Botswana BGCSE Science Double Award (0569) cheat sheet

Cell theory: all living things are made of cells; new cells arise only from division of existing cells.

Prokaryotic cells lack a membrane-bound nucleus and membrane-bound organelles; bacteria are prokaryotes.

Eukaryotic cells have a true nucleus and organelles such as mitochondria, chloroplasts and a vacuole in plants.

Plant cells uniquely possess a cellulose cell wall, large permanent vacuole and chloroplasts; animal cells lack all three.

Diffusion moves particles down a concentration gradient without energy; osmosis is diffusion of water across a partially permeable membrane.

Active transport moves substances against a concentration gradient and requires energy from respiration.

Enzymes are protein catalysts with a specific active site; rate rises with temperature up to an optimum then falls as the enzyme denatures.

Photosynthesis equation: 6CO2 + 6H2O + light energy -> C6H12O6 + 6O2, occurring in chloroplasts.

Aerobic respiration equation: C6H12O6 + 6O2 -> 6CO2 + 6H2O + energy; anaerobic in muscle gives lactic acid.

Alveoli are the gas-exchange surface: large total area, thin walls, moist surface and rich blood supply.

Blood: plasma carries dissolved substances; red cells carry oxygen via haemoglobin; white cells defend; platelets clot.

The heart has four chambers; the left ventricle has the thickest muscular wall because it pumps blood around the whole body.

Arteries carry blood away from the heart under high pressure; veins carry blood back and contain valves; capillaries are one cell thick.

Digestion: amylase breaks starch to maltose; protease breaks protein to amino acids; lipase breaks fat to fatty acids and glycerol.

Villi in the small intestine increase surface area and have thin walls and a good blood supply for absorption.

Nephron function: filtration in the glomerulus, then selective reabsorption of glucose, some salts and most water.

Homeostasis keeps internal conditions constant; insulin lowers blood glucose, glucagon raises it.

Nervous coordination: receptor -> sensory neurone -> relay neurone in the spinal cord -> motor neurone -> effector; synapses use chemical transmitters.

Pathogens: bacteria, viruses, fungi and protozoa; viruses cannot reproduce outside a host cell.

Body defences: skin and mucus as barriers; phagocytes engulf pathogens; lymphocytes produce antibodies specific to an antigen.

Vaccination introduces weakened or inactive antigen so memory cells give a faster, greater secondary response.

Antibiotics kill bacteria but are ineffective against viruses; overuse selects resistant bacterial strains.

Malaria is caused by a protozoan transmitted by female Anopheles mosquitoes; HIV is transmitted by blood, semen or breast milk.

Female reproductive hormones: FSH matures the egg and stimulates oestrogen; oestrogen thickens the uterus lining; LH triggers ovulation.

Fertilisation occurs in the oviduct; the placenta exchanges nutrients, oxygen and waste between mother and fetus.

Contraception: condoms also reduce infection risk; oral pills contain hormones that prevent ovulation.

Health risk factors include smoking, alcohol, poor diet, lack of exercise and unsafe sex.

Food chain arrows show the direction of energy flow; producers are at the first trophic level.

Only about 10% of energy passes to the next trophic level; the rest is lost as heat, in movement and in undigested waste.

Pyramids of number, biomass and energy represent trophic levels; energy pyramids are always upright.

The carbon cycle: photosynthesis removes CO2; respiration, combustion and decomposition return it.

The nitrogen cycle depends on nitrogen-fixing bacteria, decomposers and nitrifying bacteria; legumes have root nodules.

Population size is affected by competition, predation, disease, food supply and abiotic factors such as light, temperature and water.

Conservation measures include protected areas, seed banks, captive breeding, recycling and pollution control.

Bioaccumulation: persistent toxins such as DDT increase in concentration along a food chain.

Eutrophication: excess nitrate and phosphate cause algal bloom, then decomposers use oxygen and fish die.

Atom: protons and neutrons in the nucleus, electrons in shells; proton number determines the element.

Mass number = protons + neutrons; isotopes have the same protons but different neutrons.

Electron shell filling: 2, 8, 8; group number equals outer-shell electrons, period number equals shells used.

Ionic bonding: metal atoms lose electrons to form cations, non-metals gain to form anions; giant lattice with high melting point.

Covalent bonding: shared electron pairs between non-metals; simple molecules have low melting points, giant covalent structures do not.

Metallic bonding: positive ions in a sea of delocalised electrons explains conduction and malleability.

Relative atomic mass is the weighted mean of isotope masses; relative molecular mass is the sum of atomic masses.

