The Chemical Context of Life

Introduction

Understanding biology requires a strong foundation in chemistry and physics, as living organisms are composed of matter and governed by chemical principles. This section explores the chemical basis of life, focusing on the elements, atomic structure, chemical bonds, and reactions essential for biological processes.

Biology as a Multidisciplinary Science

Integration of Sciences

• Biology is deeply interconnected with chemistry, physics, and mathematics.

• Understanding biological phenomena often requires knowledge of chemical reactions, physical laws, and mathematical models.

• For example, the structure of DNA (biology) is explained by chemical bonding (chemistry) and molecular interactions (physics).

What Are Living Things Made Of?

Matter, Elements, and Compounds

• Matter: Anything that takes up space and has mass. All organisms are composed of matter.

• Element: A substance that cannot be broken down to other substances by chemical reactions (e.g., oxygen, carbon).

• Compound: A substance consisting of two or more elements in a fixed ratio (e.g., water, sodium chloride).

• Example: Sodium (Na) and chlorine (Cl) are elements; when combined, they form sodium chloride (NaCl), a compound essential for life.

Elements of Life

Major Elements in Biological Systems

• About 96% of living matter is composed of four elements: oxygen (O), carbon (C), hydrogen (H), and nitrogen (N).

• Other essential elements include calcium (Ca), phosphorus (P), potassium (K), sulfur (S), sodium (Na), chlorine (Cl), and magnesium (Mg).

• Trace elements (such as iron, zinc, and iodine) are required in minute amounts but are vital for proper biological function.

Atomic Structure and Properties

Atoms and Subatomic Particles

• Atom: The smallest unit of an element that retains its properties.

• Composed of a nucleus (containing protons and neutrons) and a cloud of electrons.

• Proton: Positively charged particle in the nucleus.

• Neutron: Electrically neutral particle in the nucleus.

• Electron: Negatively charged particle orbiting the nucleus.

Atomic Number and Mass Number

• Atomic number: Number of protons in the nucleus; defines the element.

• Mass number: Sum of protons and neutrons in the nucleus.

• Atomic mass: The atom's total mass, approximately equal to the mass number (measured in daltons).

Isotopes

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

• Radioactive isotopes: Unstable isotopes that decay spontaneously, emitting particles and energy.

• Applications: Used in medical imaging, dating fossils, and as tracers in biochemical research.

Electron Distribution and Chemical Behavior

Electron Shells and Valence Electrons

• Electrons are arranged in energy shells around the nucleus.

• Valence electrons: Electrons in the outermost shell; determine the chemical behavior of an atom.

• Atoms with incomplete valence shells tend to form chemical bonds to achieve stability.

Chemical Bonds

Covalent Bonds

• Covalent bond: Formed when two atoms share one or more pairs of valence electrons.

• Single bond: Sharing of one pair of electrons (e.g., H–H).

• Double bond: Sharing of two pairs of electrons (e.g., O=O).

• Molecule: Two or more atoms held together by covalent bonds.

Polarity of Covalent Bonds

• Electronegativity: An atom's attraction for shared electrons.

• Nonpolar covalent bond: Electrons are shared equally (e.g., O2).

• Polar covalent bond: Electrons are shared unequally, resulting in partial charges (e.g., H2O).

Ionic Bonds

• Ionic bond: Formed when one atom transfers an electron to another, creating oppositely charged ions.

• Cation: Positively charged ion (e.g., Na+).

• Anion: Negatively charged ion (e.g., Cl−).

• Ionic compound (salt): Compound formed by ionic bonds (e.g., NaCl).

Hydrogen Bonds

• Hydrogen bond: Weak bond formed when a hydrogen atom covalently bonded to one electronegative atom is attracted to another electronegative atom (usually O or N).

• Hydrogen bonds are crucial for the structure of water, proteins, and DNA.

Molecular Shape and Function

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

• Molecular shape is critical for biological function (e.g., enzyme-substrate specificity).

Chemical Reactions

Making and Breaking Bonds

• Chemical reaction: The making and breaking of chemical bonds, leading to changes in the composition of matter.

• Reactants: Starting materials in a chemical reaction.

• Products: Final materials produced by the reaction.

• All chemical reactions are, in principle, reversible.

• Chemical equilibrium: The point at which the rate of the forward reaction equals the rate of the reverse reaction.

Examples of Important Biological Reactions

• Photosynthesis:

• Cellular Respiration:

Additional info: The periodic table image highlights the main elements of life (C, H, O, N), which are the backbone of organic molecules. The sodium and chlorine example demonstrates how elements combine to form compounds with new properties, essential for understanding the chemical basis of biological systems.