GCE A/L Biology cheat sheet

Prokaryotic cells lack membrane-bound organelles and a nucleus; their DNA is a single circular chromosome plus plasmids, and ribosomes are 70S (50S + 30S subunits).

Eukaryotic ribosomes are 80S (60S + 40S); the 60S subunit contains the peptidyl transferase activity, which is ribosomal RNA, not protein.

Magnification = image size ÷ actual size; convert all lengths to the same unit before dividing, and use a scale bar when no magnification is given.

Fluid mosaic model: phospholipid bilayer with intrinsic (transmembrane) and extrinsic proteins, cholesterol for stability, and glycoproteins/glycolipids facing the extracellular side.

Diffusion rate rises with steeper concentration gradient, shorter distance, larger surface area, higher temperature, and smaller molecule size; Fick's law expresses this proportionality.

Water potential ψ = ψs (solute potential, negative) + ψp (pressure potential); water moves from higher (less negative) to lower ψ, and pure water at atmospheric pressure has ψ = 0.

Enzymes lower activation energy; they are unchanged by the reaction, are specific at the active site, and are denatured by heat or extreme pH because tertiary structure changes.

Competitive inhibitors raise apparent Km without changing Vmax; non-competitive inhibitors lower Vmax without changing Km.

Condensation joins two monosaccharides by a glycosidic bond with loss of water; hydrolysis reverses it. Triglycerides are glycerol plus three fatty acids joined by ester bonds.

Mitosis: prophase (chromosomes condense, spindle forms, nuclear envelope breaks down), metaphase (chromosomes align on the equator), anaphase (centromeres split, chromatids move to poles), telophase and cytokinesis.

Meiosis I separates homologous chromosomes (reduction division); meiosis II separates sister chromatids, so one diploid cell gives four haploid cells.

Crossing over occurs at chiasmata during prophase I between non-sister chromatids of homologous chromosomes; independent assortment occurs at metaphase I.

Meiosis produces genetic variation by crossing over, independent assortment, and random fertilisation; mitosis produces genetically identical daughter cells for growth and repair.

DNA replication is semi-conservative: helicase unwinds, DNA polymerase adds nucleotides 5' to 3', and each new molecule has one parental and one new strand.

In the cell cycle, interphase (G1, S, G2) occupies most of the time; DNA replication occurs in S phase, and mitosis is the shortest phase.

Cancer arises from mutations in genes controlling the cell cycle (oncogenes and tumour suppressor genes), causing uncontrolled mitosis and tumour formation.

Cytokinesis differs: in plant cells a cell plate forms and becomes the middle lamella; in animal cells a cleavage furrow pinches the cell in two.

A gene is a sequence of DNA bases coding for a polypeptide or functional RNA; an allele is one of the alternative forms of a gene at the same locus.

Monohybrid cross of two heterozygotes (Aa × Aa) gives a 3:1 phenotype ratio and 1:2:1 genotype ratio; a test cross with a homozygous recessive identifies an unknown genotype.

Dihybrid cross of two heterozygotes for unlinked genes gives 9:3:3:1; linked genes give ratios that deviate from this because they are inherited together.

Codominance shows both alleles in the phenotype (e.g. AB blood group); incomplete dominance gives an intermediate phenotype. Sex-linked traits are carried on the X chromosome and affect males more often.

Transcription: RNA polymerase makes mRNA from the template strand in the nucleus; splicing removes introns and joins exons before export to the cytoplasm.

Translation: mRNA codons are read by tRNA anticodons at the ribosome; each codon specifies one amino acid, and the genetic code is degenerate and non-overlapping.

DNA base sequence to mRNA: A pairs with U, T with A, G with C, C with G; the template strand is complementary to the mRNA.

Mutation types: substitution (may be silent, missense, or nonsense), insertion or deletion (causes frameshift), and these alter the polypeptide produced.

Hardy-Weinberg: p + q = 1 and p² + 2pq + q² = 1, where p and q are allele frequencies; deviations indicate selection, drift, migration, or non-random mating.

Xylem transports water and mineral ions upward; vessels are dead, lignified, hollow tubes, and transpiration pull plus cohesion and adhesion drive the water column.

Phloem translocates sucrose and amino acids bidirectionally; sieve tube elements are living, lack a nucleus at maturity, and are supported by companion cells with mitochondria.

Transpiration rate increases with higher temperature, wind speed, and lower humidity, and decreases when stomata close; the cuticle and stomata regulate water loss.

