Biology Key terms OCR 21ST CENTURY SCIENCE

Here is the transcribed glossary in a two-column table format (Term, Definition), compiled and cleaned from all provided files:

| Term | Definition |

|------|------------|

| abiotic | The non-living parts of an ecosystem (e.g., the atmosphere, water, soil, and rocks). |

| abiotic factor | A non-living factor that can affect the distribution and abundance of living organisms (e.g., temperature, light intensity, soil pH, and the availability of water). |

| abundance | How many organisms are found in each part of an ecosystem. |

| accuracy | How close a quantitative result is to the true or actual value. |

| active site | The part of an enzyme where the chemical reaction takes place. The reacting molecules (substrates) fit into the active site. |

| active transport | Molecules are moved in or out of a cell using energy. This process is used when transport needs to be faster than diffusion, and when molecules are being moved from a region where they are at low concentration to where they are at high concentration. |

| adaptation | A feature that helps an organism survive in its environment. |

| ADH | Antidiuretic hormone, which is secreted by the pituitary gland. It controls the reabsorption of water in the kidneys to control the water balance of the body. |

| adrenal gland | Gland that secretes the hormones adrenaline and cortisol. |

| adrenaline | Hormone secreted by the adrenal gland at times of stress. It prepares the body for action. |

| adult stem cell | A stem cell that can only differentiate to make one or a small number of cell types. |

| advantage | An individual organism has an advantage if it has a feature that enables it to survive and reproduce more successfully than other individuals. |

| aerobic respiration | A type of cellular respiration that uses oxygen. |

| agricultural input | A resource needed by a particular farming method. Intensive farming methods need agricultural inputs such as fertiliser and pesticide. |

| AIDS | Acquired immune deficiency syndrome, an STI caused by the HIV virus. The body’s immune system is attacked by the virus and gradually becomes weakened. |

| allele | The two copies of a gene in a pair of chromosomes are called alleles. They can be the same or different. |

| alveolus (plural alveoli) | Human lungs are divided up into millions of tiny sacs called alveoli. They form an exchange surface with a very large surface area for gaseous exchange. |

| amino acid | Amino acids are joined in long chains to make proteins. All the proteins in living organisms are made from 20 different amino acids joined in different orders. |

| anaerobic respiration | A type of cellular respiration that does not use oxygen. |

| antagonistic | Actions or effectors that have opposite effects. |

| antibiotic resistance | The ability of bacteria to survive exposure to antibiotics. The ability is caused by mutations in the bacteria’s genes. |

| antibiotics | Substances that kill or stop the growth of bacteria, but which do not work against fungi or viruses. |

| antibodies | Protein molecules made by white blood cells to fight pathogens; each antibody only recognises one specific antigen. |

| antigens | Molecules on the surface of cells and pathogens. |

| antimicrobial | A substance (e.g., an antibiotic) that kills microorganisms (or inhibits their growth). |

| archaea | One of the domains used in modern classification systems, containing prokaryotic organisms. |

| artery | Type of blood vessel that carries blood from the heart to cells and tissues. |

| aseptic techniques | Ways of working that avoid contamination of samples by microorganisms. These include sterilising equipment and growth medium in an autoclave, sterilising surfaces using alcohol, wearing personal protective equipment, working next to a Bunsen burner to create an updraft, and flaming metal tools. |

| asexual reproduction | Reproduction by simple division of a cell. The offspring are genetically identical to one another and to the parent cell. |

| ATP | Adenosine triphosphate, a substance made by cellular respiration. Cells need a constant supply of ATP for life processes. ATP is a chemical store of energy. |

| atrium (plural atria) | The human heart is made of four muscular chambers. The atria are the upper chambers. They receive blood and pump it into the lower chambers (ventricles). |

| autotroph | An organism that can make its own food. |

| auxins | A group of plant hormones that affect plant growth and development. |

| average | The average of a set of data is the single number that best represents the data. Commonly used averages include the mean, median, and mode. |

| axon | A long, thin extension of the cytoplasm of a neuron. The axon carries nerve impulses very quickly. |

| bacteria (singular bacterium) | Large group of single-celled, prokaryotic microorganisms. Some bacteria cause disease. |

| base | Part of a nucleotide. Four bases (A, C, G, and T) are found in DNA. The base U replaces T in mRNA. |

| base pairing | The bases in two strands of DNA always pair up in the same way. A always pairs with T, and C always pairs with G. |

| biconcave | The shape of a red blood cell, like a disc that has been squeezed from both sides in the middle. |

| bioaccumulation | An increase in the concentration of a substance in organisms in the higher trophic levels of a food chain. |

| biodegradable | Substances that can be broken down by microorganisms, such as bacteria and fungi. Most paper and wood items are biodegradable, but most synthetic polymers, such as plastics, are not. |

| biodiversity | The biodiversity of the Earth, or of a particular area, is the combination of the diversity of living organisms, the diversity of genetic material the organisms have, and the diversity of ecosystems in which the organisms live. |

| biofuel | A fuel produced from plants (such as bioethanol from sugar cane, and wood) or other recently living material (such as agricultural waste). |

| bioinformatics | Using computers to process and analyse biological data, including genome sequences. |

| biological control | The introduction of new species into an ecosystem to kill pests that carry plant pathogens and damage plants. |

| biomass | All the tissues that make up an organism. Some of an organism’s biomass is transferred to another organism when it is eaten. |

| biotic | The living parts of an ecosystem, i.e., all the living organisms. |

| biotic factor | A living factor that can affect the distribution and abundance of living organisms (e.g., a predator, a competitor, or pathogens). |

