Comprehensive AP Biology Course Notes

Water and Hydrogen Bonding

Water Polarity: Water is a polar molecule characterized by unequal electron sharing between oxygen and hydrogen atoms. This results in a partial negative region ( $\delta^-$ ) near the oxygen and partial positive regions ( $\delta^+$ ) near the hydrogens.
Hydrogen Bonds: These are weak, intermolecular bonds that form between molecules, specifically between the partially negative oxygen of one water molecule and the partially positive hydrogen of another. They are significantly weaker than intramolecular covalent or ionic bonds.
Universal Importance: Hydrogen bonds are not unique to water; they are essential in biology for the structural integrity of DNA (holding adenine and thymine, or cytosine and guanine together), RNA, and proteins.
Cohesion, Adhesion, and Surface Tension: - Cohesion: Hydrogen bonds between similar water molecules. It is responsible for water's high heat of vaporization (requiring significant energy to evaporate), high specific heat (ability to hold heat), and high surface tension. - Adhesion: Water molecules sticking to other substances, such as the cellulose walls in plant xylem. - Transpiration: The process where water is pulled up to the tops of trees based on water’s ability to cohere to itself and adhere to the sides of conductive tubes as it evaporates from leaves. - Surface Tension: Illustrated by a paperclip floating on water, resulting from a collective network of hydrogen bonds creating a tension "net" that supports the object.
pH and Acidity: - Acidic Solutions: Defined as having a higher concentration of hydrogen ions ( $H^+$ ) than hydroxide ions ( $OH^-$ ), resulting in a pH below $7$ . - Basic Solutions: Defined as having more hydroxide ions ( $OH^-$ ) than hydrogen ions ( $H^+$ ), resulting in a pH above $7$ . - AP Exam Context: Direct pH questions are rare, but it is an essential underlying concept often appearing in FRQs or multiple-choice scenarios.

Elements of Life

Key Elements: Life is built primarily upon Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorus, and Sulfur, often remembered by the acronym "CHNOPS."
Carbon: The central element in all molecules making up living organisms.
Hydrogen: Central to energy exchange (e.g., $\text{NAD}^+$ becoming $\text{NADH}$ ) and used to create proton gradients for $\text{ATP}$ synthesis. It is also the basis of acidity and alkalinity.
Phosphorus: Found in phosphate groups, essential for the structure of $\text{ATP}$ and $\text{DNA}$ .

Monomers, Polymers, and Functional Groups

Monomers and Polymers: Carbohydrates, proteins, and nucleic acids are built from smaller building blocks called monomers. Monomers are like Legos that combine into larger structures called polymers. The specific three-dimensional shape of these polymers determines their function.
Structural Formulas: In biological diagrams, every unspecified vertex or angle represents a carbon atom. For example, glucose ( $C_6H_{12}O_6$ ) has carbons at specific unspecified angles in its ring structure.
Dehydration Synthesis: The process of building polymers where an enzyme pulls a hydroxyl group ( $-OH$ ) from one monomer and a hydrogen ( $-H$ ) from another, removing a water molecule ( $H_2O$ ) and creating a covalent bond.
Hydrolysis: The opposite of dehydration synthesis. The suffix "-lys" means to break. An enzyme inserts a water molecule between two monomers, breaking the bond. For example, lactose (a disaccharide) is broken into galactose and glucose via the addition of a water molecule.
Functional Groups: - Phosphate Groups: Essential for energy exchange ( $\text{ATP}$ ) and energizing monomers in $\text{DNA}$ synthesis. - Methyl Groups: Used to silence $\text{DNA}$ and make molecules nonpolar/hydrophobic. - Hydroxyl and Carbonyl Groups: Polar groups that make molecules hydrophilic/water-soluble. - Carboxyl and Amino Groups: Essential components of amino acids. - Sulfhydryl Groups: Create stabilizing bonds in protein tertiary structure. - Acetyl Groups: Used to activate $\text{DNA}$ through acetylation.

