AP Biology Summer Assignment Comprehensive Notes

Chemistry

• Chemical Bonds

  • Ionic

  • Transfer of electrons → formation of oppositely-charged ions (e.g. $Na^+$ & $Cl^-$ in table salt)

  • Crystalline lattice, high melting point, soluble, conduct in solution

  • Covalent

  • Sharing of electron pairs; can be non-polar (equal sharing, e.g. $O<em>2$) or polar (unequal, e.g. $H</em>2O$)

  • Determine molecule shape → biological recognition (enzyme ↔ substrate)

  • Hydrogen bonds

  • Weak attractions between a hydrogen covalently bound to N/O/F and another electronegative atom

  • Crucial for DNA double helix stability & water’s unique traits

  • Van der Waals interactions

  • Transient, weak; drive gecko toe adhesion & lipid bilayer packing

• Water & pH

  • Polarity → cohesion (surface tension), adhesion (capillary action), high specific heat, high heat of vaporization, solid less dense than liquid

  • Dissociation: $2H<em>2O \rightleftharpoons H</em>3O^+ + OH^-$

  • $pH = -\log[H^+]$; biological systems buffered near $7.2!\text{–}!7.4$ by bicarbonate $HCO_3^-$

• Macromolecules

  • Carbohydrates

  • Empirical formula $C<em>n(H</em>2O)_n$; ring structures (α/β) influence digestibility

  • Mono- (glucose), di- (sucrose), poly- (starch, glycogen, cellulose, chitin)

  • Function: energy ((~4\text{ kcal g}^{-1})), structure (cellulose cell wall), markers (glycoproteins)

  • Lipids

  • Mostly $C$ & $H$ → non-polar; triglyceride = glycerol + 3 fatty acids (ester linkage)

  • Phospholipid: diglyceride + phosphate → amphipathic bilayers

  • Steroids: 4-ring backbone (cholesterol, cortisol)

  • Function: membranes, energy storage ((~9\text{ kcal g}^{-1})), hormones, insulation

  • Proteins

  • Monomer: amino acid $NH_2-CHR-COOH$ (20 varieties; R group chemistry dictates folding)

  • Levels: primary (sequence) → secondary (α-helix, β-sheet via H-bonds) → tertiary (R-group interactions) → quaternary (multimeric)

  • Function: enzymes, transport (hemoglobin), signaling (insulin), structure (collagen)

  • Nucleic Acids

  • Nucleotide = phosphate + pentose sugar + nitrogenous base (A, T/U, C, G)

  • DNA (deoxyribose, double-stranded, antiparallel) stores heredity; RNA (ribose, single-stranded) executes expression (mRNA, tRNA, rRNA)

• Biological Significance Examples

  • ATP is a modified nucleotide driving $ATP \rightarrow ADP + P_i$ energy coupling

  • Lipid-soluble hormones (estrogen) passively diffuse; peptide hormones require membrane receptors

Chemistry

• Chemical Bonds

  • Ionic bonds

  • Formed by the complete transfer of valence electrons between atoms, typically a metal (low electronegativity) and a non-metal (high electronegativity). This large electronegativity difference (>1.7 on the Pauling scale) leads to the formation of a positively charged cation and a negatively charged anion (e.g., $Na^+$ and $Cl^-$ in table salt, $NaCl$).

  • Characterized by forming rigid, ordered crystalline lattice structures, high melting and boiling points, and often high solubility in polar solvents like water. They conduct electricity when molten or dissolved in solution but not in solid form.

  • Covalent bonds

  • Formed by the sharing of electron pairs between atoms, typically between two non-metals. The shared electrons are attracted to the nuclei of both atoms, holding them together.

  • Can be non-polar (equal sharing, when electronegativity difference is <0.4, e.g., diatomic molecules like $O<em>2$ or $N</em>2$) or polar (unequal sharing, when electronegativity difference is $0.4-1.7$, creating partial positive $\delta^+$ and partial negative $\delta^-$ charges, e.g., $H_2O$ where oxygen is more electronegative).

  • The arrangement of electron pairs and atoms determines molecule shape, which is crucial for biological recognition processes (e.g., enzyme-substrate binding, receptor-ligand interactions).

  • Hydrogen bonds

  • Intermolecular forces (not true bonds) that are weak electrostatic attractions. They occur between a hydrogen atom covalently bonded to a highly electronegative atom (like Nitrogen (N), Oxygen (O), or Fluorine (F)) and another nearby electronegative atom already part of another polar molecule.

  • Though individually weak (about 5-10% the strength of a covalent bond), their cumulative effect is significant. They are crucial for the stability of biological macromolecules (e.g., holding the two strands of the DNA double helix together, maintaining protein secondary structure like $\alpha$-helices and $\beta$-sheets) and are responsible for many of water’s unique properties.

  • Van der Waals interactions

  • Also intermolecular forces, these are very weak, transient attractions arising from temporary, induced dipoles due to fleeting asymmetrical distributions of electrons around atoms or molecules. They include London dispersion forces (in all molecules) and dipole-dipole interactions (in polar molecules).

  • Though extremely weak individually, they become significant when many molecules are in close proximity, allowing cumulative effects to create stronger attractions. They drive gecko toe adhesion to surfaces, help stabilize the packing of lipid bilayers in cell membranes, and contribute to protein folding.

• Water & pH

  • Water possesses remarkable properties due to its polarity (bent molecular geometry, oxygen's higher electronegativity leading to partial charges, $\delta^+$ on H and $\delta^-$ on O) and extensive hydrogen bonding capabilities:

    • Cohesion: Water molecules stick to each other, creating surface tension (e.g., allowing insects to walk on water).

    • Adhesion: Water molecules stick to other polar or charged surfaces, essential for capillary action (e.g., water transport in plants).

    • High specific heat: Water absorbs or releases a large amount of heat with only a slight change in its own temperature, vital for thermoregulation in organisms and moderating global climates.

    • High heat of vaporization: A large amount of energy is required to transform liquid water into water vapor, providing an effective cooling mechanism through evaporation (e.g., sweating).

    • Solid is less dense than liquid: Ice floats because its hydrogen bonds create a more ordered, spacious lattice structure than liquid water, preventing oceans and lakes from freezing solid from the bottom up, crucial for aquatic life.