Photosynthesis: 6CO2 + 6H2O → C6H12O6 + 6O2; light-dependent reactions in the thylakoid membranes produce ATP and reduced NADP, and the Calvin cycle in the stroma fixes CO2 using RuBP and rubisco.

Limiting factors of photosynthesis are light intensity, CO2 concentration, and temperature; the rate plateaus when one factor becomes limiting.

Chlorophyll absorbs mainly red and blue-violet light and reflects green; absorption spectra explain the action spectrum of photosynthesis.

Root hair cells increase surface area for water and ion uptake; mineral ions are absorbed by active transport, and water follows by osmosis.

Stomatal opening: guard cells take up K+ and water, become turgid, and the stoma opens; wilting occurs when guard cells lose turgor.

Plant growth substances: auxins promote cell elongation and show apical dominance and phototropism; gibberellins promote stem elongation and seed germination.

Mammalian heart: blood flows right atrium → right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium → left ventricle → aorta; the left ventricle wall is thickest.

Cardiac cycle: atrial systole, ventricular systole, then diastole; the SAN sets the rhythm, and the AVN delays the impulse so atria empty before ventricles contract.

Arteries have thick muscular elastic walls and carry blood away from the heart; veins have valves and wide lumens; capillaries are one cell thick for exchange.

Haemoglobin + oxygen ⇌ oxyhaemoglobin; the dissociation curve is S-shaped, shifts right (Bohr shift) with high CO2, low pH, or high temperature, aiding oxygen unloading.

Gas exchange surfaces share features: large surface area, thin barrier, moist, and a steep diffusion gradient maintained by ventilation and blood flow.

Nephron: filtration at the glomerulus, selective reabsorption of glucose and most water in the proximal tubule, and water reabsorption controlled by ADH in the collecting duct.

Neurone impulse: depolarisation by Na+ influx, repolarisation by K+ efflux, refractory period, and saltatory conduction in myelinated axons; transmission across a synapse uses neurotransmitters.

Homeostasis uses negative feedback: receptors detect a change, a control centre coordinates, and effectors restore the set point (e.g. insulin lowers blood glucose).

Digestion: amylase breaks starch to maltose, protease breaks proteins to amino acids, and lipase breaks lipids to fatty acids and glycerol; villi increase absorption surface area.

Bacteria are prokaryotes with circular DNA and 70S ribosomes; they reproduce by binary fission, and exponential growth has lag, log, stationary, and death phases.

Viruses are not cells: they have nucleic acid (DNA or RNA) in a protein capsid and reproduce only inside a host cell using host machinery.

Pathogens cause disease by damaging tissue or releasing toxins; transmission routes include airborne droplets, water, food, vectors, and direct contact.

Innate defences: skin, mucous membranes, phagocytes, inflammation, and lysozyme; specific immunity involves lymphocytes and is slower but provides memory.

Antibodies are Y-shaped proteins (immunoglobulins) produced by plasma cells; each has specific variable regions that bind one antigen, forming an antigen–antibody complex.

B cells make antibodies (humoral response); T helper cells activate B cells and phagocytes, and T killer cells destroy infected cells.

Vaccination introduces antigens (weakened pathogen or toxoid) to stimulate memory cells, giving a faster, stronger secondary response.

Antibiotics target bacterial structures such as cell walls or ribosomes and are ineffective against viruses; misuse selects resistant bacteria.

Aseptic technique and sterilisation (autoclave, heat, filtration, UV) prevent contamination in microbiology experiments.

Natural selection requires variation, heritability, and differential survival and reproduction; advantageous alleles increase in frequency over generations.

Evidence for evolution: fossil record, homologous structures, comparative biochemistry (DNA and protein sequences), and antibiotic resistance in bacteria.

Species concept: members of a species interbreed to produce fertile offspring; speciation occurs when populations become reproductively isolated (geographic or behavioural).

Population growth: exponential (J-shaped) when resources are unlimited; logistic (S-shaped) when limiting factors set a carrying capacity K.

Food chains show energy flow; only about 10% of energy passes to the next trophic level, the rest lost as heat, movement, and excretion.

Carbon cycle: photosynthesis fixes CO2, respiration and combustion release it; nitrogen cycle: nitrogen fixation, nitrification, assimilation, denitrification.

Population size can be estimated by mark–recapture: N = (total marked × total in second sample) ÷ marked in second sample; random quadrats estimate density.

Ecological pyramids of number, biomass, and energy; energy pyramids are always upright, but number pyramids can invert (e.g. one tree, many insects).

Human impact: greenhouse gases (CO2, methane) drive global warming; eutrophication from nitrate runoff causes algal blooms and oxygen depletion.

