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 (only in green parts).
Bacterial cells are much smaller and have no nucleus - instead they have a single loop of DNA plus small rings called plasmids, a cell wall, and sometimes a flagellum for movement.
Magnification = image size divided by actual size. Always convert units carefully (1mm = 1000 micrometres).
Light microscopes can magnify up to about x1500 and resolve detail down to around 200nm.
Electron microscopes have much higher resolution (down to about 0.1nm), so they reveal smaller structures like ribosomes and detailed mitochondria structure.
Common mistake: mixing up magnification (how much bigger the image looks) with resolution (how much detail you can see) - a common exam trap.
Cells work up in a hierarchy: cells, then tissues (a group of similar cells doing the same job, eg muscular tissue), then organs (different tissues working together, eg the stomach), then organ systems (eg the digestive system), then the whole organism.
The digestive system includes glands (produce enzymes), the stomach and small intestine (digestion and absorption), the liver (makes bile) and the large intestine (absorbs water).
Enzymes are biological catalysts made of protein. Each enzyme has an active site with a specific shape that only fits its substrate (lock and key).
High temperature or wrong pH denatures the enzyme - the active site changes shape permanently so it can no longer bind its substrate.
Amylase breaks down starch into sugars, protease breaks down proteins into amino acids, lipase breaks down fats into fatty acids and glycerol.
Pathogens are microorganisms that cause disease: bacteria, viruses, fungi and protists.
Bacteria reproduce by simple cell division (binary fission) and can double roughly every 20 minutes in ideal conditions; some make you ill by producing toxins.
Viruses reproduce by invading living cells and using them to replicate, then bursting (lysing) the cell.
Common mistake: antibiotics kill bacteria, NOT viruses - this catches out a lot of students.
Vaccines contain a dead or inactive form of a pathogen to trigger the immune system to make antibodies and memory cells, giving future protection without causing the disease itself.
Respiration releases energy from glucose in every living cell.
Homeostasis is keeping the internal environment stable despite external changes, using negative feedback.
Ecosystems have interdependent populations - a common mistake is ignoring competition.
Atoms have a tiny, dense nucleus (protons + neutrons) surrounded by electrons in shells. Protons have a +1 charge, electrons -1, neutrons no charge, and atoms are neutral overall (equal protons and electrons).
Elements are arranged in order of atomic number. Groups (columns) share the same number of outer electrons and similar chemical properties. Periods (rows) show the number of electron shells. Metals lose electrons to form positive ions; non-metals gain electrons to form negative ions.
Happens between a metal and a non-metal. Electrons transfer from metal to non-metal, forming oppositely charged ions held together by strong electrostatic forces in a giant ionic lattice. This gives high melting and boiling points, and ionic compounds conduct electricity only when molten or dissolved (ions must be free to move).
Happens between non-metals. Atoms share pairs of electrons. Simple molecular substances (like water or CO2) have weak intermolecular forces so low melting points, and don't conduct electricity. Giant covalent structures (like diamond or silicon dioxide) have very high melting points due to many strong covalent bonds.
Metal atoms are held together by strong attraction between positive metal ions and a 'sea' of delocalised electrons. This explains why metals conduct electricity and heat (electrons are free to move) and why they're malleable (layers of ions can slide).
Rate measures how fast reactants turn into products. You can measure it by how fast a reactant is used up or a product is formed, over time. Units are usually g/s or cm3/s.
Four factors speed up rate: temperature, concentration (or pressure for gases), surface area, and catalysts. Higher temperature gives particles more kinetic energy, so they move faster, collide more often, and more collisions have enough energy (the activation energy) to react. Higher concentration or pressure means more particles in a given space, so more frequent collisions. Smaller pieces (bigger surface area) expose more particles to reaction, again giving more frequent collisions. A catalyst lowers the activation energy by giving reactants an alternative pathway, without being used up itself.
For a reaction to happen, particles must collide with enough energy (at least the activation energy) and the right orientation. Anything that raises collision frequency or energy raises rate.