| bladder | Organ that collects urine from the kidneys for excretion by urination. |

| blind trial | A clinical trial in which the patient does not know whether they are taking the new medicine, but their doctor does. |

| body mass index (BMI) | A calculated value that indicates whether a person’s body mass is healthy for a person of their height. |

| brain stem | Part of the brain that controls heart rate and breathing rate. |

| capillary | Type of tiny blood vessel that carries blood through tissues (from arteries to veins). |

| capture-mark-recapture | A sampling technique used to estimate the size of a population of animals. A sample of individual animals is collected from the population, and each animal given an identifying mark. Later, another sample is collected. A mathematical equation is used to estimate the population size based on the number of marked animals in the second sample. |

| carbohydrate | A substance made of carbon, hydrogen, and oxygen. Carbohydrates include sugars (e.g., glucose) and natural polymers (e.g., starch and cellulose). |

| carbon cycle | The natural processes that recycle carbon through the living and non-living parts of an ecosystem. |

| carbon neutral | Biofuels are carbon neutral because the amount of carbon dioxide used (by photosynthesis plants) to make them is equal to the amount released when the biofuel is burnt. |

| carcinogen | A substance that can cause damage to DNA. This can result in a mutation and can cause a cell to become a cancer cell. |

| cardiovascular diseases | A group of non-communicable diseases affecting the blood vessels and heart. |

| carrier | An individual who has a recessive variant but does not show the feature associated with the variant because they also have a dominant variant (they are heterozygous). They can pass the recessive variant to their offspring. |

| catalyst | A catalyst is a substance that increases the rate of a chemical reaction, but which is left unchanged by the reaction. |

| cataract | A cloudy patch that forms in the lens of the eye, causing blurred vision. |

| cause | When a change in a factor produces a particular outcome, and there is a mechanism to explain this link, then the factor is said to cause the outcome. |

| cell culture | Growing cells in carefully controlled conditions in a laboratory. The cells are grown in (or on) a growth medium, usually in warm conditions in an incubator. |

| cell cycle | Most cells go through a cycle of phases, including interphase and either one cell division (mitosis) or two cell divisions (meiosis). |

| cellular respiration | A series of chemical reactions that happens in all living cells. Glucose is used up and ATP is made for the cell to use. |

| central nervous system (CNS) | The brain and spinal cord. |

| cerebellum | Part of the brain associated with conscious movement. |

| cerebral cortex | Part of the brain associated with consciousness, intelligence, language, and memory. |

| chemical control | The use of substances, usually sprayed onto crops, to reduce the spread of plant pathogens and pests that carry them. |

| chlorophyll | A pigment that absorbs light and splits water into hydrogen and oxygen in the first stage of photosynthesis. |

| chloroplast | An organelle containing chlorophyll, found in some plant cells. The reactions of photosynthesis happen in chloroplasts. |

| chromosome | Long, thin, threadlike structure made from molecules of DNA. Chromosomes store genetic information. |

| ciliary muscle | Muscle in the eye that changes the shape of the lens to help focus light on the retina. |

| circulatory system | An organ system that includes the heart, blood vessels, and blood. The circulatory system transports substances around the body. |

| classification | Placing organisms into groups based on their similarities and differences. |

| clinical testing | A stage of drug development. A new medicine is tested on humans to find out whether it is safe and whether it works. |

| coding DNA | Regions (genes) in the genome that store the genetic code for making proteins. |

| communicable disease | A disease caused by an infection with a pathogen, which can be passed from one organism to another in body fluids, contaminated food and water, or by direct contact. |

| community | Interacting populations of organisms living in the same ecosystem. |

| competition | Organisms that require the same resource (e.g., food, water, space, light, shelter, mates, pollinators, and seed dispersers) must compete for the resource. |

| complication | A second disease that results from having a first disease. |

| computational model | A type of mathematical model in which the calculations are done by a computer. |

| control group | When testing a new treatment, a control group of people does not receive the new treatment. They may receive an existing treatment or a placebo. |

| cornea | Transparent tissue that covers the iris of the eye. It helps focus light onto the retina by refraction. |

| coronary arteries | Blood vessels that supply blood carrying oxygen and glucose directly to the muscle cells of the heart. |

| coronary heart disease | A type of cardiovascular disease in which fatty deposits cause blood clots in the coronary arteries, increasing the chance of a heart attack. |

| correlation | When an outcome happens if a specific factor is present but does not happen when it is absent, or if a measured outcome increases (or decreases) as the value of a factor increases, there is a correlation between the two. For example, there is a correlation between pollen count and the number of hay fever cases. |

| crop rotation | Crops are grown in different fields each year, so that each field is planted with a different crop each year. |

| cuticle | Layer of molecules of fat and wax, covering the outside surface of plant leaves. It helps to stop pathogens entering the leaf tissue. |

| decomposition | Process in which dead organisms are broken down (decay) and substances in their bodies are returned to the environment. |

| deforestation | Cutting down and clearing forests leaving bare ground. This damages ecosystems and causes biodiversity loss. |

| deletion mutation | A change in the DNA of an organism where one or more nucleotides are removed. This changes the way the bases are divided into triplets. |

| denatured | When the shape of an enzyme has been changed, usually as a result of high temperature or a pH change. Denatured enzymes no longer work because the shape of the active site has changed. |

| deoxygenated blood | Blood in which most or all of the haemoglobin in the red blood cells is not bound to oxygen. |