Allotropes of carbon: diamond is hard and non-conducting; graphite is soft and conducts because of delocalised electrons.

Empirical formula gives the simplest ratio; molecular formula gives the actual number of atoms per molecule.

Rate of reaction increases with higher temperature, higher concentration, greater surface area and a catalyst.

Collision theory: particles must collide with sufficient energy (activation energy) for a reaction to occur.

Acid + metal -> salt + hydrogen; acid + carbonate -> salt + water + carbon dioxide; acid + base -> salt + water.

pH below 7 is acidic, 7 neutral, above 7 alkaline; universal indicator gives a colour scale.

Neutralisation: H+ + OH- -> H2O; titration uses a burette and pipette with a suitable indicator.

Salt preparation depends on solubility: soluble salts by acid + excess insoluble base, then filtration and crystallisation.

Oxidation is loss of electrons, reduction is gain of electrons; redox reactions occur together.

Exothermic reactions release heat (temperature rises); endothermic reactions absorb heat (temperature falls).

Balanced equations must conserve atoms of each element and total charge.

Reactivity series order includes K, Na, Ca, Mg, Al, Zn, Fe, Pb, Cu, Ag, Au; more reactive metals displace less reactive ones from solution.

Metals above carbon in the series are extracted by electrolysis; those below can be reduced with carbon.

Blast furnace: coke burns to CO2, CO2 reduces to CO, and CO reduces iron oxide to iron; limestone removes impurities as slag.

Alloys such as steel and brass are harder than pure metals because different-sized atoms disrupt the layers.

Electrolysis of molten or aqueous compounds: cathode attracts cations (reduction), anode attracts anions (oxidation).

Rusting needs both oxygen and water; painting, greasing, galvanising and sacrificial protection prevent it.

Crude oil is separated by fractional distillation using differences in boiling point; fractions differ in chain length and viscosity.

Limestone (calcium carbonate) is heated to quicklime (calcium oxide), which with water gives slaked lime.

Water treatment: sedimentation, filtration and chlorination; hard water contains dissolved calcium and magnesium ions.

Speed = distance / time; velocity is speed in a stated direction; acceleration = change in velocity / time taken.

Newton's second law: resultant force = mass x acceleration (F = ma); weight = mass x gravitational field strength (W = mg).

Distance-time graph gradient gives speed; velocity-time graph gradient gives acceleration and area gives distance.

Forces act in pairs (Newton's third law); friction opposes motion and generates heat.

Moment of a force = force x perpendicular distance from pivot; principle of moments: clockwise moments equal anticlockwise moments.

Pressure = force / area; pressure in a liquid increases with depth.

Work done = force x distance moved in the direction of the force; power = work done / time.

Kinetic energy = 1/2 mv^2; gravitational potential energy = mgh; energy is conserved but degrades to heat.

Hooke's law: extension is proportional to load up to the limit of proportionality.

Wave speed = frequency x wavelength (v = f x lambda); frequency is in hertz, wavelength in metres.

Transverse waves vibrate perpendicular to travel (light); longitudinal waves vibrate parallel (sound).

Refraction occurs because speed changes between media; light bends towards the normal entering a denser medium.

Ohm's law: voltage = current x resistance (V = IR); resistors in series add, in parallel the total is less than the smallest.

Electrical power = voltage x current (P = VI); energy transferred = power x time.

Mains wiring: live (brown), neutral (blue), earth (green-yellow); fuses and earth wires protect the user.

Magnetism: like poles repel, unlike attract; a current-carrying wire in a magnetic field experiences a force.

Electromagnetic induction: a changing magnetic field near a coil induces a voltage; generators convert movement to electricity.

Radioactivity: alpha, beta and gamma differ in charge, penetration and ionising power; half-life is the time for half the nuclei to decay.

Variables: independent variable is changed, dependent variable is measured, control variables are kept constant.

A valid conclusion must be supported by the data collected and must refer back to the hypothesis.

Accuracy is closeness to the true value; precision is consistency of repeated readings; reliability improves with repeats.

Use a control group to show that the observed effect is due to the variable tested.

Mean is the sum divided by the number of values; identify anomalies and exclude them from the mean.

Line graphs suit continuous data; bar charts suit categoric data; always label axes with quantity and unit.

Gradient of a straight-line graph equals the constant rate; the intercept may indicate a systematic error.

Hazards must be identified and controlled: wear goggles, handle acids with care, and keep electrical equipment dry.

Record readings to the smallest division of the instrument and estimate the last digit.