Animal and plant cells are eukaryotic - they have a nucleus, cytoplasm and cell membrane, plus mitochondria (for respiration) and ribosomes (for protein synthesis).
Plant cells ALSO have a cell wall (made of cellulose, for support), a permanent vacuole (filled with cell sap) and chloroplasts (contain chlorophyll, site of photosynthesis) - but only in green parts.
Bacteria are prokaryotic - much smaller, no nucleus (DNA is a single loop plus small rings called plasmids), no mitochondria or chloroplasts, and the cytoplasm has no membrane-bound organelles.
Magnification = image size / actual size. Always convert units to the same before dividing (usually micrometres, um; 1mm = 1000um).
Light microscopes: max useful magnification around x2000, resolve down to about 200nm.
Electron microscopes: far higher magnification (up to x2,000,000) and better resolution (down to about 0.1nm) because they use electron beams not light.
Common mistake: forgetting to convert mm to um/nm before calculating magnification - always check units match.
Cells differentiate to become specialised for a job, e.g. sperm cells (tail for swimming), root hair cells (large surface area for water uptake), nerve cells (long, for fast signals), red blood cells (biconcave, no nucleus, packed with haemoglobin for oxygen transport).
Stem cells are undifferentiated and can become different cell types. Embryonic stem cells can become almost any cell type; adult stem cells (e.g. bone marrow) are more limited. Used in research and treatments like bone marrow transplants, but embryonic stem cell use raises ethical issues.
Cells -> tissues (group of similar cells, e.g. muscular tissue) -> organs (different tissues working together, e.g. the stomach) -> organ systems (organs working together, e.g. digestive system) -> organism.
Pathogens are microorganisms that cause disease: bacteria, viruses, fungi and protists.
Bacteria reproduce rapidly by binary fission and can produce toxins that damage cells - treat with antibiotics (NOT viruses).
Viruses are much smaller than bacteria, reproduce by invading host cells and making them replicate the virus, then bursting (lysing) the cell.
Common examples to know: measles (virus, spread by droplets), Salmonella (bacteria, food poisoning), HIV (virus, spreads via body fluids, can lead to AIDS by damaging the immune system), tuberculosis/TB (bacteria, airborne), malaria (protist, spread by mosquito vectors), rose black spot (fungus, spread by water/wind, affects plants).
The body has physical barriers (skin, nose hairs, mucus, stomach acid) and the immune system (white blood cells: phagocytes engulf pathogens; lymphocytes produce antibodies specific to antigens, and antitoxins).
Vaccination introduces a small, safe dose of dead/inactive pathogen so the body makes antibodies and memory cells, giving faster response on real infection. Common mistake: thinking vaccines give you the disease - they do not, they trigger immune memory only.
Photosynthesis: carbon dioxide + water -> glucose + oxygen, using light energy trapped by chlorophyll. It's endothermic (takes in energy). Rate is limited by light intensity, CO2 concentration and temperature - whichever is in shortest supply is the limiting factor. Above about 45C enzymes denature and rate crashes.
Respiration releases energy from glucose in every living cell, all the time. Aerobic respiration (with oxygen): glucose + oxygen -> carbon dioxide + water, releases lots of energy. Anaerobic respiration in animals (no oxygen): glucose -> lactic acid, releases much less energy and causes oxygen debt and muscle fatigue. Anaerobic respiration in plants/yeast: glucose -> ethanol + carbon dioxide (fermentation).
During exercise, heart rate, breathing rate and breath volume all increase to deliver more oxygen and glucose to muscles and remove CO2 faster.
Homeostasis keeps internal conditions constant despite external changes - key examples are blood glucose, body temperature and water levels. It always works by negative feedback: a receptor detects a change, a coordination centre (often the brain) processes it, and an effector brings the level back to normal.
Blood glucose control: insulin (from the pancreas) lowers blood glucose by causing cells to take up glucose and the liver to store it as glycogen. Glucagon raises blood glucose by causing the liver to break glycogen back down into glucose. Type 1 diabetes = the pancreas makes little/no insulin, treated with insulin injections. Type 2 diabetes = the body's cells stop responding properly to insulin, linked to obesity, managed by diet, exercise, sometimes medication.
