The Chemical Context of Life: Foundations for General Biology

The Chemical Context of Life

Introduction

Understanding the chemical basis of life is essential for studying biology. All living organisms are composed of matter, which in turn is made up of chemical elements and compounds. The properties and interactions of these substances underpin all biological processes.

Levels of Biological Organization

From Atoms to Organisms

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

• Atoms combine to form small molecules (e.g., water, carbon dioxide) and large molecules (e.g., proteins, DNA).

• Molecules make up cells, which are organized into tissues, organs, organ systems, and ultimately, organisms.

• Example: A cat is an organism composed of organ systems, which are made of tissues, cells, and ultimately atoms.

Matter and Chemical Elements

Elements and Compounds

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

• Compound: A substance consisting of two or more different elements combined in a fixed ratio (e.g., H2O, CO2).

• Living organisms are primarily composed of four essential elements: Oxygen (O), Carbon (C), Hydrogen (H), and Nitrogen (N), which make up about 96% of living matter.

Atomic Structure

Subatomic Particles

• Atom: The smallest unit of an element, retaining its chemical properties.

• Composed of subatomic particles:

  • Proton: Positively charged (+), located in the nucleus.

  • Neutron: No charge (neutral), located in the nucleus.

  • Electron: Negatively charged (−), found in electron shells surrounding the nucleus.

• Most atoms are neutral because they have equal numbers of protons and electrons.

Atomic Mass and Number

• Dalton: The unit of atomic mass, approximately equal to the mass of one proton or neutron ($1.7 \times 10^{-24}$ grams).

• Atomic number: Number of protons in an atom; identifies the element.

• Mass number: Total number of protons and neutrons in an atom.

• Atomic mass: The total mass of an atom, roughly equal to the mass number.

• Number of neutrons: $\text{Mass number} - \text{Atomic number}$

• Example (Sodium):

  • Atomic number = 11 (protons)

  • Mass number = 23 (protons + neutrons)

  • Number of neutrons = 23 - 11 = 12

  • Atomic mass ≈ 23 daltons

Isotopes

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

• Radioactive isotopes: Unstable isotopes that decay spontaneously, emitting radiation.

• Applications:

  • Medical: Diagnosing and treating diseases (e.g., cancer therapy).

  • Research: Tracing chemical pathways in organisms or the environment.

  • Dating: Carbon-14 dating for determining the age of fossils.

• Example (Carbon Isotopes):

  • Carbon-12: 6 protons, 6 neutrons

  • Carbon-13: 6 protons, 7 neutrons

  • Carbon-14: 6 protons, 8 neutrons (radioactive)

Isotope	Protons	Neutrons	Stability
Carbon-12	6	6	Stable
Carbon-13	6	7	Stable
Carbon-14	6	8	Radioactive

Additional info: Isotopes of an element have nearly identical chemical properties but may differ in physical properties due to mass differences.