IGCSE Biology Glossary
Use the glossary to check a definition, distinguish related terms and see what a complete exam statement should include. Each entry links back to the wider syllabus topic.
252 terms · Cambridge IGCSE Biology 0610 · 2026–2028 syllabus
A
Absorption
The movement of digested food molecules from the small intestine into the blood (or lymph in the case of fats). Small soluble molecules such as glucose and amino acids pass through the villus wall into blood capillaries by diffusion and active transport.
Accommodation
The process by which the eye changes the shape of the lens to focus on objects at different distances. For near objects, the lens becomes fatter; for distant objects, it becomes thinner. This is controlled by the ciliary muscles and suspensory ligaments.
Acid rain
Rain that has a pH lower than normal (below pH 5.6), caused by sulfur dioxide and nitrogen oxides dissolving in atmospheric water. These gases come from burning fossil fuels. Acid rain damages vegetation, acidifies lakes (killing aquatic life), and erodes buildings and statues.
Active immunity
Defence against a pathogen by the production of antibodies in the body, gained after an infection or by vaccination.
Active site
The specific region on an enzyme molecule where the substrate binds. The shape of the active site is complementary to the shape of the substrate, which is why each enzyme only catalyses one reaction.
Active transport
The movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration, against a concentration gradient, using energy from respiration.
Adaptive feature
An inherited feature that helps an organism to survive and reproduce in its environment.
ADH
Antidiuretic hormone, produced by the pituitary gland, that controls water reabsorption in the kidneys. When blood water concentration is low, more ADH is released, making the collecting duct walls more permeable so more water is reabsorbed, producing less and more concentrated urine.
Adrenaline
A hormone produced by the adrenal glands in response to stress or danger. Adrenaline increases heart rate, breathing rate and blood glucose concentration, preparing the body for a 'fight or flight' response by directing more oxygen and glucose to muscles.
Aerobic
A process that requires oxygen. Aerobic respiration uses oxygen to completely break down glucose into carbon dioxide and water, releasing a large amount of energy. The equation is: glucose + oxygen produces carbon dioxide + water (plus energy released).
Aerobic respiration
The chemical reactions in cells that break down glucose using oxygen to release energy. Word equation: glucose + oxygen → carbon dioxide + water.
Allele
An alternative form of a gene, found at the same position on a chromosome. For example, the gene for seed shape has a round allele and a wrinkled allele.
Alveolar wall
The thin wall of an alveolus, only one cell thick, through which gas exchange occurs between the air in the alveolus and the blood in surrounding capillaries. Its thinness provides a short diffusion distance for oxygen and carbon dioxide.
Alveoli
The tiny air sacs in the lungs where gas exchange takes place. Oxygen diffuses into the blood and carbon dioxide diffuses out.
Amino acid
A small organic molecule that is the monomer of proteins. Twenty different amino acids exist, and they are joined together by peptide bonds during protein synthesis at ribosomes. The sequence of amino acids determines the shape and function of the protein.
Amniotic fluid
The fluid that surrounds and protects the developing fetus inside the amniotic sac in the uterus. It cushions the fetus against physical shocks, supports its weight, maintains a constant temperature and allows the fetus to move freely.
Amylase
An enzyme that catalyses the breakdown of starch into maltose (a reducing sugar). Amylase is produced in the salivary glands and the pancreas, and works in slightly alkaline conditions.
Anaerobic
A process that occurs without oxygen. In anaerobic respiration, glucose is only partially broken down, releasing less energy than aerobic respiration. In animals, glucose produces lactic acid. In yeast, glucose produces ethanol and carbon dioxide (fermentation).
Anaerobic respiration
The chemical reactions in cells that break down glucose without oxygen, releasing much less energy than aerobic respiration. In muscles: glucose → lactic acid. In yeast: glucose → alcohol + carbon dioxide.
Antibiotic
A medicine that kills bacteria or prevents their reproduction without damaging human cells. Antibiotics such as penicillin are effective against bacterial infections but have no effect on viruses because viruses reproduce inside host cells.
Antibiotic resistance
The ability of a strain of bacteria to survive and reproduce in the presence of an antibiotic that would normally kill or inhibit it. Resistance arises through random mutations and spreads by natural selection: resistant bacteria survive, reproduce and pass on the resistance gene.
Antibody
A protein produced by lymphocytes that binds to specific antigens on a pathogen, marking it for destruction or causing pathogens to clump together.
Antigen
A molecule (usually a protein) found on the surface of a cell or pathogen that the immune system can recognise as foreign. Each type of pathogen has unique antigens, which trigger a specific immune response involving antibody production by lymphocytes.
