LOWKEY SMART

Write one line of working for every step: it is how you catch your own slips.

All living things are made of cells, the smallest units of life. Most cells are too small to see, so we use a light microscope to magnify them. Animal and plant cells both have: a nucleus, which contains the genetic material (DNA) and controls the cell; a cell membrane, which controls what enters and leaves the cell; cytoplasm, a jelly where most chemical reactions happen; and mitochondria, where respiration releases energy. Plant cells also have a cell wall made of cellulose, which strengthens the cell; chloroplasts, which contain green chlorophyll and are where photosynthesis happens; and a permanent vacuole filled with cell sap, which helps keep the cell firm. Animal cells do not have a cell wall, chloroplasts or a permanent vacuole.

Many cells are specialised: their shape and parts are suited to one job. A red blood cell has no nucleus, which leaves more room to carry oxygen. A sperm cell has a tail so it can swim to the egg. A nerve cell is long and thin so it can carry electrical signals around the body. A root hair cell has a long, thin extension that gives it a large surface area for absorbing water from the soil. Living things made of many cells are organised in levels: similar cells working together form a tissue, different tissues working together form an organ, and organs working together form an organ system. For example, the heart is an organ in the circulatory system. The full order is cell, tissue, organ, organ system, organism.

All matter is made of tiny particles. In a solid, the particles are touching, in a regular pattern, and only vibrate in fixed positions, so a solid has a fixed shape and cannot be compressed (squashed). In a liquid, the particles are touching but arranged randomly and can move past each other, so a liquid flows, takes the shape of its container and cannot be compressed. In a gas, the particles are far apart and move quickly in random directions, so a gas can be compressed and spreads out to fill its container. Diffusion is when particles spread out from where there are lots of them (a high concentration) to where there are fewer (a low concentration). Diffusion happens in liquids and gases, and it is faster when they are hotter. Gas pressure is caused by gas particles hitting the walls of their container.

Changing state is a physical change: no new substance is made, and it can be reversed. Melting is solid to liquid, and freezing is liquid to solid. Evaporation and boiling are liquid to gas, and condensation is gas to liquid. Sublimation is when a solid turns straight into a gas without melting first. A pure substance melts and boils at fixed temperatures: pure water melts at 0°C and boils at 100°C. When a substance is heated, its particles gain energy and move faster. Mass is conserved when a substance changes state, so 50 g of ice melts into 50 g of water.

A force is a push or a pull. Forces are measured in newtons (N) using a newtonmeter. Contact forces act when objects touch, for example friction, air resistance and upthrust. Non-contact forces act at a distance, for example gravity, magnetic force and electrostatic force. We draw forces as arrows: the length of the arrow shows the size of the force, and the arrow points in its direction. If the forces on an object are balanced (equal and opposite), a still object stays still and a moving object keeps the same speed and direction. If the forces are unbalanced, the object speeds up, slows down or changes direction. The resultant force is the overall force: for two forces in opposite directions, subtract the smaller from the larger, and it acts in the direction of the larger one. Weight = mass × gravitational field strength, and on Earth the gravitational field strength is 10 N/kg.

Speed tells you how far something travels in a certain time: speed = distance ÷ time. If distance is in metres (m) and time is in seconds (s), speed is in metres per second (m/s). If distance is in kilometres (km) and time is in hours (h), speed is in km/h. You can rearrange the formula: distance = speed × time, and time = distance ÷ speed. The formula gives the average speed for a journey, because the real speed usually changes along the way. On a distance-time graph, a horizontal (flat) line means the object is stationary (not moving), and a steeper line means a faster speed.