  • Dissociation of Water: Water molecules can spontaneously dissociate into hydronium ions ($H_3O^+$ or simply $H^+$) and hydroxide ions ($OH^-$). In pure water at $25^ {o}C$, $[H^+] = [OH^-] = 10^{-7} M$.

  • The equilibrium is represented as: $2H<em>2O \rightleftharpoons H</em>3O^+ + OH^-$

  • pH scale: A measure of the hydrogen ion concentration ($[H^+]$) in a solution, defined as $pH = -\log[H^+]$. A lower pH indicates higher acidity (more $H^+$), while a higher pH indicates higher alkalinity (more $OH^-$).

  • Biological systems are highly sensitive to pH changes. Most cellular processes, particularly enzyme activity, function optimally within a narrow pH range, typically buffered near $pH = 7.2 \text{–} 7.4$. The bicarbonate buffer system ($H<em>2CO</em>3 \rightleftharpoons HCO_3^- + H^+$) is crucial for maintaining blood pH within this physiological range.

• Macromolecules

  • Carbohydrates (Saccharides)

  • Organic molecules composed of Carbon ($C$), Hydrogen ($H$), and Oxygen ($O$), typically with an empirical formula of $C<em>n(H</em>2O)_n$. They exist primarily as ring structures in aqueous solutions, and the orientation of the hydroxyl ($-OH$) group on the anomeric carbon ($\alpha$ or $\beta$ linkage) significantly influences their digestibility (e.g., humans can digest $\alpha$-linked starch but not $\beta$-linked cellulose).

  • Types:

    • Monosaccharides: Simple sugars, monomers (e.g., glucose, fructose, galactose). Primary energy source.

    • Disaccharides: Formed from two monosaccharides joined by a glycosidic linkage via dehydration synthesis (e.g., sucrose (glucose + fructose), lactose (glucose + galactose), maltose (glucose + glucose)).

    • Polysaccharides: Long chains of many monosaccharides, can be branched or unbranched.

      • Energy Storage: Starch (plants), Glycogen (animals, highly branched).

      • Structural: Cellulose (plant cell walls, provides rigidity, $\beta$ linkages), Chitin (exoskeletons of insects and fungi cell walls, contains nitrogen).

  • Function: Primary source of quick energy (approximately $\sim 4\text{ kcal g}^{-1}$), structural components (cellulose in plants, chitin in fungi/arthropods), and cell surface markers (glycoproteins and glycolipids involved in cell recognition and signaling).

  • Lipids

  • Diverse group of hydrophobic (water-fearing) molecules, primarily composed of Carbon ($C$) and Hydrogen ($H$) atoms with very few oxygen atoms, making them non-polar.

  • Triglycerides: The most common type of fat; consists of a glycerol molecule esterified to three fatty acid chains (via ester linkages).

    • Fatty acids can be saturated (no double bonds, solid at room temperature, straight chains) or unsaturated (one or more carbon-carbon double bonds, liquid at room temperature, often kinked due to cis double bonds).

  • Phospholipids: Modified diglycerides where one fatty acid is replaced by a phosphate group (often with another small polar molecule attached). This renders them amphipathic, meaning they have both a hydrophilic (water-loving) head (phosphate group) and a hydrophobic (water-fearing) tail (fatty acid chains). They spontaneously form lipid bilayers in aqueous environments, which are the fundamental structure of cell membranes.

  • Steroids: Characterized by a distinctive carbon skeleton of four fused rings (e.g., cholesterol, a precursor for other steroids like cortisol, estrogen, and testosterone). Cholesterol is a crucial component of animal cell membranes, influencing fluidity.

  • Function: Major components of cell membranes, long-term energy storage (approximately $\sim 9\text{ kcal g}^{-1}$, more than double carbohydrates), hormones (steroid hormones), and thermal insulation/organ protection.

  • Proteins

  • Extremely diverse and versatile macromolecules, essential for virtually all biological processes. Their fundamental monomer unit is the amino acid ($\text{NH}_2\text{-CHR-COOH}$), linked together by peptide bonds.

  • There are 20 common varieties of amino acids, each characterized by a unique R group (side chain), which dictates its chemical properties (polar, nonpolar, electrically charged, acidic, basic) and ultimately influences how the protein folds into its specific 3D structure.

  • Proteins exhibit four levels of structural organization:

    • Primary structure: The unique, linear sequence of amino acids, determined by genetic information. This sequence dictates all higher-order structures.

    • Secondary structure: Localized, regularly repeating configurations formed by hydrogen bonds between the backbone (not R-groups) atoms of amino acids. Common forms include the $\alpha$-helix (a coiled structure) and the $\beta$-sheet (pleated, accordion-like structure).

    • Tertiary structure: The overall three-dimensional shape of a single polypeptide chain, resulting from various R-group interactions (e.g., hydrophobic interactions, ionic bonds, disulfide bridges, hydrogen bonds, Van der Waals forces) that fold the secondary structures into a compact globular or fibrous form.

    • Quaternary structure: Occurs in proteins composed of two or more polypeptide chains (subunits). It describes the arrangement and interactions of these multiple subunits to form a functional multimeric protein (e.g., hemoglobin, which has four subunits).

  • The precise 3D shape of a protein is critical for its function; denaturation (loss of native shape due to changes in pH, temperature, or salinity) often leads to loss of function.

  • Function: Enormous range of functions, including acting as enzymes (catalyzing biochemical reactions), transport (e.g., hemoglobin transports oxygen), signaling (e.g., insulin hormone), structure (e.g., collagen in connective tissues, keratin in hair/nails), defense (antibodies), and movement (actin/myosin).

  • Nucleic Acids

  • Polymers specialized for the storage, transmission, and expression of genetic information.

  • The monomer unit is the nucleotide, composed of three parts: a phosphate group, a five-carbon pentose sugar (deoxyribose in DNA, ribose in RNA), and a nitrogenous base (adenine (A), guanine (G), cytosine (C), thymine (T - in DNA), or uracil (U - in RNA)).

  • DNA (Deoxyribonucleic Acid):

    • Contains deoxyribose sugar.

    • Typically exists as a double helix, consisting of two antiparallel polynucleotide strands.