| depth of field | When a microscope is focused on a structure being observed, objects outside the depth of field (above and below the structure) appear blurred. |

| descriptive model | A type of scientific model that uses words to identify features of a system and to describe how they interact. One example of a descriptive model is a simple account of the inputs and outputs of photosynthesis. |

| desertification | The process in which an ecosystem turns to desert (a place where no crops can grow). |

| development | How an organism changes as it grows and matures. Its cells are organised into different tissues and organs, and they take on specific jobs. |

| diagnostic test | A test to help diagnose or detect disease. The tests can involve observing symptoms, cell culture, microscopy, staining, testing with antimicrobials, genome analysis, and use of monoclonal antibodies. |

| differentiation | The process by which a cell becomes specialised to take on a specific job. |

| diffusion | Passive movement of molecules. The net movement is from a region of their higher concentration to a region of their lower concentration. |

| digestive system | An organ system that includes the mouth, oesophagus, stomach, liver, pancreas, and intestines. The digestive system breaks down the biomass in the food we eat. |

| distribution | Where particular organisms are found within an ecosystem. |

| DNA | Deoxyribonucleic acid, a natural polymer made of nucleotides. DNA carries the genetic code, which controls how an organism develops and functions. |

| domain | The largest group of organisms used in modern classification systems. The three domains are bacteria, archaea and eukaryotes (including animals, plants, fungi, and protists). |

| domesticated | Organisms that have been selectively bred to live and work with humans. |

| dominant | You only need to have one copy of a dominant genetic variant to have the feature it produces. A dominant genetic variant will always show its associated feature in the phenotype. |

| double helix | The shape of the DNA molecule, with two strands twisted together. |

| double-blind trial | A clinical trial in which neither the doctor nor the patient knows whether the patient is taking the new medicine. |

| ecosystem | A community of organisms and the environment in which it lives (and with which it interacts). |

| ecosystem services | Benefits that humans receive from ecosystems, including provisions (e.g., food, clean water, materials, and medicines), fertile soil and crop pollination, the control of climate, the breaking down of waste and cycling of substances, and cultural benefits (e.g., recreation, discovery, and aesthetics). |

| effector | Muscle or gland that carries out the response to a stimulus. |

| efficiency (of biomass transfer) | The percentage of biomass from a trophic level that is transferred to a higher trophic level in the same food chain. |

| electron | A tiny, negatively charged particle, which is part of an atom. Electrons are found outside the atoms’ nucleus. |

| electro microscope | A type of microscope that uses a beam of electrons, rather than a beam of light, to produce an image. |

| embryonic stem cells | Unspecialised cells in an embryo before the eight-cell stage. They can divide and differentiate to make all the specialised cells in the body. |

| endocrine system | A communication system in the body that uses hormones released from glands to control cells and tissues elsewhere in the body. |

| endothermic | An endothermic process transfers energy from surroundings, making them cooler. |

| energy stores | An energy store is something (such as the Sun or a chemical substance) that enables you to account for the energy at the start and end of a transfer by doing a calculation. |

| enzyme | A protein with an active site that catalyses (speeds up) a chemical reaction. |

| epidemic | An outbreak of a disease affecting many individuals in a population. |

| erector muscles | Muscles in the skin that make hairs stand up to trap a layer of warm air next to the skin. |

| ethene | A plant hormone that affects plant growth and development. |

| ethical | Concerned with what is right or wrong. Ethical questions cannot be answered by science alone. |

| eukaryotic organism | A type of organism that has cells with a nucleus and organelles with membranes. Includes all animals, plants, fungi, and protists. |

| eutrophication | A type of environmental damage. Excess nitrate or phosphate in water causes rapid growth of algae, followed by death and decomposition of water plants (due to lack of light), and a reduction of oxygen levels in the water. This usually leads to the death of water animals (due to lack of oxygen). |

| evolution | The process by which species gradually change over time. Evolution can produce new species. |

| exchange surface | Surface where substances are exchanged between an organism and its external environment. |

| excretory system | An organ system that includes the lungs, kidneys, bladder, intestines, rectum, and anus. The excretory system gets rid of undigested food, excess water, urea (in urine), and carbon dioxide. |

| exothermic | An exothermic process transfers energy to its surroundings, making them warmer. |

| extinction | The permanent disappearance of a species because all the members of the species have died. |

| factor | A variable that influences the outcome of an experiment. A variable that changes and may affect something else (the outcome). |

| false negative | A wrong test result. The test result says that a person does not have a medical condition but this is incorrect. |

| false positive | A wrong test result. The test result says that a person has a medical condition but this is incorrect. |

| family tree | A genetic diagram that is used as a model of inheritance. It shows which members of a family have inherited a particular feature. |

| fatty sheath | Insulating layer of fat around the outside of an axon. |

| fermentation | Anaerobic respiration in yeast cells, which breaks down glucose, and makes ethanol (alcohol) and carbon dioxide. |

| fertile soil | Soil in which crops and other plants can be grown. |

| fertilisation | The fusing of gametes during sexual reproduction. |

| fluorescent | A fluorescent substance absorbs electromagnetic radiation (e.g., from ultraviolet light) and emits it as visible light, so appears to glow. |

| food chain | A model of the feeding relationships between populations of organisms. A food chain shows one route of biomass transfer through a community. |

| food web | A model of the feeding relationships between populations of organisms. A food web shows multiple routes of biomass transfer through a community. |

| fossil | The remains of an organism that lived millions of years ago that has been preserved in rock. |

| fossil record | A collection of millions of fossils, which provides evidence of the evolution of species. |

