Biology cheat sheet

Prokaryotic cells lack membrane-bound organelles and a nucleus; eukaryotic cells compartmentalize DNA in a nucleus and house oxidative phosphorylation in mitochondria.

Endosymbiotic theory: mitochondria and chloroplasts derive from engulfed bacteria, evidenced by double membranes, circular DNA, and 70S ribosomes.

Membrane transport decision rule: small nonpolar molecules diffuse freely; polar solutes need channels or carriers; ions and large molecules require pumps or vesicular transport; water uses aquaporins.

Enzyme kinetics: Michaelis-Menten v = Vmax[S]/(Km + [S]); Km equals the substrate concentration at half Vmax and is inversely related to substrate affinity.

Competitive inhibitors raise apparent Km with unchanged Vmax; noncompetitive inhibitors lower Vmax with unchanged Km; uncompetitive inhibitors lower both.

ATP yield per glucose: about 30-32 ATP in eukaryotes (2 glycolysis, 2 Krebs substrate-level, rest oxidative phosphorylation); prokaryotes yield slightly more due to no transport cost.

Glycolysis occurs in cytoplasm (net 2 ATP, 2 NADH, 2 pyruvate); Krebs cycle in mitochondrial matrix; electron transport chain on inner mitochondrial membrane.

Cell cycle checkpoints: G1/S (DNA damage, size), G2/M (replication complete, damage), spindle assembly checkpoint in M; failure of checkpoints permits aneuploidy and tumorigenesis.

Central dogma: DNA to RNA via transcription, RNA to protein via translation; retroviruses reverse this with reverse transcriptase.

DNA base pairing: A-T (2 hydrogen bonds), G-C (3 hydrogen bonds); higher GC content raises melting temperature.

Mendelian ratios: monohybrid F2 phenotype 3:1, dihybrid F2 9:3:3:1; testcross with homozygous recessive reveals unknown genotype.

Chi-square test: X^2 = sum of (observed - expected)^2 / expected; compare to critical value at df = categories - 1 to accept or reject the null hypothesis.

Sex-linked recessive traits appear more often in males (XY) because a single recessive allele on X is expressed; affected fathers pass the allele to all daughters.

Hardy-Weinberg equilibrium requires no mutation, no migration, random mating, infinite population, and no selection; p^2 + 2pq + q^2 = 1 and p + q = 1.

Natural selection requires heritable variation in fitness; genetic drift is random allele frequency change strongest in small populations; gene flow reduces divergence between populations.

Speciation: allopatric requires geographic isolation; sympatric occurs without it, often via polyploidy or disruptive selection; reproductive isolation defines species under the biological species concept.

Surface area to volume ratio decreases as organism size increases; this constrains diffusion-only exchange and drives circulatory and respiratory systems.

Mammalian heart: right side pumps deoxygenated blood to lungs, left side pumps oxygenated blood to body; systemic pressure exceeds pulmonary pressure.

Gas exchange in alveoli relies on thin walls, large surface area, and partial pressure gradients; hemoglobin oxygen affinity falls as CO2 rises or pH drops (Bohr effect).

Plant vascular tissue: xylem transports water and minerals upward under tension from transpiration; phloem transports sugars bidirectionally via pressure flow.

Stomata open when guard cells take up K+ and water, becoming turgid; closure occurs with ABA signaling during drought.

Homeostasis via negative feedback: a sensor detects deviation, an integrator compares to a set point, and an effector reverses the change; positive feedback amplifies (e.g., oxytocin in labor).

Neuron action potential: depolarization opens voltage-gated Na+ channels, repolarization opens K+ channels, refractory period prevents backward propagation.

Osmoregulation strategies: osmoconformers match internal osmolarity to environment; osmoregulators maintain constant internal osmolarity using kidneys, gills, or salt glands.

Population growth: exponential dN/dt = rN; logistic dN/dt = rN(K - N)/K, where K is carrying capacity and growth is fastest at N = K/2.

Trophic levels transfer roughly 10% of energy to the next level; biomass pyramids invert in some aquatic systems due to rapid phytoplankton turnover.

Interspecific interactions: competition (-/-), predation and parasitism (+/-), mutualism (+/+), commensalism (+/0), amensalism (-/0).

Primary succession begins on bare substrate lacking soil; secondary succession follows disturbance that leaves soil intact and proceeds faster.

Carbon cycle: photosynthesis fixes CO2, respiration and combustion release it; nitrogen cycle relies on fixation, nitrification, assimilation, ammonification, and denitrification.

Keystone species exert disproportionate community influence relative to abundance; removal triggers cascading changes; foundation species create habitat structure.

Biodiversity indices: species richness counts species; Shannon index H = -sum(p_i ln p_i) weights both richness and evenness.

Biogeochemical principle: matter cycles while energy flows one way and is ultimately lost as heat; net primary productivity = gross primary productivity minus respiration.

Experimental design: independent variable is manipulated, dependent variable is measured, controlled variables are held constant, and a control group receives no treatment or a placebo.

Validity versus reliability: validity means measuring the intended quantity; reliability means repeatable results; precision is repeatability of a measurement, accuracy is closeness to true value.

Random assignment reduces confounding in experiments; random sampling improves generalizability of survey and observational data.

Correlation does not establish causation; confounding variables and reverse causation are alternative explanations for an observed association.

Graph selection: line graphs for continuous change over time or dose, bar graphs for categorical comparisons, scatter plots for correlation between two continuous variables.

Error analysis: percent error = |measured - accepted| / accepted x 100; standard deviation quantifies spread; standard error of the mean decreases with sample size.

Statistical significance: p-value below alpha (commonly 0.05) rejects the null hypothesis; a small p-value does not indicate a large effect size.

Microscopy: magnification = ocular x objective; resolution improves with shorter wavelength and higher numerical aperture; immersion oil increases light collection at high power.

Laboratory safety: know location of eyewash, shower, and fire extinguisher; never pipette by mouth; label all containers; dispose of biohazards and chemicals per protocol.