OSAT Biological Sciences (110) cheat sheet

Hierarchy of organization: atom → molecule → organelle → cell → tissue → organ → organ system → organism → population → community → ecosystem → biosphere; each level has emergent properties not predictable from lower levels alone.

Scientific method sequence: observation → question → hypothesis → prediction → controlled experiment → data analysis → conclusion; a hypothesis must be falsifiable and testable.

Independent variable is manipulated by the researcher; dependent variable is measured; controlled variables are held constant; a control group receives no treatment or a placebo.

Four classes of biological macromolecules: carbohydrates (C,H,O; 1:2:1 ratio), lipids (C,H,O; mostly nonpolar), proteins (C,H,O,N, sometimes S), nucleic acids (C,H,O,N,P).

Water properties trace to polarity and hydrogen bonding: high specific heat (~1 cal/g/°C), high heat of vaporization, cohesion/adhesion (capillary action), universal solvent, ice less dense than liquid water.

pH scale is logarithmic: pH = −log[H+]; each unit is a tenfold change; buffers resist pH change by absorbing or releasing H+; pH 7 is neutral at 25 °C.

Cell theory: all living things are made of cells, the cell is the basic unit of life, and all cells arise from pre-existing cells.

Biological magnification concentrates fat-soluble toxins at successively higher trophic levels; only about 10% of energy transfers between trophic levels.

Prokaryotic cells lack a membrane-bound nucleus and membrane-bound organelles; eukaryotic cells have a nucleus, mitochondria, and endomembrane system.

Endosymbiotic theory: mitochondria and chloroplasts derive from engulfed bacteria; evidence includes double membranes, own circular DNA, and 70S ribosomes.

Membrane structure is a fluid mosaic: phospholipid bilayer with embedded proteins, cholesterol (animals), and glycoproteins; small nonpolar molecules diffuse freely, ions and polar molecules need channels or carriers.

Transport decision rule: simple diffusion (down gradient, no energy), facilitated diffusion (down gradient, via protein, no ATP), active transport (against gradient, requires ATP), osmosis (water across a semipermeable membrane toward higher solute concentration).

Enzymes lower activation energy, are not consumed, and are specific to substrates at the active site; competitive inhibitors compete for the active site, noncompetitive inhibitors bind elsewhere and alter shape.

Cellular respiration: glycolysis (cytoplasm, 2 ATP net, 2 NADH) → pyruvate oxidation → Krebs cycle (matrix, NADH, FADH2, CO2) → oxidative phosphorylation (inner membrane, ~30–32 ATP total, O2 as final electron acceptor).

Photosynthesis: light reactions in thylakoid membranes split water, release O2, and produce ATP and NADPH; the Calvin cycle in the stroma fixes CO2 via rubisco into G3P.

Cell cycle: G1 → S (DNA replication) → G2 → M; mitosis phases are prophase, metaphase, anaphase, telophase; checkpoints at G1, G2, and M regulate progression; cancer results from loss of checkpoint control.

DNA structure: antiparallel double helix, sugar-phosphate backbone, complementary base pairing A–T (two H bonds) and G–C (three H bonds); replication is semiconservative.

Central dogma: DNA → RNA (transcription, RNA polymerase) → protein (translation at ribosomes); codons are triplet, degenerate, and read 5'→3' on mRNA.

Mendel's laws: segregation (alleles separate during meiosis) and independent assortment (genes on different chromosomes assort independently); Punnett squares predict genotype and phenotype ratios.

Incomplete dominance yields an intermediate phenotype; codominance expresses both alleles; multiple alleles (ABO blood) and polygenic traits (height, skin color) produce continuous variation.

Sex-linked traits carried on the X chromosome appear more often in males because males are hemizygous; a carrier mother × unaffected father gives 50% of sons affected.

Evolution mechanisms: natural selection, mutation, gene flow, genetic drift (bottleneck and founder effects), and nonrandom mating; Hardy–Weinberg equilibrium requires no mutation, no migration, no selection, infinite population, and random mating.

Speciation requires reproductive isolation; allopatric speciation involves geographic separation, sympatric speciation occurs without it (polyploidy, habitat shifts).

Evidence for evolution: fossil record, homologous structures (common ancestry), analogous structures (convergent evolution), vestigial structures, molecular/DNA similarity, and biogeography.

Plant tissue systems: dermal (epidermis, cuticle), vascular (xylem transports water and minerals up; phloem transports sugars bidirectionally), and ground (parenchyma, collenchyma, sclerenchyma).

Transpiration pull and cohesion-tension explain water movement in xylem; stomata open when guard cells take up water and K+, closing to conserve water.

Animal tissue types: epithelial (covering/lining), connective (bone, blood, cartilage), muscle (skeletal, cardiac, smooth), and nervous (neurons, glia).

Homeostasis uses negative feedback (reverses deviation, e.g., insulin lowering blood glucose) and positive feedback (amplifies deviation, e.g., oxytocin in childbirth).

Population growth: exponential when resources are unlimited (dN/dt = rN); logistic when carrying capacity K limits growth (dN/dt = rN(K−N)/K), producing an S-shaped curve.

Species interactions: mutualism (+,+), commensalism (+,0), parasitism (+,−), predation (+,−), competition (−,−); competitive exclusion occurs when two species compete for the same limiting resource.

Succession: primary begins on bare substrate with no soil (lichens, mosses); secondary follows disturbance with soil intact; pioneer species give way to a climax community.

Biogeochemical cycles: carbon (photosynthesis, respiration, combustion), nitrogen (fixation by Rhizobium and lightning, nitrification, denitrification), water, and phosphorus (no atmospheric phase; cycles via rock and soil).