GCE O/L Science cheat sheet

Cell organelles: nucleus (genetic material), cytoplasm (metabolic reactions), cell membrane (selective permeability), mitochondria (aerobic respiration), chloroplasts (photosynthesis, plant cells only), cell wall (cellulose, plant cells only), vacuole (plant cells, maintains turgor).

Prokaryotic cells (bacteria) lack a nucleus and membrane-bound organelles; eukaryotic cells (plants, animals, fungi, protoctists) have a nucleus and organelles.

Diffusion: net movement of particles from high to low concentration down a concentration gradient; passive, no energy required; rate increases with steeper gradient, higher temperature, larger surface area, shorter distance.

Osmosis: diffusion of water through a partially permeable membrane from a dilute solution (high water potential) to a concentrated solution (low water potential); animal cells may lyse or shrink, plant cells become turgid or plasmolysed.

Active transport: movement of ions or molecules against a concentration gradient using energy from ATP and carrier proteins; example: mineral uptake by root hair cells.

Biological molecules: carbohydrates (C, H, O; e.g., glucose C6H12O6, starch, cellulose), proteins (C, H, O, N; amino acids), lipids (C, H, O; glycerol + fatty acids), vitamins and minerals.

Food tests: starch with iodine solution (blue-black), reducing sugar with Benedict's solution (brick-red on heating), protein with Biuret reagent (purple), lipid with ethanol emulsion test (cloudy white).

Enzymes are biological catalysts (proteins) that lower activation energy; they are specific to substrates; activity is affected by temperature (denaturation above optimum) and pH (optimum varies: pepsin acidic, trypsin alkaline).

Surface area to volume ratio decreases as cells get larger; exchange surfaces (alveoli, villi, root hairs) have large area, thin walls, good blood supply and ventilation to maximise diffusion.

Photosynthesis equation: 6CO2 + 6H2O → C6H12O6 + 6O2 (light energy absorbed by chlorophyll; occurs in chloroplasts).

Limiting factors of photosynthesis: light intensity, CO2 concentration, temperature; graphs show plateau when another factor becomes limiting.

Leaf adaptations: large surface area, thin, transparent cuticle and epidermis, palisade mesophyll rich in chloroplasts, spongy mesophyll with air spaces, stomata for gas exchange, vascular bundles for transport.

Aerobic respiration: C6H12O6 + 6O2 → 6CO2 + 6H2O + energy (ATP); occurs mainly in mitochondria; releases more energy per glucose than anaerobic respiration.

Anaerobic respiration in humans: glucose → lactic acid + small energy; in yeast: glucose → ethanol + CO2 + small energy (fermentation).

Human digestive system: ingestion, digestion (mechanical and chemical), absorption in small intestine villi, assimilation, egestion; enzymes: amylase (starch→maltose), protease (protein→amino acids), lipase (lipids→glycerol + fatty acids).

Balanced diet components: carbohydrates (energy), proteins (growth and repair), lipids (energy storage, insulation), vitamins (A, C, D), minerals (calcium, iron), water, dietary fibre (prevents constipation).

Gas exchange in humans: alveoli have large surface area, thin walls (one cell thick), moist lining, good blood supply; ventilation moves air in and out; breathing controlled by diaphragm and intercostal muscles.

Plant gas exchange: stomata open in light for CO2 uptake and O2 release; guard cells regulate opening; lenticels in stems allow gas exchange.

Blood components: red blood cells (haemoglobin transports O2), white blood cells (defence), platelets (clotting), plasma (transports dissolved substances, hormones, CO2, urea).

Blood vessels: arteries (thick muscular walls, carry blood away from heart, high pressure), veins (valves, carry blood to heart, low pressure), capillaries (one-cell-thick walls, exchange).

Human heart: double circulation; right side pumps deoxygenated blood to lungs (pulmonary circuit), left side pumps oxygenated blood to body (systemic circuit); valves prevent backflow.

Transpiration: loss of water vapour from leaves through stomata; rate increased by high temperature, low humidity, high light intensity, wind; cohesion-tension pulls water up xylem.