    • Bases pair specifically: Adenine with Thymine (A-T) via two hydrogen bonds, and Guanine with Cytosine (G-C) via three hydrogen bonds (Chargaff's rules).

    • Primarily functions to store heredity information and instructions for building proteins and RNA molecules.

  • RNA (Ribonucleic Acid):

    • Contains ribose sugar.

    • Typically single-stranded, though it can fold into complex 3D structures.

    • Uracil (U) replaces Thymine (T), so pairing rules are A-U and G-C.

    • Plays diverse roles in the execution of genetic expression:

      • mRNA (messenger RNA): Carries genetic information from DNA in the nucleus to ribosomes in the cytoplasm.

      • tRNA (transfer RNA): Carries specific amino acids to the ribosome during protein synthesis (translation).

      • rRNA (ribosomal RNA): A structural and catalytic component of ribosomes, where protein synthesis occurs.

• Biological Significance Examples

  • ATP (Adenosine Triphosphate) is a modified nucleotide (adenine + ribose + 3 phosphate groups) serving as the primary energy currency of the cell. Energy is released when the terminal phosphate bond is hydrolyzed: $ATP \rightarrow ADP + P_i$ (adenosine diphosphate + inorganic phosphate). This energy is used to power virtually all cellular work through a process called energy coupling.

  • Hormone Action: The chemical nature of hormones dictates their mode of action.

    • Lipid-soluble hormones (e.g., steroid hormones like estrogen, testosterone, cortisol; thyroid hormones) are non-polar and small enough to passively diffuse directly across the non-polar cell membrane. They bind to intracellular receptors (in the cytoplasm or nucleus) and typically alter gene expression.

    • Peptide/protein hormones (e.g., insulin, growth hormone) and most neurotransmitters are hydrophilic and cannot pass directly through the cell membrane. They must bind to specific membrane receptors on the cell surface, initiating a signal transduction pathway that triggers a response inside the cell without the hormone itself entering.

Cells

• Key Organelles & Functions

  • Cell membrane (Plasma membrane): A dynamic, fluid structure primarily composed of a phospholipid bilayer with diverse proteins embedded within or associated with it (Fluid Mosaic Model).

    • Function: Controls what enters and leaves the cell (selective permeability), maintains cell homeostasis, facilitates cell-to-cell communication and recognition (via glycoproteins and glycolipids), and plays a role in signal transduction (receiving and transmitting extracellular signals).

  • Mitochondrion (plural: mitochondria): Often called the "powerhouse" of the cell. It is a double-membraned organelle; the inner membrane is highly folded into structures called cristae, which increase its surface area.

    • Function: Site of most stages of aerobic respiration, where glucose and oxygen are used to produce ATP. The Krebs cycle occurs in the matrix, and the electron transport chain and ATP synthase are located on the inner mitochondrial membrane (cristae).

    • Overall simplified reaction: $C<em>6H</em>{12}O<em>6 + 6 O</em>2 \rightarrow 6 CO<em>2 + 6 H</em>2O + \sim 36 \text{ ATP}$

  • Chloroplast: Found in plant cells and algae. It is also a double-membraned organelle, containing an internal system of flattened sacs called thylakoids (stacked into grana) and a fluid-filled space called the stroma.

    • Function: Site of photosynthesis, the process that converts light energy into chemical energy.

      • Light-dependent reactions: Occur on the thylakoid membranes, converting light energy into ATP and NADPH, and releasing $O_2$.

      • Light-independent reactions (Calvin cycle): Occur in the stroma, using ATP and NADPH from the light reactions to fix atmospheric $CO_2$ into glucose.

    • Overall simplified reaction: $6CO<em>2 + 6H</em>2O \xrightarrow{\text{light}} C<em>6H</em>{12}O<em>6 + 6O</em>2$

  • Ribosome: Non-membranous organelles composed of ribosomal RNA (rRNA) and ribosomal proteins, consisting of two subunits (large and small).

    • Function: The primary site of protein synthesis (translation), where the genetic code carried by messenger RNA (mRNA) is converted into a polypeptide chain.

    • Free ribosomes: Suspended in the cytosol, synthesize proteins that function within the cytosol.

    • Bound ribosomes: Attached to the endoplasmic reticulum (ER) or nuclear envelope, synthesize proteins destined for secretion, insertion into membranes, or delivery to certain organelles (e.g., lysosomes).

  • Nucleus: The most prominent organelle in eukaryotic cells, typically enclosed by a double membrane called the nuclear envelope. This envelope is perforated by nuclear pores that regulate the passage of molecules between the nucleus and the cytoplasm.

    • Function: Houses the cell's genetic material (DNA), organized into chromosomes. It controls gene expression and mediates DNA replication. Contains the nucleolus, a dense region where ribosomal RNA (rRNA) is synthesized and assembled with proteins to form ribosomal subunits.

• Membrane Transport

  • Processes by which substances move across the cell membrane, essential for maintaining cellular homeostasis.

  • Passive Transport (Requires no direct ATP expenditure)

    • Diffusion: The net movement of substances from an area of higher concentration to an area of lower concentration (down their concentration gradient) until equilibrium is reached. Occurs directly across the lipid bilayer for small, nonpolar molecules (e.g., $O<em>2$ into cells, $CO</em>2$ out of cells).

    • Osmosis: The specific diffusion of water across a selectively permeable membrane from an area of higher water potential (lower solute concentration) to an area of lower water potential (higher solute concentration). Water often moves through specialized protein channels called aquaporins.

      • Hypertonic solution: Higher solute concentration outside the cell; cell loses water and shrinks (crenates in animal cells, plasmolysis in plant cells).

      • Hypotonic solution: Lower solute concentration outside the cell; cell gains water and swells/bursts (lyses in animal cells, causes turgor pressure in plant cells).

      • Isotonic solution: Equal solute concentration inside and outside; no net water movement.

    • Facilitated diffusion: Movement of substances down their concentration gradient, but requiring the assistance of specific transport proteins (channels or carriers) embedded in the membrane for molecules that cannot directly cross the lipid bilayer (e.g., ions, polar molecules, large molecules).

      • Channels: Provide a hydrophilic pore for specific ions or small polar molecules (e.g., K⁺ leak channels).

