein composed of four polypeptide subunits.

Nucleic Acids

Types and Functions of Nucleic Acids

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

• DNA (Deoxyribonucleic Acid): Double-stranded, contains deoxyribose sugar, uses thymine as a base.

• RNA (Ribonucleic Acid): Usually single-stranded, contains ribose sugar, uses uracil instead of thymine.

• Nucleotides: Monomers of nucleic acids, each consisting of a phosphate group, a five-carbon sugar, and a nitrogenous base.

Example: ATP (adenosine triphosphate) is a nucleotide that serves as the primary energy carrier in cells.

Comparison Table: Major Biological Molecules

Type	Monomer	Polymer	Main Functions	Examples
Carbohydrates	Monosaccharide	Polysaccharide	Energy storage, structure	Glucose, starch, cellulose
Lipids	Fatty acid, glycerol	Triglyceride, phospholipid	Energy storage, membranes	Fats, oils, phospholipids
Proteins	Amino acid	Polypeptide	Catalysis, structure, transport	Enzymes, hemoglobin
Nucleic Acids	Nucleotide	DNA, RNA	Genetic information, energy	DNA, RNA, ATP

Additional Academic Context

• Hydrophobic vs. Hydrophilic: Hydrophobic molecules repel water (e.g., lipids), while hydrophilic molecules interact well with water (e.g., carbohydrates, proteins).

• Enzyme-Substrate Interaction: Enzymes bind substrates in a specific manner, often described as a "lock-and-key" or "induced fit" model, to catalyze biochemical reactions.

• Condensation and Hydrolysis:

  • Condensation (Dehydration Synthesis): Joins monomers by removing water.

  • Hydrolysis: Breaks polymers into monomers by adding water.

Equation Example:

• General formula for a monosaccharide: $C_nH_{2n}O_n$

• Hydrolysis reaction: $\text{Disaccharide} + H_2O \rightarrow \text{Monosaccharide}_1 + \text{Monosaccharide}_2$

• ATP hydrolysis: $ATP + H_2O \rightarrow ADP + P_i + \text{energy}$