The Chemical Context of Life

Introduction

Understanding the chemical basis of life is fundamental to biology. Living organisms are composed of matter, which is made up of elements and compounds. The properties and interactions of these chemical substances underpin all biological processes.

A Chemical Connection to Biology

Biology and Chemistry

• Biology is the study of life, and all living things are composed of matter.

• Organisms and their environments are subject to the basic laws of physics and chemistry.

• Biological processes, such as growth and metabolism, depend on chemical reactions.

• Example: The use of formic acid by ants for protection and communication demonstrates the chemical basis of biological functions.

Matter, Elements, and Compounds

Definitions and Properties

• Matter is anything that takes up space and has mass.

• Element: A substance that cannot be broken down into other substances by chemical reactions.

• Compound: A substance consisting of two or more elements combined in a fixed ratio.

• Compounds have characteristics different from those of their constituent elements (emergent properties).

• Example: Sodium (Na) and chlorine (Cl) combine to form sodium chloride (NaCl), which has properties distinct from either element.

Table: Comparison of Matter, Elements, and Compounds

The Elements of Life

Essential and Trace Elements

• About 20-25% of the 92 natural elements are essential for life.

• Essential elements: Required by an organism for healthy life (e.g., C, H, O, N, P, S).

• Trace elements: Required in minute quantities (e.g., iron, iodine).

• Most living matter is composed of four elements: carbon, hydrogen, oxygen, and nitrogen.

Table: Major Elements in the Human Body

Atomic Structure and Properties

Structure of Atoms

• Atoms are composed of protons (positive charge), neutrons (no charge), and electrons (negative charge).

• Protons and neutrons form the atomic nucleus; electrons form a "cloud" around the nucleus.

• Atomic mass is measured in daltons.

Atomic Number and Atomic Mass

• Atomic number (Z): Number of protons in the nucleus.

• Mass number (A): Sum of protons and neutrons.

• Atomic mass: The atom's total mass, approximately equal to the mass number.

• Isotopes: Atoms of the same element with different numbers of neutrons.

Electron Distribution and Chemical Properties

• The chemical behavior of an atom is determined by the distribution of electrons in electron shells.

• Valence electrons (in the outermost shell) are most important for chemical bonding.

• Elements with a full valence shell are chemically inert (e.g., noble gases like neon).

Chemical Bonds and Interactions

Types of Chemical Bonds

• Covalent bonds: Atoms share pairs of electrons; can be single, double, or triple bonds.

• Ionic bonds: Electrons are transferred from one atom to another, resulting in oppositely charged ions that attract each other.

• Hydrogen bonds: Weak attractions between a hydrogen atom covalently bonded to an electronegative atom and another electronegative atom.

• Van der Waals interactions: Weak attractions due to transient local partial charges.

Covalent Bonds

• Strongest type of bond in biological molecules.

• Can be represented by structural formulas (e.g., H–H for H2).

• Single covalent bond: One pair of shared electrons.

• Double covalent bond: Two pairs of shared electrons.

Electronegativity and Polarity

• Electronegativity: An atom's attraction for shared electrons in a covalent bond.

• If atoms have similar electronegativity, the bond is nonpolar (electrons shared equally).

• If one atom is more electronegative, the bond is polar (electrons shared unequally), resulting in partial charges.

• Example: Water (H2O) is a polar molecule.

Ionic Bonds

• Formed when one atom donates an electron to another, creating ions.

• Cation: Positively charged ion.

• Anion: Negatively charged ion.

• Ionic compounds (salts) often form crystals (e.g., NaCl).

Weak Chemical Interactions

• Weak bonds (hydrogen bonds, van der Waals interactions) are important for the structure and function of large biological molecules.

• These interactions are reversible, allowing for dynamic biological processes.

Hydrogen Bonds

• Form when a hydrogen atom covalently bonded to an electronegative atom is attracted to another electronegative atom.

• Common in water and in the structure of DNA and proteins.

Van der Waals Interactions

• Occur when electrons are distributed asymmetrically, creating temporary "hot spots" of charge.

• Important for interactions between molecules, such as the adhesion of gecko feet to surfaces.

Molecular Shape and Function

Importance of Molecular Shape

• The shape of a molecule is determined by the positions of its atoms' orbitals.

• Molecular shape is crucial for function, especially in biological molecules like enzymes and DNA.

• Shape determines how molecules recognize and respond to one another.

• Example: The double helix structure of DNA is stabilized by hydrogen bonds between base pairs.

Table: Types of Bonds in DNA

Examples of Molecular Shape

• Natural endorphins and morphine have similar shapes, allowing both to bind to the same brain receptors.

• Shape complementarity is essential for molecular recognition in biological systems.

Summary

• Life depends on the unique properties of elements and compounds.

• Atomic structure determines chemical properties and bonding behavior.

• Chemical bonds (covalent, ionic, hydrogen, van der Waals) are essential for the structure and function of biological molecules.

• Molecular shape is key to biological function and interaction.