Thermoregulation: the thermoregulatory centre in the brain monitors blood temperature. Too hot: vasodilation (blood vessels widen near skin) and sweating cool you down. Too cold: vasoconstriction and shivering warm you up.
Common mistake: don't say insulin 'makes' glucose - it makes cells absorb glucose. Don't confuse vasodilation (blood vessels widen = lose heat) with vasoconstriction (narrow = keep heat).
A community is all the different species living in a habitat; an ecosystem includes the physical environment too. Organisms are interdependent (competition for resources, predator-prey relationships).
Energy is lost between trophic levels (as heat from respiration, in waste, in parts not eaten) which is why food chains rarely have more than 4-5 levels and why eating lower down the chain (e.g. plants) is more energy-efficient.
Human impacts on biodiversity: deforestation, land use for farming/building, pollution (air, water, land) and global warming all reduce biodiversity. Maintaining biodiversity is essential for the future of the human race - it keeps ecosystems stable.
Common mistake: energy is not 'created' or 'destroyed' in food chains, it's transferred and much of it is lost as heat, not passed on.
Atoms have a tiny, dense nucleus (protons + neutrons) surrounded by electrons in shells. Protons have a relative charge of +1, electrons -1, neutrons 0. Protons and neutrons both have a relative mass of 1; electrons are almost massless (1/1836).
Elements are arranged in order of atomic number. Group number = number of outer electrons. Period number = number of occupied shells. Group 0 (noble gases) have full outer shells, so they are unreactive.
Metals lose electrons to form positive ions; non-metals gain electrons to form negative ions. Opposite charges attract, giving a giant ionic lattice held by strong electrostatic forces in all directions. This explains why ionic compounds have high melting and boiling points and conduct electricity only when molten or dissolved (ions become free to move).
Non-metal atoms share pairs of electrons to get a full outer shell. Simple molecules (like H2O or CO2) have strong covalent bonds inside the molecule but weak intermolecular forces between molecules, so they have low melting/boiling points and don't conduct electricity. Giant covalent structures (diamond, graphite, silicon dioxide) have millions of strong covalent bonds, giving very high melting points.
Metals consist of a giant lattice of positive ions surrounded by a 'sea' of delocalised electrons. This explains conductivity (electrons move freely) and malleability (layers of ions slide over each other).
Relative atomic mass (Ar) is the average mass of an atom compared to 1/12 of a carbon-12 atom. Relative formula mass (Mr) is the sum of all the Ar values in a formula.
Moles: moles = mass (g) / Mr. One mole of any substance contains 6.02 x 10^23 particles (Avogadro's constant).
Conservation of mass: in a closed system, the mass of reactants equals the mass of products, because atoms are only rearranged, never created or destroyed. In open systems (like gas escaping), the measured mass can appear to change.
Percentage yield = (actual yield / theoretical yield) x 100. Yield is never 100% due to incomplete reactions, side reactions, or losses during separation.
Rate = amount of reactant used up or product formed, divided by time. Measured in g/s or cm3/s.
Four factors speed up a reaction:
You can measure rate by: loss of mass (gas escaping, on a balance), volume of gas collected (gas syringe), or time taken for a solution to go cloudy (disappearing cross method).
Some reactions go both ways, shown by the symbol with two half arrows. At equilibrium, the forward and reverse reactions happen at the same rate, so concentrations of reactants and products stay constant (not equal).
Exothermic reactions transfer energy to the surroundings - temperature goes up. Examples: combustion, neutralisation, oxidation (like rusting).
Endothermic reactions take in energy from the surroundings - temperature goes down. Examples: thermal decomposition, citric acid and sodium hydrogencarbonate.
On a reaction profile, exothermic has products lower than reactants; endothermic has products higher than reactants. Activation energy is the minimum energy needed for a reaction to start - shown as the peak on the profile.