      • Carriers: Bind to specific molecules, undergo a conformational change, and release the molecule on the other side (e.g., GLUT4 transporter for glucose uptake).

  • Active Transport (Requires ATP expenditure)

    • Primary Active Transport: Directly uses energy from ATP hydrolysis to pump substances against their concentration gradient. The Na⁺/K⁺ pump (sodium-potassium pump) is a crucial example, pumping $3Na^+$ ions out of the cell and $2K^+$ ions into the cell for each ATP consumed, which helps establish and maintain the membrane potential in animal cells.

    • Secondary Active Transport (Cotransport): Uses the potential energy stored in an existing electrochemical gradient (often established by primary active transport) to move another substance against its own concentration gradient. It does not directly use ATP.

      • Symport: Both substances move in the same direction across the membrane (e.g., Na⁺-glucose cotransporter, where glucose is cotransported into the cell with Na⁺).

      • Antiport: Substances move in opposite directions (e.g., Na⁺-Ca²⁺ exchanger).

    • Bulk Transport: For very large molecules or particles that cannot pass through trans-membrane proteins, involving the formation of vesicles.

      • Endocytosis: The cell takes in macromolecules or particles by forming vesicles from the plasma membrane.

        • Phagocytosis: "Cellular eating"; engulfment of large particles, often by pseudopods (e.g., amoeba feeding, white blood cells engulfing bacteria).

        • Pinocytosis: "Cellular drinking"; engulfment of extracellular fluid containing dissolved solutes in small vesicles.

        • Receptor-mediated endocytosis: Highly specific uptake process where specific molecules (ligands) bind to receptors on the cell surface, triggering the formation of a coated vesicle (e.g., uptake of cholesterol via LDL).

      • Exocytosis: The cell secretes macromolecules by fusing vesicles with the plasma membrane, releasing their contents outside the cell (e.g., release of neurotransmitters, hormones, waste products).

• Mitosis (Somatic Cell Division)

  • Purpose: A form of asexual cell division that produces two genetically identical daughter cells from a single parent cell. It is essential for growth, tissue repair, and asexual reproduction in multicellular organisms.

  • Results in identical $2n$ (diploid) cells, maintaining the chromosome number of the parent cell.

  • Phases of the M (Mitotic) Phase (preceded by Interphase: G1, S, G2):

    • Prophase: Chromatin fibers condense into visible chromosomes, each consisting of two identical sister chromatids joined at the centromere. The mitotic spindle (composed of microtubules) begins to form from the centrosomes. The nucleoli disappear.

    • Prometaphase: The nuclear envelope fragments. Kinetochore microtubules (a type of spindle fiber) attach to the kinetochores (protein structures) at the centromeres of each chromatid, pulling the chromosomes toward the center of the cell.

    • Metaphase: All duplicated chromosomes are precisely aligned on the metaphase plate (an imaginary plane equidistant from the spindle's two poles). The centromeres of all chromosomes are perfectly aligned.

    • Anaphase: The shortest stage. Sister chromatids suddenly separate at the centromeres, becoming individual chromosomes. These newly separated chromosomes are pulled by shortening microtubules towards opposite poles of the cell.

    • Telophase: Chromosomes arrive at the poles and begin to decondense. New nuclear envelopes form around the two sets of chromosomes. Nucleoli reappear. The mitotic spindle disassembles.

    • Cytokinesis: The division of the cytoplasm, which usually overlaps with telophase. In animal cells, a cleavage furrow forms by a contractile ring of actin and myosin filaments. In plant cells, a cell plate forms in the middle, which eventually develops into a new cell wall separating the daughter cells.

  • End Result: Two diploid cells that are genetically identical to the parent cell.

• Meiosis (Germ Cell Division)

  • Purpose: A specialized type of cell division that reduces the number of chromosomes by half, producing four genetically unique haploid ($n$) gametes (sperm or egg cells) from a single diploid ($2n$) parent cell. It is crucial for sexual reproduction and introduces genetic diversity.

  • Involves two successive divisions: Meiosis I and Meiosis II, each with its own set of prophase, metaphase, anaphase, and telophase stages.

  • Meiosis I (Reductional Division): Homologous chromosomes separate, reducing the chromosome number from diploid to haploid.

    • Prophase I: The longest and most complex meiotic phase. Chromatin condenses. Homologous chromosomes pair up side-by-side, forming a structure called a tetrad (bivalent), with four chromatids. Synapsis (the precise alignment of homologous chromosomes) occurs. Crossing-over (exchange of genetic material between non-sister chromatids of homologous chromosomes) takes place, leading to recombination and increased genetic diversity.

    • Metaphase I: Tetrads (pairs of homologous chromosomes) align randomly at the metaphase plate, with each homolog facing opposite poles. This independent assortment of homologous chromosomes contributes significantly to genetic variation.

    • Anaphase I: Homologous chromosomes separate and move towards opposite poles. Sister chromatids remain attached at their centromeres.

    • Telophase I & Cytokinesis: Chromosomes arrive at the poles; each pole now has a haploid set of chromosomes, each still consisting of two sister chromatids. Cytokinesis usually occurs, forming two haploid daughter cells.

  • Meiosis II (Equational Division): Sister chromatids separate, similar to mitosis.

    • Prophase II: Spindle apparatus forms in each haploid cell.

    • Metaphase II: Chromosomes (each still composed of two sister chromatids) align at the metaphase plate of each haploid cell.

    • Anaphase II: Sister chromatids finally separate and move as individual chromosomes toward opposite poles.

    • Telophase II & Cytokinesis: Nuclei reform at opposite poles. Cytokinesis occurs, resulting in a total of four daughter cells, each haploid and genetically unique.

  • End Result: Four genetically unique haploid cells, leading to substantial genetic diversity among gametes, which is critical for evolutionary adaptation.

Genetics

• Vocabulary

  • Gene: A unit of heredity, a segment of DNA that codes for a specific protein or functional RNA molecule.

  • Allele: Alternative forms of a gene (e.g., for flower color, one allele might code for purple, another for white).

  • Locus: The specific physical location of a gene on a chromosome.

  • Homozygous: Having two identical alleles for a particular gene (e.g., $TT$ for tall or $tt$ for short).