Genetic engineering steps: isolate the gene (restriction enzymes), insert into a vector (plasmid), transform a host cell, and select and clone the recombinants.

Restriction endonucleases cut DNA at specific palindromic recognition sites, producing sticky ends; DNA ligase joins fragments by forming phosphodiester bonds.

PCR amplifies DNA in cycles: denaturation (~95 °C), primer annealing (~55 °C), and extension by Taq polymerase (~72 °C).

Gel electrophoresis separates DNA fragments by size: negatively charged DNA moves toward the positive electrode, and smaller fragments travel further.

Micropropagation (tissue culture) uses sterile nutrient agar with auxins and cytokinins to clone plants from explants; aseptic conditions prevent contamination.

Fermenters (bioreactors) maintain optimum temperature, pH, oxygen, and nutrient supply for microorganisms producing antibiotics, enzymes, or single-cell protein.

Selective breeding and artificial insemination improve livestock; plant breeding uses cross-pollination, and GM crops may carry pest resistance or herbicide tolerance.

Experimental design: independent and dependent variables, controlled variables, replicates, and a control group; reliability improves with repeats, validity with controls.

Data handling: calculate means, plot suitable graphs with labelled axes and units, and use standard deviation or error bars to judge significance.

Soil fertility depends on humus, mineral ions (nitrate, phosphate, potassium), pH, and aeration; fertilisers replace lost nutrients but excess causes eutrophication.

Nitrogen-fixing bacteria (Rhizobium) in root nodules and nitrifying bacteria in soil convert nitrogen into usable nitrates for plants.

Crop yield is increased by irrigation, fertiliser application, pest control, selective breeding, and greenhouse control of temperature and CO2.

Pests are controlled biologically (predators, parasites), chemically (pesticides), or culturally (crop rotation); biological control avoids bioaccumulation but is slower.

Food preservation: refrigeration slows microbial growth, canning uses heat sterilisation, salting and drying reduce water availability, and pickling lowers pH.

Yoghurt production uses Lactobacillus to ferment lactose into lactic acid, lowering pH and coagulating milk proteins.

Bread making uses yeast fermentation: sugars are converted to ethanol and CO2, and the gas expands the gluten network.

Beer and wine production rely on anaerobic fermentation by yeast; distillation concentrates ethanol produced this way.

Monoclonal antibodies and insulin produced by recombinant bacteria are examples of industrial biotechnology using genetically modified microorganisms.

Carbohydrates: monosaccharides (glucose, fructose), disaccharides (maltose, sucrose, lactose), and polysaccharides (starch, glycogen, cellulose); cellulose is a structural β-glucose polymer.

Proteins are polymers of amino acids joined by peptide bonds; primary, secondary (α-helix, β-pleated sheet), tertiary, and quaternary structures determine function.

Lipids are hydrophobic; phospholipids are amphipathic and form bilayers, while triglycerides store energy and steroids include hormones.

Tests: Benedict's for reducing sugars (blue to brick red), iodine for starch (brown to blue-black), Biuret for protein (blue to purple), emulsion test for lipid (milky white).

Osmosis is the movement of water through a partially permeable membrane from higher to lower water potential; animal cells lyse in pure water, plant cells become turgid.

Active transport moves substances against a concentration gradient using ATP and carrier proteins; facilitated diffusion uses channel or carrier proteins down a gradient without ATP.

Surface area to volume ratio falls as cells get larger, limiting diffusion; this explains why exchange surfaces are folded or thin.

Cell organelles: mitochondria for aerobic respiration, chloroplasts for photosynthesis, rough ER for protein synthesis, Golgi for modification and packaging, lysosomes for hydrolysis.

Pedigree analysis: a recessive trait can skip generations and appear in offspring of unaffected parents; a dominant trait appears in every generation.

Sex determination in humans: XX female, XY male; the father determines the sex of offspring because he contributes either X or X/Y.

Sex-linked recessive conditions (e.g. haemophilia, red-green colour blindness) affect males more because males have only one X chromosome.

Multiple alleles: the ABO blood group has alleles IA, IB, and i; IA and IB are codominant, and i is recessive.

Codominance and incomplete dominance produce intermediate or combined phenotypes; test crosses and ratios identify the mode of inheritance.

Chi-squared test compares observed and expected ratios: χ² = Σ (O − E)² ÷ E; compare with the critical value at the appropriate degrees of freedom.

Genetic screening and counselling use pedigree, karyotype, and DNA tests to estimate the probability of inherited disorders.

Mutations in somatic cells are not inherited; mutations in gametes can be passed to offspring and cause inherited disorders.