Chapter 2: The Chemical Context of Life

2.1 Matter consists of chemical elements in pure form and in combinations called compounds

This section introduces the basic chemical principles underlying biological systems, focusing on matter, elements, and compounds.

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

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

• Compound: A substance consisting of two or more elements combined in a fixed ratio. Example: Water (H2O).

• Key Fact: Four elements—carbon, hydrogen, oxygen, and nitrogen—make up about 96% of living matter.

2.2 The structure of an atom determines its properties

Atoms are the smallest units of elements that retain their properties. The arrangement of subatomic particles within atoms determines their chemical behavior.

• Subatomic Particles:

  • Proton: Positively charged particle found in the nucleus.

  • Neutron: Neutral particle found in the nucleus.

  • Electron: Negatively charged particle orbiting the nucleus.

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

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

• Isotope: Atoms of the same element with different numbers of neutrons. Example: Carbon-12 and Carbon-14.

• Radioactive Isotope: An isotope with an unstable nucleus that decays, emitting radiation. Used in medical imaging (e.g., PET scans).

• Electron Shells: Electrons are arranged in shells around the nucleus; the outermost shell determines chemical reactivity.

• Valence Electrons: Electrons in the outermost shell; involved in chemical bonding.

• Energy: The capacity to cause change; electrons have potential energy based on their position relative to the nucleus.

2.3 Chemical bonds: The formation and function of molecules and ionic compounds

Chemical bonds form when atoms interact to achieve stable electron configurations. The type of bond affects molecular structure and function.

• Molecule: Two or more atoms held together by covalent bonds.

• Types of Chemical Bonds:

  • Covalent Bond: Atoms share pairs of electrons. Can be single, double, or triple bonds.

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

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

  • Ionic Bond: One atom donates an electron to another, forming oppositely charged ions (e.g., NaCl).

  • Hydrogen Bond: Weak attraction between a hydrogen atom (covalently bonded to an electronegative atom) and another electronegative atom.

  • van der Waals Interactions: Weak attractions due to transient local partial charges; important in large molecules and biological surfaces.

• Anion: Negatively charged ion (gains electrons).

• Cation: Positively charged ion (loses electrons).

• Bond Strength (from strongest to weakest): Covalent > Ionic > Hydrogen > van der Waals.

2.4 Chemical reactions make and break chemical bonds

Chemical reactions involve the making and breaking of bonds, resulting in the formation of new substances.

• Chemical Reaction: The process by which reactants are transformed into products.

• Example: Photosynthesis: $6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$

Chapter 3: Water and Life

3.1 Polar covalent bonds in water molecules result in hydrogen bonding

Water's unique properties arise from its molecular structure and hydrogen bonding.

• Polarity: Water is a polar molecule; oxygen is more electronegative than hydrogen, creating partial charges.

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

3.2 Four emergent properties of water contribute to Earth's suitability for life

Hydrogen bonding gives water several unique properties essential for life.

• Cohesion: Water molecules stick together due to hydrogen bonding; important for transport in plants.

• Adhesion: Water molecules stick to other substances; helps water move against gravity in plants.

• Surface Tension: Cohesion at the surface allows insects to walk on water.

• Specific Heat: Water has a high specific heat, meaning it resists temperature changes. This moderates Earth's climate and stabilizes temperatures in organisms.

• Heat of Vaporization: Water requires a lot of energy to evaporate, which helps cool organisms (e.g., sweating).

• Ice Floats: Solid water (ice) is less dense than liquid water, so it floats. This insulates bodies of water and protects aquatic life in winter.

Chapter 4: Carbon and the Molecular Diversity of Life

4.2 Carbon atoms can form diverse molecules by bonding to four other atoms

Carbon's ability to form four covalent bonds allows for a diversity of stable organic molecules.

• Electron Configuration: Carbon has four valence electrons, allowing it to form four covalent bonds.

• Hydrocarbons: Molecules consisting only of carbon and hydrogen; hydrophobic and store energy.

• Isomers: Compounds with the same molecular formula but different structures and properties.

  • Structural Isomers: Differ in covalent arrangement of atoms.

  • Cis-trans Isomers: Differ in spatial arrangement around double bonds.

  • Enantiomers: Mirror images of each other; important in pharmacology.

4.3 A few chemical groups are key to molecular function

Certain chemical groups attached to carbon skeletons are critical for the function of biological molecules.

• Functional Group: A specific group of atoms within a molecule responsible for characteristic chemical reactions.

• Key Functional Groups:

  • Hydroxyl (–OH): Alcohols; polar, forms hydrogen bonds.

  • Carbonyl (C=O): Aldehydes and ketones; found in sugars.

  • Carboxyl (–COOH): Acts as an acid; found in amino acids and fatty acids.

  • Amino (–NH2): Acts as a base; found in amino acids.

  • Sulfhydryl (–SH): Forms disulfide bonds in proteins.

  • Phosphate (–OPO32–): Involved in energy transfer (e.g., ATP).

  • Methyl (–CH3): Affects gene expression.

• Example: The difference between estradiol and testosterone is due to different functional groups, resulting in different biological effects.

• ATP (Adenosine Triphosphate): A molecule that stores and releases energy for cellular work. The reaction: $ATP \rightarrow ADP + P_i + \text{energy}$

Additional info: These notes synthesize and expand upon the provided study guide, ensuring all key concepts are explained with definitions, examples, and relevant tables for General Biology students.