Every Biology concept in the high school Science Bowl study set (grades 9-12), with a short definition for each. Read through them here, then drill them as flashcards or practise real Biology questions.
Double-membraned organelle that produces ATP via aerobic cellular respiration. The Krebs cycle runs in the matrix; the electron transport chain runs on the inner membrane (cristae). Contains its own circular DNA, evidence of endosymbiotic origin.
Nuclear division producing two genetically identical diploid daughter cells. Phases (PMAT): Prophase (chromatin condenses), Metaphase (chromosomes align at cell plate), Anaphase (sister chromatids separate), Telophase (nuclei reform). Used for growth and repair.
Two-stage division producing four haploid gametes with genetic variation. Crossing over (recombination) occurs during Prophase I. Meiosis I separates homologous pairs; Meiosis II separates sister chromatids. Results in genetic diversity.
DNA → RNA → Protein. Transcription: RNA polymerase reads the template DNA strand to produce mRNA in the nucleus. Translation: ribosomes read mRNA codons and tRNA anticodons to assemble polypeptides. rRNA is synthesized in the nucleolus.
Protein catalysts that lower activation energy without being consumed. Substrates bind at the active site. Classes: oxidoreductase (redox), hydrolase (water addition), transferase (group transfer), lyase (bond breaking without water), isomerase, ligase.
Converts light energy to chemical energy in chloroplasts. Light reactions (thylakoid membranes): split water, produce ATP and NADPH, release O₂. Calvin cycle (stroma): fixes CO₂ into G3P using ATP and NADPH. Net: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
Breaks down glucose to ATP: Glycolysis (cytoplasm, 2 net ATP), Krebs/Citric Acid Cycle (matrix, 2 ATP), Electron Transport Chain (inner membrane, ~32 ATP). Anaerobic fermentation produces lactic acid (animals) or ethanol + CO₂ (yeast).
Dominant alleles mask recessive ones in heterozygotes. F2 monohybrid cross: 3:1 phenotype ratio. Dihybrid cross (F2): 9:3:3:1 ratio. Law of Segregation: allele pairs separate during gamete formation. Law of Independent Assortment: genes on different chromosomes assort independently.
Mutualism (+/+): both benefit. Commensalism (+/0): one benefits, other unaffected. Parasitism (+/−): parasite benefits, host is harmed. Competition (−/−): both harmed. Predation (+/−): predator benefits. Niche: the ecological role of an organism in its community.
Three domains: Bacteria (no nucleus, peptidoglycan cell walls, single RNA polymerase), Archaea (no nucleus, no peptidoglycan, multiple RNA polymerases, can have introns), Eukarya (nucleus, complex organelles). Six-kingdom system places Monera, Protista, Fungi, Plantae, and Animalia under domains.
Nucleus: DNA storage and transcription. Ribosome: protein synthesis (80S in eukaryotes, 70S in prokaryotes). Rough ER: protein folding with ribosomes. Smooth ER: lipid synthesis. Golgi: modifies and packages proteins. Lysosome: hydrolytic digestion. Vacuole: storage (large in plant cells).
CNS (brain + spinal cord) processes information. PNS: sensory (afferent) carries signals to CNS; motor (efferent) carries signals away. Neurons transmit via action potentials (all-or-nothing); neurotransmitters cross the synaptic cleft. Reflex arcs bypass the brain for fast responses.
Fluid mosaic model: phospholipid bilayer with embedded proteins. Hydrophilic heads face outward; hydrophobic tails face inward. Selectively permeable — small nonpolar molecules pass freely, large or charged molecules require transport proteins. Cholesterol stabilizes membrane fluidity.
Diffusion: net movement of molecules from high to low concentration (passive, no energy). Osmosis: diffusion of water across a semipermeable membrane toward higher solute concentration. Hypertonic solution: cell loses water (crenation in animals, plasmolysis in plants). Hypotonic: cell gains water.
Heart has 4 chambers: right atrium → right ventricle → lungs (pulmonary circuit) → left atrium → left ventricle → body (systemic circuit). Oxygenated blood enters the left atrium via pulmonary veins. Arteries carry blood away from heart; veins return blood to heart.
Innate immunity: non-specific (skin barrier, phagocytes, inflammation). Adaptive immunity: specific responses. B cells produce antibodies (humoral immunity). T cells: helper T cells (CD4+) coordinate response; cytotoxic T cells (CD8+) kill infected cells. Memory cells enable faster secondary responses.
