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.

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.

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.

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.

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.

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.