Chapter 2: Atoms, Molecules, and Life

Introduction to Chemistry in Biology

Chemistry is fundamental to biology because the interactions between atoms form the basis of biological organization. The atom is the smallest unit of matter that retains the physical and chemical properties of an element. Understanding atomic structure and chemical bonds is essential for grasping how biological molecules form and function.

• Atom: Smallest unit of matter retaining element properties.

• Subatomic particles: Protons, neutrons, and electrons.

Subatomic Particles

Atoms are composed of three main subatomic particles, each with distinct properties and locations within the atom.

The Atomic Nucleus and Elements

Atomic Number and Isotopes

The atomic nucleus contains protons and neutrons. The atomic number is the number of protons in an atom and defines the element. Isotopes are atoms of the same element with different numbers of neutrons.

• Atomic number: Number of protons in an atom.

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

Electron Shells

Electrons occupy shells around the nucleus. The further from the nucleus, the higher the energy level. The chemical behavior of an atom is determined by the number of electrons in its outermost shell.

• Atoms are arranged in electron shells based on their energy level.

• Atoms with incomplete outer shells are reactive.

• Atoms with full outer shells are stable.

Chemical Bonds

Types of Chemical Bonds

Chemical bonds join atoms to make molecules. The main types of chemical bonds are ionic, covalent, and hydrogen bonds.

• Ionic bond: Attraction between oppositely charged ions after electron transfer.

• Covalent bond: Atoms share electrons to fill their outer shells.

• Hydrogen bond: Weak attraction between polar molecules, especially involving hydrogen.

Formation of Ions

• Anion: Negatively charged atom or molecule.

• Cation: Positively charged atom or molecule.

Types of Covalent Bonds

• Single covalent bond: Shares one pair of electrons.

• Double covalent bond: Shares two pairs of electrons.

• Triple covalent bond: Shares three pairs of electrons.

Polar vs. Nonpolar Covalent Bonds

• Nonpolar covalent bond: Electrons are shared equally.

• Polar covalent bond: Electrons are shared unequally, resulting in charge disparity (one end positive, one end negative).

Hydrogen Bonds

• Form between polar molecules (e.g., water).

• Weaker than covalent bonds but crucial for biological systems.

• Help stabilize the three-dimensional shape of large molecules.

Water and Life

Properties of Water

Water is essential for life, making up 70-95% of cells and covering most of Earth's surface. Its unique properties arise from hydrogen bonding between molecules.

• Cohesion: Attraction between molecules of the same kind (water sticks together).

• Surface tension: Force that increases the ability of a liquid to stretch without breaking.

• Polarity: Water is a polar molecule, making it a versatile solvent.

Hydrophilic and Hydrophobic Substances

• Hydrophilic: Substances with affinity for water; dissolve easily (e.g., salts, sugars).

• Hydrophobic: Substances lacking affinity for water; do not dissolve easily (e.g., oils).

Thermal Properties of Water

• High specific heat: Water resists temperature changes.

• High heat of vaporization: Water absorbs large amounts of heat when evaporating.

Specific heat is the amount of energy required to raise the temperature of 1 gram of a substance by 1 degree Celsius.

Heat of vaporization is the energy required to convert 1 gram of liquid to gas.

Solutions and Concentrations

Solute Concentration and Molarity

Solutions are mixtures where solutes are dissolved in solvents. Concentration measures how much solute is present in a given volume.

• Solute concentration: Amount of substance in a given volume.

• Molarity (M): Moles of solute per liter of solution.

Mole and Molecular Weight

• Mole: Specific quantity, particles.

• Molecular weight: Mass of one mole of a molecule, calculated by summing atomic masses.

Acids, Bases, and pH

Acids and Bases

Water can dissociate into hydrogen ions (H+) and hydroxide ions (OH-). The concentration of these ions determines acidity or basicity.

• Acid: Increases H+ concentration in solution.

• Base: Reduces H+ concentration, often by increasing OH-.

The pH Scale

The pH scale measures the acidity of solutions, ranging from 0 (most acidic) to 14 (most basic).

Because the pH scale is logarithmic, a change of one pH unit represents a tenfold change in H+ concentration.

Buffers

Buffers help maintain a constant pH in biological systems by accepting or donating H+ ions as needed. They are combinations of H+ donors and acceptors.

• Example: Bicarbonate buffer system in blood.

Summary Table: Key Terms and Definitions

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