Translocation: movement of sucrose and amino acids in phloem from sources (leaves) to sinks (roots, fruits, growing points).

Excretion: removal of metabolic waste; lungs excrete CO2, kidneys excrete urea and excess water/ions as urine, skin excretes sweat (water, salts, urea).

Nephron: filtration in glomerulus, selective reabsorption of glucose, some ions and water in proximal tubule, urine formation; ADH controls water reabsorption.

Homeostasis: maintenance of constant internal environment; negative feedback regulates body temperature (37 °C), blood glucose (insulin lowers, glucagon raises), water balance (ADH).

Thermoregulation: vasodilation and sweating cool the body; vasoconstriction, shivering and hair erection conserve heat.

Asexual reproduction produces genetically identical offspring (clones) from one parent; sexual reproduction involves gametes and produces genetic variation.

Human reproductive system: testes produce sperm and testosterone; ovaries produce eggs and oestrogen; fertilisation in oviduct; implantation in uterus; placenta exchanges nutrients and wastes.

Menstrual cycle: FSH stimulates egg maturation, oestrogen repairs uterus lining, LH triggers ovulation, progesterone maintains lining; approximately 28 days.

Plant reproduction: flower parts (sepals, petals, stamens, carpel); pollination transfers pollen; fertilisation forms zygote; seeds disperse by wind, water, animals; germination needs water, oxygen, suitable temperature.

DNA is a double helix of nucleotides; a gene is a section of DNA coding for a protein; chromosomes occur in pairs (23 pairs in humans).

Mitosis produces two genetically identical diploid cells for growth and repair; meiosis produces four haploid gametes with genetic variation (crossing over, independent assortment).

Genetics: dominant and recessive alleles; genotype (alleles present) vs phenotype (observable characteristic); homozygous/heterozygous; Punnett squares predict offspring ratios (e.g., 3:1).

Sex determination: human female XX, male XY; sex-linked characteristics carried on X chromosome (e.g., red-green colour blindness).

Variation: genetic (caused by genes, mutation, meiosis) and environmental (diet, exercise, climate); natural selection acts on variation; mutations are random changes in DNA.

Ecosystem = community (all living organisms) + habitat (physical environment); population = all organisms of one species in an area.

Food chains and webs show feeding relationships; arrows show direction of energy flow; producers (plants) convert light energy to chemical energy; consumers are primary, secondary, tertiary.

Energy transfer between trophic levels is inefficient (~10% passed on); energy lost as heat, movement, excretion, uneaten parts; pyramids of biomass/numbers represent trophic levels.

Carbon cycle: photosynthesis removes CO2; respiration, combustion and decomposition release CO2; fossil fuels store carbon.

Water cycle: evaporation, transpiration, condensation, precipitation, run-off; nitrogen cycle: nitrogen fixation, decomposition, nitrification, denitrification.

Biotic factors: competition, predation, disease, food availability; abiotic factors: light, temperature, water, pH, oxygen, soil type.

Human impact: deforestation, pollution (air, water, land), greenhouse gases (CO2, methane) cause global warming; eutrophication from fertiliser run-off; acid rain from sulfur dioxide.

Conservation: protected areas, recycling, renewable energy, sustainable fishing, captive breeding; indicator species (lichens for air quality, mayfly larvae for clean water).

Population growth: birth rate, death rate, immigration, emigration; carrying capacity limited by resources; competition and predation regulate populations.

Atom: nucleus (protons + neutrons) surrounded by electrons in shells; atomic number = number of protons; mass number = protons + neutrons; isotopes have same protons, different neutrons.

Electron configuration: shells fill 2, 8, 8, ...; group number = outer shell electrons; period number = number of shells.

Ionic bond: transfer of electrons from metal to non-metal forming oppositely charged ions (e.g., NaCl); high melting points, conduct when molten/aqueous.

Covalent bond: sharing of electron pairs between non-metals; simple molecular substances have low melting points; giant covalent structures (diamond, graphite, silicon dioxide) have high melting points.

Metallic bonding: positive ions in a sea of delocalised electrons; explains conductivity, malleability and ductility.