| FSH | Follicle-stimulating hormone, which is important in controlling the menstrual cycle. |

| functional MRI (fMRI) | Imaging technique that uses magnets to produce an image of tissues inside the body. An fMRI scan shows areas of the brain that are active as a patient carries out a specific task. MRI stands for magnetic resonance imaging. |

| fungi (singular fungus) | Large group of eukaryotic organisms that cannot make their own food. Most are multicellular, but some (yeasts) are single-celled. Some fungi cause disease. |

| gamete | Sex cell used in sexual reproduction. Male and female gametes fuse during fertilisation. In humans, the male gamete is the sperm and the female gamete is the ovum (egg cell). |

| gas exchange | The exchange of oxygen and carbon dioxide between an organism and its surroundings. Occurs in the lungs of humans and the stomata of plants. |

| gaseous exchange system | An organ system that includes the nose and mouth, trachea, and lungs. The gaseous exchange system takes in oxygen from the air we breathe in, and excretes carbon dioxide into the air we breathe out. |

| gender | An individual’s gender depends on whether they feel masculine or feminine, and whether they are happy to identify themselves as a man or a woman. It is affected by a person’s feelings, society, and biological factors such as their genome and hormones. |

| gene | A region of DNA containing the instructions for a cell that give the order of amino acids in a protein. |

| gene expression | When the instructions in genes are read to make proteins. Controlling gene expression controls which proteins are made by a cell. |

| gene probe | A short section of single-stranded DNA, which has a sequence of bases that will pair up with the bases in a particular genetic variant. The probe will bind to the target variant in a DNA sample. |

| gene technology | Techniques such as genetic testing and genetic engineering that are based on our understanding of the genome. |

| genetic engineering | Altering the characteristics of an organism by modifying the DNA in its genome. |

| genetic testing | Testing an individual for particular genetic variants, including those that cause diseases. |

| genetic variant | A different version of a gene, caused by a change (mutation) in the DNA. |

| genome | The entire genetic material of an organism. |

| genome sequencing | A technique that works out the complete sequence of bases in an individual’s DNA. |

| genomics | The study of the structure and function of genomes, which includes genome sequencing and bioinformatics. |

| genotype | The genetic variants that an organism has made up its genotype. |

| genus | A group used in the classification of organisms, containing several similar species. |

| gibberellins | A group of plant hormones that affect plant growth and development. |

| gland | Part of the body that makes hormones, enzymes, and other secretions (e.g., the pituitary gland, the salivary gland, and sweat glands). |

| global warming | An increase in the average temperature of the whole Earth. The average global surface temperature increased by about 0.89 °C from 1901 to 2012. |

| globalisation | The increase of international travel, trade, and cooperation, including the transportation of people and cargo (including plants and animals) around the world. |

| glucagon | Hormone secreted by the pancreas that causes the liver to break down carbohydrate to release glucose. |

| gradient (of a graph) | The gradient, or slope, of a graph is a measure of its steepness. It is calculated by choosing two points on the graph and calculating: the change in the value of the y-axis variable / the change in the value of the x-axis variable. |

| graduation | A line on a container, ruler, or meter that marks a measurement. |

| graft-versus-host disease | When bone marrow cells have been donated to a patient, the new bone marrow makes white blood cells. The white blood cells from the new bone marrow recognize the patient’s body cells as non-self, and attack them. |

| Gram staining | A procedure for staining bacteria to distinguish between those that are Gram positive and those that are Gram negative. |

| gravitropism | When a plant grows towards or against the pull of gravity. |

| greenhouse gas | A gas that contributes to the greenhouse effect, in which the atmosphere absorbs infrared radiation from the Earth’s surface and radiates some of it back to the surface, making it warmer than it would otherwise be. Greenhouse gases include carbon dioxide, methane, and water vapour. |

| growth medium | A liquid or jelly (e.g., agar) used in cell culture. It contains nutrients to encourage the growth of cells. |

| habitat | The place where an organism lives within an ecosystem. The habitat provides conditions in which the organism can thrive. |

| haemoglobin | Protein molecule in red blood cells. Haemoglobin binds to oxygen and carries it around the body. It also gives blood its red colour. |

| hazard | A source of potential harm to health or the environment. |

| heart attack | Death of heart muscle that occurs when a coronary artery is blocked, starving the heart muscle cells of oxygen and food. |

| herd immunity | The protection given to a population against an outbreak of a disease when a very high percentage of the individuals in the population have been vaccinated against the disease. |

| heterotroph | An organism that cannot make its own food, so must eat other organisms. |

| heterozygous | The two alleles of a gene are different genetic variants. |

| HIV | Human immunodeficiency virus, the virus that causes AIDS. |

| homeostasis | The processes in your body that constantly work to keep the body’s internal environment the same. |

| homologous | The two alleles of a gene are the same genetic variant. |

| hormone | A substance secreted by specialised glands in the body. Hormones bring about changes in cells or tissues in different parts of the animal or plant. |

| hygiene | Behaviours and conditions that reduce the spread of pathogens. |

| hypothalamus | Part of the brain that controls many different functions, including processes that regulate body temperature. |

| hypothesis | A tentative explanation for an observation. A hypothesis is used to make a prediction that can be tested. |

| identification key | A guide that helps to identify the species of an organism. A key may include a series of steps or questions, for example, about flower colour or the arrangement of leaves. It may also include pictures. |

| immune system | A system of tissues and cells in an organism that fights infections using non-specific defences and (in animals) white blood cells that respond to specific pathogens. |

