Biomolecules: Structure and Function in Living Systems

Biomolecules in Living Systems

Introduction to Biomolecules

Biomolecules are essential chemical compounds that form the basis of life. They are involved in the structure, function, and regulation of the body's tissues and organs. The four major classes of biomolecules are carbohydrates, lipids, proteins, and nucleic acids.

• Carbohydrates: Serve as energy sources and structural components.

• Lipids: Function in energy storage, membrane structure, and signaling.

• Proteins: Perform a wide range of functions including catalysis, transport, and structural support.

• Nucleic acids: Store and transmit genetic information.

Fundamental Chemical Principles

Atomic Structure and Chemical Bonds

Atoms interact to form molecules through chemical bonds, which are crucial for the structure and function of biomolecules.

• Covalent bonds: Formed by sharing electrons; strongest type of bond. Example: H2O, CH4.

• Ionic bonds: Formed by transfer of electrons; moderate strength.

• Hydrogen bonds: Weak interactions between partially charged atoms, especially involving hydrogen and electronegative atoms like oxygen and nitrogen. These are critical for the structure of water and biological macromolecules.

Valence electrons determine bonding capacity. For example:

• Oxygen: Valence = 2

• Nitrogen: Valence = 3

• Carbon: Valence = 4; can form four covalent bonds, making it highly versatile.

Organic Molecules and Carbon-Based Life

Properties of Carbon

Organic molecules are primarily composed of carbon and hydrogen. Carbon's ability to form four covalent bonds allows for a diversity of molecular structures, including chains and rings.

• Hydrocarbon chains: Chains of covalently bonded carbon and hydrogen atoms; can be straight or branched.

• Isomers: Molecules with the same chemical formula but different structures and properties.

• Functional groups: Specific groups of atoms within molecules that confer particular chemical properties (e.g., hydroxyl, carboxyl, amino, phosphate).

Polarity and Hydrophobicity

Organic molecules may have hydrophobic (nonpolar) or hydrophilic (polar) regions, affecting their solubility and interactions in biological systems.

• Hydrophobic: Water-insoluble, typically nonpolar hydrocarbon chains.

• Hydrophilic: Water-soluble, typically polar functional groups.

Macromolecules: Polymers and Monomers

Polymer Formation and Breakdown

Biological macromolecules are often polymers, formed by linking monomers through chemical reactions.

• Monomer: A single subunit (e.g., glucose).

• Polymer: A chain of repeating monomers (e.g., starch, proteins).

• Dehydration synthesis: Joins monomers by removing water.

• Hydrolysis: Breaks polymers into monomers by adding water.

Equations:

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

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

Carbohydrates

Structure and Types

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, typically in a ratio of 1:2:1. They exist as monosaccharides, disaccharides, and polysaccharides.

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

• Disaccharides: Two monosaccharides joined by a glycosidic bond (e.g., sucrose).

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

Biological Functions

• Energy source: Glucose is a primary energy source for cells.

• Energy storage: Starch in plants, glycogen in animals.

• Structural support: Cellulose in plant cell walls, chitin in arthropod exoskeletons.

Lipids

Structure and Types

Lipids are hydrophobic molecules, including fats, phospholipids, and steroids.

• Fats: Composed of glycerol and fatty acids; used for energy storage.

• Phospholipids: Major component of cell membranes; have hydrophilic heads and hydrophobic tails.

• Steroids: Four fused carbon rings; include cholesterol and hormones like estrogen and testosterone.

Biological Functions

• Energy storage: Fats store energy efficiently.

• Membrane structure: Phospholipids form bilayers in cell membranes.

• Signaling: Steroid hormones regulate physiological processes.

Proteins

Structure and Levels of Organization

Proteins are polymers of amino acids, each with a unique side chain (R group) that determines its properties. Protein structure is organized into four levels:

• Primary structure: Linear sequence of amino acids.

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

• Tertiary structure: Three-dimensional shape formed by interactions among side chains.

• Quaternary structure: Association of multiple polypeptide chains (e.g., hemoglobin).

Biological Functions

• Enzymes: Catalyze biochemical reactions.

• Transport proteins: Move substances across membranes (e.g., hemoglobin transports oxygen).

• Structural proteins: Provide support (e.g., collagen, keratin).

• Motor proteins: Enable movement (e.g., actin, myosin).

Nucleic Acids

Structure and Types

Nucleic acids are polymers of nucleotides, each consisting of a sugar, phosphate group, and nitrogenous base. The two main types are DNA and RNA.

• DNA (deoxyribonucleic acid): Double-stranded helix; stores genetic information.

• RNA (ribonucleic acid): Single-stranded; involved in protein synthesis and gene regulation.

Nucleotide structure:

• Sugar: Deoxyribose in DNA, ribose in RNA.

• Phosphate group: Links nucleotides via phosphodiester bonds.

• Nitrogenous base: Adenine (A), Thymine (T, in DNA), Uracil (U, in RNA), Cytosine (C), Guanine (G).

Base Pairing Rules

• DNA: A pairs with T (2 hydrogen bonds), C pairs with G (3 hydrogen bonds).

• RNA: A pairs with U.

Biological Functions

• Genetic information storage: DNA encodes instructions for protein synthesis.

• Gene expression: RNA transcribes and translates genetic information.

• Ribosomal RNA: Forms ribosomes.

• Messenger RNA: Carries genetic code from DNA to ribosomes.

• Transfer RNA: Brings amino acids to ribosomes during protein synthesis.

Applications in Molecular Biology

Genetic Code and Protein Synthesis

The genetic code connects DNA to RNA to amino acids. Codons (groups of three nucleotides) in mRNA are translated into specific amino acids during protein synthesis.

• Redundancy: Multiple codons can code for the same amino acid.

• Example: The codon UGC codes for cysteine.

Genetic Variation and Human Evolution

Genetic mutations and variations can lead to differences in traits, such as lactose tolerance in humans. These adaptations are influenced by evolutionary pressures and cultural practices.

• Lactase persistence: Continued production of lactase enzyme into adulthood, common in populations with a history of dairy consumption.

• Sickle-cell anemia: Caused by a mutation in the hemoglobin gene, leading to altered protein structure and function.

Biotechnology Applications

Techniques such as DNA sequencing and amplification are widely used in research, medicine, agriculture, and forensic science.

• Genome sequencing: Used to study genetic diseases, track viral mutations, and improve crop strains.

• Forensic analysis: DNA profiling links individuals to biological samples.

Summary Table: Major Biomolecules

Biomolecule	Monomer	Polymer	Main Functions	Examples
Carbohydrates	Monosaccharide	Polysaccharide	Energy, structure	Glucose, starch, cellulose
Lipids	Fatty acid, glycerol	Triglyceride, phospholipid	Energy storage, membranes	Fats, phospholipids, steroids
Proteins	Amino acid	Polypeptide	Catalysis, transport, structure	Enzymes, hemoglobin, collagen
Nucleic acids	Nucleotide	DNA, RNA	Genetic information	DNA, mRNA, tRNA

Additional info: Some explanations and examples have been expanded for clarity and completeness.