Key Concepts in the Chemical Context of Life, Water and Life, Carbon Chemistry, Biological Macromolecules, and Cell Structure

Chapter 2: The Chemical Context of Life

Elements, Compounds, and Atoms

Understanding the chemical basis of life begins with the study of elements and compounds, which are the building blocks of matter.

• Element: A substance that cannot be broken down into other substances by chemical means (e.g., Oxygen, Carbon).

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

• Essential elements: Elements required for an organism to survive, grow, and reproduce. About 20-25% of the 92 natural elements are essential for life.

• Trace elements: Elements required by an organism in only minute quantities (e.g., Iron, Iodine).

• Atom: The smallest unit of matter that retains the properties of an element.

• Subatomic particles: Atoms are composed of protons (positive charge), neutrons (no charge), and electrons (negative charge).

Elements making up ~96% of living matter: Carbon (C), Hydrogen (H), Oxygen (O), and Nitrogen (N).

Atomic Structure and Properties

• Atomic number: Number of protons in the nucleus of an atom.

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

• Valence electron: Electron in the outermost shell, important for chemical bonding.

• Valence shell: The outermost electron shell of an atom.

Isotopes and Radioactivity

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

• Radioactive isotopes: Isotopes that decay spontaneously, giving off particles and energy.

Chemical Bonds

• Covalent bond: A chemical bond formed by the sharing of a pair of valence electrons between atoms.

• Single bond: Sharing of one pair of electrons ($A - B$).

• Double bond: Sharing of two pairs of electrons ($A = B$).

• Electronegativity: The attraction of a particular atom for the electrons of a covalent bond.

• Polar covalent bond: Electrons are shared unequally due to differences in electronegativity (e.g., in H2O).

• Nonpolar covalent bond: Electrons are shared equally (e.g., in O2).

• Ionic bond: Formed when one atom transfers an electron to another, resulting in oppositely charged ions that attract each other.

• Ion: A charged atom or molecule.

• Cation: Positively charged ion.

• Anion: Negatively charged ion.

• Hydrogen bond: Weak bond between a hydrogen atom covalently bonded to one electronegative atom and another electronegative atom.

• van der Waals interaction: Weak attractions between molecules or parts of molecules that result from transient local partial charges.

Chemical Reactions and Equilibrium

• Chemical equilibrium: The point at which the forward and reverse reactions offset each other exactly.

• Reactants: Starting materials in a chemical reaction.

• Products: Ending materials in a chemical reaction.

Chapter 3: Water and Life

Properties of Water

Water's unique properties are essential for life and are largely due to its ability to form hydrogen bonds.

• Cohesion: The linking together of like molecules, often by hydrogen bonds.

• Surface tension: A measure of how difficult it is to stretch or break the surface of a liquid.

• Adhesion: The clinging of one substance to another.

Temperature Moderation

• Specific heat: The amount of heat that must be absorbed or lost for 1 g of a substance to change its temperature by 1°C.

• Heat of vaporization: The quantity of heat a liquid must absorb for 1 g of it to be converted from the liquid to the gaseous state.

• Evaporative cooling: The process by which the surface of an object becomes cooler during evaporation.

Solutions and Solubility

• Solution: A liquid that is a homogeneous mixture of two or more substances.

• Solvent: The dissolving agent of a solution (water is the most versatile solvent).

• Solute: The substance that is dissolved.

• Aqueous solution: A solution in which water is the solvent.

• Hydrophilic: Substances that have an affinity for water.

• Hydrophobic: Substances that do not have an affinity for water.

Acids, Bases, and pH

• Hydrogen ion (H+): A single proton with a charge of +1.

• Hydroxide ion (OH-): A water molecule that has lost a proton.

• Hydronium ion (H3O+): A water molecule with an extra proton.

• Acid: A substance that increases the hydrogen ion concentration of a solution.

• Base: A substance that reduces the hydrogen ion concentration of a solution.

• pH: A measure of hydrogen ion concentration, defined as $pH = -\log_{10}[H^+]$

• Buffer: A substance that minimizes changes in the concentrations of H+ and OH- in a solution.

pH Scale: Ranges from 0 (most acidic) to 14 (most basic). Each unit represents a tenfold difference in H+ concentration.

Buffer action: Buffers work by accepting H+ ions when they are in excess and donating H+ ions when they are depleted.

Chapter 4: Carbon and the Molecular Diversity of Life

Organic Compounds and Carbon

Organic chemistry is the study of carbon compounds, which are the basis of life’s molecular diversity.

• Organic compounds: Compounds containing carbon and hydrogen, often with oxygen, nitrogen, sulfur, or phosphorus.

• Hydrocarbons: Organic molecules consisting only of carbon and hydrogen.

• Complexity and variety: Carbon's ability to form four covalent bonds allows for a diversity of stable structures (chains, rings, branches).

• Valence electrons: Carbon has 4 valence electrons and can form 4 covalent bonds.

Isomers

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

• Structural isomers: Differ in the covalent arrangements of their atoms.

• Cis-trans (geometric) isomers: Differ in arrangement around a double bond.

• Enantiomers: Isomers that are mirror images of each other.

Functional Groups

Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules.

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

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

• Carboxyl group (-COOH): Acts as an acid.

• Amino group (-NH2): Acts as a base.

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

• Phosphate group (-OPO32-): Contributes negative charge, involved in energy transfer.

• Methyl group (-CH3): Affects gene expression.

Energy source for cellular processes: Adenosine triphosphate (ATP) is the primary energy carrier in cells.

