VCE Biology cheat sheet

Prokaryotic cells lack membrane-bound organelles and a nucleus; their DNA is a single circular chromosome in a nucleoid region, plus plasmids.

Endosymbiotic theory: mitochondria and chloroplasts derive from engulfed bacteria - evidence includes double membranes, own circular DNA, 70S ribosomes, and binary fission.

Surface area to volume ratio decreases as cell size increases; cells stay small because exchange of materials depends on this ratio.

Compartmentalisation by membrane-bound organelles allows incompatible reactions (e.g. hydrolysis in lysosomes, ATP synthesis in mitochondria) to occur simultaneously.

Plant cell features distinguishing them from animal cells: cellulose cell wall, large permanent vacuole, chloroplasts, and no centrioles in most higher plants.

Cell fractionation: homogenise in isotonic buffer, then differential centrifugation - heavier organelles (nuclei) pellet at lower speeds than lighter ones (ribosomes).

The fluid mosaic model: phospholipid bilayer with embedded proteins, cholesterol, glycolipids and glycoproteins; membranes are selectively permeable and dynamic.

Transport decision rule: simple diffusion and osmosis are passive (down gradient, no energy); facilitated diffusion uses channels/carriers passively; active transport uses carriers and ATP against the gradient.

Asexual reproduction produces genetically identical offspring by mitosis; sexual reproduction uses meiosis and fertilisation to generate genetic variation.

Meiosis I separates homologous chromosomes (halving chromosome number); meiosis II separates sister chromatids, giving four haploid cells.

Crossing over in prophase I and independent assortment in metaphase I are the two main sources of meiotic genetic variation.

Genotype describes alleles present; phenotype is the observable trait resulting from genotype plus environmental influence.

Monohybrid cross decision rule: assign alleles, determine parental gametes, use a Punnett square, then state genotype and phenotype ratios.

Punnett square ratios for a cross of two heterozygotes (Aa x Aa) are 1:2:1 genotypically and 3:1 phenotypically for a dominant trait.

Homeostasis maintains a relatively stable internal environment via negative feedback loops: stimulus, receptor, control centre, effector, response.

Insulin lowers blood glucose by promoting cellular uptake and glycogen synthesis; glucagon raises it by promoting glycogen breakdown - an antagonistic pair.

A signal transduction pathway: receptor binding, transduction via a cascade (often phosphorylation), then a cellular response such as gene expression or enzyme activation.

Receptors are specific to signal molecules; hydrophilic signals (e.g. insulin) bind surface receptors, while hydrophobic signals (e.g. steroid hormones) enter the cell and bind intracellular receptors.

Enzymes lower activation energy by binding substrate at the active site; they are not consumed and are reused.

Enzyme rate factors: substrate concentration, temperature, pH, inhibitors, and enzyme concentration; above optimum temperature or wrong pH, denaturation disrupts the active site.

Competitive inhibitors bind the active site and are overcome by more substrate; non-competitive inhibitors bind elsewhere and are not overcome by added substrate.

Photosynthesis: light-dependent reactions in thylakoids produce ATP and NADPH and split water; the Calvin cycle in the stroma uses these to fix CO2 into glucose.

Aerobic cellular respiration: glycolysis (cytosol), Krebs cycle (matrix), and electron transport chain (inner membrane) yielding roughly 30-32 ATP per glucose.

Anaerobic respiration in animals yields lactate and in yeast yields ethanol plus CO2; both regenerate NAD+ so glycolysis can continue.

Adaptive immunity: B cells produce antibodies (humoral), T cells mediate cell death (cell-mediated); memory cells give a faster, larger secondary response.

First line of defence: intact skin, mucus, cilia, stomach acid and other physical and chemical barriers that are non-specific.

Second line: inflammation, phagocytes, fever and antimicrobial proteins - non-specific and rapid.

Third line: lymphocytes with specific antigen receptors, clonal selection and expansion, and immunological memory.

Antibodies are Y-shaped proteins with variable antigen-binding sites; they agglutinate pathogens and mark them for destruction.

Vaccination exposes the immune system to antigens (or instructions to make them) so memory cells form without disease symptoms.

Herd immunity threshold depends on pathogen transmissibility; higher R0 requires a higher proportion of immune individuals.

Autoimmune disease occurs when self-tolerance fails; allergy is an exaggerated response to a harmless antigen (e.g. IgE-mediated).

Evolution is a change in allele frequencies in a population over generations; it acts on populations, not individuals.

Natural selection requires heritable variation, differential reproductive success, and selection pressure; it is not goal-directed.

Allele frequency change requires mutation, gene flow, genetic drift or selection; mutation is the ultimate source of new alleles.

Genetic drift is random change in allele frequency, most pronounced in small populations (bottleneck and founder effects).

Speciation: allopatric involves geographic isolation; sympatric involves reproductive isolation without geographic separation (e.g. polyploidy, behavioural isolation).

Evidence for evolution includes fossils, homologous structures, comparative DNA and protein sequences, and observed antibiotic or pesticide resistance.

Hardy-Weinberg conditions: no mutation, no gene flow, random mating, no selection, large population; deviations indicate evolution is occurring.

Human evolution trends: bipedalism (foramen magnum position), increasing brain size, reduced jaw and brow ridges, and use of tools and fire.

Point mutations: substitution may be silent, missense or nonsense; frameshift indels usually disrupt protein function more severely.

Chromosomal mutations include deletion, duplication, inversion and translocation; these affect many genes at once.

CRISPR-Cas9 uses a guide RNA to target a sequence and Cas9 to cut DNA, allowing gene knockout or insertion via repair.

Gel electrophoresis separates DNA fragments by size using an electric field; smaller fragments travel further through the gel.

PCR amplifies DNA using primers, nucleotides and Taq polymerase through denaturation, annealing and extension cycles.

Biological implications of gene technologies include ethical debates, privacy of genetic data, and risks of unintended off-target edits.

Biomacromolecules: carbohydrates (C,H,O), proteins (C,H,O,N, sometimes S), lipids (C,H,O, fewer O), nucleic acids (C,H,O,N,P).

Protein structure levels: primary (amino acid sequence), secondary (alpha helix, beta sheet), tertiary (3D folding), quaternary (multiple polypeptides).

Denaturation disrupts hydrogen and ionic bonds, altering tertiary structure and function without breaking peptide bonds.

Osmosis is the net movement of water across a semipermeable membrane from low solute concentration to high solute concentration.

In a hypertonic solution, animal cells shrink (crenation) and plant cells plasmolyse; in a hypotonic solution, animal cells may lyse while plant cells become turgid.

Indicators: iodine tests for starch (blue-black), Benedict's for reducing sugars (brick-red on heating), Biuret for protein (purple), and emulsion test for lipids (white emulsion).