LOWKEY SMART

For simultaneous equations, put your x and y back into BOTH equations to check them.

Homeostasis is keeping internal conditions such as body temperature, blood glucose and water level constant, so that cells and enzymes work properly. In the nervous system, a receptor detects a stimulus, a sensory neurone carries the impulse to the central nervous system (CNS: the brain and spinal cord), and a motor neurone carries it on to an effector, which is a muscle or a gland. A reflex is fast and automatic because it does not involve the conscious part of the brain: stimulus → receptor → sensory neurone → relay neurone (in the spinal cord) → motor neurone → effector → response. The gap between two neurones is called a synapse, and a chemical diffuses across it to start an impulse in the next neurone. Muscles respond by contracting, and glands respond by releasing hormones.

The suffixes -ible and -able both mean 'able to be'. Use -able after a complete word (enjoy + able = enjoyable) and after most words ending in a hard c or g (comfort + able). Use -ible when the root is not a complete English word on its own (possible, visible, terrible). There is no reliable rule, so these words must be learned.

Hormones are chemical messengers released by glands into the blood, which carries them to target organs; their effects are slower but last longer than nerve impulses. Blood glucose is controlled by the pancreas. If blood glucose is too high, the pancreas releases insulin, which makes cells take in glucose and makes the liver and muscles store the extra as glycogen. If it is too low, the pancreas releases glucagon, which makes the liver turn glycogen back into glucose. In Type 1 diabetes the pancreas makes too little insulin, so it is treated with insulin injections; in Type 2 diabetes the body's cells no longer respond properly to insulin, and it is treated with a carbohydrate-controlled diet and exercise. The pituitary gland in the brain is called the master gland because its hormones control other glands.

In the active voice, the subject does the action: 'The scientist discovered the virus.' In the passive voice, the subject receives the action, and the verb becomes a form of 'to be' plus a past participle: 'The virus was discovered by the scientist.' Writers use the passive to focus attention on the action or result rather than who did it.

The rate of a reaction is how fast reactants are used up or products are made: mean rate = quantity of product formed ÷ time taken. Particles only react when they collide with enough energy, and the minimum energy needed is called the activation energy. The rate increases if you raise the temperature (particles move faster, so they collide more often and with more energy), increase the concentration of a solution or the pressure of a gas (particles are more crowded, so collisions happen more often), or increase the surface area of a solid by using a powder instead of a lump. A catalyst speeds up a reaction without being used up, by giving a different pathway with a lower activation energy, and enzymes are biological catalysts.

The platform was almost empty by the time the last train was due. A single overhead light buzzed and flickered, throwing long shadows across the cracked tarmac. The ticket office had closed hours earlier, its shutter pulled down and padlocked for the night. An elderly man sat alone on the metal bench, his suitcase resting against his leg like a loyal dog. He checked his watch for the third time in a minute, though the train was not yet late. Somewhere beyond the tracks, a fox called out, sharp and sudden, and the man's head turned towards the sound before settling back to stillness. The wind picked up, dragging a crisp packet noisily along the platform edge until it vanished into the dark. When the train finally appeared, its headlights cutting twin beams through the mist, the man rose slowly, gathering his suitcase with the care of someone carrying something far more precious than clothes. The doors hissed open. For a moment he hesitated on the threshold, glancing back down the empty platform as though leaving something behind that he could never quite name. Then the doors closed, and the station returned to its silence, waiting for a morning that felt a very long way off.

Crude oil is a finite resource formed from the remains of ancient sea creatures, and it is a mixture of hydrocarbons: compounds of hydrogen and carbon only. Most are alkanes, with the general formula CₙH₂ₙ₊₂, for example methane CH₄, ethane C₂H₆, propane C₃H₈ and butane C₄H₁₀. Fractional distillation separates crude oil into fractions of molecules with similar numbers of carbon atoms; bigger molecules have higher boiling points, are more viscous (thicker) and are less flammable. Complete combustion of a hydrocarbon in plenty of oxygen produces carbon dioxide and water. Cracking breaks long-chain alkanes into shorter, more useful alkanes and alkenes, and alkenes turn bromine water from orange to colourless.

A pair of dashes can add extra information into the middle of a sentence, with more emphasis than commas: 'The bridge - ancient and crumbling - had stood for centuries.' Brackets add information more quietly, as an aside the reader could skip: 'The bridge (built in 1850) had stood for centuries.' A single dash can also show an abrupt change: 'She opened the door and screamed - the room was empty.'

Speed = distance ÷ time, measured in m/s; velocity is speed in a given direction, so velocity is a vector and speed is a scalar. Acceleration = change in velocity ÷ time taken, a = (v - u)/t, where u is the starting velocity and v the final velocity, measured in m/s²; a negative acceleration means slowing down. On a distance-time graph the gradient gives the speed, and a horizontal line means the object is stationary. On a velocity-time graph the gradient gives the acceleration and the area under the graph gives the distance travelled. Another useful equation is v² - u² = 2as, where s is the distance.

Writers choose words for their connotations, the feelings and ideas attached to them beyond their literal meaning. Hyperbole is deliberate exaggeration ('I've told you a million times'). Juxtaposition places two contrasting ideas close together for effect. Pathetic fallacy gives weather or nature human emotions to match the mood of a scene ('the sky wept with grief').

A resultant force is the single force that has the same effect as all the forces acting together: add forces that act in the same direction and subtract opposite ones. Newton's first law: if the resultant force is zero, a still object stays still and a moving object keeps moving at the same speed in the same direction. Newton's second law: resultant force = mass × acceleration (F = ma), with force in newtons (N), mass in kg and acceleration in m/s². Weight = mass × gravitational field strength (W = mg), with g = 10 N/kg on Earth; mass does not change from place to place, but weight does. Newton's third law: when two objects interact, they exert equal and opposite forces on each other.

I can hear the clock in the hallway louder than usual tonight, as if it wants me to notice every second slipping past. My notes are spread across the desk in overlapping piles, colour coded until the colours themselves stopped meaning anything. I keep telling myself that panicking about three subjects at once will not help, yet here I am, flicking between them anyway, achieving very little except a growing headache. Downstairs, I can hear my family moving quietly, careful not to disturb me, which somehow makes the silence in my room feel heavier rather than lighter. I tried testing myself on quotations an hour ago and blanked on the easiest one, which did nothing for my confidence. Still, when I stop and actually think about it, I know most of this material better than I am giving myself credit for tonight. Tomorrow will not be perfect, and it does not need to be; it only needs me to walk in and do what I have already practised a hundred times. I am closing the books now, before exhaustion undoes everything the revision achieved. Whatever happens in the morning, I got here through months of steady work, not a single frantic night.