Chapter 5: The Structure and Function of Large Biological Molecules

Macromolecules and Polymers

• Macromolecule: A large molecule formed by the joining of smaller molecules, usually by a condensation reaction.

• Polymer: A long molecule consisting of many similar or identical building blocks (monomers) linked by covalent bonds.

• Monomer: The subunit that serves as the building block of a polymer.

• Enzyme: A macromolecule serving as a catalyst, increasing the rate of a reaction without being consumed.

Polymerization Reactions

• Dehydration reaction: Two monomers bond together through the loss of a water molecule.

• Hydrolysis: Polymers are disassembled to monomers by the addition of water.

Carbohydrates

• Carbohydrates: Sugars and polymers of sugars.

• Monosaccharide: Single sugar molecule (e.g., glucose).

• Disaccharide: Two monosaccharides joined by a glycosidic linkage (e.g., sucrose).

• Polysaccharide: Polymers of many monosaccharides (e.g., starch, glycogen, cellulose).

• Starch: Storage polysaccharide in plants; consists of glucose monomers.

• Glycogen: Storage polysaccharide in animals.

• Cellulose: Structural polysaccharide in plant cell walls; differs from starch in glycosidic linkages.

Lipids

• Lipids: Hydrophobic molecules, including fats, phospholipids, and steroids.

• Glycerol: A three-carbon alcohol with a hydroxyl group attached to each carbon.

• Fatty acid: A carboxylic acid with a long carbon chain.

• Triglyceride: Three fatty acids linked to one glycerol molecule.

• Phospholipid: Two fatty acids and a phosphate group attached to glycerol; major component of cell membranes.

• Saturated fatty acid: No double bonds between carbon atoms; solid at room temperature.

• Unsaturated fatty acid: One or more double bonds; liquid at room temperature.

Proteins

• Proteins: Polymers of amino acids; perform a vast array of functions (enzymatic, structural, transport, etc.).

• Amino acid: Organic molecule with an amino group, carboxyl group, hydrogen atom, and R group (side chain).

• Levels of protein structure:

  • Primary: Sequence of amino acids.

  • Secondary: Coils and folds (α-helix, β-pleated sheet) due to hydrogen bonding.

  • Tertiary: Overall 3D shape due to interactions among R groups.

  • Quaternary: Association of multiple polypeptide chains.

Nucleic Acids

• Nucleic acids: Polymers made of nucleotide monomers; store and transmit genetic information.

• Nucleotide: Composed of a nitrogenous base, a pentose sugar, and a phosphate group.

• Nucleoside: Nitrogenous base plus sugar (no phosphate).

• DNA (deoxyribonucleic acid): Double-stranded helix; stores genetic information.

• RNA (ribonucleic acid): Usually single-stranded; involved in protein synthesis.

• Nitrogenous bases in DNA: Adenine (A), Thymine (T), Cytosine (C), Guanine (G).

• Two families of nitrogenous bases: Purines (A, G) and Pyrimidines (C, T, U).

• Structure of DNA: Double helix with complementary base pairing (A-T, G-C).

Chapter 6: A Tour of the Cell

Microscopy and Cell Types

• Types of microscopes: Light microscope (LM), Scanning electron microscope (SEM), Transmission electron microscope (TEM).

• Resolution: The clarity of the image; electron microscopes have higher resolution than light microscopes.

• Prokaryotic cells: Lack a nucleus and membrane-bound organelles (e.g., bacteria, archaea).

• Eukaryotic cells: Have a nucleus and membrane-bound organelles (e.g., plants, animals, fungi, protists).

Cellular Components and Organelles

• Organelle: Specialized subunit within a cell with a specific function.

• Nucleus: Contains most of the cell's DNA; site of RNA synthesis.

• Ribosomes: Sites of protein synthesis.

• Rough endoplasmic reticulum (RER): Studded with ribosomes; synthesizes proteins.

• Smooth endoplasmic reticulum (SER): Lacks ribosomes; synthesizes lipids, detoxifies drugs.

• Golgi apparatus: Modifies, sorts, and packages proteins and lipids for storage or transport.

• Lysosomes: Contain hydrolytic enzymes for digestion.

• Vacuoles: Storage, waste disposal, and maintaining turgor pressure in plant cells.

• Plasma/cell membrane: Selectively permeable barrier that regulates what enters and leaves the cell.

• Mitochondria: Sites of cellular respiration; generate ATP.

• Chloroplasts: Sites of photosynthesis in plant cells.

• Peroxisomes: Break down fatty acids and detoxify harmful substances.

• Cytoskeleton: Network of fibers that maintains cell shape, secures organelles, and enables movement.

• Cell wall: Rigid structure outside the plasma membrane in plants, fungi, and some protists.

Genetic Material and Cell Organization

• Chromosome: DNA molecule with associated proteins; carries genetic information.

• Chromatin: Complex of DNA and proteins that makes up chromosomes.

• Endomembrane system: Includes the nuclear envelope, ER, Golgi apparatus, lysosomes, vesicles, and plasma membrane; involved in synthesis and transport of proteins and lipids.

• Cell junctions: Structures that connect cells to one another (e.g., tight junctions, desmosomes, gap junctions in animals; plasmodesmata in plants).

Animal vs. Plant Cells

Feature	Animal Cell	Plant Cell
Cell Wall	Absent	Present
Chloroplasts	Absent	Present
Vacuole	Small or absent	Large central vacuole
Lysosomes	Present	Rare

Example: Plant cells can perform photosynthesis due to the presence of chloroplasts, while animal cells cannot.