Chapter 2: Water and Carbon – The Chemical Basis of Life

Chemistry in Biology

Chemistry is intimately linked to the evolution and function of life. Understanding the chemical properties of atoms and molecules is essential for studying biological processes.

• Atoms: Smallest units of matter that retain the properties of an element.

• Elements: Substances that cannot be broken down into other substances by chemical means. Examples: Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N).

• Life is a series of chemical reactions: Biological processes depend on the transformation and interaction of molecules.

• Atoms consist of:

  • Protons: Positively charged particles in the nucleus.

  • Neutrons: Neutral particles in the nucleus.

  • Electrons: Negatively charged particles orbiting the nucleus.

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

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

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

Electron Shells and Chemical Behavior

The arrangement of electrons in shells around the nucleus determines how atoms interact and bond with each other.

• Electron Shells: Energy levels where electrons are found. The first shell holds up to 2 electrons, the second up to 8, and so on.

• Valence Electrons: Electrons in the outermost shell; determine chemical reactivity.

• Octet Rule: Atoms tend to gain, lose, or share electrons to achieve a full outer shell (usually 8 electrons).

• Example: Sodium (Na) has 11 electrons: 2 in the first shell, 8 in the second, and 1 in the third (valence shell).

Chemical Bonds

Atoms form chemical bonds to achieve stable electron configurations. The main types of bonds in biology are covalent, ionic, and hydrogen bonds.

• Covalent Bonds: Atoms share pairs of electrons. These are strong and common in biological molecules.

  • Single Covalent Bond: One pair of electrons shared (e.g., H–H in H2).

  • Double Covalent Bond: Two pairs of electrons shared (e.g., O=O in O2).

  • Polar Covalent Bond: Electrons are shared unequally, leading to partial charges (e.g., H2O).

  • Nonpolar Covalent Bond: Electrons are shared equally (e.g., H2, O2).

• Ionic Bonds: Formed when one atom donates an electron to another, resulting in oppositely charged ions that attract each other (e.g., NaCl).

• Hydrogen Bonds: Weak attractions between a hydrogen atom (covalently bonded to an electronegative atom like oxygen or nitrogen) and another electronegative atom. Important in water and biological molecules like DNA.

Properties of Water

Water is essential for life due to its unique chemical and physical properties, which arise from its molecular structure and hydrogen bonding.

• Polarity: Water is a polar molecule, with a partial negative charge near the oxygen atom and partial positive charges near the hydrogen atoms.

• Hydrogen Bonding: Each water molecule can form up to four hydrogen bonds with neighboring water molecules.

• Cohesion: Water molecules attract each other due to hydrogen bonding, resulting in surface tension.

• Adhesion: Water molecules stick to other polar or charged surfaces.

• High Specific Heat: Water can absorb or release large amounts of heat with little temperature change. This helps stabilize temperatures in organisms and environments.

• High Heat of Vaporization: Water requires a lot of energy to change from liquid to gas, which is important for cooling mechanisms like sweating.

• Density of Ice vs. Liquid Water: Ice is less dense than liquid water because hydrogen bonds hold water molecules in a lattice structure, causing ice to float.

Acids, Bases, and pH

The pH scale measures the concentration of hydrogen ions (H+) in a solution, indicating its acidity or basicity.

• Acid: Substance that increases the concentration of H+ ions in solution.

• Base: Substance that decreases the concentration of H+ ions (often by increasing OH–).

• pH Scale: Ranges from 0 (most acidic) to 14 (most basic), with 7 being neutral.

• Formula:

• Water Dissociation: Water can dissociate into hydronium (H3O+) and hydroxide (OH–) ions:

  • Or, simplified:

Summary Table: Types of Chemical Bonds

Additional info:

• These notes cover foundational chemistry concepts essential for understanding biological molecules and processes.

• Understanding the properties of water is crucial for topics such as enzyme function, cell structure, and metabolism.