Glands secrete hormones into the bloodstream. Hypothalamus controls the pituitary (master gland). Key glands: thyroid (thyroxine — metabolism), adrenal (epinephrine, cortisol — stress), pancreas (insulin lowers blood glucose; glucagon raises it), gonads (sex hormones).
Mechanical and chemical breakdown of food. Mouth (amylase), esophagus, stomach (pepsin, HCl, pH ~2), small intestine (site of most absorption; villi and microvilli increase surface area), large intestine (water reabsorption). Liver produces bile; pancreas secretes digestive enzymes and bicarbonate.
Oxygen enters via nose/mouth → trachea → bronchi → bronchioles → alveoli (site of gas exchange). CO₂ diffuses out; O₂ diffuses into capillaries. Hemoglobin in red blood cells carries O₂ as oxyhemoglobin. Diaphragm contraction increases lung volume (inhalation = active process).
Kidneys filter blood to produce urine. Functional unit: nephron. Glomerulus (capillary ball in Bowman's capsule) filters blood by pressure. Loop of Henle concentrates filtrate. Collecting duct reabsorbs water (regulated by ADH). Nitrogenous wastes: urea (mammals), uric acid (birds/reptiles), ammonia (fish).
Non-living infectious agents consisting of nucleic acid (DNA or RNA) surrounded by a protein coat (capsid); some have a lipid envelope. Lytic cycle: virus hijacks host, replicates, lyses cell (releases new viruses). Lysogenic cycle: viral DNA integrates into host genome (prophage) and replicates with host.
Hardy-Weinberg equilibrium: allele frequencies stable if no mutation, migration, genetic drift, natural selection, or non-random mating. Equations: p + q = 1; p² + 2pq + q² = 1. Natural selection acts on phenotypes (determined by genotypes). Genetic drift: random allele frequency change in small populations.
Vascular plants have xylem (water/minerals upward) and phloem (sugars bidirectionally). Angiosperms (flowering plants) produce seeds in fruit; gymnosperms produce naked seeds (cones). Monocots: parallel leaf veins, fibrous roots; dicots: branching veins, taproot. Bryophytes (mosses) lack vascular tissue.
Skeletal muscle: striated, voluntary, attached to bones via tendons, multinucleated. Smooth muscle: non-striated, involuntary, found in organ walls and blood vessels. Cardiac muscle: striated, involuntary, found only in the heart, connected by intercalated discs. Sarcomere is the basic contractile unit.
Maintenance of stable internal conditions despite external changes. Negative feedback: response opposes the stimulus (e.g., insulin lowers high blood glucose). Positive feedback: response amplifies the stimulus (e.g., oxytocin during childbirth, blood clotting). Temperature, pH, and blood glucose are key regulated variables.
Double helix: two antiparallel strands of nucleotides. Each nucleotide: deoxyribose sugar, phosphate group, nitrogenous base. Complementary base pairing: A–T (2 hydrogen bonds), G–C (3 hydrogen bonds). 5'→3' directionality. Chargaff's rule: %A = %T and %G = %C in any organism.
Natural selection: heritable variation + differential reproductive success = adaptation over generations. Darwin's observations: overproduction, variation, heritability, differential survival. Speciation: allopatric (geographic isolation) vs sympatric. Evidence: fossil record, homologous structures, molecular phylogenetics.
ATP (adenosine triphosphate) is the universal energy currency of cells. Structure: adenine base + ribose sugar + three phosphate groups. Hydrolysis of ATP → ADP + Pᵢ releases ~7.3 kcal/mol. ATP is regenerated by substrate-level phosphorylation (glycolysis, Krebs) and oxidative phosphorylation (ETC). The cell maintains a high ATP:ADP ratio to drive endergonic reactions.
Process by which cells respond to external signals. Steps: ligand binds receptor → second messenger generated → cascade of phosphorylation events → cellular response. Types: G-protein coupled receptors (GPCRs) use cAMP or IP₃/Ca²⁺ as second messengers; receptor tyrosine kinases (RTKs) phosphorylate proteins directly. Amplification: one ligand triggers many downstream effectors.
Phases: G1 (cell growth), S (DNA replication), G2 (preparation for division), M (mitosis + cytokinesis). Checkpoints: G1/S (DNA damage), G2/M (replication complete), spindle checkpoint (chromosomes attached). Cyclins and CDKs drive progression. p53 is a tumor suppressor that halts the cycle at damage. Mutations disabling checkpoints or activating proto-oncogenes lead to cancer.
