ILTS Science: Biology (239) cheat sheet

Macromolecule building blocks: carbohydrates from monosaccharides joined by glycosidic bonds; proteins from amino acids joined by peptide bonds; nucleic acids from nucleotides joined by phosphodiester bonds; lipids assembled from glycerol plus fatty acids (triglycerides) or isoprenoid units (steroids).

Four levels of protein structure: primary = amino acid sequence; secondary = local alpha-helix and beta-pleated sheet stabilized by backbone hydrogen bonds; tertiary = overall 3D fold from side-chain interactions (hydrophobic, ionic, disulfide, hydrogen); quaternary = assembly of multiple polypeptide subunits.

Enzyme kinetics: Michaelis-Menten rate v = Vmax[S]/(Km+[S]); Km equals substrate concentration at half-maximal velocity and is inversely related to substrate affinity; competitive inhibitors raise apparent Km with unchanged Vmax, noncompetitive inhibitors lower Vmax with unchanged Km.

Membrane transport decision rule: simple diffusion follows the concentration gradient without protein; facilitated diffusion uses a channel or carrier down the gradient; primary active transport (e.g., Na+/K+-ATPase, 3 Na+ out per 2 K+ in) hydrolyzes ATP against the gradient; secondary active transport couples to an existing ion gradient.

Cell cycle checkpoints: G1/S checkpoint assesses cell size and DNA integrity; G2/M checkpoint verifies complete replication and repair; the spindle assembly checkpoint blocks anaphase until all kinetochores attach. p53 halts the cycle for repair or triggers apoptosis.

Mendelian ratios: monohybrid cross of heterozygotes gives 3:1 phenotype and 1:2:1 genotype; dihybrid cross of independent heterozygotes gives 9:3:3:1; a testcross with a homozygous recessive reveals an unknown genotype; deviations signal linkage, epistasis, or lethality.

Hardy-Weinberg equilibrium requires no mutation, no migration, random mating, no selection, and infinite population size; allele frequencies p+q=1 and genotype frequencies p^2+2pq+q^2=1 describe an un evolving population.

Central dogma flow: DNA replication (semiconservative, DNA polymerase reads 3' to 5' and synthesizes 5' to 3') then transcription (RNA polymerase builds mRNA 5' to 3' off the template strand) then translation (ribosome reads mRNA codons 5' to 3' while tRNAs deliver amino acids).

Ecology energy flow: only about 10% of energy transfers between trophic levels, so biomass pyramids narrow upward; the 10% rule explains why food chains rarely exceed four or five links; net primary productivity = gross primary productivity minus respiration.

Photosynthesis and respiration are complementary: photosynthesis fixes CO2 in the Calvin cycle using ATP and NADPH from light reactions; cellular respiration oxidizes glucose through glycolysis, the Krebs cycle, and the electron transport chain, yielding roughly 30-32 ATP per glucose in aerobic eukaryotes.

Natural selection requires heritable variation in a population, differential reproductive success, and a selective pressure; individuals do not evolve, populations do; allele frequencies shift across generations.

Speciation mechanisms: allopatric speciation follows geographic isolation; sympatric speciation arises without isolation through polyploidy, habitat differentiation, or sexual selection. A species is defined by reproductive isolation under the biological species concept.

Homeostasis via negative feedback: a sensor detects deviation, an integrating center compares to a set point, and an effector reverses the change (e.g., insulin lowers blood glucose; glucagon raises it). Positive feedback amplifies change and is rare (e.g., oxytocin in childbirth).

Genetics probability rules: the multiplication rule applies to independent events occurring together; the addition rule applies to mutually exclusive outcomes. Use a Punnett square for single traits and forked-line or product rules for multiple independent traits.

Biotechnology tools: restriction endonucleases cut specific palindromic sequences; DNA ligase joins fragments; gel electrophoresis separates by size with smaller fragments migrating farther; PCR uses denaturation, annealing, and extension cycles with Taq polymerase; CRISPR-Cas9 targets DNA by guide RNA complementarity.

Experimental design in biology: an independent variable is manipulated, the dependent variable is measured, controlled variables are held constant, and a control group establishes baseline. Correlation does not establish causation; sample size and replication affect reliability.

Classification hierarchy: Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species; binomial nomenclature italicizes genus and species with the genus capitalized; phylogenies are inferred from shared derived characters and molecular data.

Water properties tied to hydrogen bonding: high specific heat, high heat of vaporization, cohesion and adhesion supporting transpiration, and solvent capacity for polar solutes; these properties stabilize temperature and transport in organisms.