| immunity | The ability to produce antibodies against a pathogen very quickly, to destroy it before it causes symptoms of disease. Immunity depends on memory cells. |

| incubation period | The delay between infection with a pathogen and the appearance of symptoms of disease. |

| indicator species | A species whose abundance can be used to indicate the level of pollution in an area. |

| industrialisation | The process of change in a society, from farming and hand-production methods to mass production using machines and chemical processes. |

| infertility | A state in which an organism is not able to reproduce using sexual reproduction. |

| inherited | A feature is inherited when genetic information that affects the feature is passed from parents to offspring in the DNA of gametes. |

| insertion mutation | A change in the DNA of an organism where one or more nucleotides are added. This changes the way the bases are divided into triplets. |

| insulin | Hormone secreted by the pancreas that helps to control the level of sugar (glucose) in the blood. It causes cells to absorb glucose from the blood. |

| interdependence | The size of a population of organisms affects, and is affected by, the sizes of other populations in an ecosystem. Feeding relationships are one way in which populations are interdependent. |

| interphase | The longest phase of the cell cycle, in which the cell grows larger, organelles are copied, and each chromosome is copied. |

| inverse square law | A mathematical relationship between light intensity and the distance from a point light source. The relative light intensity at any distance from the light source is inversely proportional to the square of the distance from the light source. |

| ionising radiation | Radiation with sufficient energy to remove electrons from atoms in its path. Ionising radiation, such as ultraviolet, X-rays, and gamma rays, can damage cells. |

| iris | Ring of coloured tissue in the eye, which controls the diameter of the pupil. |

| kick sampling | A sampling technique used to collect and count organisms from a river. |

| kidney | Organ in the body that removes water and waste from the blood, then reabsorbs water to balance the blood plasma concentration. |

| kidney tubules | Tubes in the kidney. Water and waste are diffused out of the blood into the kidney tubules to make urine. |

| kingdom | A group used in the classification of organisms, containing millions of species. There are kingdoms of plants, animals, fungi, protists, and bacteria. |

| lactic acid | The product of anaerobic respiration in animal cells. Lactic acid is toxic at high concentrations and must be removed from the body quickly. |

| large intestine | Tube-like organ that receives digested food from the small intestine, where water is absorbed (leaving faeces). |

| lens (eye) | Transparent tissue behind the pupil in the eye. It helps focus light onto the retina by refraction. |

| LH | Luteinising hormone, which is important in controlling the menstrual cycle. |

| light intensity | The amount of light reaching a given area in a given time. |

| light microscope | A type of microscope that uses a beam of light to produce an image. |

| limiting factor | The factor that prevents the rate of a reaction or process from increasing. For photosynthesis, the limiting factor may be light intensity, temperature, carbon dioxide concentration, or water availability. |

| lipid | A substance made from glycerol and fatty acids. Lipids are used for storage and making cell membranes. |

| lock-and-key model | In chemical reactions catalysed by enzymes, molecules taking part in the reaction fit exactly into the enzyme’s active site. The active site will not fit other molecules – it is specific. This is like a key fitting into a lock. |

| longsighted | People who are longsighted can see distant objects clearly, but near objects look blurred. |

| lung | Organ made of tissues that are adapted to absorb oxygen from the air and remove carbon dioxide from the blood. |

| magnification | Use of a lens to make an object appear larger. |

| mathematical model | A type of scientific model that uses patterns in data of past events, along with known scientific relationships, to predict what will happen to one variable when another is changed. A simple example of a mathematical model is the relationship between speed, distance, and time. |

| mean value | A type of average, found by adding up a set of measurements and then dividing by the number of measurements. You can have more confidence in the mean of a set of measurements than in a single measurement. The mean can be used as the best estimate of the true value. |

| median | A type of average. The median is the middle value in a set of values ordered from smallest to largest. |

| medicine | A substance, or mixture of substances, that treats the symptoms and/or cause of a disease. |

| meiosis | A type of cell division that halves the number of chromosomes to produce gametes. |

| memory cells | White blood cells that stay in the blood after infection and create immunity. They respond very quickly by producing antibodies against an antigen the second time it is encountered. |

| meniscus | The water surface in a narrow tube curves to form a meniscus. |

| menstrual cycle | The cycle of changes in a woman’s body associated with ovulation. The cycle takes approximately 28 days. |

| menstruation | The shedding of part of the uterus lining each month as part of the menstrual cycle. |

| meristem | A region of unspecialised cells in a plant, which can develop into any kind of specialised cell. |

| messenger RNA (mRNA) | A substance involved in making proteins in cells. RNA stands for ribonucleic acid. The mRNA molecule is similar to DNA but is single stranded. It carries the genetic code from the DNA in the nucleus to the ribosomes in the cytoplasm. |

| mitochondria | Organelles in animal and plant cells where some of the reactions of cellular respiration take place. |

| mitosis | A short phase of the cell cycle in which chromosome copies separate, the nucleus divides, and the cell divides in two. |

| mode | A type of average. The mode is the value that occurs most often in a set of values. |

| model | A scientific model is a way of representing something from the real world, such as a system of interacting parts. It includes some, but not necessarily all, of the features of the system it represents. It can show how these features are connected or interact, and can be used to explain scientific ideas, answer questions, and make predictions. |

| model organism | An organism that is extensively studied by scientists to help develop explanations that may also apply to other species. Examples include Drosophila (fruit flies) and the tobacco mosaic virus. |

| monoclonal antibodies | Antibodies against a specific antigen, made in a laboratory from clones of a single white blood cell. |