Common methods: timing how long a precipitate takes to block a cross (mass/volume vs time gives a curve that levels off when a reactant runs out), or collecting gas volume in a syringe over time. The steeper the graph line, the faster the rate at that point. Tangents give rate at a specific time.
Some reactions go both ways, shown by the symbol with two arrows (⇌). At equilibrium, forward and reverse reactions happen at the same rate, so concentrations stay constant (not equal). Le Chatelier's principle: if you change conditions (temperature, pressure, concentration), the equilibrium shifts to oppose the change and partially cancel it out.
Crude oil is a mixture of hydrocarbons, mostly alkanes (general formula CnH2n+2), separated by fractional distillation using differences in boiling point. Short-chain fractions are more flammable, less viscous, and have lower boiling points than long-chain fractions.
Cracking breaks long-chain, less useful hydrocarbons into shorter, more useful alkanes and alkenes, using heat with a catalyst, or steam. This matches supply to demand for smaller molecules like fuels.
Energy is stored in eight ways: kinetic, gravitational potential, elastic potential, thermal, chemical, magnetic, electrostatic and nuclear.
Energy transfers happen by four methods: mechanically (a force doing work), electrically (a current doing work), by heating, or by radiation (light or sound).
Energy cannot be created or destroyed, only transferred, stored or dissipated - this is conservation of energy. Wasted energy usually ends up as thermal energy that spreads out (dissipates) and becomes less useful.
Current is measured in amps with an ammeter connected in series. Potential difference (voltage) is measured in volts with a voltmeter connected in parallel.
Ohm's law: potential difference = current x resistance (V = IR). Resistance is measured in ohms.
In series circuits current is the same everywhere and voltages add up; in parallel circuits voltage is the same across each branch and current splits between branches.
Electrical power = potential difference x current (P = VI), also power = current squared x resistance (P = I2R).
UK mains electricity is alternating current (AC) at about 230V; a battery supplies direct current (DC).
Atoms have a tiny, dense, positively charged nucleus (protons and neutrons) surrounded by electrons in shells. Atomic (proton) number = number of protons; mass number = protons + neutrons.
Isotopes are atoms of the same element with different numbers of neutrons.
Radioactive decay is random and cannot be predicted for a single atom. Alpha particles are stopped by paper, beta by aluminium, gamma needs thick lead or concrete.
Half-life is the time for the number of radioactive nuclei (or the activity) to halve.
A force is a push or pull, measured in newtons (N). Contact forces need touching (friction, air resistance, tension, normal force); non-contact forces act at a distance (gravity, magnetic, electrostatic).
Resultant force is the single force that has the same effect as all forces combined. If forces are balanced, resultant is zero and the object stays still or moves at constant velocity (Newton's First Law).
Speed (m/s) = distance (m) / time (s). Typical walking speed is about 1.5 m/s, running about 3 m/s, cycling about 6 m/s, cars around 13-30 m/s.
Acceleration (m/s squared) = change in velocity / time taken. On a distance-time graph, gradient equals speed; a curve means acceleration. On a velocity-time graph, gradient equals acceleration and the area under the line equals distance travelled.
Stopping distance = thinking distance + braking distance. Thinking distance increases with speed and reaction time (alcohol, drugs, tiredness, phone use all increase it). Braking distance increases with speed, poor tyre tread, wet or icy roads, and poor brakes.
Waves transfer energy without transferring matter. Transverse waves (like light and all EM 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.
Magnets have two poles, north and south. Like poles repel, unlike poles attract. Magnetic fields point from north to south outside the magnet, shown by field lines with arrows.
A current-carrying wire creates a magnetic field around it; this is the basis of an electromagnet, made stronger by adding more coil turns, increasing current, or adding an iron core. Electromagnets are useful because they can be switched on and off, unlike permanent magnets.
Induced magnets (like iron near a magnet) only keep their magnetism while in the field; permanent magnets keep it. Common mistake: confusing magnetic materials (iron, steel, cobalt, nickel) with all metals - most metals are not magnetic.