Carbohydrates and Lipids

The Four Macromolecules: Carbohydrates, lipids, proteins, and nucleic acids.
Carbohydrates: - Monosaccharides: Simple sugars like glucose (the fuel of life). - Disaccharides: Such as lactose, composed of two linked monosaccharides. - Polysaccharides: Used for energy storage (Starch in plants, Glycogen in animals) and structural roles (Cellulose in plant cell walls).
Cellulose Digestion: Humans cannot digest cellulose because we lack the enzymes to hydrolyze its specific bonds. Ruminants (cows, goats) and termites can digest it due to symbiotic relationships with microorganisms that produce the necessary enzymes.
Lactose Tolerance vs. Intolerance: Lactase is the enzyme that hydrolyzes lactose. Most mammals stop producing it after infancy. However, some human groups (pastoralists in Europe, Africa, and the Indian subcontinent) evolved a mutation for lactase persistence, allowing milk consumption into adulthood.
Lipids: - Characteristics: Wholly or partly nonpolar, hydrophobic, and not composed of repeating monomers. - Triglycerides (Fats): Used for energy storage; solid in animals (fats), liquid in plants (oils). - Waxes: Used for waterproofing. - Phospholipids: Composed of a hydrophilic polar head and a hydrophobic nonpolar tail connected by glycerol. In water, they spontaneously form a bilayer, the structural framework of cell membranes. - Steroid Hormones: Used for signaling, such as testosterone or estrogen.

Proteins and Protein Structure

Amino Acid Monomers: Consist of a central carbon, an amino group, a carboxyl group, a hydrogen, and one of $20$ variable "R" groups (side chains) which can be polar, nonpolar, acidic, or basic.
Four Levels of Protein Structure: - Primary Structure: The linear sequence of amino acids linked by peptide bonds, determined genetically. - Secondary Structure: Interactions in the polypeptide backbone (carbon-carbon-nitrogen) forming Alpha Helices (corkscrews) or Beta Pleated Sheets, stabilized by hydrogen bonds. - Tertiary Structure: Interactions between R groups, including hydrogen bonds, ionic bonds, covalent bonds (disulfide bridges between sulfhydryl groups), and hydrophobic clustering. - Quaternary Structure: Interactions between multiple folded polypeptide chains (e.g., Hemoglobin, Spike protein of $\text{SARS-CoV-2}$ ).
Sickle Cell Disease Case Study: An inherited disorder caused by a recessive point mutation substituting a nonpolar valine for an acidic glutamic acid in the hemoglobin beta chain. This causes hemoglobin to form hydrophobic fibers when deoxygenated, resulting in sickled red blood cells that clog arteries.
Evolutionary Benefit: Carrying one copy of the sickle cell allele provides resistance to malaria.

Nucleic Acids

Biological Importance: DNA is the molecule of heredity. RNA is used for information transfer (mRNA) and can act as an enzyme (ribosomes, spliceosomes).
Nucleotide Monomers: Composed of a five-carbon sugar, a phosphate group (connected to the $5'$ carbon), and a nitrogenous base (connected to the $1'$ carbon).
DNA vs. RNA: - DNA: Sugar is deoxyribose; bases are adenine, thymine, cytosine, guanine ( $A, T, C, G$ ). Double-stranded. - RNA: Sugar is ribose; bases are adenine, uracil, cytosine, guanine ( $A, U, C, G$ ). Single-stranded.
DNA Structure: Two nucleotide strands in an antiparallel orientation ( $5' \rightarrow 3'$ vs. $3' \rightarrow 5'$ ) connected by hydrogen bonds ( $A$ with $T$ , $C$ with $G$ ).
Directionality: DNA polymerase only adds new nucleotides at the $3'$ end of a growing strand, meaning synthesis always occurs in the $5' \rightarrow 3'$ direction.

Cell Structure and Prokaryotes vs. Eukaryotes

Fundamental Parts: All cells have a membrane, genetic information (DNA), and ribosomes for protein synthesis controlled by enzymes.
Prokaryotic Cells: Small, simple, no nucleus, circular chromosomes, and plasmids. Found in domains Bacteria and Archaea.
Eukaryotic Cells: Larger, complex, have a nucleus, linear chromosomes wrapped in proteins, mitochondria, and membrane-bound organelles. Bound in domain Eukarya.
Cell Size and Area/Volume Ratio: Cells must be small to maximize surface area for diffusion relative to volume. - Math: For a $1\, \mu\text{m}$ cube, $SA = 6$ , $V = 1$ , Ratio = $6:1$ . For a $10\, \mu\text{m}$ cube, $SA = 600$ , $V = 1000$ , Ratio = $0.6:1$ .
Strategies to Increase Surface Area: Thin sheets (fish gills or elephant ears) and highly folded membranes (mitochondrial interior, intestinal villi).
Whale Size Evolution: A larger size decreases SA:V ratio, resulting in less heat loss to cold ocean water, which increased evolutionary fitness for these endothermic mammals.
Metabolic Scaling: Smaller mammals (shrews) have higher relative metabolic rates (energy per gram of tissue) than larger mammals (elephants) because they lose heat more quickly due to higher SA:V ratios.

