Free Praxis Biology Exam Flashcards

Hypothesis = testable, falsifiable prediction about a single observation. Theory = well-substantiated explanation supported by repeated experiments and multiple lines of evidence (e.g., cell theory, evolution). Teach students that 'theory' in science is NOT a guess.

Independent variable is what the experimenter changes (manipulated, plotted on x-axis). Dependent variable is what is measured in response (plotted on y-axis). Control variables stay constant. Only one independent variable should change per experimental trial.

Safety Data Sheets (formerly MSDS, renamed under OSHA's 2012 GHS adoption) document chemical hazards, handling, storage, and first aid. Required for every hazardous chemical in a school lab and must be accessible to all students and teachers before use.

Always start with lowest-power objective (4x scanning lens) to locate specimen, then switch to higher magnification. Total magnification = objective × ocular (10x). Use coarse focus only on low power; fine focus only on high power to avoid cracking slides.

Live microbe cultures must be autoclaved (121°C, 15 psi, 15+ min) before disposal. Preserved specimens with formaldehyde require chemical-waste pickup, not sink disposal. Dissection tissue goes in biohazard bags. Never pour ethidium bromide or other mutagens down the drain.

Accuracy = how close a measurement is to the true value. Precision = how close repeated measurements are to each other. A scale can be precise (consistent) but inaccurate (off the true value) if it has a systematic calibration error.

Prokaryotes (bacteria, archaea): no membrane-bound nucleus, no organelles, circular DNA, 70S ribosomes, smaller (1–10 μm). Eukaryotes (protists, fungi, plants, animals): true nucleus, membrane-bound organelles, linear DNA with histones, 80S ribosomes, larger (10–100 μm).

Plant cells uniquely contain: cell wall (cellulose), chloroplasts, large central vacuole, and plasmodesmata. Animal cells uniquely contain: centrioles and lysosomes (rare in plants). Both share nucleus, mitochondria, ER, Golgi, ribosomes, and plasma membrane.

Site of aerobic cellular respiration — produces ATP via the Krebs cycle (matrix) and electron transport chain (inner membrane cristae). Contains own circular DNA and 70S ribosomes (endosymbiotic theory). Inner membrane is folded to maximize surface area for ATP synthesis.

Site of photosynthesis in plants and algae. Contains chlorophyll in thylakoid membranes (stacked in grana). Light reactions occur on thylakoid membranes; Calvin cycle occurs in surrounding stroma. Like mitochondria, has its own circular DNA (endosymbiotic origin).

Mitochondria and chloroplasts evolved from free-living prokaryotes engulfed by ancestral eukaryotic cells ~2 billion years ago. Evidence: double membranes, circular DNA, 70S ribosomes, independent binary fission, sizes matching modern bacteria. Proposed by Lynn Margulis (1967).

Passive: no ATP needed, moves with concentration gradient (diffusion, osmosis, facilitated diffusion). Active: requires ATP, moves against gradient (Na+/K+ pump, endocytosis, exocytosis). Sodium-potassium pump moves 3 Na+ out and 2 K+ in per ATP hydrolyzed.

Hypotonic = lower solute outside cell → water enters → animal cells lyse, plant cells become turgid. Hypertonic = higher solute outside → water exits → animal cells crenate, plant cells plasmolyze. Isotonic = equal solute → no net water movement.

Glycolysis splits one 6-carbon glucose into two 3-carbon pyruvates in the cytoplasm. Net yield: 2 ATP (substrate-level phosphorylation) and 2 NADH. Occurs anaerobically and is the first step of both fermentation and aerobic respiration.

Per glucose molecule: glycolysis 2 ATP, Krebs cycle 2 ATP, electron transport chain ~28 ATP — total ~30–32 ATP (older textbooks cite 36–38). Variation depends on which shuttle moves NADH from cytoplasm into mitochondria. Anaerobic fermentation yields only 2 ATP.

Light reactions (thylakoid membrane): use H2O and light to produce ATP, NADPH, and O2. Calvin cycle (stroma, light-independent): uses ATP and NADPH to fix CO2 into G3P via RuBisCO enzyme. Both stages happen in chloroplasts but only the first requires light directly.

6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2. CO2 is reduced to glucose; water is oxidized to release O2. The oxygen we breathe originates from the H2O split during the light reactions, NOT from CO2.

Prophase: chromatin condenses, nuclear envelope breaks down. Metaphase: chromosomes align at metaphase plate. Anaphase: sister chromatids separate to opposite poles. Telophase: nuclear envelopes reform. Cytokinesis follows, producing 2 genetically identical diploid daughter cells.