Periodic table: metals on left, non-metals on right; group 1 alkali metals (reactive, form +1 ions), group 7 halogens (form -1 ions), group 0 noble gases (unreactive, full outer shell).

Group trends: reactivity increases down group 1 (outer electron lost more easily), decreases down group 7 (electron gained less easily).

Ion charges: group 1 +1, group 2 +2, group 3 +3, group 5 -3, group 6 -2, group 7 -1; transition metals form variable charges (e.g., Fe2+, Fe3+, Cu2+).

Relative atomic mass (Ar) is the average mass of isotopes relative to 1/12 of carbon-12; relative formula mass (Mr) is sum of Ar of atoms in formula.

Chemical equations must be balanced; state symbols (s), (l), (g), (aq) indicate physical state.

Mole = amount of substance containing 6.02 × 10^23 particles (Avogadro constant); molar mass = mass of one mole in g/mol.

Formulas: moles n = m / M; concentration c = n / V (V in dm³); gas volume at room temperature and pressure: 1 mol occupies 24 dm³.

Percentage yield = (actual yield / theoretical yield) × 100; atom economy = (Mr of desired product / sum of Mr of reactants) × 100.

Reaction types: combination, decomposition, displacement (more reactive metal displaces less reactive), neutralisation (acid + base → salt + water), combustion, redox (oxidation = loss of electrons, reduction = gain).

Exothermic reactions release energy (temperature rises); endothermic absorb energy (temperature falls); activation energy is minimum energy for reaction; catalysts lower activation energy.

Rates of reaction increased by higher temperature, higher concentration, larger surface area, catalyst; collisions must have sufficient energy.

Empirical formula from percentage composition; molecular formula = empirical formula × n; water of crystallisation can be determined by heating.

Limiting reactant determines maximum product; excess reactant remains after reaction.

Acid: proton (H+) donor; base: proton acceptor; strong acids (HCl, HNO3, H2SO4) fully ionise; weak acids (ethanoic acid) partially ionise.

pH scale 0–14: pH < 7 acidic, pH 7 neutral, pH > 7 alkaline; universal indicator colours (red, orange, yellow, green, blue, purple).

Neutralisation: acid + base → salt + water; acid + metal → salt + hydrogen; acid + carbonate → salt + water + carbon dioxide.

Salt preparation: soluble salts by acid + excess insoluble base/carbonate or titration; insoluble salts by precipitation (mixing two solutions).

Tests: hydrogen (lighted splint pops), oxygen (relights glowing splint), carbon dioxide (limewater milky), chlorine (bleaches damp litmus), ammonia (turns damp red litmus blue).

Flame tests: lithium crimson, sodium yellow, potassium lilac, calcium orange-red, copper blue-green.

Precipitation tests: chloride + silver nitrate → white precipitate; sulfate + barium chloride → white precipitate; carbonate + acid → CO2.

Indicators: litmus (red in acid, blue in alkali), methyl orange (red in acid, yellow in alkali), phenolphthalein (colourless in acid, pink in alkali).

Acid strength vs concentration: strength is degree of ionisation; concentration is amount of acid per volume; pH measures H+ concentration.

Reactivity series (decreasing): potassium, sodium, calcium, magnesium, aluminium, zinc, iron, tin, lead, copper, silver, gold; metals above carbon extracted by electrolysis, below by reduction with carbon.

Metal properties: good conductors of heat and electricity, malleable, ductile, high melting points; alloys (e.g., steel, brass) are harder than pure metals.

Corrosion: rusting of iron requires oxygen and water; prevented by painting, oiling, galvanising (zinc coating), sacrificial protection.

Extraction of iron in blast furnace: iron ore (haematite), coke, limestone; coke reduces iron oxide; limestone removes impurities as slag.

Non-metals: carbon (diamond, graphite, graphene, fullerenes), oxygen, nitrogen, sulfur, chlorine; properties vary; noble gases are inert.

Hydrocarbons: alkanes (single bonds, general formula CnH2n+2, e.g., methane, ethane, propane, butane); alkenes (double bond, CnH2n, e.g., ethene) are unsaturated and decolourise bromine water.