Organelles and Endosymbiosis

Compartmentalization: Internal division of space allowing for distinct internal chemistries (e.g., lysosomal hydrolytic enzymes) and increased internal surface area for membrane-bound reactions.
Endosymbiosis: Proposes that mitochondria and chloroplasts arose from free-living bacteria engulfed by an ancestral archaeal cell about $1.8$ billion years ago. - Evidence: These organelles have circular DNA, double membranes, replicate via binary fission, and have bacteria-like ribosomes.
Organelle Functions: - Nucleus: Protects DNA; contains the nucleolus (ribosome assembly). - Ribosomes: Translate mRNA into amino acid sequences. Can be free (cytoplasm) or bound (Rough ER). - Mitochondria: Convert food energy into $\text{ATP}$ via the Krebs cycle and oxidative phosphorylation. - Rough ER: Synthesizes proteins for export or membrane inclusion. - Smooth ER: Synthesizes lipids, detoxifies toxins, and breaks down carbohydrates. - Golgi Complex: Chemically modifies and packages proteins into vesicles. - Lysosomes: Contain hydrolytic enzymes for intracellular digestion and apoptosis (programmed cell death). - Vacuoles: In plants, the central vacuole stores water and maintains turgor pressure. - Chloroplasts: Create carbohydrates via photosynthesis. - Cytoskeleton: Protein fiber network for structural support and movement (e.g., amoeboid crawling).

Membrane Structure and Transport

Fluid Mosaic Model: Membranes consist of phospholipids, proteins, and cholesterol in constant lateral motion.
Proteins: Can be transmembrane (crossing the bilayer), integral (partially embedded), or peripheral (attached to surface).
Passive Transport (Diffusion): - Simple Diffusion: Small nonpolar molecules ( $O_2, N_2, CO_2$ ) or steroids pass through the bilayer down their concentration gradient. - Facilitated Diffusion: Polar molecules and ions pass through specific protein channels.
Active Transport: Pumping molecules against a concentration gradient using energy ( $\text{ATP}$ or electron flow).
Bulk Transport: Endocytosis (membrane pinches in) and Exocytosis (material dumped out).
Osmosis: The diffusion of water from a hypotonic solution (high water/low solute) to a hypertonic solution (low water/high solute). - Tonicity in Animals: Cells must be in isotonic solutions; they shrivel in hypertonic and burst in hypotonic environments. - Tonicity in Plants: Prefer hypotonic environments to maintain turgor pressure against the cell wall to prevent wilting (plasmolysis).
Water Potential ( $\Psi$ ): A quantitative measure of water's tendency to move. $\Psi = \Psi_s + \Psi_p$ (water potential = solute potential + pressure potential). Water always moves from higher $\Psi$ to lower $\Psi$ .

Enzymes and Metabolism

Enzymes: Usually proteins that catalyze reactions by lowering activation energy ( $E_a$ ). They are specific to their substrate based on active site shape.
Factors Affecting Enzymes: Changes in pH or temperature can cause denaturation (loss of shape). Higher temperature increases activity up to a point due to kinetic energy.
Inhibition: - Competitive: Foreign molecule blocks the active site. - Non-competitive: Molecule binds to the allosteric site, changing the active site shape.
Metabolic Pathways: Linked series of reactions (e.g., Glycolysis, Krebs, Calvin Cycle).
Autotrophs: Produce their own food. Phototrophs use light; Chemoautotrophs use inorganic chemicals (sulfur, iron).
Energy Coupling: Linking exergonic reactions (energy-releasing) to endergonic reactions (energy-requiring), such as using $\text{ATP}$ hydrolysis to power muscle contraction.

Photosynthesis

Equation: $6CO_2 + 6H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6O_2$ .
Light Reactions: Occur in the thylakoid membranes. Convert light into chemical energy ( $\text{ATP}$ and $\text{NADPH}$ ). Involves Photosystem II (splits water, releases $O_2$ ) and Photosystem I (produces $\text{NADPH}$ ). Protons are pumped into the thylakoid space to power $\text{ATP}$ synthase.
Calvin Cycle: Occurs in the stroma. Three phases: Carbon Fixation (catalyzed by Rubisco), Energy Investment (using $\text{ATP/NADPH}$ to make $G3P$ ), and Regeneration of $RuBP$ .
Carbon Counting: $3 \text{ molecules of } RuBP \text{ (15C)} + 3 \text{ molecules of } CO_2 \text{ (3C)} = 18 \text{ carbons}$ . One $G3P \text{ (3C)}$ is harvested; the remaining $15$ carbons regenerate the $3 RuBP$ .