Programmed cell death — an orderly, energy-dependent process essential for development and homeostasis. Intrinsic pathway: mitochondria release cytochrome c in response to DNA damage → activates caspase cascade. Extrinsic pathway: death receptors (e.g., Fas) activated by external signals. Apoptosis eliminates damaged, infected, or unneeded cells without triggering inflammation. Defective apoptosis → cancer or autoimmune disease.
Undifferentiated cells capable of self-renewal and differentiation into specialized cell types. Totipotent: can form any cell (fertilized egg). Pluripotent: most cell types (embryonic stem cells, iPSCs). Multipotent: limited types (bone marrow stem cells → blood cells). Induced pluripotent stem cells (iPSCs): adult cells reprogrammed to pluripotency by Yamanaka factors (Oct4, Sox2, Klf4, c-Myc).
PCR (polymerase chain reaction): amplifies DNA exponentially using primers and Taq polymerase. Gel electrophoresis: separates DNA fragments by size. Restriction enzymes cut DNA at specific sequences; ligase joins fragments (recombinant DNA). CRISPR-Cas9: guide RNA directs Cas9 nuclease to cut target DNA; allows precise gene editing. Southern blot: detects specific DNA sequences; Northern: RNA; Western: proteins.
Primary: sequence of amino acids linked by peptide bonds (determined by DNA). Secondary: local folding into α-helices and β-sheets via hydrogen bonds between backbone atoms. Tertiary: overall 3-D shape stabilized by hydrophobic interactions, disulfide bonds, ionic bonds, H-bonds between R groups. Quaternary: multiple polypeptide subunits (e.g., hemoglobin has 4). Denaturation disrupts 3-D structure without breaking peptide bonds.
Hydrophobic biomolecules. Triglycerides: glycerol + 3 fatty acids; energy storage (9 kcal/g). Saturated fats: no double bonds, solid at room temp. Unsaturated (mono/poly): one or more double bonds, liquid at room temp. Phospholipids: form cell membranes (hydrophilic head, hydrophobic tails). Steroids: four-ring structure; cholesterol is membrane component and precursor to steroid hormones (testosterone, estrogen, cortisol).
Monosaccharides: simplest sugars (glucose, fructose, galactose — C₆H₁₂O₆). Disaccharides: two monosaccharides joined by glycosidic bond (sucrose = glucose + fructose; lactose = glucose + galactose; maltose = glucose + glucose). Polysaccharides: starch and glycogen (branched glucose polymers for energy storage); cellulose (β-1,4 linkages; structural; humans cannot digest); chitin (structural in fungi/arthropods).
Nitrogen fixation: N₂ → NH₃ by Rhizobium (in legume root nodules) and free-living bacteria (Azotobacter). Nitrification: NH₃ → NO₂⁻ (Nitrosomonas) → NO₃⁻ (Nitrobacter). Assimilation: plants absorb NH₄⁺ or NO₃⁻ to make amino acids and nucleic acids. Ammonification: decomposers convert organic N back to NH₄⁺. Denitrification: NO₃⁻ → N₂ by anaerobic bacteria (Pseudomonas). Humans disrupt the cycle via fertilizers and combustion.
The gradual process of community change over time. Primary succession: begins on bare rock with no soil (volcanic island, glacier retreat); pioneer species (lichens, mosses) weather rock and build soil. Secondary succession: disturbance of an existing ecosystem (fire, flood); soil remains so recovery is faster. Climax community: stable endpoint determined by climate. Facilitation: early species modify the environment enabling later species.
Innate (fixed action patterns, FAPs): genetically determined, triggered by specific stimuli (releaser). Learned: habituation (decreased response to repeated stimulus), classical conditioning (Pavlov), operant conditioning (Skinner), imprinting (Lorenz — critical period), insight learning. Altruism explained by kin selection (Hamilton's rule: rb > c, where r = relatedness, b = benefit, c = cost). Foraging theory: optimal energy gain per unit time.
Heterotrophic decomposers with cell walls of chitin. Body plan: hyphae (filaments) forming a mycelium. Reproduction: asexual (conidia spores) and sexual (zygospores, ascospores, basidiospores). Major phyla: Zygomycota (bread molds), Ascomycota (yeasts, morels, Penicillium), Basidiomycota (mushrooms, rusts, smuts). Mycorrhizae: mutualistic symbiosis with plant roots (provide phosphorus; receive sugars). Lichens: mutualism with algae/cyanobacteria.