Mitosis: 1 division → 2 diploid (2n) daughter cells genetically identical to parent (somatic cells). Meiosis: 2 divisions → 4 haploid (n) daughter cells genetically unique (gametes). Humans: mitosis 46 → 46, 46; meiosis 46 → 23, 23, 23, 23.

Occurs during prophase I of meiosis at chiasmata, when homologous chromosomes exchange segments of DNA. Creates genetic recombination, increasing offspring variation. This is one of the three sources of genetic diversity in sexual reproduction, along with independent assortment and random fertilization.

Double helix of antiparallel strands held by hydrogen bonds. Sugar-phosphate backbones run 5'→3' and 3'→5'. Base pairing: A=T (2 H-bonds), G≡C (3 H-bonds). Chargaff's rule: %A=%T and %G=%C. Discovered 1953 with Rosalind Franklin's X-ray diffraction.

Each parent strand serves as template for a new complementary strand; each daughter helix has 1 old + 1 new strand. Demonstrated by Meselson-Stahl 1958 using N15 isotope. Leading strand synthesized continuously, lagging strand in Okazaki fragments by DNA polymerase III (5'→3' direction only).

DNA → RNA → Protein. Transcription (nucleus): DNA copied to mRNA by RNA polymerase. Translation (ribosome): mRNA codons (3 bases) read to assemble amino acids carried by tRNA. Reverse transcription (retroviruses like HIV) and prions are exceptions to the original Crick formulation.

Codon = 3-nucleotide mRNA sequence coding for one amino acid (64 total). Anticodon = complementary tRNA triplet. Start codon: AUG (methionine). Stop codons: UAA, UAG, UGA (no amino acid). The code is degenerate (multiple codons per amino acid) and nearly universal across life.

Silent: codon changes but amino acid stays same (due to degenerate code). Missense: one amino acid swapped (e.g., sickle-cell). Nonsense: premature stop codon, truncates protein. Frameshift: insertion/deletion shifts reading frame, usually catastrophic. Frameshifts are typically more harmful than substitutions.

Each individual carries two alleles for a trait; during gamete formation (meiosis I), the two alleles separate so each gamete carries only one. Why a heterozygous parent (Aa) produces 50% A and 50% a gametes. Forms the basis for the 3:1 monohybrid ratio.

Alleles of different genes (on different chromosomes) segregate independently during meiosis. Produces the 9:3:3:1 dihybrid ratio. Exception: linked genes on the same chromosome assort together unless separated by crossing over — frequency of recombination is used to map gene distances.

Genes on sex chromosomes (usually X). X-linked recessive traits (color blindness, hemophilia, Duchenne muscular dystrophy) appear more often in males because males (XY) need only one recessive allele while females (XX) need two. Carrier mothers pass the trait to ~50% of sons.

p² + 2pq + q² = 1 and p + q = 1, where p and q are allele frequencies. Conditions for equilibrium (no evolution): (1) large population, (2) random mating, (3) no mutation, (4) no migration, (5) no natural selection. Violations of any condition cause allele frequencies to change.

Requires: (1) heritable variation, (2) overproduction of offspring, (3) differential survival/reproduction tied to traits. Individuals with favorable traits leave more offspring; allele frequencies shift over generations. Natural selection acts on phenotypes, NOT directly on genotypes, and it cannot create new mutations — only filter existing variation.

Directional: one extreme favored (peppered moths darkening). Stabilizing: intermediate phenotype favored, extremes selected against (human birth weight). Disruptive: both extremes favored over the mean, can drive speciation (African seedcracker finches with small or large beaks).

Allopatric: new species form when a population is geographically isolated (Galápagos finches). Sympatric: speciation occurs in the same area, often via polyploidy in plants or behavioral/sexual isolation (cichlids in African lakes). Both require reproductive isolation to be complete.

Fossil record showing transitional forms (Tiktaalik, Archaeopteryx), homologous structures (vertebrate forelimbs), vestigial organs (human appendix, whale pelvis), embryological similarities, biogeography (marsupials of Australia), molecular evidence (cytochrome c sequence similarity), direct observation (antibiotic resistance, Darwin's finches).

Bacteria (true bacteria, peptidoglycan walls), Archaea (extremophiles, branched lipid membranes, no peptidoglycan), and Eukarya (membrane-bound nucleus). Proposed by Carl Woese (1977) based on rRNA sequence comparison. Archaea are genetically more similar to Eukarya than to Bacteria.