| monoclonal antibody therapy | A cancer treatment that uses monoclonal antibodies against a cancer cell antigen. |

| monoculture | A farming technique where an area is planted with a single crop. The area has very low biodiversity. |

| motor neuron | Neuron that carries nerve impulses from the CNS to an effector. |

| multicellular | An organism made of two or (usually) more cells. |

| mutation | A change in the DNA of an organism. It may or may not affect the organism’s phenotype. |

| natural selection | When individual organisms are better adapted to their environment they are more likely to survive and reproduce, passing on their features to the next generation. |

| negative feedback | Actions that reverse any change in a system, so that the system returns to a steady state. |

| nerve impulse | Electrical signal carried by a neuron. |

| net movement | The overall movement of molecules by diffusion, when more molecules move in one direction than another. If equal numbers of molecules move in all directions, there is no net movement. |

| neuron | A specialised cell in the nervous system that transmits nerve impulses. |

| non-coding DNA | Regions in the genome that do not store code for making proteins, but that can affect gene expression. |

| non-communicable disease | A disease not caused by an infection with a pathogen, but by a person’s genes, their environment, or unhealthy lifestyle. |

| non-specific defences | Physical, chemical, and bacterial defences that are part of the immune system; they are always present, rather than being produced in response to a specific pathogen. |

| nucleotide | The monomer of DNA and RNA. Each nucleotide is made of a common sugar, a phosphate group, and a base. |

| nucleus (cell) | Cell organelle that contains the chromosomes in cells of plants, animals, fungi, and protists. |

| obese | A person who is very overweight is described as obese. An adult with a BMI greater than 30 kg/m² is classed as obese. |

| oesophagus | Tube that joins the mouth to the stomach. |

| oestrogen | Hormone that is important in puberty and in controlling the menstrual cycle. |

| open-label trial | A clinical trial in which both the patient and their doctor know whether the patient is taking the new medicine. |

| optic nerve | Bundle of sensory neurons that transmit nerve impulses from the retina in the eye to the brain. |

| optimum | The most favourable condition for an enzyme, at which it catalyses a reaction at its fastest rate. |

| order of magnitude | If two numbers differ by an order of magnitude, then one number is about ten times bigger than the other. A value given to the nearest order of magnitude will be given to the nearest power of ten. |

| osmosis | The diffusion of water molecules from an area where they are in high concentration (a dilute solution) to an area where they are at lower concentration (a concentrated solution) through a partially permeable membrane. |

| outcome | A variable that changes as a result of something else changing. |

| outlier | A measured result that seems very different from other repeat measurements, or from the value you would expect. The measurement should be treated as data, unless there is a reason to reject it (e.g., when it is known that a mistake was made when the measurement was taken or recorded). |

| overgrazing | A type of ecosystem damage caused when animals eat almost all of the plant life. |

| ovulation | The release of an ovum (egg cell) from a mature follicle in an ovary. |

| oxygenated blood | Blood in which most or all of the haemoglobin in the red blood cells is bound to oxygen. |

| pancreas | Organ that secretes digestive enzymes that are released into the small intestine. |

| pandemic | An outbreak of a disease affecting many populations over a wide geographical area. |

| partially permeable membrane | A membrane that acts as a barrier to some molecules but allows others to diffuse through freely. |

| passive | A passive process does not require an input of energy. For example, diffusion is a passive process. |

| pathogen | An organism that causes disease. Some bacteria, viruses, fungi, and protists are pathogens. |

| pre-implantation genetic diagnosis (PGD) | A type of genetic technology. Embryos fertilised outside the body are tested for genetic variants that could cause disease. Only healthy embryos are put into the mother’s uterus. |

| producer | An organism at the start of a food chain. A producer is an autotroph, able to make its own food. |

| progesterone | Hormone that is important in controlling the menstrual cycle. |

| prokaryotic organism | An organism made of cells that do not contain a nucleus or organelles with membranes. Includes the simplest living organisms, such as bacteria. |

| protein | A natural polymer made from amino acids. Proteins can be structural (e.g., collagen) or functional (e.g., enzymes). |

| protist | Large group of eukaryotic microorganisms that can be single-celled or multicellular. Some protists cause disease. |

| Punnett square | A genetic diagram that is used as a model of inheritance. It shows all the possible combinations of alleles of a gene, for the offspring of two individuals. It is used to make mathematical predictions about the possible genotypes of the offspring. |

| pyramid of biomass | A model of the feeding relationships between populations of organisms. Trophic levels in a food chain are represented by bars. The width of each bar depends upon how much biomass is present, on average, per unit of area. Usually, this is measured in grams per square metre (g/m²). |

| quadrat | A square grid of a known area that is used when measuring the abundance of non-moving or slow-moving organisms in a location. |

| random error | A measurement error due to results varying in an unpredictable way, for example, due to the scientist having to make a judgement about timing or colour. |

| range | Describes the spread between the highest and the lowest of a set of measurements. |

| rate | A measure of how quickly something changes. For example, the rate of a reaction can be investigated by measuring the formation of a product, such as a gas (in dm³/s). |

| receptor | A specialised cell that detects changes in an organism’s environment. Also called a sensory receptor. Also a specialised molecule on the surface of a cell that recognises and binds to a specific substance. |

| recessive | A recessive genetic variant will only show its associated feature when the genotype is homozygous. |

| rectum | The final part of the digestive system before the anus. |

| red blood cell | Type of blood cell containing haemoglobin, which transports oxygen around the body. |