Paraphyletic group of mostly unicellular eukaryotes. Algae (photoautotrophic): diatoms (silica shells), dinoflagellates (red tides, bioluminescence), green algae (ancestor of plants). Animal-like protozoa: amoeba (pseudopods), paramecium (cilia), trypanosoma (flagella, causes sleeping sickness), Plasmodium (causes malaria — transmitted by Anopheles mosquito). Slime molds: form multicellular fruiting bodies for reproduction.
Auxin (IAA): cell elongation, phototropism (shoots toward light), apical dominance (suppresses lateral buds); made in shoot tips. Gibberellins: stem elongation, seed germination, fruit development (seedless fruits). Cytokinins: promote cell division, delay senescence; synthesized in roots. Ethylene (gas): fruit ripening, leaf/flower/fruit abscission, response to mechanical stress. Abscisic acid (ABA): stomatal closure during drought, seed dormancy, stress response.
Angiosperms (flowering plants): flowers contain stamens (anther produces pollen = male gametophyte) and pistil (stigma, style, ovary with ovules). Pollination: transfer of pollen to stigma (wind, insects, birds). Double fertilization (unique to angiosperms): one sperm + egg → zygote (2n); second sperm + polar nuclei → endosperm (3n, nutrient tissue). Fruit = mature ovary. Seed dispersal: wind, water, animals (ingestion or attachment).
Regulation of water and solute balance. Kidney nephron: glomerular filtration (pressure-driven), tubular reabsorption (glucose, amino acids, Na⁺ reabsorbed; loop of Henle concentrates filtrate), tubular secretion (H⁺, K⁺ added). ADH (vasopressin): increases water permeability of collecting duct → concentrated urine. Aldosterone: increases Na⁺ reabsorption → retains water. Freshwater fish: hypotonic environment; lose salts, gain water — excrete dilute urine.
Ectotherms (poikilotherms): body temperature varies with environment; rely on behavioral regulation (basking, burrowing). Endotherms (homeotherms): maintain constant core temperature via metabolic heat; include birds and mammals. Mechanisms: shivering (muscle thermogenesis), sweating/panting (evaporative cooling), vasodilation (radiates heat) vs. vasoconstriction (conserves heat). Brown adipose tissue (BAT): non-shivering thermogenesis. Countercurrent heat exchange in extremities.
External fertilization: sperm and eggs released into water (fish, amphibians); requires aquatic environment. Internal fertilization: sperm deposited inside female; required for terrestrial life. Oviparous: eggs develop outside (birds, reptiles, fish). Ovoviviparous: eggs retained internally (some sharks, snakes). Viviparous: live birth with placenta (most mammals). r-strategy: many small offspring, little parental care (insects). K-strategy: few offspring, heavy parental investment (elephants, humans).
Prokaryotes — operons: lac operon (glucose absent + lactose present → CAP activates; lac repressor inactivated → transcription on). Eukaryotes: transcription factors bind enhancers/promoters; histone acetylation opens chromatin (active); methylation silences genes; RNA processing (alternative splicing → multiple proteins from one gene); miRNA: small RNAs that base-pair with mRNA → degradation or translation inhibition. Epigenetics: heritable changes in gene expression without DNA sequence change.
Transgenic organisms: GMO crops (Bt toxin gene from Bacillus thuringiensis inserted into corn/soy). Gene therapy: correcting defective genes (e.g., AAV vectors for SCID, hemophilia). Monoclonal antibodies: produced by hybridoma cells; used in cancer treatment (trastuzumab) and diagnostics. DNA fingerprinting: uses STR (short tandem repeat) analysis; used in forensics and paternity testing. Human Genome Project (completed 2003): sequenced ~3 billion base pairs across 23 chromosome pairs.
Mechanoreceptors: detect pressure/vibration (Meissner's, Pacinian corpuscles in skin; hair cells in cochlea for hearing). Photoreceptors: rods (dim light, monochromatic, peripheral) and cones (color vision, S/M/L for blue/green/red, fovea for acuity). Chemoreceptors: olfactory neurons (smell, ~400 receptor types) and taste buds (sweet, salty, sour, bitter, umami). Nociceptors: pain. Vestibular system: semicircular canals detect rotational acceleration; utricle/saccule detect linear acceleration.
Allele frequencies remain stable in a population across generations when: no mutation, no migration (gene flow), no genetic drift, random mating, no natural selection. Equations: p + q = 1 (allele frequencies); p² + 2pq + q² = 1 (genotype frequencies). Hardy-Weinberg is used as a null model to detect evolution: any deviation from expected frequencies indicates one of the five forces is acting on the population.