LOWKEY SMART

Check an expansion or a solution by putting a small number (like x = 2) back into both sides.

Animal and plant cells are eukaryotic: their DNA is inside a nucleus, which controls the cell's activities. Both also have cytoplasm (where most chemical reactions happen), a cell membrane (controls what goes in and out), mitochondria (where aerobic respiration releases energy) and ribosomes (where proteins are made). Plant cells also have a cell wall made of cellulose that strengthens the cell, a permanent vacuole filled with cell sap, and chloroplasts, which absorb light for photosynthesis. Bacteria are prokaryotic: they are much smaller, have no nucleus, and their DNA is a single loop in the cytoplasm, often with extra small rings of DNA called plasmids. Bacteria also have a cell wall, but it is not made of cellulose.

Some words sound alike but have different meanings and spellings. 'Affect' is usually a verb meaning to influence; 'effect' is usually a noun meaning a result. 'Practise' is the verb; 'practice' is the noun. 'Stationary' means not moving; 'stationery' means paper and pens. Choosing the right one shows precise control of language, which examiners reward.

Magnification = image size ÷ real size, so real size = image size ÷ magnification, and image size = magnification × real size. Always put both sizes in the same units first: 1 mm = 1000 micrometres (µm). A light microscope magnifies up to about ×2000 and shows cells and nuclei; an electron microscope has a much higher magnification and resolution, so it shows tiny structures such as ribosomes. Resolution is the ability to tell two points that are close together apart. The total magnification of a light microscope = eyepiece lens magnification × objective lens magnification.

A relative clause adds extra information about a noun, introduced by a relative pronoun: who (people), which (things), that (people or things) or whose (belonging to). In 'The scientist who discovered the cure was interviewed', 'who discovered the cure' is the relative clause.

An atom has a tiny central nucleus containing protons (relative charge +1, relative mass 1) and neutrons (charge 0, mass 1), with electrons (charge -1, almost no mass) in shells around it. The atomic number is the number of protons, and in a neutral atom the number of electrons equals the number of protons, so the atom has no overall charge. The mass number is the number of protons + neutrons, so neutrons = mass number - atomic number. Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons, such as carbon-12 and carbon-14. Electrons fill shells in order: up to 2 in the first shell, then up to 8 in the second and 8 in the third, so sodium (11 electrons) is 2,8,1. The nucleus is about 10 000 times smaller across than the whole atom.

The storm arrived without warning. One moment the harbour lay flat and silver under a pale sky; the next, black clouds rolled in from the west like smoke pouring from an unseen fire. Fishing boats strained against their ropes, groaning as the swell lifted and dropped them. Rain came in stinging sheets, turning the cobbled quay into a dark mirror. Market stalls along the front had already been abandoned, their awnings flapping wildly, tins and crates rolling loose across the cobbles. An old fisherman stood at the harbour wall, watching the horizon with the calm of someone who had seen a hundred storms before. His weathered hands gripped the railing as spray leapt over the wall and soaked his coat. Further along, a young sailor fought to secure a loose sail, the canvas snapping like a whip in the wind. Gulls wheeled overhead, screeching as though warning the harbour of worse still to come. Somewhere out past the harbour mouth, a horn sounded twice, faint and urgent, swallowed almost instantly by the roar of the waves. The old fisherman did not move. He simply narrowed his eyes, counted the seconds between the flashes of lightning and the crack of thunder, and waited, as though the sea itself might tell him what to do next.

An element contains only one type of atom, and a compound contains two or more elements chemically bonded in fixed proportions, such as water (H₂O) or carbon dioxide (CO₂). A mixture contains substances that are not chemically joined, so it can be separated by physical methods. Filtration separates an insoluble solid from a liquid, and crystallisation gets a soluble solid back from a solution by evaporating some of the water and leaving it to form crystals. Simple distillation separates a liquid from a solution (for example pure water from salt water) by boiling it and then cooling the vapour so it condenses; fractional distillation separates liquids with different boiling points. Paper chromatography separates dissolved substances such as the dyes in ink: the more soluble a dye is in the solvent, the further it travels up the paper.

A semicolon joins two closely related independent clauses without a joining word: 'The rain stopped; the sun broke through.' A colon introduces an explanation, a list or a dramatic single idea: 'She had one goal: to win.' Using these well shows control over sentence structure at GCSE.

Energy is held in stores such as kinetic (moving objects), gravitational potential (raised objects), elastic potential (stretched or squashed objects), thermal, chemical and nuclear. Kinetic energy = ½ × mass × speed², or E = ½mv², with mass in kg and speed in m/s. Gravitational potential energy = mass × gravitational field strength × height, or E = mgh, where g = 10 N/kg on Earth; all energy is measured in joules (J). Heating an object transfers energy to its thermal store. Energy cannot be created or destroyed, only transferred, stored or dissipated, which means spread out and wasted, usually as heat to the surroundings.

Writers use language techniques to create effect. A metaphor says one thing is another ('the exam was a mountain'). A simile compares using 'like' or 'as' ('as brave as a lion'). Personification gives human qualities to something non-human ('the wind whispered'). Alliteration repeats a starting sound ('soft, silent snow').

Power is the rate of energy transfer: power = energy transferred ÷ time, or P = E/t, measured in watts (W), where 1 W = 1 J per second and 1 kW = 1000 W. Efficiency = useful output energy ÷ total input energy, and you multiply by 100 to give a percentage; the same formula works with useful output power ÷ total input power. Wasted energy = total input energy - useful output energy. No device is 100% efficient, because some energy is always dissipated, usually as heat. Lubrication reduces friction between moving parts, and thermal insulation reduces heat loss, so both make things more efficient.

Local councils across the country are being forced to make difficult choices about which services to protect. In many towns, the public library is now open for only four days a week, and weekend opening has been cut entirely. This decision might save money in the short term, but it ignores who actually uses the library most. Working parents, students revising for exams and older residents without home internet all rely on Saturday opening far more than midweek slots. This is not a small number of people; it adds up to thousands of visits every single week. Closing the doors at the weekend does not remove the need for these services; it simply pushes people towards more expensive alternatives, or towards no alternative at all. Supporters of the cuts argue that visitor numbers have fallen since the pandemic, and that resources are better spent elsewhere. Yet libraries have already adapted, offering free wifi, homework clubs and job-search support that many families cannot get anywhere else nearby. A library is not simply a room full of books; it is one of the last free public spaces left in many towns. Before councils close another door, they should ask a harder question: what replaces a library once it is gone? The answer, in most cases, is nothing.