Cellular Respiration

Equation: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{ATP}$ .
Glycolysis: Cytoplasm. Breaks glucose into two pyruvate, netting $2 \text{ ATP}$ and $2 \text{ NADH}$ .
Link Reaction: Moves pyruvate into mitochondrial matrix; removes $CO_2$ and produces Acetyl-CoA and $NADH$ .
Krebs Cycle: Matrix. Per Acetyl-CoA: produces $1 \text{ ATP}$ , $3 \text{ NADH}$ , $1 \text{ FADH}_2$ , and releases $2 \text{ CO}_2$ .
Oxidative Phosphorylation: Inner membrane. Electrons from $NADH/FADH_2$ flow through the $\text{ETC}$ , pumping protons into the intermembrane space. Oxygen is the final electron acceptor. Protons flow back through $\text{ATP}$ synthase to generate the majority of oxygen-dependent $\text{ATP}$ .
Heat Generation: Uncoupling proteins (thermogenin) in brown fat allow protons to bypass $\text{ATP}$ synthase, converting the energy directly into heat.
Fermentation: Allows glycolysis to continue without oxygen by regenerating $\text{NAD}^+$ . - Alcohol Fermentation: Yields ethanol and $CO_2$ (yeast). - Lactic Acid Fermentation: Yields lactic acid (animals).

Cell Signaling and Homeostasis

Three Phases: Reception, Transduction (amplification via second messengers like $cAMP$ and kinase cascades), and Response.
Epinephrine Pathway: Epinephrine (ligand) binds G-protein coupled receptor. G-protein binds $GTP$ , activates Adenylyl Cyclase. Adenylyl Cyclase converts $\text{ATP}$ to $cAMP$ . Phosphorylation cascade activates glycogen phosphorylase to release glucose.
Feedback Groups: - Negative Feedback: Returns system to set point (e.g., Insulin lowers blood glucose, Glucagon raises it). - Positive Feedback: Accelerates change towards completion (e.g., Childbirth via oxytocin, ethylene in fruit ripening).

Cell Cycle and Cancer

Phases: Interphase ( $G_1$ growth, $S$ DNA synthesis, $G_2$ final prep) and M-phase (Mitosis and Cytokinesis).
Mitosis Phases: Prophase (condense), Metaphase (align), Anaphase (separate chromatids), Telophase (nuclei reform).
Checkpoints: Controlled by cyclins and cyclin-dependent kinases ( $CDKs$ ).
Cancer: Results from overactive proto-onco genes (step on the gas) or mutated tumor suppressor genes (broken brakes, like $p53$ ).

Meiosis and Heredity

Meiosis: Two rounds of division to produce four unique haploid gametes. Meiosis I separates homologous pairs; Meiosis II separates sister chromatids.
Diversity Sources: Crossing over (Prophase I), Independent Assortment ( $2^{23}$ combinations in humans), and random fertilization.
Chromosomal Variations: Nondisjunction (failure to separate) leads to aneuploidy (Trisomy $21$ or Turner syndrome).
Hardy-Weinberg Equations: - $p + q = 1$ - $p^2 + 2pq + q^2 = 1$
Mendel's Laws: Segregation (pass one allele) and Independent Assortment (genes for different traits separate independently).
Non-Mendelian: Linked genes (same chromosome), sex-linked traits (on $X$ chromosome), and non-nuclear inheritance (mitochondria passed via mother).

Evolution and Ecology

Natural Selection: Survival of the fittest (fit = number of offspring).
Evidence: Homologies (anatomical, molecular, embryonic), Vestigial structures, Biogeography, and the fossil record.
Speciation: Allopatric (geographical barrier) and Sympatric (same area, behavioral or polyploidy).
Population Growth: Exponential ( $\frac{dN}{dt} = rN$ ) and Logistic ( $\frac{dN}{dt} = rN \frac{K-N}{K}$ ).
Trophic Structures: 10% energy transfer rule. Producers at the base.
Community Interactions: Mutualism ( $+ / +$ ), Commensalism ( $+ / 0$ ), Parasitism/Predation ( $+ / -$ ).
Keystone Species: Influence community structure disproportionately (e.g., Sea stars, wolves, beavers).
Environmental Issues: Eutrophication (nutrient runoff causing dead zones), Biomagnification (toxins concentrating in top predators), and Climate Change ( $CO_2$ trapping heat).