  • Heterozygous: Having two different alleles for a particular gene (e.g., $Tt$ for tall).

  • Purebred: Organisms that are homozygous for a specific trait and consistently produce offspring with that same trait when self-fertilized or crossed with similar purebreds.

  • Hybrid: Offspring resulting from the cross of two genetically different parents, typically heterozygous for one or more traits.

  • Genotype: The genetic makeup of an organism, referring to the set of alleles it possesses for a particular gene or genes (e.g., $TT$, $Tt$, $tt$).

  • Phenotype: The observable physical or biochemical characteristics of an organism, resulting from the interaction of its genotype and the environment (e.g., Tall, Short).

  • Dominant allele: An allele that expresses its phenotypic effect even when heterozygous with a recessive allele (e.g., $T$ in $Tt$).

  • Recessive allele: An allele whose phenotypic effect is only expressed when it is homozygous (e.g., $t$ in $tt$) and is masked by a dominant allele in a heterozygote.

• Punnett Squares

  • A diagram used to predict the genotypes and phenotypes of offspring resulting from a genetic cross. It illustrates Mendel's Law of Segregation (alleles for a trait separate during gamete formation) and Law of Independent Assortment (alleles for different genes assort independently).

  • Monohybrid cross: A cross involving one trait (e.g., $Tt \times Tt$).

    • Example: $Tt \times Tt$ (two heterozygous tall pea plants) →

      • Genotypic ratio: 1 $TT$ : 2 $Tt$ : 1 $tt$

      • Phenotypic ratio: 3 tall : 1 short (since $TT$ and $Tt$ are tall, $tt$ is short)

  • Dihybrid cross: A cross involving two different traits (e.g., $RrYy \times RrYy$).

    • Example: $RrYy \times RrYy$ (heterozygous for round/wrinkled seeds and yellow/green seeds) →

      • Phenotypic ratio: Typically results in a 9:3:3:1 ratio in the F2 generation for independently assorting dominant/recessive traits (9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green).

• Non-Mendelian Patterns of Inheritance

  • Genetic patterns that do not follow Mendel's classic dominant/recessive, single-gene inheritance.

  • Codominance: Both alleles in a heterozygote are fully and distinctly expressed in the phenotype, without blending. (e.g., ABO blood group system, where A and B alleles are codominant, resulting in AB blood type when both are present).

  • Incomplete dominance: The heterozygote exhibits a phenotype that is intermediate between the phenotypes of the two homozygous parents, appearing as a blend. (e.g., Red (RR) snapdragons crossed with White (WW) snapdragons produce Pink (RW) snapdragons).

  • Multiple alleles: More than two alleles exist for a given gene within a population (though an individual still only carries two, one on each homologous chromosome). (e.g., The ABO blood group system has three alleles: $I^A$, $I^B$, and $i$).

  • Epistasis: A gene at one locus alters the phenotypic expression of a gene at a second locus (e.g., in Labrador retrievers, the gene for pigment deposition (E/e) can mask the gene for fur color (B/b)).

  • Polygenic inheritance: Multiple genes contribute to a single phenotypic trait, often resulting in continuous variation (e.g., human height, skin color, intelligence), forming a bell-shaped curve of phenotypes.

  • Environmental influence: The environment can interact with the genotype to influence the phenotype (e.g., hydrangeas flower color varies with soil pH).

• Pedigrees

  • A family tree diagram that tracks the inheritance of a specific genetic trait over several generations. Used to determine the mode of inheritance (autosomal vs. X-linked, dominant vs. recessive) of genetic disorders.

  • Symbols: Squares represent males, circles represent females. A horizontal line connecting a square and a circle indicates a mating. Offspring are shown branching off the mating line. Shaded squares or circles indicate individuals who express the trait (affected). Unshaded indicates unaffected. Generations are often denoted by Roman numerals (I, II, III…). Individual members within a generation are numbered.

  • Autosomal dominant: Appears in every generation; affected individuals have at least one affected parent; males and females are equally affected.

  • Autosomal recessive: Skips generations; affected individuals can have unaffected parents (carriers); males and females equally affected. (e.g., cystic fibrosis).

  • X-linked recessive: More common in males (who need only one copy of the recessive allele to be affected); affected sons typically have unaffected mothers who are carriers; no father-to-son transmission.

• Common Genetic Disorders (Examples)

  • Autosomal recessive disorders: Require two copies of the recessive allele to be affected. Carriers are heterozygous and usually asymptomatic.

    • Cystic Fibrosis (CF): Affects chloride ion transport, leading to abnormally thick mucus build-up in the lungs, pancreas, and other organs, causing respiratory and digestive problems.

    • Sickle-Cell Anemia: A single amino acid substitution in the hemoglobin protein causes red blood cells to deform into a sickle shape under low oxygen conditions, leading to clogged blood vessels, pain, and anemia.

  • Autosomal dominant disorders: Only one copy of the dominant allele is needed to be affected.

    • Huntington’s Disease: A progressive, neurodegenerative disorder that typically begins in middle age, leading to uncontrolled movements, cognitive decline, and psychiatric problems.

    • Achondroplasia: A common form of dwarfism resulting from a dominant allele that affects bone growth.

  • X-linked disorders: Caused by genes located on the X chromosome.

    • Hemophilia A: A blood clotting disorder primarily affecting males, due to a deficiency in Factor VIII, a clotting protein.

    • Duchenne Muscular Dystrophy (DMD): A severe form of muscular dystrophy that primarily affects males, causing progressive muscle degeneration and weakness.

  • Chromosomal disorders: Result from abnormalities in chromosome number or structure.

    • Down Syndrome (Trisomy 21): Caused by an extra copy of chromosome 21, leading to characteristic facial features, intellectual disability, and developmental delays.

    • Turner Syndrome (XO): Affects females, characterized by the presence of only one X chromosome (XO) instead of the usual two (XX), leading to developmental problems including short stature, heart defects, and infertility.

    • Klinefelter Syndrome (XXY): Affects males, characterized by an extra X chromosome (XXY), leading to reduced fertility, smaller testes, and sometimes gynecomastia.

Ecology

• Biomes (Sample Highlights & Characteristics)

  • Large-scale ecosystems characterized by distinct climate patterns, dominant vegetation types, and adapted animal life.

