Biology, in plain words
Plain-language definitions of school Biology terms, A to Z, each with an example, where marks go, and the board's own source.
68 terms
Terms starting with A
- Acid rainAcid rain is rain made acidic by sulphur dioxide and nitrogen oxides in the air, which come mainly from burning coal, oil and petrol.
- Aerobic and anaerobic respirationRespiration is the breakdown of food such as glucose inside cells to release energy; it is aerobic when it uses oxygen and anaerobic when it does not.
- AlleleAn allele is one of the different forms of the same gene, found at the same position on a pair of chromosomes.
- AlveoliAlveoli are tiny balloon-like sacs at the ends of the air tubes in the lungs, where oxygen passes into the blood and carbon dioxide passes out.
- Antidiuretic hormone (ADH)Antidiuretic hormone, or vasopressin, is a pituitary hormone that makes the kidney tubules take back water, so the body loses less water in urine.
- Arteries and veinsArteries are blood vessels that carry blood away from the heart, and veins are blood vessels that carry blood back to the heart.
Terms starting with B
- Binomial nomenclatureBinomial nomenclature is the system of giving every kind of living thing a two-part scientific name: its genus, then its species.
- Biodegradable and non-biodegradableBiodegradable substances are broken down by living things such as bacteria and fungi, while non-biodegradable ones, such as many plastics, are not and stay in the environment.
- Biological magnificationBiological magnification is the build-up of harmful chemicals, such as pesticides, at each step of a food chain, so the top level holds the most.
- Birth rateBirth rate, also called natality, is the number of live births per thousand people in one year.
- Blood coagulationBlood coagulation is the clotting of blood at a wound, where a mesh of fibrin threads traps blood cells and stops the bleeding.
Terms starting with C
- CataractA cataract is an eye defect in which the eye lens becomes cloudy, or opaque, so that vision grows blurred.
- CentromereThe centromere is the point where the two chromatids of a copied chromosome are held together, and where spindle fibres attach during cell division.
- ChromosomeA chromosome is a thread-like body in the nucleus, made of DNA and protein, that carries a cell's genes; body cells have them in pairs.
Terms starting with D
- Diabetes mellitusDiabetes mellitus is a disorder in which blood sugar stays too high, because the body makes too little insulin or does not respond to it.
- Dihybrid crossA dihybrid cross follows two pairs of contrasting traits at the same time, such as plant height and fruit colour, across two generations.
- Dominant and recessive traitsA dominant trait shows even when only one copy of its gene is present, while a recessive trait shows only when both copies are the recessive kind.
- Double circulationDouble circulation means blood passes through the heart twice in one trip round the body: once going to the lungs, once going to the body.
Terms starting with E
Terms starting with F
- Feedback mechanismA feedback mechanism is the way the body senses the effect of a hormone and then adjusts how much of that hormone is released, and when.
- FertilisationFertilisation is the fusion of a male germ-cell with a female germ-cell, forming a zygote that can grow into a new individual.
- FissionFission is a way single-celled organisms reproduce, in which one cell divides to form two or more new individuals.
- Food chainA food chain is a series of living things in which each one is eaten by the next, showing how food and energy pass from one to another.
Terms starting with G
- GeneA gene is a section of DNA that carries the information for making one protein, and through that protein it controls a trait.
- Greenhouse effect and global warmingThe greenhouse effect is the trapping of the Earth's heat by gases such as carbon dioxide and methane; more of these gases causes global warming.
Terms starting with H
- Heart valvesHeart valves are flaps inside the heart that let blood flow only one way, from auricles to ventricles and from ventricles into the arteries.
- Hepatic portal systemThe hepatic portal system is the route that takes blood from the stomach and intestines to the liver first, before it returns to the heart.
- HormoneA hormone is a chemical made by an endocrine gland and carried by the blood to a target organ, whose work it controls.
Terms starting with I
- Independent inheritanceIndependent inheritance means that two different traits, such as seed shape and seed colour, are passed on separately, so offspring can show new combinations of them.
- Involuntary actionAn involuntary action is a movement of muscles that we cannot control by thinking, such as the heartbeat or salivation, run mainly by the mid-brain and hind-brain.
Terms starting with M
- MeiosisMeiosis is the cell division that makes gametes, the egg and sperm, and halves the chromosome number so each gamete gets one of every pair.
