General Biology: Foundations of Chemistry, Macromolecules, and Cell Structure

Valence, Bonding, and Chemical Properties

Valence and Electron Sharing

Valence refers to the bonding capacity of an atom, determined by the number of electrons needed to fill its outer shell. This property is crucial for understanding how atoms interact to form molecules.

• Valence Electrons: Electrons in the outermost shell that participate in bonding.

• Hydrogen: 1 valence electron; forms 1 bond.

• Oxygen: 6 valence electrons; forms 2 bonds.

• Nitrogen: 5 valence electrons; forms 3 bonds.

• Carbon: 4 valence electrons; forms 4 bonds.

Example: Carbon forms four covalent bonds, allowing it to serve as the backbone of organic molecules.

Chemical Bonds and Molecular Polarity

Ionic and Covalent Bonds

Atoms bond to achieve stable electron configurations. The main types of bonds are ionic and covalent.

• Ionic Bonds: Formed when electrons are transferred from one atom to another (e.g., NaCl).

• Covalent Bonds: Formed when atoms share electrons.

• Polar Covalent Bonds: Unequal sharing of electrons, resulting in partial charges (e.g., H2O).

• Nonpolar Covalent Bonds: Equal sharing of electrons (e.g., O2).

Example: In a water molecule, oxygen is more electronegative than hydrogen, creating a polar covalent bond.

Hydrogen Bonds

Hydrogen bonds are weak attractions between a hydrogen atom covalently bonded to an electronegative atom (like O or N) and another electronegative atom.

• Importance: Stabilize the structure of proteins and nucleic acids, and give water its unique properties.

Example: Hydrogen bonds between water molecules contribute to high surface tension and boiling point.

Water and Life

Hydrophilic and Hydrophobic Substances

Substances interact with water based on their polarity.

• Hydrophilic: "Water-loving"; substances that dissolve in water (e.g., salts, sugars).

• Hydrophobic: "Water-fearing"; substances that do not dissolve in water (e.g., oils, fats).

Example: Cell membranes are composed of phospholipids with hydrophilic heads and hydrophobic tails.

Macromolecules: Structure and Function

Carbohydrates

Carbohydrates are energy sources and structural components. They are classified by the number of sugar units.

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

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

• Polysaccharides: Long chains of monosaccharides (e.g., starch, cellulose).

Example: Cellulose provides structural support in plant cell walls.

Lipids

Lipids are hydrophobic molecules, including fats, phospholipids, and steroids.

• 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; include cholesterol and hormones.

Example: Unsaturated fats have double bonds, making them liquid at room temperature.

Proteins

Proteins are polymers of amino acids, joined by peptide bonds. They perform a wide range of functions, including catalysis, structure, and transport.

• Primary Structure: Sequence of amino acids.

• Secondary Structure: Alpha helices and beta sheets formed by hydrogen bonding.

• Tertiary Structure: 3D folding due to side chain interactions.

• Quaternary Structure: Association of multiple polypeptide chains (not found in all proteins).

Example: Hemoglobin is a quaternary protein that carries oxygen in blood.

Nucleic Acids

Nucleic acids (DNA and RNA) store and transmit genetic information.

• DNA: Double helix; stores genetic code.

• RNA: Single-stranded; involved in protein synthesis.

• Nucleotides: Monomers composed of a sugar, phosphate group, and nitrogenous base.

Example: The percentage of adenine (A) equals thymine (T) in double-stranded DNA (Chargaff's rule).

Cell Structure and Function

Prokaryotic vs. Eukaryotic Cells

Cells are classified as prokaryotic (bacteria, archaea) or eukaryotic (plants, animals, fungi, protists).

• Prokaryotic Cells: Lack a nucleus; DNA is in the nucleoid region.

• Eukaryotic Cells: Have a nucleus and membrane-bound organelles.

Example: Both cell types have ribosomes, plasma membrane, and cytoplasm.

Cell Organelles and Their Functions

Organelle	Function
Nucleus	Stores genetic material (DNA)
Ribosomes	Protein synthesis
Endoplasmic Reticulum (ER)	Protein and lipid synthesis
Golgi Apparatus	Modifies, sorts, and packages proteins/lipids
Lysosomes	Digestion of macromolecules
Mitochondria	ATP production (cellular respiration)
Chloroplasts	Photosynthesis (plants/algae)
Plasma Membrane	Selective barrier; regulates entry/exit

Example: The rough ER is studded with ribosomes and synthesizes proteins for secretion.

Cytoskeleton

The cytoskeleton provides structural support, cell shape, and facilitates movement.

• Microtubules: Hollow rods; shape and support the cell.

• Microfilaments: Thin rods; involved in cell movement and shape.

• Intermediate Filaments: Provide mechanical support.

Example: Cilia and flagella are composed of microtubules.

Biological Membranes and Transport

Membrane Structure

Biological membranes are composed of a phospholipid bilayer with embedded proteins.

• Amphipathic Nature: Hydrophilic heads face outward; hydrophobic tails face inward.

• Fluid Mosaic Model: Membranes are dynamic and proteins move laterally within the bilayer.

Example: Cholesterol modulates membrane fluidity in animal cells.

Transport Across Membranes

• Passive Transport: Diffusion and facilitated diffusion (no energy required).

• Active Transport: Movement against a concentration gradient (requires energy, e.g., Na+/K+ pump).

Example: Aquaporins facilitate water movement across membranes.

Summary Table: Types of Bonds in Biology

Bond Type	Strength	Example
Covalent	Strong	Peptide bonds in proteins
Ionic	Moderate	NaCl (table salt)
Hydrogen	Weak	Between water molecules
Van der Waals	Very weak	Lipid interactions

Additional info:

• Some explanations and examples were expanded for clarity and completeness.

• Tables were inferred and constructed to summarize key concepts.