| reflex arc | A neuron pathway that brings about a reflex response. A reflex arc involves a sensory neuron, a relay neuron, and a motor neuron. |

| reflex response | A rapid, involuntary response to a stimulus that uses nerve impulses transmitted along a reflex arc. |

| refraction | The wavelength of a light wave changes as it travels from one medium to another in which its speed is different. This can cause the direction of travel to change (the light ray bends). For example, when travelling from air into water, the light ray changes direction. |

| regulation | The legal control of applications of science by a government or non-governmental body. |

| relay neuron | A neuron that connects the sensory neuron to the motor neuron in a reflex arc. |

| repeatable | Data are said to be repeatable when the same investigator finds the same or similar results under the same conditions. We can have more confidence in data that are repeatable. |

| representational model | A type of scientific model that uses simple shapes or analogies to represent the interacting parts of a system. One example is the particle model – it helps us to visualise the tiny particles (atoms and molecules) that make up substances. |

| representative sample | The characteristics of a representative sample are very similar to the characteristics of the whole population. |

| reproducible | Data are said to be reproducible when other investigators have found the same or similar results under similar conditions. We can have more confidence in data that are reproducible. |

| resolution | A microscope that lets you see details that are very close together has high resolution (resolving power). |

| response | Action or behaviour that is caused by a stimulus. |

| retina | Layer of light receptor cells (rods and cones) at the back of the eye, which transmit nerve impulses to the brain via sensory neurons in the optic nerve. |

| ribosome | A cell organelle where amino acids are joined together to make a protein. |

| risk | An estimate of the probability that an unwanted outcome will happen. The size of the risk can be estimated from the chance of it occurring in a large sample over a given period of time. |

| risk factors | Variables linked to an increased risk of disease (though they may not be the cause of the disease). |

| root hair cell | Cell in a plant root that increases the surface area for absorption of water by osmosis and for mineral salts by active transport. |

| salivary gland | Gland in the mouth that secretes digestive enzymes and saliva. |

| sample | It is usually not possible to collect data about a whole population of organisms or other specimens. A study usually collects data about a proportion of them. This is a sample. Conclusions about a sample can only be applied to the whole population if it is a representative sample. |

| sanitation | The hygienic disposal of waste, including faeces. |

| screening | Testing large libraries of substances to find out which ones could be developed into new medicines. |

| secondary data | Data collected by somebody else, which can be compared to the primary data collected in the lab or field by the person doing the investigation. |

| secretion | The release of a hormone or other substance from a gland. |

| selective breeding | The process of humans choosing organisms with certain characteristics and mating them to produce offspring with favourable characteristics. |

| sensory neuron | Neuron that carries nerve impulses from a receptor to the CNS. |

| sex | Whether an individual is male or female. Sex is determined by the inheritance of X and Y chromosomes. Sex should not be confused with gender. |

| sex chromosomes | Chromosomes that determine an individual’s sex. In humans it is chromosome pair 23 (the X and Y chromosomes). |

| sex hormone | Hormones that control the development of male or female features. Androgens are one type of male sex hormone. |

| sexual reproduction | Reproduction using gametes that fuse during fertilisation. This creates genetic variation in the offspring. |

| sexually transmitted infection (STI) | A communicable disease caused by a pathogen, which can be caught and passed on during unprotected sex. |

| shivering | Very quick muscle contractions. The increased rate of cellular respiration helps to raise body temperature when it is cold. |

| shortsighted | People who are shortsighted can see near objects clearly, but distant objects look blurred. |

| sickle-cell trait | If a person is heterozygous for the recessive allele that causes sickle-cell disease, they have sickle-cell trait. This gives them some resistance to the protist pathogen that causes malaria. |

| significant figures | The number of significant figures shows the precision that can be claimed for a piece of data. The first significant figure in a number is the first non-zero digit from the left. |

| sink | In a plant, sugar is transported to a sink (e.g., a root or developing fruit) from a source (e.g., a leaf or storage organ). |

| small intestine | Tube-like organ that receives food from the stomach, where digestion is completed and molecules from food are absorbed into the blood. |

| soil erosion | Soil removal by wind or rain into rivers or the sea. This damages ecosystems. It is more likely after deforestation. |

| source (plants) | In a plant, sugar is transported from a source (e.g., a leaf or storage organ) to a sink (e.g., a root or developing fruit). |

| spatial model | A type of mathematical model in which a computer is used to make a model of one or more objects in a three-dimensional space. The model can be used to predict the outcome of changing a variable (e.g., temperature) in a given space (e.g., a landscape and the atmosphere above it). |

| specialised | A cell that has differentiated to take on a specific job. |

| species | A group of organisms that can breed to produce fertile offspring. |

| spores | Resistant structures made by some bacteria and fungi that can survive in unfavourable conditions (e.g., cooking) and will grow again when conditions are more favourable. |

| stem cell | Unspecialised animal cell that can divide and differentiate into specialised cells. |

| stem cell treatment | Use of stem cells in medicine to treat damage or disease. |

| sterile | Free from bacteria, fungi, protists, and viruses that could cause disease. |

| stimulus | A change in an organism’s environment that causes a response. |

| stomach | Hollow organ that contains acid, and churns and breaks down food. |

| stomata (singular stoma) | Tiny holes in the outer surface of a leaf that enable a plant to exchange gases (carbon dioxide, oxygen, and water vapour) with its surroundings. |

| subsistence farming | Farming that produces only enough crop to support the farmer, the farmer’s family, and the continuation of the farm. |

