New Zealand Scholarship Biology cheat sheet

Levels of organisation run molecule → organelle → cell → tissue → organ → organ system → organism → population → community → ecosystem → biosphere; each level has emergent properties not predictable from the level below.

Cell theory: all living things are composed of one or more cells, the cell is the basic unit of structure and function, and all cells arise from pre-existing cells by division.

Prokaryotes lack membrane-bound organelles and a nucleus; eukaryotes have a nuclear envelope, mitochondria, and (in plants, algae, fungi) a cell wall - a reliable first split in classification questions.

Surface area to volume ratio scales as 1/length, so doubling cell size quarters the SA:V ratio; this constrains diffusion-only exchange and drives features such as villi, alveoli, and flattened leaves.

Homeostasis is maintenance of a variable around a set point by negative feedback: receptor detects deviation, control centre compares to set point, effector reverses it; positive feedback amplifies deviation (e.g. oxytocin in labour).

Biological drawing conventions: sharp pencil, unbroken lines, no shading, labels with straight ruled lines not crossing, magnification stated as a ratio or with a scale bar, and title identifying the specimen and view.

Magnification = image size ÷ actual size; convert units before dividing (1 mm = 1000 µm). A 50 µm object drawn 10 mm long is at ×200.

Validity, reliability and accuracy are distinct: validity means the measurement or design actually tests the hypothesis; reliability is repeatability of results; accuracy is closeness to the true value.

Correlation is not causation: an association between two variables can arise from a confounding third variable, reverse causation, or chance, so controlled manipulation or a natural experiment is needed to infer cause.

Fluid mosaic model: a phospholipid bilayer with hydrophilic heads outward, integral and peripheral proteins, cholesterol modulating fluidity, glycolipids and glycoproteins for recognition, and a hydrophobic core restricting polar solutes.

Simple diffusion is passive down a concentration gradient and proportional to gradient steepness and membrane permeability; facilitated diffusion uses channel or carrier proteins down the gradient with no energy input.

Active transport moves solutes against their gradient using ATP-driven pumps (e.g. Na⁺/K⁺ ATPase, 3 Na⁺ out per 2 K⁺ in), enabling cells to maintain gradients that power secondary transport.

Osmosis is net water movement across a semipermeable membrane from higher to lower water potential; water potential ψ = ψs (solute) + ψp (pressure), and pure water at atmospheric pressure has ψ = 0.

In plant cells, turgor pressure (ψp) rises as water enters, reducing the water potential gradient; plasmolysis occurs when the protoplast pulls away from the wall in hypertonic solution.

Enzyme kinetics: rate rises with substrate concentration until Vmax; Km is the substrate concentration at half Vmax and inversely indicates enzyme–substrate affinity; competitive inhibitors raise apparent Km, non-competitive lower Vmax.

Enzymes lower activation energy, are unchanged by the reaction, and are sensitive to temperature and pH because these alter tertiary structure and hence active-site shape (denaturation is usually irreversible).

Metabolic pathways are controlled by end-product inhibition of an early enzyme (allosteric regulation), which prevents wasteful accumulation of intermediates.

ATP couples exergonic and endergonic reactions: hydrolysis to ADP + Pi releases energy that drives synthesis, transport, and movement; ATP is regenerated by respiration or photophosphorylation.

DNA structure: two antiparallel sugar–phosphate backbones, deoxyribose, phosphate, and four bases (A, T, G, C) with A–T and G–C base pairs held by 2 and 3 hydrogen bonds respectively; the double helix has major and minor grooves.

Semiconservative replication: helicase unwinds, DNA polymerase adds nucleotides 5′→3′ to a primer on each template strand, producing a leading strand continuously and a lagging strand in Okazaki fragments joined by ligase.

Transcription: RNA polymerase makes mRNA from the template strand 3′→5′ (mRNA 5′→3′); in eukaryotes introns are spliced out and a 5′ cap and poly-A tail added before export.

Translation: ribosomes read mRNA codons 5′→3′, tRNA anticodons pair with them, and amino acids are joined by peptide bonds; the genetic code is degenerate, non-overlapping, and near-universal.

Mutation types: substitution (silent, missense, nonsense), insertion/deletion causing frameshift, and chromosomal changes (duplication, deletion, inversion, translocation); only germline mutations are heritable.

Mendel's laws: segregation of alleles at meiosis I anaphase, and independent assortment of unlinked loci; test crosses and dihybrid ratios (9:3:3:1 for unlinked heterozygotes) reveal genotype.

Monohybrid cross of two heterozygotes gives 1:2:1 genotype and 3:1 phenotype for a dominant allele; a test cross with a recessive gives 1:1 if the unknown is heterozygous.

Sex-linked traits show different frequencies in males and females because males are hemizygous for X-linked genes; carrier females can transmit a recessive allele to affected sons.

Gene expression is regulated at transcription (promoters, enhancers, transcription factors), post-transcription (splicing, mRNA stability), and post-translation (protein modification); operons such as lac in bacteria are coordinately controlled.

Pedigree analysis: a trait appearing in every generation with affected offspring from an affected parent suggests dominance; unaffected parents with an affected child indicates recessive inheritance.