Crude oil is a mixture of hydrocarbons, mainly alkanes, formed from ancient plankton over millions of years. It is separated by fractional distillation - the oil is heated, vapours rise up a column that is cooler at the top, and fractions condense at different heights according to boiling point. Short-chain hydrocarbons have low boiling points, are runny and very flammable; long-chain hydrocarbons have high boiling points, are thick and burn less easily.
Cracking breaks long-chain alkanes into shorter, more useful alkanes and alkenes, using heat with a catalyst (catalytic cracking) or high pressure steam. This meets demand for smaller molecules like petrol. Alkenes have a C=C double bond and are more reactive than alkanes - they decolourise orange bromine water to colourless, which is the test for unsaturation. Alkanes are saturated (all single bonds) and stay orange with bromine water.
Energy is never created or destroyed, only transferred between stores: kinetic, gravitational potential, elastic potential, thermal, chemical, magnetic, electrostatic and nuclear.
Total energy before an event equals total energy after. Useful energy is the energy transferred to where you want it; wasted energy usually ends up as heat in the surroundings (dissipated). Efficiency = useful output energy divided by total input energy, then x100 for a percentage. No machine is 100% efficient because some energy always dissipates as heat through friction or resistance.
Power (W) = energy transferred (J) / time (s), also power (W) = work done (J) / time (s). 1 watt = 1 joule per second. Common mistake: forgetting power is a rate, not a total amount of energy.
Series circuits: same current everywhere, voltages add up, resistances add up. Parallel circuits: same voltage across each branch, current splits, total resistance is lower than the smallest branch resistance.
UK mains supply is 230 V a.c. at 50 Hz. The live wire (brown) carries the alternating potential difference, neutral (blue) completes the circuit near 0 V, earth (green and yellow) is a safety wire that carries fault current to protect people. Fuses melt and break the circuit if current is too high; a fuse rating should be just above normal operating current. Common mistake: mixing up live and neutral, or forgetting the earth wire only carries current during a fault.
Atoms have a tiny, dense, positively charged nucleus (protons and neutrons) surrounded by electrons in shells. Atomic number = number of protons; mass number = protons plus neutrons. Isotopes have the same proton number but different neutron numbers. Radioactive decay is random and cannot be predicted for a single atom, but half-life (the time for half the nuclei in a sample to decay) is predictable for large numbers. Alpha particles are stopped by paper, beta by thin aluminium, gamma needs thick lead or concrete.
A force is a push or pull measured in newtons (N). Speed = distance / time. Velocity is speed in a given direction. Acceleration = change in velocity / time, in m/s squared. A negative acceleration means the object is slowing down.
Newton's laws matter a lot for exams. First law: an object stays at rest or constant velocity unless a resultant force acts on it. Second law: force = mass x acceleration (F = m x a). Third law: every action has an equal and opposite reaction.
Weight = mass x gravitational field strength (W = m x g). On Earth g is 9.8 N/kg, sometimes rounded to 10 N/kg in questions. Mass stays the same everywhere but weight changes with gravity.
Stopping distance = thinking distance + braking distance. Thinking distance increases with speed and reaction time (alcohol, drugs, tiredness, distraction increase it). Braking distance increases with speed, poor road conditions, and worn brakes or tyres. Doubling speed more than doubles braking distance because kinetic energy increases with speed squared.
Waves transfer energy, not matter. Transverse waves (like light and water waves) vibrate at right angles to the direction of travel. Longitudinal waves (like sound) vibrate parallel to the direction of travel, forming compressions and rarefactions.
Wave speed = frequency x wavelength (v = f x lambda). Frequency is measured in hertz (Hz), wavelength in metres (m). The electromagnetic spectrum, in order of increasing frequency, is: radio, microwave, infrared, visible light, ultraviolet, X-ray, gamma. All EM waves travel at the same speed in a vacuum (3 x 10^8 m/s).
Magnets have a north and south pole. Like poles repel, unlike poles attract. Magnetic fields point from north to south outside the magnet. A current-carrying wire creates a magnetic field around it, which is the basis of electromagnets. Increasing current or adding more coil turns strengthens an electromagnet.