| substrate | A substance that is changed by the action of a particular enzyme. Each molecule of the substrate must be the correct shape to fit into the enzyme’s active site. |

| surface area | The total area of a surface available for a chemical reaction or absorption to take place. |

| surface area:volume ratio | The ratio of an organism’s or object’s surface area to its volume. |

| sustainability | Using resources and the environment to meet the needs of people today without damaging Earth for people of the future. One way to do this is to use resources at the same rate as they can be replaced. |

| sweat gland | Gland in the skin that secretes sweat through pores onto the skin surface. |

| symptoms | Feelings or changes experienced during illness (e.g., sore throat or runny nose). |

| synapse | A tiny gap between neurons that transmits nerve impulses from one neuron to another using transmitter substances that diffuse across the gap. |

| system | A collection of interacting parts. |

| systematic error | A measurement error that differs from the true value by the same amount each time a measurement is made. A systematic error may be due to the environment, methods of observation, or the instrument used. |

| target | An antigen or receptor molecule on a pathogen or host cell that could be affected by a medicine. |

| testosterone | Hormone that is important in puberty in human males. |

| theory | A scientific theory is a general explanation that applies to a large number of situations or examples (perhaps to all possible ones). It has been tested and used successfully, and is widely accepted by scientists. An example is the theory of evolution by natural selection. |

| thyroid gland | Gland in the human body that secretes the hormone thyroxine. |

| thyroxine | A hormone in the human body. It is secreted by the thyroid gland; it regulates growth and the rates of chemical reactions in cells. |

| transect | A technique used to measure the distribution and abundance of organisms at regular intervals along a straight line through an ecosystem. Abiotic factors, including pollution levels, may also be measured. |

| transgenic | An organism that has been genetically engineered to contain DNA from another organism. |

| transitional species | A species that shows some features of an older species and some of a newer species. Fossils of transitional species show one possible way that the newer species could have evolved from the older one. |

| translocation | The process of moving sugars, amino acids, and other substances through a plant. |

| transmitter substance | Substance that diffuses across a synapse to transmit a nerve impulse from one neuron to another. |

| transpiration | The process of water movement through a plant and its evaporation from cells in the leaves. |

| triplet code | A sequence of three bases in DNA or mRNA that codes for a particular amino acid. |

| trophic level | A feeding level in a food chain. The lowest trophic level is always producers. Biomass is transferred to higher trophic levels (including primary consumers, secondary consumers, and tertiary consumers) when organisms are eaten. |

| tropism | When a plant responds to its environment by growing in a particular direction. |

| true value | The actual value. |

| tumour | A lump or ball of cells observed in patients with cancer. Changes (mutations) in genes that usually control the cell cycle cause a cell to divide many times by mitosis, forming a tumour. |

| turgid | When a plant cell takes in water, it bulges and becomes stretched. It is said to be turgid. The strong cell wall prevents the cell from bursting. |

| type 1 diabetes | Non-communicable disease in which the blood sugar level cannot be controlled effectively because pancreas cells do not make enough insulin. |

| type 2 diabetes | Non-communicable disease in which the blood sugar level cannot be controlled effectively because pancreas cells don’t make enough insulin, or the body’s cells stop responding to it. Starts later in life, usually in people who are obese. |

| uncertainty | An indication of the confidence a scientist has in the accuracy of a measurement. It can be expressed as a range of values within which the true value must lie. |

| unicellular | An organism made of a single cell. |

| unspecialised | A cell that has not yet differentiated to take on a specific job. Only stem cells are unspecialised. |

| urea | Waste product of the breakdown of proteins and amino acids in the liver. |

| vaccination | Introducing a substance (vaccine) into an organism to make it immune to a disease. |

| vaccine | A vaccine contains dead or inactive pathogens, or parts of pathogens, used to trigger an immune response in an organism and establish immunity (without the organism catching the disease). |

| valid | The conclusions from an experiment are valid if the procedures ensure that the effects observed are due to the cause claimed, and if the analysis has taken account of other possible factors. |

| valve | Flap of tissue that acts like a one-way gate, only letting blood flow in one direction. Valves are found in the heart and veins. |

| variation | Differences between the members of a group. For example, there is usually variation within a population of organisms and within repeated measurements of a quantity. |

| vasoconstriction | Narrowing of arteries that supply blood to blood capillaries in the skin. As a result, less blood flows into the capillaries. |

| vasodilation | Widening of arteries that supply blood to capillaries in the skin. As a result, more blood flows into the capillaries. |

| vector (biology) | Used to transfer genes from one organism to another in genetic engineering. A plasmid, a bacterium, or a virus can be used as a vector. |

| vein | Type of blood vessel that carries blood from cells and tissues to the heart. |

| ventricles | The human heart is made of four muscular chambers. The ventricles are the lower chambers. The right ventricle pumps blood to the lungs and the left ventricle pumps blood to the rest of the body. |

| villi (singular villus) | Finger-like projections on the wall of the human small intestine. Villi increase the surface area for absorption of substances from the gut. |

| viruses | Large group of infectious agents that can only reproduce inside living cells. Some viruses cause disease. |

| water balance | Having the right amount of water in your body fluids so that cells are not damaged and chemical reactions and other processes can take place. |

| water cycle | Processes that cycle water through the abiotic and biotic parts of an ecosystem. |

| white blood cells | Cells in the blood that are a major part of the immune system; one type of white blood cell ingests and digests pathogens; another type makes antibodies. |

| xylem | Plant tissue that transports water and dissolved minerals through a plant. |