Natural selection requires heritable variation, differential reproductive success, and a selective pressure that makes some variants more likely to survive and reproduce in the current environment.

Allele frequency change is evolution; the Hardy–Weinberg model (p² + 2pq + q² = 1, p + q = 1) describes a non-evolving population and its assumptions are no selection, no mutation, no migration, random mating, and large population size.

Genetic drift is random change in allele frequency, strongest in small populations; the founder effect and bottleneck effect are drift-driven reductions in diversity.

Speciation: allopatric speciation follows geographic isolation; sympatric speciation can follow polyploidy (common in plants) or disruptive selection with assortative mating, and requires reproductive isolation.

Reproductive isolating mechanisms are prezygotic (habitat, temporal, behavioural, mechanical, gametic) or postzygotic (hybrid inviability, hybrid sterility, hybrid breakdown).

Evidence for evolution includes the fossil record, homologous structures, vestigial features, biogeography, comparative embryology, and molecular sequence similarity (e.g. cytochrome c, ribosomal RNA).

Convergent evolution produces analogous structures from different ancestries; divergent evolution produces homologous structures from a common ancestor; both are distinguished by ancestry, not function.

Coevolution is reciprocal selection between interacting species (predator–prey, host–parasite, plant–pollinator); it can produce matched traits such as long proboscis and deep corolla tube.

The biological species concept (interbreeding populations producing fertile offspring) does not apply to asexual organisms or fossils, so palaeontological and ecological species concepts are used instead.

Population growth: exponential dN/dt = rN occurs with unlimited resources; logistic dN/dt = rN(K − N)/K adds carrying capacity K, producing an S-shaped curve with fastest growth at N = K/2.

r-selected species have high reproductive rate, small body size, many small offspring, little parental care, and boom–bust dynamics; K-selected species have low fecundity, large size, and stable populations near K.

Interspecific interactions: competition (−/−), predation and parasitism (+/−), mutualism (+/+), commensalism (+/0), and amensalism (−/0); realised niche is narrower than fundamental niche because of competition.

Energy flows one way through trophic levels and is lost as heat and in undigested material; typically only about 10% transfers to the next level, so food chains rarely exceed 4–5 links.

Pyramids of numbers can invert (one tree supporting many insects) but pyramids of energy never do; biomass pyramids can invert seasonally in aquatic systems where producers turn over fast.

Carbon cycle: photosynthesis fixes CO₂, respiration and combustion release it, oceans and carbonate rocks are major stores, and decomposers return carbon from dead organic matter.

Nitrogen cycle: nitrogen fixation (Rhizobium, cyanobacteria), ammonification, nitrification (Nitrosomonas, Nitrobacter), assimilation, and denitrification; legumes use root nodules with bacteroids.

Primary succession starts on bare substrate with pioneer species and soil formation; secondary succession follows disturbance with soil and seed bank intact, so it proceeds faster toward a stable community.

Biodiversity has genetic, species, and ecosystem components; ecosystem services include provisioning, regulating, cultural, and supporting services, and keystone species have disproportionate community effects.

Mammalian circulation is a double circuit: pulmonary to the lungs and systemic to the body, with the left ventricle thicker-walled because it pumps against higher systemic resistance.

Gas exchange requires a moist, thin, large-surface-area membrane with a steep diffusion gradient; haemoglobin's oxygen affinity falls as CO₂ and H⁺ rise (Bohr effect), promoting oxygen unloading in active tissue.

Nephron function: filtration at the glomerulus, selective reabsorption of glucose and most water in the proximal tubule, and variable water reabsorption in the collecting duct under ADH control.

Nerve impulse: resting potential about −70 mV from Na⁺/K⁺ pump and K⁺ leak; depolarisation by Na⁺ influx, repolarisation by K⁺ efflux, refractory period enforces one-way conduction, and myelin enables saltatory conduction.

Synaptic transmission is chemical: Ca²⁺ influx causes vesicle fusion, neurotransmitter diffuses across the cleft, binds receptors, and is removed by reuptake or enzymatic breakdown.

Plant transport: xylem carries water and mineral ions upward by transpiration pull (cohesion–tension), while phloem translocates sucrose from source to sink under pressure flow.

Transpiration rate rises with temperature, wind speed, and lower humidity, and falls with higher humidity; stomatal closure via abscisic acid limits water loss but also cuts CO₂ uptake.

Photosynthesis: light-dependent reactions in thylakoid membranes split water, produce O₂, ATP and NADPH; the Calvin cycle in the stroma fixes CO₂ using rubisco to make triose phosphate.

Plant hormones: auxin drives apical dominance and phototropism, gibberellin promotes stem elongation and seed germination, cytokinin promotes cell division, ABA promotes dormancy and stomatal closure, ethylene promotes ripening and abscission.

Identify the manipulated (independent) variable, measured (dependent) variable, and controlled variables; a control group differs only in the treatment whose effect is being tested.

Error types: random error scatters results around the true value and is reduced by replication and averaging; systematic error shifts all results in one direction and is not fixed by more replicates.