  • Tundra: Found at high latitudes and altitudes. Characterized by permafrost (permanently frozen subsoil) which prevents deep rooted plants, very low annual precipitation, extremely cold temperatures, short growing season.

    • Vegetation: Dwarf shrubs, sedges, grasses, mosses, lichens (e.g., caribou moss).

    • Animals: Caribou, arctic fox, polar bears, migratory birds; adaptations include thick fur, blubber, hibernation.

  • Tropical Rainforest: Located near the equator. Characterized by consistently warm temperatures, high humidity, and heavy rainfall year-round. Has a thin, nutrient-poor soil layer (nutrients are rapidly recycled from decaying organic matter).

    • Vegetation: Tall, broadleaf evergreen trees forming dense canopies (emergent, canopy, understory, forest floor layers), many epiphytes (plants growing on other plants).

    • Animals: Exceptionally high biodiversity, including insects, monkeys, birds, reptiles, amphibians; many specialized niches.

  • Desert: Found in regions with very low annual precipitation (less than $25 \text{ cm}$). Experiences extreme temperature variations between day (very hot) and night (cold).

    • Vegetation: Sparse, well-adapted to aridity (e.g., succulents like cacti with CAM photosynthesis, deep roots, reduced leaves), often spiny.

    • Animals: Nocturnal animals (e.g., fennec fox, kangaroo rats), reptiles, insects; adaptations for water conservation (e.g., burrowing, specialized kidneys, dry feces).

  • Freshwater (Lakes/Ponds): Standing bodies of freshwater. Often exhibit zonation:

    • Littoral zone: Shallow, well-lit water close to shore; rooted plants (e.g., reeds, lilies).

    • Limnetic zone: Open, well-lit surface water away from shore; dominated by phytoplankton and zooplankton.

    • Profundal zone: Deep, aphotic (little or no light) water below the limnetic zone; relies on organic matter sinking from above; low oxygen.

    • Benthic zone: Bottom substrate.

    • Characterized by phytoplankton, various fish species, amphibians. Experience seasonal mixing (turnover) of nutrients and oxygen, especially in temperate regions.

  • Marine (Coral Reef): Diverse underwater ecosystems built by colonies of tiny polyps that secrete calcium carbonate skeletons, forming large structures. Found in warm, clear, shallow tropical waters that receive sufficient sunlight.

    • Organisms: Symbiotic dinoflagellates called zooxanthellae live within coral polyps, providing food through photosynthesis and giving corals their vibrant colors. Exceptionally high biodiversity of fish, invertebrates, and algae. Vulnerable to ocean acidification and rising temperatures.

• Food Chains/Webs & Energy Pyramid

  • Trophic levels: Describes the feeding positions of organisms in an ecosystem.

    • Producers (Autotrophs): First trophic level; produce their own food, usually through photosynthesis (e.g., plants, algae, cyanobacteria).

    • Primary Consumers (Herbivores): Second trophic level; eat producers (e.g., deer, rabbits).

    • Secondary Consumers (Carnivores/Omnivores): Third trophic level; eat primary consumers (e.g., fox, cat, humans).

    • Tertiary Consumers (Top Carnivores/Omnivores): Fourth trophic level; eat secondary consumers (e.g., eagles, sharks).

    • Decomposers (Detritivores): Break down dead organic matter from all trophic levels, returning nutrients to the soil (e.g., bacteria, fungi, earthworms).

  • Food Chain: A simple, linear representation of how energy flows through an ecosystem.

  • Food Web: A more complex and realistic model showing the interconnected feeding relationships among multiple organisms in an ecosystem.

  • 10% Rule (Energy Transfer): On average, only about $\sim 10\%$ of the energy from one trophic level is transferred to the next higher trophic level. The remaining 90% is lost as metabolic heat (due to respiration), or used for metabolic processes, or remains undigested/unassimilated. This inefficiency is explained by the Second Law of Thermodynamics (entropy), meaning ecosystems have limited high-level consumers.

  • Energy Pyramid (Ecological Pyramid): A graphical representation of the energy content at each trophic level, showing a progressive decrease in energy at higher levels, typically forming a pyramid shape.

  • Keystone species: A species that has a disproportionately large effect on its environment relative to its abundance. Its removal would cause a cascade of effects and could significantly alter the ecosystem (e.g., sea otters controlling sea urchin populations, preventing overgrazing of kelp forests; wolves in Yellowstone).

• Species Interactions

  • Various ways species interact within a community, influencing their population dynamics and evolution.

  • Mutualism ($+/+$): Both species benefit from the interaction (e.g., bees pollinating flowering plants while feeding on nectar; mycorrhizal fungi and plant roots).

  • Commensalism ($+/0$): One species benefits, while the other is neither significantly helped nor harmed (e.g., barnacles living on whales; cattle egrets feeding near grazing livestock).

  • Parasitism ($+/\text{–}$): One species (the parasite) benefits by deriving nourishment from another species (the host), which is harmed in the process (e.g., tapeworm living in a dog's intestine; ticks feeding on deer).

  • Predation ($+/\text{–}$): One species (predator) kills and consumes another species (prey) (e.g., lion hunting a zebra).

  • Competition ($-/\text{–}$): Species compete for limited resources (food, water, space, mates). Can be intraspecific (within the same species) or interspecific (between different species).

  • Symbiosis: A broad term referring to a long-term, close association between two or more different biological species. It encompasses mutualism, commensalism, and parasitism.

• Biogeochemical Cycles

  • The recycling pathways for essential chemical elements (like water, carbon, nitrogen, phosphorus) between the biotic (living) and abiotic (non-living) components of an ecosystem. Driven by biological, geological, and chemical processes.

  • Water Cycle (Hydrologic Cycle):

    • Processes: Evaporation (liquid to gas from bodies of water/land), Transpiration (evaporation of water from plant leaves, especially significant, accounting for $\sim 90\%$ of terrestrial evaporation), Condensation (gas to liquid, forming clouds), Precipitation (water falling as rain, snow, etc.), Runoff (water flowing over land surfaces into rivers/lakes/oceans), Infiltration (water seeping into the ground to form groundwater/aquifers).

