Origins of Life and the Chemical Basis of Biology

Unit 1 – Life Starts Small: Origins of Life

Introduction

This unit explores the chemical foundations of life, focusing on the elements that compose living organisms, the structure of atoms, the formation of molecules, and the origins of life on Earth. Understanding these concepts is essential for grasping how biological systems function at the molecular level.

Composition of Living Organisms

Elements in Biological Systems

• Element: A pure substance consisting of only one kind of atom, which cannot be broken down into other substances by chemical means.

• Atom: The smallest unit of matter that retains the properties of an element. Atoms are composed of protons (positively charged), neutrons (neutral), and electrons (negatively charged).

• Living organisms are primarily composed of a small number of elements, with oxygen (O), carbon (C), hydrogen (H), and nitrogen (N) making up the majority of biological matter.

Element	Symbol	Function in the Human Body
Oxygen	O	Component of water and organic molecules; required for cellular respiration
Carbon	C	Backbone of all organic molecules
Hydrogen	H	Component of water and most organic molecules
Nitrogen	N	Component of proteins and nucleic acids
Calcium	Ca	Bone and teeth structure, muscle function
Phosphorus	P	Component of nucleic acids and ATP
Potassium	K	Nerve signaling, fluid balance
Sulfur	S	Component of some amino acids
Sodium	Na	Nerve signaling, fluid balance
Chlorine	Cl	Fluid balance, stomach acid
Magnesium	Mg	Enzyme cofactor

Atomic Structure and Isotopes

Structure of Atoms

• Atoms consist of a nucleus (containing protons and neutrons) surrounded by a cloud of electrons.

• Protons have a positive charge, electrons have a negative charge, and neutrons are neutral.

• Atoms are electrically neutral when the number of protons equals the number of electrons.

Isotopes

• Isotopes: Atoms of the same element with the same number of protons but different numbers of neutrons, resulting in different mass numbers.

• Example: Carbon has three naturally occurring isotopes:

Isotope	Protons	Neutrons	Mass Number
Carbon-12	6	6	12
Carbon-13	6	7	13
Carbon-14	6	8	14

• Some isotopes are radioactive and can be used in dating fossils (e.g., Carbon-14 dating).

Chemical Bonds and Molecules

Types of Chemical Bonds

• Covalent Bonds: Atoms share pairs of electrons to achieve stability. Covalent bonds can be nonpolar (equal sharing) or polar (unequal sharing due to differences in electronegativity).

• Ionic Bonds: Formed when one atom donates an electron to another, resulting in oppositely charged ions that attract each other.

• Hydrogen Bonds: Weak attractions between a hydrogen atom (partially positive) and an electronegative atom (such as oxygen or nitrogen) in another molecule.

Electronegativity and Polarity

• Electronegativity: The ability of an atom to attract shared electrons in a covalent bond.

• Polar covalent bonds occur when atoms with different electronegativities share electrons unequally, creating partial charges (e.g., in water molecules).

Water: The Solvent of Life

• Water's polarity allows it to dissolve many substances, making it an excellent solvent for biological reactions.

• Hydrogen bonding between water molecules leads to unique properties such as cohesion, adhesion, and high specific heat.

Carbon and Molecular Diversity

Properties of Carbon

• Carbon has four valence electrons, allowing it to form up to four covalent bonds with other atoms.

• This versatility enables the formation of a wide variety of complex organic molecules, including chains, branched structures, and rings.

• Hydrocarbons are compounds composed only of carbon and hydrogen.

Isomers

• Isomers: Compounds with the same molecular formula but different structures and properties.

• Types of isomers include:

  • Structural isomers: Differ in the covalent arrangement of atoms.

  • Cis-trans isomers: Differ in spatial arrangement around a double bond.

  • Enantiomers: Mirror-image isomers, important in biological systems (e.g., R- and S- forms of a drug).

Functional Groups

• Functional groups are specific groups of atoms attached to carbon skeletons that confer unique chemical properties to organic molecules.

• Examples include hydroxyl (-OH), methyl (-CH3), carboxyl (-COOH), amino (-NH2), and phosphate (-PO4).

• The presence and arrangement of functional groups determine the reactivity and function of organic molecules.

Origins of Life: Hypotheses and Experiments

Stages in the Origin of Life

1. Abiotic (nonliving) synthesis of small organic molecules such as amino acids and nucleotides.

2. Polymerization of these small molecules into macromolecules like proteins and nucleic acids.

3. Formation of protocells—membrane-bound structures that maintained an internal environment distinct from their surroundings.

4. Origin of self-replicating molecules (e.g., RNA) that enabled inheritance and evolution.

Miller-Urey Experiment

• Simulated early Earth conditions by exposing a mixture of gases to electrical sparks, resulting in the formation of amino acids and other organic molecules.

• Supported the hypothesis that organic molecules necessary for life could form under prebiotic conditions.

Importance of Organic Molecules

• Organic molecules, especially those based on carbon, are fundamental to the structure and function of living organisms.

• The diversity and complexity of organic molecules arise from the versatility of carbon bonding and the presence of functional groups.

Summary Table: Types of Chemical Bonds

Bond Type	Description	Example
Covalent	Atoms share electrons	H2O (water), CH4 (methane)
Ionic	Transfer of electrons creates ions	NaCl (sodium chloride)
Hydrogen	Weak attraction between polar molecules	Between water molecules

Key Concepts for Review

• Describe the structure of atoms and the significance of isotopes.

• Differentiate between covalent, ionic, and hydrogen bonds.

• Explain why water is essential for life.

• Discuss the unique properties of carbon and how they contribute to molecular diversity.

• Summarize the main hypotheses for the origin of life and the experimental evidence supporting them.

Additional info: Some explanations and examples have been expanded for clarity and completeness based on standard biology textbooks.