Domain → Kingdom → Phylum → Class → Order → Family → Genus → Species. Mnemonic: 'Dear King Philip Came Over For Good Soup.' Binomial nomenclature uses genus + species, italicized (e.g., Homo sapiens). Genus is capitalized, species is lowercase.

Bryophytes (mosses, liverworts): nonvascular, no seeds. Pteridophytes (ferns): vascular, no seeds, spore reproduction. Gymnosperms (conifers): vascular, naked seeds in cones. Angiosperms (flowering plants): vascular, seeds enclosed in fruit, dominant land plants today. Vascular tissue (xylem/phloem) was a key evolutionary innovation.

Xylem: transports water and dissolved minerals UP from roots to leaves; composed of dead, hollow cells (tracheids, vessels) using transpiration pull. Phloem: transports sugars (photosynthate) from sources to sinks, can flow up or down; composed of living sieve-tube cells aided by companion cells.

Plant life cycle alternates between diploid sporophyte and haploid gametophyte phases. Sporophyte produces haploid spores via meiosis; gametophyte produces gametes via mitosis; fertilization regenerates sporophyte. Sporophyte dominates in vascular plants; gametophyte dominates in mosses.

Agnatha (jawless fish — lampreys), Chondrichthyes (cartilaginous fish — sharks), Osteichthyes (bony fish), Amphibia (frogs, salamanders), Reptilia (snakes, lizards), Aves (birds), Mammalia (mammals). Key innovations along the lineage: jaws, bony skeleton, four limbs, amniotic egg, endothermy.

Four-chambered heart with complete double circulation: right atrium → right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium → left ventricle → aorta → body → vena cava → right atrium. Prevents mixing of oxygenated and deoxygenated blood, enabling high metabolic rate.

Nervous: fast (milliseconds), short-lived, electrical impulses through neurons targeting specific cells. Endocrine: slow (seconds to hours), longer-lasting, hormones travel through bloodstream to target tissues with matching receptors. Hypothalamus links the two via the pituitary gland.

Producers (autotrophs) → Primary consumers (herbivores) → Secondary consumers (carnivores) → Tertiary consumers → Decomposers. Only ~10% of energy is transferred between levels (10% rule); the rest is lost as heat via cellular respiration. Limits food chains to typically 4–5 levels.

Photosynthesis removes atmospheric CO2 (sink). Cellular respiration, decomposition, combustion of fossil fuels, and volcanic activity release CO2 (sources). Oceans absorb ~25% of human CO2 emissions, causing ocean acidification. Anthropogenic burning has raised atmospheric CO2 from ~280 to over 420 ppm.

Atmospheric N2 is converted to usable forms by: nitrogen fixation (Rhizobium bacteria in legume root nodules or lightning) → ammonia (NH3) → nitrification (Nitrosomonas, Nitrobacter) → nitrate (NO3-). Denitrifying bacteria return N2 to the atmosphere. Nitrogen is often the limiting nutrient in terrestrial ecosystems.

Exponential (J-curve): unlimited resources, dN/dt = rN. Logistic (S-curve): limited by carrying capacity (K), dN/dt = rN(K-N)/K; growth slows as N approaches K. Real populations show logistic growth long-term because density-dependent factors (food, space, disease) intensify with density.

Type I (convex): low mortality early, high late (humans, elephants — few offspring, heavy parental care). Type II (linear): constant death rate at all ages (many birds, small mammals). Type III (concave): very high early mortality, survivors live long (oysters, fish, oak trees — many offspring, no parental care).

Mutualism (+/+): both species benefit (clownfish/anemone, mycorrhizae). Commensalism (+/0): one benefits, other unaffected (barnacles on whales). Parasitism (+/-): one benefits, other harmed (tapeworms in mammals). Competition (-/-): both harmed when sharing limited resource.

Primary: starts on bare rock with no soil (after glacial retreat, volcanic eruption); pioneer species like lichens build soil over centuries. Secondary: occurs where soil remains after disturbance (fire, abandoned farm); proceeds faster — grasses → shrubs → fast-growing trees → climax community.

Tundra (cold, low precipitation, permafrost), Taiga/boreal forest (coniferous), Temperate deciduous forest, Temperate grassland, Desert (<25 cm rainfall/year), Tropical rainforest (highest biodiversity, near equator), Savanna, Chaparral. Biomes are classified primarily by precipitation and temperature ranges.

A species whose impact on the community is disproportionately large relative to its abundance. Removal causes dramatic ecosystem changes. Classic example: sea otters control sea-urchin populations, protecting kelp forests. Other examples: gray wolves in Yellowstone, beavers as ecosystem engineers, starfish (Pisaster) in tidal communities.