    • Key reservoir: Oceans, ice caps, groundwater, atmosphere.

    • Human impact: Deforestation reduces transpiration and increases runoff; groundwater depletion; climate change altering precipitation patterns.

  • Carbon Cycle:

    • Carbon is the backbone of all organic molecules.

    • Photosynthesis: $6CO<em>2 + 6H</em>2O \rightarrow C<em>6H</em>{12}O<em>6 + 6O</em>2$ (fixation of atmospheric $CO_2$ into organic compounds by producers).

    • Cellular Respiration: Organic compounds are broken down by living organisms (producers, consumers, decomposers), releasing $CO_2$ back into the atmosphere and water.

    • Decomposition: Decomposers break down dead organic matter, returning carbon to the soil and atmosphere.

    • Ocean uptake: Oceans serve as a major carbon sink, absorbing $CO<em>2$ from the atmosphere, forming carbonic acid ($H</em>2CO_3$).

    • Fossil fuel combustion: Burning of coal, oil, and natural gas (stored carbon from ancient organic matter) releases large amounts of $CO_2$ into the atmosphere, significantly contributing to the greenhouse effect and climate change.

    • Reservoirs: Atmosphere ($CO<em>2$), oceans (dissolved $CO</em>2$, carbonates), fossil fuels, biomass, soils.

  • Nitrogen Cycle:

    • Nitrogen ($N_2$) is the most abundant gas in the atmosphere but is unusable by most organisms in this form. It's a key component of proteins and nucleic acids.

    • Nitrogen Fixation: Conversion of atmospheric nitrogen gas ($N<em>2$) into ammonia ($NH</em>3$) or ammonium ($NH_4^+$). This process is primarily carried out by nitrogen-fixing bacteria (e.g., Rhizobium in legume root nodules) or by lightning.

    • Ammonification: Decomposers convert organic nitrogen (from dead plants/animals and waste) back into ammonia ($NH_3$).

    • Nitrification: Ammonia ($NH<em>3$) or ammonium ($NH</em>4^+$) is oxidized by nitrifying bacteria into nitrites ($NO<em>2^-$) and then into nitrates ($NO</em>3^-$). Nitrates are the primary form of nitrogen absorbed by plants.

    • Assimilation: Plants absorb nitrates ($NO<em>3^-$) or ammonium ($NH</em>4^+$) from the soil and incorporate them into amino acids, proteins, and nucleic acids. Animals obtain nitrogen by eating plants or other animals.

    • Denitrification: Denitrifying bacteria convert nitrates ($NO<em>3^-$) back into nitrogen gas ($N</em>2$), which is released into the atmosphere, completing the cycle.

    • Human impact: Excessive use of synthetic nitrogen fertilizers leads to eutrophication of aquatic ecosystems (excessive nutrient enrichment causing algal blooms and oxygen depletion).

  • Phosphorus Cycle: Unique because it does not have a significant atmospheric gaseous phase; it is primarily a sedimentary cycle.

    • Weathering: Phosphate ($PO_4^{3-}$) is released from rocks through weathering and erosion.

    • Uptake: Plants absorb dissolved phosphate from soil or water.

    • Transfer: Phosphate moves through the food web when animals eat plants or other animals.

    • Return: Decomposers return phosphate to the soil from dead organic matter. Sedimentation can lock phosphorus into rocks, slowing its cycling.

    • Human impact: Runoff from agricultural fertilizers (containing phosphates) and detergents contributes to eutrophication.

Connections & Significance

• The polarity of water (Chemistry) and its ability to form hydrogen bonds are fundamental to the creation of phospholipid bilayers that form cell membranes (Cells) and drive the transpiration pull in plants, essential for water movement through ecosystems (Ecology).

• ATP, a modified nucleotide whose energy is released from chemical bonds (Chemistry), is predominantly produced in the mitochondria through cellular respiration (Cells). This ATP fuels crucial active transport processes like the Na⁺/K⁺ pump, which establishes the electrochemical gradient necessary for nervous impulses. Dysregulation of these impulses or related cellular processes can manifest in genetic disorders traced using pedigrees (Genetics).

• The processes of crossing-over during Prophase I of meiosis and independent assortment during Metaphase I (Genetics) are primary sources of genetic variation within a population. This variation is the raw material upon which natural selection acts, allowing species to adapt and evolve in response to changing environmental conditions across diverse biomes (Ecology).

• The Carbon cycle (Ecology) provides a direct link between the biochemical processes of photosynthesis (occurring in Chloroplasts, Cells) and cellular respiration (occurring in Mitochondria, Cells). This interdependency showcases a fundamental chemistry $\leftrightarrow$ ecology feedback loop, where the products of one process are the reactants of the other, illustrating the continuous flow of matter and energy.

Ethical / Practical Notes

• Genetically Modified Organisms (GMOs) involve the manipulation of nucleic acids (DNA) in living organisms to introduce desired traits, such as disease resistance or increased yield. While offering potential benefits for food security and crop resilience, this technology raises complex ethical questions regarding environmental impact (e.g., potential ecological release of modified genes) and equity (access to these technologies for all farmers).

• CRISPR gene-editing is a revolutionary technology derived from bacterial defense systems that allows for precise editing of DNA sequences. It offers unprecedented potential for curing genetic disorders (Genetics) and improving human health. However, its power necessitates careful ethical consideration regarding unintended consequences (e.g., off-target edits), potential for germline editing (changes passed to offspring), and societal equity in access to such advanced medical interventions.

