All living things are made of cells. Animal cells have a nucleus, cytoplasm, cell membrane, mitochondria and ribosomes. Plant cells have all of these PLUS a cell wall (made of cellulose), a permanent vacuole (filled with cell sap) and chloroplasts (for photosynthesis). Bacterial cells are different again: no nucleus (DNA is a single loose loop plus small rings called plasmids), a cell wall (not cellulose), and sometimes flagella for movement.
Magnification = image size divided by actual (real) size. Rearrange to find real size when needed. Always convert units carefully: 1mm = 1000 micrometres (um). Light microscopes can magnify up to about x2000; electron microscopes give much higher magnification and resolution, letting you see organelles like ribosomes and mitochondria in detail.
Cells differentiate to become specialised for a job. Examples: sperm cells (tail for swimming, lots of mitochondria for energy), root hair cells (large surface area for water/mineral uptake), nerve cells (long axon, branched endings to carry signals), red blood cells (no nucleus, biconcave shape, packed with haemoglobin to carry oxygen).
Cells -> Tissues -> Organs -> Organ systems -> Organism. A tissue is a group of similar cells with the same function (e.g. muscular tissue, glandular tissue, epithelial tissue). An organ is a group of different tissues working together (e.g. the stomach contains muscular, glandular and epithelial tissue). The digestive system is a key organ system: mouth, oesophagus, stomach, small intestine, large intestine, liver and pancreas all work together to break down food.
Pathogens are microorganisms that cause disease: bacteria, viruses, fungi and protists. Bacteria are living cells that can reproduce rapidly and release toxins; viruses are not cells, they invade living cells and reprogram them to replicate. Common mistake: students often say viruses are 'living' — they are not classed as living cells, they need a host cell to reproduce. Diseases spread by: direct contact, water, air (droplet infection) or via vectors (e.g. mosquitoes for malaria, a protist disease).
Photosynthesis is an endothermic reaction: carbon dioxide + water --(light)--> glucose + oxygen. It happens in chloroplasts, using the green pigment chlorophyll to absorb light energy.
Respiration releases energy from glucose and happens in every living cell, all the time.
Homeostasis is the regulation of internal conditions to maintain a stable internal environment, responding to changes using negative feedback.
Ecosystems involve interdependence between organisms and their environment.
Atoms have a tiny, dense nucleus (protons and neutrons) surrounded by electrons in shells. Protons carry a +1 charge, electrons -1, neutrons no charge. Atomic number = number of protons (and electrons in a neutral atom). Mass number = protons + neutrons.
Isotopes are atoms of the same element with the same proton number but a different neutron number, so different mass numbers. Relative atomic mass (Ar) is the weighted mean mass of all isotopes compared to carbon-12.
Electron shells fill up with 2, 8, 8 electrons (lowest energy shell first). The group number of a main-group element equals its outer-shell electron count; the period number equals the number of shells in use.
Conservation of mass: no atoms are lost or made in a reaction, so the total mass of reactants equals the total mass of products in a closed system. If a gas escapes, the measured mass can appear to decrease.
Relative formula mass (Mr) is found by adding up the Ar values of all atoms in the formula.
Moles: the mole is the unit for amount of substance. Use mass (g) = moles x Mr to convert between mass and moles. Avogadro's number (6.02 x 10^23) is the number of particles in one mole, though OCR Combined Science focuses mainly on mass-mole calculations rather than particle counts.
Percentage yield = (actual yield / theoretical yield) x 100. Yield is never 100% in real reactions because of side reactions, incomplete reactions, or loss during transfer/purification.
Rate = amount of reactant used or product formed ÷ time. Units can be g/s, cm3/s or mol/s.
Four factors speed up a reaction: higher temperature, higher concentration (or pressure for gases), smaller particle size (bigger surface area), and adding a catalyst.
Collision theory explains this: particles must collide with enough energy (at least the activation energy) and the right orientation. More collisions or more energetic collisions mean a faster rate.
Catalysts speed up a reaction without being used up and without changing the products. They work by lowering the activation energy, providing an alternative reaction pathway.
Rate can be measured by: change in mass (gas escaping, use a balance), volume of gas produced (gas syringe), or time taken for a precipitate to obscure a mark (the 'disappearing cross' method with sodium thiosulfate and hydrochloric acid).
Gradient of a graph (amount vs time) gives the rate at that point. A steeper line means faster rate; the line flattens when the reaction finishes (a reactant is used up).
A reversible reaction can go forwards and backwards, shown by the symbol ⇌.
In a closed system, a reversible reaction reaches dynamic equilibrium, where the forward and reverse rates are equal and concentrations of reactants and products stay constant (but not necessarily equal).
Le Chatelier's principle: if you change the conditions (temperature, pressure, concentration), the equilibrium shifts to oppose the change.
Exothermic reactions release energy to the surroundings, so temperature increases (e.g. combustion, neutralisation, respiration).