- MeningesThe meninges are the three protective membranes that cover the brain and the spinal cord.
- MitosisMitosis is the cell division in which one cell splits into two new cells, each with as many chromosomes as the parent cell.
- Monohybrid crossA monohybrid cross is a cross between two parents that differ in one pair of contrasting traits, such as tall and dwarf pea plants.
Terms starting with N
- NephronA nephron is one of the many tiny filtering units in a kidney; it cleans the blood of waste such as urea and makes urine.
- NeuronA neuron is a nerve cell, the basic unit of the nervous system, which carries messages through the body as electrical impulses.
- Nitrogen cycleThe nitrogen cycle is the way nitrogen moves from the air into soil and living things, and back to the air again.
- Noise pollutionNoise pollution is loud or unwanted sound, from sources such as traffic, factories, building work and loudspeakers, that harms health and comfort.
Terms starting with O
- OsmosisOsmosis is the movement of water from a weaker solution to a stronger one through a selectively permeable membrane.
- OvulationOvulation is the release of a mature egg, or ovum, from a Graafian follicle in the ovary.
- Ozone layerThe ozone layer is ozone gas high up in the atmosphere that shields the Earth's surface from the Sun's harmful ultraviolet (UV) radiation.
Terms starting with P
- PhotosynthesisPhotosynthesis is the process in which green plants use light energy, trapped by chlorophyll, to make glucose from carbon dioxide and water, giving out oxygen.
- PlacentaThe placenta is a disc-shaped organ in the uterus wall that links the baby to the mother, passing food and oxygen in and wastes out.
- Plant hormonesPlant hormones are chemicals made in one part of a plant that move to another part to control its growth, development and responses to its surroundings.
- PlasmolysisPlasmolysis is the shrinking of a plant cell's contents away from its cell wall when the cell loses water to a stronger solution outside it.
- PollinationPollination is the transfer of pollen grains from the anther of a stamen to the stigma of a flower.
- Population controlPopulation control means the steps a family or country takes to slow population growth, mainly by planning how many children to have and when.
- Population densityPopulation density is the number of people living in each unit of area, usually each square kilometre.
- Population explosionPopulation explosion is a very fast rise in the number of people in a place, faster than its food, water and jobs can support.
- Prokaryotic and eukaryotic cellsA prokaryotic cell has no true nucleus wrapped in a membrane, while a eukaryotic cell keeps its genetic material inside a nucleus with a membrane.
Terms starting with R
- Red blood cells (RBCs)Red blood cells are the blood cells that carry oxygen from the lungs to every part of the body, using a red pigment called haemoglobin.
- Reflex actionA reflex action is a quick, automatic response to a stimulus, made without thinking first, such as pulling your hand back from a flame.
- Root hairA root hair is a thin outgrowth of one outer cell of a young root, through which the root takes in water and minerals.
Terms starting with S
- Sex determinationSex determination is how a new individual's sex is decided; in humans, the father's sperm fixes it at fertilisation by carrying an X or a Y.
- Sex-linked inheritanceSex-linked inheritance is the passing on of a trait whose gene lies on a sex chromosome, as in the X-linked disorders haemophilia and colour blindness.
- Starch testThe starch test checks whether a leaf has made food by photosynthesis: a leaf with starch turns blue-black when iodine solution is added.
- Static and dynamic equilibriumStatic equilibrium is balance while the head is still, and dynamic equilibrium is balance while the head or body moves; the inner ear controls both.
- StomataStomata are tiny pores in a leaf's surface, each opened and closed by two guard cells to let gases and water vapour pass.
- Sympathetic nervous systemThe sympathetic nervous system is the part of the autonomic nervous system that gets the body ready to face danger or stress.
- SynapseA synapse is the meeting point between two neurons, where a message passes from the axon ending of one to the dendrites of the next.
Terms starting with T
- TranspirationTranspiration is the loss of water as water vapour from the parts of a plant above the ground, mainly through the stomata of its leaves.
- Trophic levelA trophic level is one step in a food chain, such as producer, herbivore or carnivore, set by where an organism gets its food.
- TropismA tropism is a slow movement of a plant part by growth, towards or away from a stimulus such as light, gravity, water, touch or a chemical.
- TurgidityTurgidity is the firm, swollen state of a plant cell that has taken in so much water that its contents press on the cell wall.