When interpreting a graph, quote specific values with units, describe trends (increase, plateau, decrease), and compare experimental with control rather than describing each line in isolation.

Standard deviation describes spread of data; standard error of the mean (SD/√n) describes precision of the estimate, and overlapping error bars suggest a difference may not be statistically significant.

A conclusion must be limited to the range of the data: extrapolating beyond tested concentrations, temperatures, or species is invalid, and a claim about mechanism requires evidence beyond correlation.

Ethical considerations in biological investigation include informed consent, animal welfare and the 3Rs (replacement, reduction, refinement), minimising environmental impact, and honest reporting including unexpected results.

Models (e.g. population growth, enzyme kinetics, Hardy–Weinberg) are simplifications with stated assumptions; critique a model by naming which assumption is violated by real data and predicting the direction of the resulting error.

Scholarship-level answers integrate multiple contexts (molecular, organismal, ecological), evaluate competing explanations, and use evidence to justify conclusions rather than merely describing processes.

Recombinant DNA workflow: restriction enzymes cut DNA at palindromic recognition sites, ligase joins fragments into a vector (often a plasmid with an antibiotic-resistance selectable marker), and the vector is introduced into host cells.

PCR uses denaturation (~95 °C), primer annealing (~55 °C), and extension by a thermostable DNA polymerase (~72 °C); each cycle doubles the target sequence.

Gel electrophoresis separates DNA fragments by size because the negatively charged phosphate backbone migrates toward the anode through a gel mesh; smaller fragments travel further.

CRISPR-Cas9 uses a guide RNA complementary to a target sequence and Cas9 nuclease to create a double-strand break, which is repaired by error-prone non-homologous end joining or by homology-directed repair.

Sanger sequencing terminates chain extension with dideoxynucleotides, producing fragments differing in length by one base that are separated to read the sequence.

Innate immunity is non-specific and immediate (barriers, phagocytes, inflammation); adaptive immunity is specific and slower on first exposure, with humoral (B cells, antibodies) and cell-mediated (T cells) branches.

Antibodies are Y-shaped immunoglobulins with variable antigen-binding sites; memory B and T cells give a faster, larger secondary response, which is the basis of vaccination.

Monoclonal antibodies come from a single B-cell clone fused with a myeloma cell (hybridoma), giving a uniform antibody used in diagnostics and targeted therapy.

Selective breeding and marker-assisted selection accelerate genetic gain but reduce genetic diversity; gene flow between modified and wild populations is a recognised ecological risk.

Three-domain classification (Bacteria, Archaea, Eukarya) rests on ribosomal RNA sequence comparison; Archaea resemble eukaryotes in some molecular features and often occupy extreme environments.

Binomial nomenclature: genus capitalised, species lower case, both italicised or underlined; taxonomic ranks are domain, kingdom, phylum, class, order, family, genus, species.

Viruses are not cells: they have a nucleic acid core (DNA or RNA) and a protein capsid, reproduce only inside a host cell, and are classified separately from cellular organisms.

Bacterial growth curve phases: lag, exponential (log), stationary, and death; antibiotic resistance spreads by mutation under selection plus horizontal gene transfer via conjugation, transformation, or transduction.

Koch's postulates link a microorganism to a disease: the organism is present in all cases, can be isolated and cultured, causes the disease when introduced to a healthy host, and can be re-isolated.

Pathogen transmission routes include airborne droplets, direct contact, faecal–oral, vector-borne, and blood-borne; control targets the route (ventilation, hand hygiene, water treatment, vector control).

Antibiotics target bacterial structures absent in eukaryotes (cell wall synthesis, bacterial ribosomes, DNA gyrase), which is why they do not treat viral infections.

Dichotomous keys use paired, mutually exclusive, observable characteristics; each couplet must be unambiguous and lead to a named taxon.

Mitosis produces two genetically identical diploid daughter cells through prophase, metaphase, anaphase, and telophase with cytokinesis; it supports growth, repair, and asexual reproduction.

Meiosis halves chromosome number through two divisions: homologous chromosomes separate in meiosis I (crossing over in prophase I), sister chromatids separate in meiosis II, giving four haploid cells.

Independent assortment and crossing over generate genetic variation; non-disjunction produces gametes with an extra or missing chromosome (e.g. trisomy 21).

Asexual reproduction (binary fission, budding, fragmentation, vegetative propagation) produces genetically uniform offspring adapted to a stable environment but vulnerable to a single selective pressure.

Sexual reproduction generates variation at the cost of requiring two parents, mate finding, and slower reproduction; variation is advantageous under changing or unpredictable conditions.

Human gametogenesis: spermatogenesis produces four sperm from each primary spermatocyte continuously; oogenesis produces one ovum and polar bodies, with meiosis II completed only on fertilisation.

Embryonic development follows cleavage, gastrulation (three germ layers: ectoderm, mesoderm, endoderm), and organogenesis; cell differentiation arises from differential gene expression, not loss of genes.

Apoptosis is programmed cell death essential for development (e.g. removal of webbing between digits) and is distinct from necrosis, which is uncontrolled and provokes inflammation.