Endothermic reactions take in energy, so temperature decreases (e.g. thermal decomposition, citric acid + sodium bicarbonate).
Bond breaking requires energy (endothermic); bond making releases energy (exothermic). Overall energy change = energy to break bonds minus energy released making bonds.
Crude oil is a mixture of hydrocarbons, mostly alkanes, separated by fractional distillation using differences in boiling point. Shorter chains have lower boiling points and rise higher in the column (e.g. petrol gases); longer chains (bitumen) are collected lower down.
Alkanes are saturated hydrocarbons with the general formula CnH2n+2 (e.g. methane CH4, ethane C2H6).
Cracking breaks long-chain hydrocarbons into shorter, more useful alkanes and alkenes, using heat with a catalyst (catalytic cracking) or steam at high temperature.
Alkenes are unsaturated, contain a C=C double bond, general formula CnH2n, and decolourise orange bromine water (a key test).
Don't say a catalyst 'speeds up the reaction by increasing temperature' — it lowers activation energy instead.
Don't confuse exothermic (temperature rises) with endothermic (temperature falls) — always link to surroundings, not the reacting chemicals.
Remember increasing pressure only affects rate for gases, not solids or liquids.
Always state that catalysts are chemically unchanged and can be reused.
Energy is never created or destroyed, only transferred between stores: kinetic, thermal, chemical, elastic potential, gravitational potential, magnetic, electrostatic and nuclear.
Useful energy transfers always lose some energy to the surroundings, usually as heat, through friction, air resistance or electrical resistance. This is called dissipation. Insulation (like loft insulation, u-values, cavity wall fill) reduces unwanted transfers by thermal conduction and convection.
Current (A) is the rate of flow of charge. Charge (C) = current (A) x time (s). Potential difference, or voltage (V), is the energy transferred per unit charge: V = energy (J) / charge (C).
Students often confuse energy and power, forget to square velocity or extension in the equations, or think resistance splits equally in parallel circuits (it does not unless the resistors are identical). Always keep units consistent (kg, m, s, J, W, V, A, ohms).
Particles in solids, liquids and gases differ in arrangement, movement and energy. Heating increases particle kinetic energy, which can cause a change of state without a temperature change during the change itself (specific latent heat). Density (kg/m3) = mass (kg) / volume (m3): solids and liquids are usually denser than gases because particles are closer together.
A force is a push or pull measured in newtons (N). Forces are vectors, so direction matters.
Contact forces (friction, air resistance, tension, normal contact) need touching objects.
Non-contact forces (gravity, magnetic, electrostatic) act at a distance.
Resultant force is the single force that has the same effect as all forces combined.
Weight (N) = mass (kg) x gravitational field strength (N/kg). On Earth g is about 10 N/kg (sometimes 9.8).
Mass is the amount of matter and stays constant; weight changes with gravitational field strength.
Newton's First Law: an object stays at rest or constant velocity unless a resultant force acts on it.
Newton's Second Law: force (N) = mass (kg) x acceleration (m/s squared), written F = ma.
Newton's Third Law: every action force has an equal and opposite reaction force.
Speed (m/s) = distance (m) / time (s). Acceleration (m/s squared) = change in velocity / time taken.
A common mistake is confusing speed and velocity - velocity includes direction, speed does not.
Distance-time graphs: gradient is speed; a flat line means stationary; a curve means changing speed.
Velocity-time graphs: gradient is acceleration; area under the graph is distance travelled.
Stopping distance = thinking distance + braking distance.
Thinking distance increases with speed and reaction time (alcohol, drugs, tiredness, distraction make it worse).
Braking distance increases with speed, poor tyre tread, wet or icy roads, and worn brakes.
A key exam trap: doubling speed more than doubles braking distance because kinetic energy increases with speed squared.
Waves transfer energy and information, not matter, from one place to another.
Transverse waves (like light and all electromagnetic waves) vibrate at right angles to the direction of travel.
Longitudinal waves (like sound) vibrate parallel to the direction of travel, with compressions and rarefactions.
Wave speed (m/s) = frequency (Hz) x wavelength (m), written v = f x lambda.
Frequency is measured in hertz (Hz) - the number of waves per second.
Amplitude is the maximum displacement from the rest position; it relates to energy carried, not speed.
Sound cannot travel through a vacuum because it needs particles to vibrate; light can travel through a vacuum.
The electromagnetic spectrum in order of increasing frequency: radio, microwave, infrared, visible light, ultraviolet, X-ray, gamma.
Magnetic fields are strongest at the poles and their direction runs from north to south outside the magnet.
Like poles repel, unlike poles attract.
A permanent magnet always produces its own field; an induced magnet only becomes magnetic when placed in a field and loses most magnetism when removed.
A current-carrying wire creates a magnetic field around it - this is the basis of electromagnets.
An electromagnet's strength increases with more coil turns, higher current, or an iron core.
Common mistake: students forget field lines always point from north to south outside the magnet, and that magnetic field strength decreases with distance from the magnet.