Everything from the 3 lessons of this topic on a few pages: the key idea of each lesson, the diagram to remember, the points to learn and the traps examiners set. Read it, cover it, test yourself.
Every living organism shows all seven characteristics of life, often remembered as MRS GREN: movement, respiration, sensitivity, growth, reproduction, excretion and nutrition. Each has a precise definition. Respiration happens in cells and releases energy; growth is a permanent increase in size and dry mass; excretion removes the waste products of metabolism. Non-living things may seem to show one or two, but never all seven, and never by the processes that happen in cells.
Respiration is not breathing. Breathing moves air in and out of the lungs; respiration is the chemical reactions in cells that release energy. A plant has no lungs, but every one of its living cells respires all the time.
Growth means a permanent increase in size and dry mass. A seed that swells by absorbing water gets bigger, but it has not grown, because water is not part of dry mass and can be lost again. Always write both parts of the definition.
Organisms are classified into groups by the features that they share. A species is a group of organisms that can reproduce to produce fertile offspring. The binomial system is an internationally agreed system in which the scientific name has two parts: the genus then the species, as in Panthera leo. A dichotomous key identifies an organism through pairs of statements about features you can see. Extended: classification aims to reflect evolutionary relationships, and organisms that share a more recent ancestor have more similar DNA base sequences.
A species is not defined by appearance. Animals that look alike can be separate species, and animals that look very different, like dog breeds, can be one species. The test is whether they produce fertile offspring. Offspring alone is not enough: a mule is offspring, but it is sterile.
Base every statement on a feature you can see and that does not change. “Is large”, “is brown” or “lives in a pond” lose marks: size and colour vary between individuals and with age, and habitat is not a feature of the organism.
Animals and plants are both multicellular, but plant cells have cellulose cell walls and chloroplasts while animal cells have neither. Vertebrates are grouped by skin covering, breathing and reproduction: fish, amphibians, reptiles, birds, mammals. Arthropods have jointed legs, a segmented body and an exoskeleton, and are grouped by counting legs, body parts and antennae: insects, arachnids, crustaceans, myriapods. Extended: every organism is in one of five kingdoms (animal, plant, fungus, prokaryote, protoctist); flowering plants are monocotyledons or dicotyledons; a virus is only genetic material in a protein coat.
Fungi are not plants. A mushroom grows out of the ground, but fungal cell walls are made of chitin, not cellulose, and fungi have no chloroplasts, so they cannot photosynthesise.
Classify by features, not by habitat or lifestyle. A whale and a dolphin live in the sea but breathe with lungs and feed their young on milk: mammals. A penguin cannot fly but has feathers: a bird. A bat flies, but it has fur and its wings are skin: a mammal.
Read whether a question gives legs or pairs of legs. Eight legs is four pairs (arachnid); eight pairs would point to a myriapod or crustacean. A spider is not an insect: it has four pairs of legs, two body parts and no antennae.