The Chemistry of Living Things

Introduction

The chemistry of living things forms the foundation for understanding biological processes. This section explores the basic chemical principles essential for life, including the nature of matter, atomic structure, chemical bonds, and the molecules that compose living organisms.

Matter and Elements

Definition of Matter and Elements

• Matter: Anything that has mass and occupies space. All living and non-living things are composed of matter.

• Element: A fundamental (pure) form of matter that cannot be broken down into a simpler substance by ordinary chemical means. Each element is made up of only one kind of atom.

• The Periodic Table of Elements lists all known elements in order of increasing atomic number.

Energy

• Energy: The capacity to do work. In biological systems, energy is required for growth, movement, and cellular processes.

Atoms: Structure and Properties

Atomic Structure

• Atoms are the smallest functional units of an element.

• Atoms consist of a nucleus (central core) and shells (surrounding the nucleus).

• The nucleus contains:

  • Protons: Positively charged particles with mass.

  • Neutrons: Uncharged particles with mass.

• Shells contain:

  • Electrons: Negatively charged particles with negligible mass.

Atomic Number, Mass, and Symbols

• Atomic symbol: One or two letters representing an element (e.g., Na for sodium, O for oxygen).

• Atomic number: The number of protons in an atom; unique for each element.

• Atomic mass: Approximately equal to the sum of protons and neutrons.

• In a neutral atom: number of protons = number of electrons.

Isotopes

• Isotopes: Atoms of the same element with different numbers of neutrons, resulting in different atomic masses.

• Radioisotopes: Unstable isotopes that emit energy (radiation) as they decay. Used in dating ancient materials, diagnostic imaging, and cancer treatment.

Table: Comparison of Subatomic Particles

Chemical Bonds and Molecules

Types of Chemical Bonds

• Covalent bonds: Atoms share electrons to fill their outermost shells. These are strong bonds.

  • Nonpolar covalent bonds: Electrons are shared equally (e.g., H2, O2).

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

• Ionic bonds: Formed between oppositely charged ions (atoms that have gained or lost electrons). Example: NaCl (sodium chloride).

• Hydrogen bonds: Weak attractions between the slightly positive hydrogen atom of one polar molecule and the slightly negative atom of another. Important in water and biological molecules.

Table: Summary of Chemical Bonds

Water: The Essential Biological Molecule

Properties of Water

• Excellent solvent for polar and ionic substances.

• Liquid at body temperature, facilitating transport and chemical reactions.

• High heat capacity: absorbs and holds heat energy, helping regulate body temperature.

• Participates in essential chemical reactions (e.g., hydrolysis and dehydration synthesis).

Hydrophilic vs. Hydrophobic Substances

• Hydrophilic: Polar molecules that dissolve easily in water.

• Hydrophobic: Nonpolar molecules that do not dissolve in water.

pH and Buffers

• pH scale: Measures hydrogen ion concentration in solution, ranging from 0 (acidic) to 14 (basic). Neutral pH is 7.

• Acids: Donate hydrogen ions (H+), increasing [H+].

• Bases: Accept hydrogen ions, decreasing [H+].

• Buffers: Minimize changes in pH by accepting or donating H+ as needed. Example: carbonic acid-bicarbonate buffer system in blood.

Equation:

Organic Molecules in Living Organisms

Overview

• Organic molecules contain carbon and hydrogen, often with oxygen, nitrogen, phosphorus, or sulfur.

• Four major classes: carbohydrates, lipids, proteins, and nucleic acids.

Carbohydrates

• General formula:

• Functions: Energy source and structural support.

• Monosaccharides: Simple sugars (e.g., glucose, fructose, ribose).

• Disaccharides: Two monosaccharides joined (e.g., sucrose, lactose).

• Polysaccharides: Long chains for energy storage (starch in plants, glycogen in animals) and structure (cellulose in plants).

Lipids

• Hydrophobic molecules, insoluble in water.

• Classes:

  • Triglycerides: Energy storage (fats and oils).

  • Phospholipids: Main component of cell membranes; have hydrophilic heads and hydrophobic tails.

  • Steroids: Four-ring structures (e.g., cholesterol, hormones).

Proteins

• Polymers of amino acids (20 types).

• Structure:

  • Primary: Amino acid sequence.

  • Secondary: Alpha helices and beta sheets (hydrogen bonds).

  • Tertiary: 3D folding (various bonds and interactions).

  • Quaternary: Multiple polypeptide chains.

• Functions: Enzymes, structural support, transport, signaling.

• Denaturation: Loss of structure (and function) due to heat or pH changes.

Enzymes

• Proteins that act as biological catalysts, speeding up chemical reactions without being consumed.

• Enzyme activity depends on shape, which is sensitive to temperature, pH, and inhibitors.

Nucleic Acids

• Polymers of nucleotides (monomers with a sugar, phosphate, and nitrogenous base).

• DNA: Double-stranded, contains deoxyribose, bases A, T, G, C. Stores genetic information.

• RNA: Single-stranded, contains ribose, bases A, U, G, C. Involved in protein synthesis.

• Information flow: DNA → RNA → Protein

Adenosine Triphosphate (ATP)

• ATP is the universal energy currency of the cell.

• Structure: Adenine base, ribose sugar, three phosphate groups.

• Energy is released when ATP is converted to ADP and inorganic phosphate:

• ATP is regenerated from ADP using energy from cellular respiration.

Summary Table: Major Classes of Biological Molecules

Additional info: Some details, such as the specific structure of amino acids and the mechanisms of dehydration synthesis and hydrolysis, were expanded for clarity and completeness.