• Climate change is largely driven by human perturbation of biogeochemical cycles, particularly the carbon cycle, through the excessive release of $CO<em>2$ from fossil fuel combustion. Understanding the chemical bonds and pH of water is crucial for tracking impacts like ocean acidification, where increased atmospheric $CO</em>2$ dissolves into seawater, forming carbonic acid: $CO<em>2 + H</em>2O \rightarrow H<em>2CO</em>3$, which further dissociates to produce hydrogen

Cells

• Key Organelles & Functions

  • Cell membrane: phospholipid bilayer + proteins → selective permeability, signal transduction

  • Mitochondrion: double membrane; inner folds = cristae; site of aerobic respiration $C<em>6H</em>{12}O<em>6 + 6 O</em>2 \rightarrow 6 CO<em>2 + 6 H</em>2O + \sim36ATP$

  • Chloroplast: thylakoids (light rxn) + stroma (Calvin cycle); photosynthesis $6CO<em>2 + 6H</em>2O \xrightarrow{light} C<em>6H</em>{12}O<em>6 + 6O</em>2$

  • Ribosome: rRNA + protein; translation of mRNA → polypeptide; free (cytosolic) vs. bound (rough ER)

  • Nucleus: double membrane (nuclear envelope) + pores; houses DNA, nucleolus builds ribosomal subunits

• Membrane Transport

  • Passive (no ATP)

  • Diffusion: down gradient (O₂ into cells)

  • Osmosis: water via aquaporins; hyper/hypo/isotonic effects (plant turgor, animal lysis)

  • Facilitated diffusion: channels (K⁺ leak) & carriers (GLUT4)

  • Active (ATP required)

  • Primary: Na⁺/K⁺ pump moves $3Na^+$ out/$2K^+$ in; sets membrane potential

  • Secondary: symport/antiport using stored gradient (Na⁺-glucose cotransporter)

  • Bulk: endocytosis (phagocytosis, pinocytosis, receptor-mediated) & exocytosis

• Mitosis

  • Purpose: asexual division → identical $2n$ cells for growth/repair

  • Phases: Prophase (chromatin condenses, spindle forms) → Metaphase (chromosomes line up at equator) → Anaphase (sister chromatids split) → Telophase (nuclei reform) + Cytokinesis (cleavage furrow/cell plate)

  • End: 2 diploid cells genetically identical to parent

• Meiosis

  • Purpose: gametogenesis → genetic diversity; produces haploid $n$ gametes

  • Two divisions

  • Meiosis I (reductional): homologs pair (synapsis) forming tetrads, crossing-over (prophase I), independent assortment (metaphase I) → 2 haploid cells

  • Meiosis II (equational): resembles mitosis; chromatids separate → 4 genetically unique haploid cells

Genetics

• Vocabulary

  • Homozygous (TT / tt); heterozygous (Tt); purebred (same alleles); hybrid (mixed alleles)

• Punnett Squares

  • Monohybrid cross $Tt \times Tt$ → genotype 1 TT : 2 Tt : 1 tt; phenotype 3 tall : 1 short

  • Dihybrid cross $RrYy \times RrYy$ → 9:3:3:1 phenotypic ratio (law of independent assortment)

• Non-Mendelian Patterns

  • Codominance: both alleles fully expressed (AB blood type)

  • Incomplete dominance: heterozygote intermediate (red × white snapdragons → pink)

  • Multiple alleles: >2 forms at locus (ABO, rabbit coat)

• Pedigrees

  • Squares = male, circles = female; shaded = affected; horizontal line = mating; generations = Roman numerals; track inheritance (autosomal/x-linked, dominant/recessive)

• Common Genetic Disorders

  • Autosomal recessive: cystic fibrosis, sickle-cell anemia

  • Autosomal dominant: Huntington’s disease, achondroplasia

  • X-linked: hemophilia A, Duchenne muscular dystrophy

  • Chromosomal: Down syndrome (trisomy 21), Turner (XO)

Ecology

• Biomes (sample highlights)

  • Tundra: permafrost soil, lichens, mosses, caribou, arctic fox; low precipitation, cold

  • Tropical rainforest: thin nutrient-poor soil, broadleaf evergreen, epiphytes, high biodiversity, constant warm temp & heavy rainfall

  • Desert: sandy soil, succulents (CAM photosynthesis), nocturnal animals, high temp variation, low precipitation

  • Freshwater (lakes/ponds): zonation (littoral, limnetic), phytoplankton, fish, nutrient turnover (seasonal mixing)

  • Marine (coral reef): calcium carbonate skeleton; symbiotic zooxanthellae; clear warm water

• Food Chains/Webs & Energy Pyramid

  • Trophic levels: producer → primary consumer → secondary → tertiary

  • 10% rule: only ~10 % of energy passes up; explained by entropy (2nd law of thermodynamics)

  • Keystone species: disproportionate ecological impact (sea otters controlling urchins in kelp forests)

• Species Interactions

  • Mutualism (++): bees & flowering plants

  • Commensalism (+0): barnacles on whales

  • Parasitism (+–): tapeworm in dog

  • Symbiosis = long-term close association encompassing all three above

• Biogeochemical Cycles

  • Water cycle: evaporation → condensation → precipitation → runoff; transpiration by plants ((~90\%) of terrestrial evaporation)

  • Carbon cycle

  • Photosynthesis: $6CO<em>2 + 6H</em>2O \rightarrow C<em>6H</em>{12}O<em>6 + 6O</em>2$ (fixation)

  • Respiration & decomposition return $CO_2$; ocean uptake, fossil fuel combustion

  • Nitrogen cycle

  • Nitrogen fixation: $N<em>2 \xrightarrow{nitrogenase} NH</em>3$ (by Rhizobium, lightning)

  • Nitrification: $NH<em>3 \rightarrow NO</em>2^- \rightarrow NO_3^-$

  • Assimilation into amino acids/nucleotides

  • Denitrification: $NO<em>3^- \rightarrow N</em>2$ returning gas to atmosphere

  • Human impact: fertilizer runoff → eutrophication

Connections & Significance

• Water’s polarity (Chemistry) underlies membrane formation (Cells) & transpiration pull (Ecology)

• ATP produced in mitochondria (Cells) fuels active transport of Na⁺/K⁺ that sets the nervous impulses traced in pedigree disorders (Genetics)

• Crossing-over in meiosis → genetic variation that natural selection acts on across biomes (Ecology)

• Carbon cycle links photosynthesis (Chloroplasts) to respiration (Mitochondria) showing chemistry ↔ ecology feedback

Ethical / Practical Notes

• GMOs manipulate nucleic acids for crop resilience—ties genetics to food-web stability

• CRISPR gene-editing offers cures for disorders but raises ecological release & equity questions

• Climate change perturbs biogeochemical cycles; understanding bonds/pH helps track ocean acidification $CO<em>2 + H</em>2O \rightarrow H<em>2CO</em>3 \rightarrow H^+ + HCO_3^-$