Crude oil is separated by fractional distillation based on boiling points; fractions used as fuels and feedstock.

Polymers: addition polymerisation of alkenes (e.g., polyethene); condensation polymerisation forms nylon, PET; plastics are non-biodegradable causing environmental issues.

Combustion of hydrocarbons: complete combustion gives CO2 + H2O; incomplete gives CO (toxic) and soot; greenhouse gases and acid rain from sulfur impurities.

Speed = distance / time; velocity is speed in a given direction; acceleration = change in velocity / time taken (m/s²).

Distance-time graph: gradient = speed; velocity-time graph: gradient = acceleration, area under graph = distance travelled.

Newton's laws: (1) object remains at rest or uniform motion unless acted on by resultant force; (2) F = m × a; (3) action and reaction forces equal and opposite.

Weight = mass × gravitational field strength (W = m × g; g ≈ 10 N/kg on Earth); mass is amount of matter (kg), weight is force (N).

Friction opposes motion; air resistance increases with speed; terminal velocity reached when air resistance equals weight.

Moments: moment = force × perpendicular distance from pivot; principle of moments: sum of clockwise moments = sum of anticlockwise moments for equilibrium.

Pressure = force / area (Pa = N/m²); pressure in liquids increases with depth and density (P = ρgh); atmospheric pressure decreases with height.

Hooke's law: extension is directly proportional to applied force up to the limit of proportionality (F = kx).

Centre of mass: point where weight acts; stability increased by lower centre of mass and wider base.

Energy stores: kinetic (½mv²), gravitational potential (mgh), elastic potential, chemical, thermal, nuclear; energy is conserved but can be transferred usefully or dissipated.

Work done = force × distance moved in direction of force (W = F × d); power = work done / time (P = W/t); efficiency = useful output / total input × 100%.

Wave equation: speed = frequency × wavelength (v = fλ); frequency in hertz (Hz), wavelength in metres.

Transverse waves (light, water waves) oscillate perpendicular to direction of travel; longitudinal waves (sound) oscillate parallel; sound needs a medium, light does not.

Reflection: angle of incidence = angle of reflection; refraction: change of speed at boundary, bending towards normal when entering denser medium.

Lenses: convex lens converges light (real images), concave diverges; focal length; real images can be projected, virtual cannot.

Dispersion: white light splits into spectrum through prism because different colours refract by different amounts; red least, violet most.

Sound: produced by vibrations, travels faster in solids than gases, cannot travel through vacuum; pitch depends on frequency, loudness on amplitude.

Electromagnetic spectrum (increasing frequency): radio, microwave, infrared, visible, ultraviolet, X-rays, gamma rays; all travel at speed of light in vacuum.

Current = charge / time (I = Q/t); potential difference = energy transferred per unit charge (V = W/Q); resistance = voltage / current (R = V/I).

Ohm's law: current through a conductor is proportional to voltage across it at constant temperature (ohmic conductors); filament lamp and diode are non-ohmic.

Series circuits: same current, voltages add, total resistance = sum of resistances; parallel circuits: same voltage, currents add, reciprocal resistance formula.

Electrical power: P = VI = I²R = V²/R; energy transferred = power × time (E = Pt); domestic electricity measured in kilowatt-hours (kWh).

Magnetism: like poles repel, unlike attract; magnetic field lines from north to south; electromagnet formed by current-carrying coil with iron core; strength increased by more turns, higher current.

Motor effect: force on current-carrying conductor in magnetic field (F = BIL sinθ); direction given by Fleming's left-hand rule; electric motor converts electrical to kinetic energy.

Electromagnetic induction: changing magnetic field induces voltage; generator converts kinetic to electrical; transformers change AC voltage (Vp/Vs = Np/Ns).

Heat transfer: conduction (solids, particle vibrations), convection (fluids, density changes), radiation (infrared, no medium needed); dark, matt surfaces emit and absorb more radiation.

Specific heat capacity: energy = mass × specific heat capacity × temperature change (E = mcΔT); latent heat: energy to change state without temperature change (E = mL).