4  Chemical Basis of Life (Optional)

Note: This chapter is included as optional foundational material for students needing chemistry review. Most introductory biology courses assume basic chemistry knowledge, but this chapter provides explicit connections between chemistry and biology.

4.1 Learning Objectives

By the end of this chapter, you should be able to:

  1. Identify the major elements found in living organisms and their biological roles
  2. Explain how molecular structure determines biological function
  3. Describe the properties of water that make it essential for life
  4. Distinguish between the four major classes of biological macromolecules
  5. Explain how weak chemical interactions contribute to biological structure and function
  6. Apply chemical principles to explain biological phenomena at the molecular level

4.2 Introduction

Biology is chemistry in action. The properties and interactions of atoms and molecules determine the structure and function of all living systems. This chapter reviews essential chemical concepts with a focus on their biological applications, establishing the molecular foundation for understanding cellular processes, metabolism, genetics, and evolution. While not exhaustive, this overview highlights the chemical principles most relevant to biological systems.

4.3 Elements of Life

4.3.1 Major Elements

Six elements constitute about 99% of living matter by weight:

Carbon (C): 18.5%

  • Forms four covalent bonds
  • Backbone of organic molecules
  • Can form chains, rings, and complex structures

Hydrogen (H): 9.5%

  • Forms single covalent bonds
  • Component of water and organic molecules
  • Participates in hydrogen bonding

Oxygen (O): 65%

  • Component of water and organic molecules
  • Highly electronegative
  • Final electron acceptor in aerobic respiration

Nitrogen (N): 3.3%

  • Component of proteins, nucleic acids
  • Forms three covalent bonds
  • Important in base pairing and protein structure

Phosphorus (P): 1.0%

  • Component of nucleic acids, ATP, phospholipids
  • Forms energy-rich bonds
  • Important in energy transfer and genetic material

Sulfur (S): 0.3%

  • Component of some amino acids (cysteine, methionine)
  • Forms disulfide bonds in proteins
  • Important in protein structure

4.3.2 Trace Elements

Essential in minute quantities for specific functions:

  • Calcium (Ca): Bone structure, muscle contraction, signaling
  • Potassium (K): Nerve function, osmotic balance
  • Sodium (Na): Nerve function, osmotic balance
  • Chlorine (Cl): Osmotic balance, digestion
  • Magnesium (Mg): Chlorophyll, enzyme cofactor
  • Iron (Fe): Hemoglobin, electron transport
  • Iodine (I): Thyroid hormones -Zinc (Zn): Enzyme cofactor, protein structure

4.4 Chemical Bonds and Interactions

4.4.1 Covalent Bonds

Definition: Sharing of electron pairs between atoms

Strength: 50-200 kcal/mol

Biological importance: Form backbone of organic molecules

Polar vs. Nonpolar Covalent Bonds:

  • Polar: Unequal electron sharing (O-H, N-H)
  • Nonpolar: Equal electron sharing (C-C, C-H)

4.4.2 Ionic Bonds

  • Definition: Transfer of electrons, creating charged ions that attract
  • Strength: 3-7 kcal/mol in water (weakened by hydration)
  • Biological importance: Mineral structure, membrane potentials

4.4.3 Hydrogen Bonds

  • Definition: Attraction between hydrogen atom (bound to electronegative atom) and another electronegative atom
  • Strength: 1-5 kcal/mol
  • Biological importance: DNA base pairing, protein folding, water properties

4.4.4 van der Waals Interactions

  • Definition: Weak attractions between transient dipoles
  • Strength: 0.5-1 kcal/mol
  • Biological importance: Protein folding, substrate binding, membrane structure

4.4.5 Hydrophobic Effect

  • Definition: Nonpolar molecules aggregating in water to minimize surface area
  • Not actually a bond: Entropy-driven process
  • Biological importance: Membrane formation, protein folding, enzyme-substrate interactions

Note: This optional chapter continues with sections on water properties, pH and buffers, organic chemistry basics, and the four macromolecule classes (carbohydrates, lipids, proteins, nucleic acids). Due to space constraints and the fact that most introductory biology textbooks include this material, the full chapter is not provided here, but the structure above shows how it would fit into the foundational concepts part.

End of Part I: Foundational Concepts

You have now established the fundamental principles that underlie all biological systems:

  1. What life is and how we recognize it
  2. How energy flows through living systems
  3. How information directs biological organization

These concepts will provide the framework for understanding specific biological systems and processes in the remaining parts of this book.