General Biology Study Notes: Chemical Context of Life & Molecular Diversity

Chapter 2 – The Chemical Context of Life

Structure of the Atom

The atom is the fundamental unit of matter, composed of a nucleus and electron shells. Understanding atomic structure is essential for grasping chemical properties and reactions.

• Subatomic Particles: Atoms consist of protons (positive charge), neutrons (neutral), and electrons (negative charge).

• Atomic Number: The number of protons in the nucleus; defines the element.

• Atomic Mass: The sum of protons and neutrons in the nucleus. $\text{Atomic Mass} = \text{Number of Protons} + \text{Number of Neutrons}$

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

• Example: Carbon-12 and Carbon-14 are isotopes of carbon.

Valence Electrons and Chemical Bonding

Valence electrons are the electrons in the outermost shell of an atom and play a crucial role in chemical bonding and reactivity.

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

• Location: Found in the highest energy level (outer shell) of an atom.

• Periodic Table: The group number often indicates the number of valence electrons for main-group elements.

• Importance: Atoms bond to achieve a full valence shell, often following the octet rule.

• Example: Oxygen has 6 valence electrons; needs 2 more to complete its shell.

Water Molecule Geometry and Hydrogen Bonding

The unique geometry of water molecules and the nature of their covalent bonds lead to hydrogen bonding, which is responsible for many of water’s properties.

• Covalent Bonds: Oxygen and hydrogen share electrons unequally, forming polar covalent bonds.

• Electron Distribution: Shared electrons spend more time near oxygen, making it partially negative and hydrogen partially positive.

• Hydrogen Bonds: The partial charges allow water molecules to attract each other via hydrogen bonds.

• Example: Water’s high boiling point is due to hydrogen bonding.

Emergent Properties of Water

Water exhibits several emergent properties due to hydrogen bonding, each vital for life.

• Cohesion: Water molecules stick together, aiding transport in plants.

• Adhesion: Water molecules stick to other surfaces, important for capillary action.

• High Specific Heat: Water resists temperature changes, stabilizing environments.

• Versatility as a Solvent: Water dissolves many substances, facilitating biochemical reactions.

• Example: Ice floats because solid water is less dense than liquid water.

pH, Acidity, and Alkalinity

The concentration of hydrogen ions ($[H^+]$) and hydroxide ions ($[OH^-]$) determines the pH of a solution, affecting its acidity or alkalinity.

• pH Scale: Ranges from 0 (acidic) to 14 (basic); $\text{pH} = -\log_{10}[H^+]$

• Acidic Solution: High $[H^+]$, low pH.

• Basic Solution: High $[OH^-]$, high pH.

• Example: Lemon juice is acidic; soap solution is basic.

Electronegativity and Bond Polarity

Electronegativity is an atom’s ability to attract electrons in a bond, influencing whether a bond is polar or nonpolar.

• Oxygen vs. Hydrogen: Oxygen is more electronegative than hydrogen, resulting in polar covalent bonds in water.

• Carbon vs. Hydrogen: Carbon is slightly more electronegative than hydrogen, but C-H bonds are considered nonpolar.

• Example: Water is polar; methane ($CH_4$) is nonpolar.

Chapter 3 – Carbon and the Molecular Diversity of Life

Atomic Structure and Bonding of Carbon

Carbon’s atomic structure allows it to form diverse and complex molecules essential for life.

• Valence Electrons: Carbon has 4 valence electrons.

• Bonding: Can form up to 4 covalent bonds, allowing for a variety of molecular shapes.

• Example: Carbon forms chains, rings, and branched structures in organic molecules.

Functional Groups and Hydrophilicity/Hydrophobicity

Functional groups determine the chemical properties and solubility of biological molecules.

• Hydroxyl Group (-OH): Hydrophilic

• Carbonyl Group (C=O): Hydrophilic

• Carboxyl Group (-COOH): Hydrophilic

• Amino Group (-NH2): Hydrophilic

• Sulfhydryl Group (-SH): Hydrophobic

• Phosphate Group (-PO4): Hydrophilic

• Methyl Group (-CH3): Hydrophobic

• Example: Amino acids contain both amino and carboxyl groups.

Classes of Biological Molecules

There are four main classes of biological macromolecules, each with distinct monomers, polymers, and linkages.

Monomer	Polymer	Type of linkage
Carbohydrates: Monosaccharides	Polysaccharides	Glycosidic bond
Lipids: Fatty acids & glycerol	Triglycerides, phospholipids	Ester bond
Proteins: Amino acids	Polypeptides	Peptide bond
Nucleic Acids: Nucleotides	DNA, RNA	Phosphodiester bond

Polymerization and Depolymerization

Biological macromolecules are assembled and disassembled by specific chemical reactions.

• Dehydration Synthesis: Joins monomers by removing water.

• Hydrolysis: Breaks polymers into monomers by adding water.

• Example: Digestion of starch into glucose monomers.

Carbohydrate Structures and Functions

Carbohydrates serve as energy sources and structural components in cells.

• Cellulose: Structural polysaccharide in plants; monomer is glucose; linkage is β-1,4 glycosidic bond.

• Chitin: Structural polysaccharide in fungi and arthropods; monomer is N-acetylglucosamine.

• Starch: Energy storage polysaccharide in plants; monomer is glucose; linkage is α-1,4 glycosidic bond.

• Comparison: Cellulose and chitin are structural; starch is for energy storage.

Lipid Structures and Properties

Lipids are diverse molecules with roles in energy storage, membrane structure, and signaling.

• Fats: Composed of glycerol and fatty acids; used for energy storage.

• Phospholipids: Major component of cell membranes; amphipathic (hydrophilic head, hydrophobic tails).

• Steroids: Four fused rings; includes cholesterol and hormones.

• Cholesterol: Classified as a steroid; important for membrane fluidity.

• Saturated vs. Unsaturated: Saturated fats have no double bonds; unsaturated fats have one or more double bonds, affecting membrane fluidity.

Protein Structure and Levels of Organization

Proteins have four levels of structure, each stabilized by specific molecular forces.

• Primary Structure: Sequence of amino acids; stabilized by peptide bonds.

• Secondary Structure: Local folding (α-helix, β-sheet); stabilized by hydrogen bonds.

• Tertiary Structure: Overall 3D shape; stabilized by hydrophobic interactions, ionic bonds, hydrogen bonds, and disulfide bridges.

• Quaternary Structure: Association of multiple polypeptides; stabilized by similar forces as tertiary structure.

• Example: Hemoglobin has quaternary structure.

DNA and RNA: Structure and Function

DNA and RNA are nucleic acids with distinct structures and functions in cells.

• Differences:

  1. DNA contains deoxyribose; RNA contains ribose.

  2. DNA is double-stranded; RNA is single-stranded.

  3. DNA uses thymine; RNA uses uracil.

• Polymerization: Nucleotides are joined by phosphodiester bonds.

• Functions: DNA stores genetic information; RNA is involved in protein synthesis and gene regulation.

• Example: mRNA carries genetic code from DNA to ribosomes.

Additional info:

• Reviewing homework, quizzes, and class notes is essential for mastering these topics.

• Practice application and synthesis of concepts for exam success.