IB Biology SL cheat sheet

Binomial nomenclature: genus capitalized, species lowercase, both italicized (Homo sapiens); domains are Bacteria, Archaea, Eukarya.

Three-domain classification rests on rRNA sequence differences; Archaea and Eukarya share a more recent common ancestor than either does with Bacteria.

Cladistics groups organisms by shared derived characters (homologous traits from a common ancestor); analogous traits from convergent evolution are excluded.

Species definition (biological): a group whose members can interbreed to produce fertile offspring; mules (horse × donkey) are sterile, showing limits.

Natural classification reflects evolutionary relationships; artificial classification uses arbitrary features such as habitat or colour.

Dichotomous keys are built from paired, mutually exclusive, observable statements; each choice leads to a new pair until one taxon remains.

Evidence for evolution: fossil record (transitional forms), homologous structures, selective breeding, and molecular (DNA/protein) sequence comparison.

Kingdom-to-species hierarchy: Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species - increasing similarity down the list.

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

Prokaryotes: no membrane-bound organelles, 70S ribosomes, single circular DNA, peptidoglycan wall (bacteria); eukaryotes have 80S ribosomes and membrane-bound compartments.

Surface area to volume ratio falls as cell size rises; small cells and flattened shapes keep diffusion adequate.

Membranes are phospholipid bilayers; small non-polar molecules diffuse freely, ions and polar molecules need channels or pumps.

Vesicle transport: exocytosis fuses vesicles with the plasma membrane to secrete; endocytosis invaginates the membrane to take material in.

Enzyme activity peaks at optimum pH and temperature; above optimum, denaturation of the active site reduces rate irreversibly.

Photosynthesis: light-dependent reactions in thylakoids produce ATP and NADPH; the Calvin cycle in the stroma fixes CO2 using rubisco.

Pulmonary gas exchange: alveoli give a large surface area, thin walls, moist lining, and a rich capillary network maintaining a steep concentration gradient.

Autotrophs fix carbon by photosynthesis or chemosynthesis; heterotrophs obtain carbon from other organisms, and saprotrophs digest dead matter externally.

Food chains transfer energy; only about 10% passes to the next trophic level, so chains rarely exceed four or five levels.

Trophic levels: producers, primary consumers, secondary consumers; decomposers recycle nutrients but occupy no fixed trophic level.

Population growth: exponential when resources are unlimited; logistic when a carrying capacity (K) is reached and growth slows.

Symbiosis types: mutualism (both benefit), commensalism (one benefits, other unaffected), parasitism (one benefits, host harmed).

Greenhouse effect: gases such as CO2, methane, and water vapour absorb and re-emit outgoing infrared radiation, warming the atmosphere.

Carbon cycle paths: photosynthesis, respiration, combustion, decomposition, and fossilization; oceans act as a major carbon sink.

Mesocosms and sampling: quadrats estimate population density; capture–mark–recapture estimates population size as (marked × total caught) ÷ recaptures.

DNA structure: two antiparallel strands, sugar–phosphate backbone, complementary base pairs A–T and G–C joined by hydrogen bonds.

Semi-conservative replication uses helicase to unwind, DNA polymerase to add nucleotides 5′→3′, and ligase to join fragments.

Transcription: RNA polymerase makes mRNA from the template strand; translation at ribosomes reads codons 5′→3′ using tRNA anticodons.

Meiosis: one replication followed by two divisions; homologous pairs separate in meiosis I, sister chromatids in meiosis II, giving four haploid cells.

Crossing over in prophase I and independent assortment in metaphase I generate genetic variation in gametes.

Monohybrid cross of heterozygotes gives 3:1 phenotype and 1:2:1 genotype; test cross with a recessive parent reveals an unknown genotype.

Mendel's law of segregation: alleles separate during gamete formation; codominance and multiple alleles (ABO blood groups) extend simple dominance.

Mutations are random changes in base sequence; most are neutral or harmful, and gene mutations supply the variation on which selection acts.