ILEARN Biology ECA cheat sheet

Cell theory: all living things are made of cells, the cell is the basic unit of life, and new cells come only from existing cells by division.

Prokaryotes lack a membrane-bound nucleus and membrane-bound organelles (bacteria, archaea); eukaryotes have a nucleus plus organelles such as mitochondria and chloroplasts.

Organelle roles: ribosome builds proteins, rough ER folds and transports them, Golgi modifies and packages them, mitochondrion makes ATP, chloroplast runs photosynthesis, lysosome digests macromolecules.

Membrane transport decision rule: high to low with no protein is simple diffusion; high to low through a channel or carrier is facilitated diffusion; low to high with ATP is active transport; water crossing a membrane is osmosis.

Photosynthesis: 6CO2 + 6H2O + light yields C6H12O6 + 6O2; the light reactions in thylakoid membranes split water to release O2 and make ATP and NADPH, while the Calvin cycle in the stroma fixes CO2.

Cellular respiration: C6H12O6 + 6O2 yields 6CO2 + 6H2O + ATP, with glycolysis in the cytoplasm (net 2 ATP), the Krebs cycle in the mitochondrial matrix, and the electron transport chain on the inner membrane.

Fermentation follows glycolysis when oxygen is absent, regenerating NAD+ so glycolysis continues; it yields 2 ATP per glucose plus either lactate (muscle cells) or ethanol and CO2 (yeast).

Negative feedback reverses a deviation from a set point (insulin lowers high blood glucose); positive feedback amplifies a change (oxytocin in childbirth, clotting cascades).

Vascular tissue: xylem carries water and dissolved minerals upward from roots through dead cells driven by transpiration pull; phloem carries sugars from source to sink in living sieve-tube cells.

Mendel's law of segregation: the two alleles of a gene separate during meiosis so each gamete receives one; law of independent assortment: genes on different chromosomes are inherited independently of each other.

A monohybrid cross of two heterozygotes (Aa x Aa) gives a 1:2:1 genotype ratio and a 3:1 phenotype ratio; a testcross to a homozygous recessive individual reveals an unknown genotype.

A dihybrid cross of two heterozygotes (AaBb x AaBb) with independent assortment gives the phenotype ratio 9:3:3:1.

Incomplete dominance produces an intermediate phenotype (red x white gives pink); codominance expresses both alleles fully at once (AB blood); ABO blood type involves three alleles, IA, IB, and i.

DNA is a double helix with antiparallel sugar-phosphate backbones; adenine pairs with thymine by two hydrogen bonds and guanine pairs with cytosine by three, making the strands complementary.

Central dogma: semi-conservative DNA replication, then transcription to mRNA in the nucleus, then translation at the ribosome, where tRNA anticodons match mRNA codons to deliver amino acids.

A substitution point mutation may be silent, missense, or nonsense; insertions or deletions not in multiples of three cause frameshifts that change every downstream codon.

Meiosis generates variation by crossing over in prophase I, independent assortment in metaphase I, and random fertilization; it produces four haploid cells, whereas mitosis produces two identical diploid cells.

Human karyotype: 46 chromosomes in 23 pairs, with XX female and XY male; nondisjunction during meiosis produces aneuploid gametes, as in trisomy 21.

Natural selection requires heritable variation, competition for limited resources, and differential reproductive success; the outcome is rising frequency of advantageous alleles over generations.

Evidence for common ancestry includes the fossil record, homologous structures, vestigial structures, similar embryological development, shared DNA and protein sequences, and biogeography.

Homologous structures share underlying structure and origin but may differ in function (common ancestry); analogous structures share function but differ in origin (convergent evolution).

Hardy-Weinberg: p + q = 1 and p^2 + 2pq + q^2 = 1 hold only with no mutation, no gene flow, no selection, random mating, and a large population; violating a condition means the population is evolving.

Mechanisms of microevolution are natural selection, mutation, gene flow (migration), and genetic drift, including bottleneck and founder effects in small populations.

Selection types: directional selection shifts the mean, stabilizing selection favors the intermediate, and disruptive selection favors both extremes; sexual selection acts through mate choice or competition.

Speciation requires reproductive isolation; allopatric speciation follows geographic separation, while sympatric speciation occurs without it, for example through polyploidy in plants.

Classification uses binomial nomenclature (genus plus species, italicized) and groups organisms by shared derived traits into the three domains Bacteria, Archaea, and Eukarya.

Endosymbiotic theory: mitochondria and chloroplasts descend from engulfed prokaryotes, supported by their own circular DNA, double membranes, and bacteria-sized ribosomes.

Energy flows one way: producers (autotrophs) to primary consumers to secondary consumers; only about 10% of energy passes to the next trophic level, and the rest is lost mainly as heat.

Energy pyramids are always upright, but biomass and numbers pyramids can be inverted, as when one large tree supports many insects.

Symbiosis types: mutualism (+/+), commensalism (+/0), parasitism (+/-); predation (+/-) and competition (-/-) also shape community structure.

Carbon cycle: photosynthesis removes CO2 while respiration, decomposition, and combustion release it; the nitrogen cycle requires bacterial fixation, nitrification, assimilation, and denitrification.

Population change equals (births + immigration) minus (deaths + emigration); exponential growth is J-shaped, while logistic growth levels off at the carrying capacity K.

Density-dependent limits such as food, disease, and predation intensify as population size rises; density-independent limits such as fire, drought, and storms act regardless of population size.

Primary succession begins on bare rock without soil, with lichens and mosses as pioneers; secondary succession begins where soil remains after a disturbance such as fire or land clearing.

Biodiversity increases ecosystem stability and resilience; the leading threats are habitat loss, invasive species, pollution, overharvesting, and climate change.

Persistent toxins concentrate at higher trophic levels (biomagnification); fertilizer runoff causes eutrophication, in which algal blooms and decomposition deplete dissolved oxygen and kill aquatic animals.