Artery
A blood vessel that carries blood away from the heart, usually oxygenated (except the pulmonary artery). Arteries have thick muscular walls, a narrow lumen relative to wall thickness, and elastic tissue that stretches and recoils to maintain blood pressure between heartbeats.
Asexual reproduction
A process resulting in the production of genetically identical offspring from one parent.
Assimilation
The movement of digested food molecules into the cells of the body where they are used, becoming part of the cells.
Auxin
A plant growth hormone (specifically indole-3-acetic acid, IAA) that controls cell elongation and is responsible for phototropism and gravitropism. Auxin accumulates on the shaded side of a shoot, causing cells there to elongate more, bending the shoot towards light.
B
Balanced diet
A diet that contains all the necessary nutrients in the correct proportions for the body to function properly: carbohydrates, proteins, fats, vitamins, minerals, water and fibre (roughage). It provides enough energy but not too much.
Base pair
The specific pairing of nitrogenous bases in DNA held together by hydrogen bonds: adenine (A) always pairs with thymine (T), and cytosine (C) always pairs with guanine (G). This complementary base pairing ensures accurate replication of DNA.
Bile
An alkaline fluid produced by the liver, stored in the gall bladder, and released into the duodenum. Bile emulsifies fats into smaller droplets to increase surface area for lipase action, and neutralises stomach acid to provide the alkaline pH needed by intestinal enzymes.
Binomial system
An internationally agreed system in which the scientific name of an organism is made up of two parts: the genus and the species.
Biodiversity
The variety of living organisms in an area, including the diversity of species, genetic diversity within species, and the diversity of ecosystems. High biodiversity indicates a healthy, stable ecosystem, while low biodiversity may indicate environmental stress.
Bowman's capsule
A cup-shaped structure at the start of a nephron that surrounds the glomerulus. It collects the filtrate produced by ultrafiltration: a liquid containing water, glucose, amino acids, urea and mineral ions, but not blood cells or large proteins.
Bronchiole
A small airway that branches from the bronchi inside the lungs. Bronchioles have smooth muscle in their walls but no cartilage, allowing them to constrict or dilate to control airflow. They end in clusters of alveoli.
Bronchus
One of two large airways that branch from the trachea and enter the left and right lungs. The bronchi have walls supported by cartilage rings and are lined with ciliated cells and mucus-producing goblet cells. The plural is bronchi.
C
Capillary
The smallest type of blood vessel, with walls only one cell thick. Capillaries form dense networks in tissues, providing a large surface area and short diffusion distance for the exchange of substances (oxygen, glucose, carbon dioxide, urea) between blood and cells.
Carbon cycle
The biogeochemical cycle in which carbon is recycled between the atmosphere, living organisms, the ocean and the Earth's crust. Carbon dioxide is removed from the atmosphere by photosynthesis and returned by respiration, combustion and decomposition.
Carbon monoxide
A toxic gas in tobacco smoke that binds irreversibly to haemoglobin, forming carboxyhaemoglobin. This reduces the oxygen-carrying capacity of the blood because the haemoglobin can no longer bind oxygen, forcing the heart to work harder.
Carnivore
An animal that feeds on other animals. Carnivores can be secondary consumers (eating herbivores), tertiary consumers (eating other carnivores) or top predators. Examples include foxes, hawks and lions.
Carrier
An individual who is heterozygous for a recessive genetic condition, carrying one copy of the recessive allele but not showing the condition because the dominant allele masks its effect. Carriers can pass the recessive allele to their offspring.
Carrier protein
A membrane protein that binds to specific molecules or ions and transports them across the cell membrane. Carrier proteins are involved in active transport, using energy from respiration to move substances against the concentration gradient.
Catalyst
A substance that increases the rate of a chemical reaction without being used up or permanently changed in the process. In biological systems, enzymes act as biological catalysts.
Cell membrane
A thin, partially permeable barrier surrounding every living cell that controls which substances enter and leave. It is made of a phospholipid bilayer with embedded proteins and is present in both plant and animal cells.
Cell wall
A rigid outer layer made of cellulose that surrounds the cell membrane of plant cells, bacterial cells and fungal cells. It provides structural support and prevents the cell from bursting when water enters by osmosis.
Cellulose
A complex carbohydrate (polysaccharide) made of long chains of beta-glucose molecules. It forms the main structural component of plant cell walls, providing rigidity and strength.
Chlorophyll
The green pigment found in chloroplasts that absorbs light energy for photosynthesis. Chlorophyll absorbs red and blue wavelengths of light most effectively and reflects green light, which is why leaves appear green.
Chloroplast
An organelle found in plant cells and some protoctist cells that contains the green pigment chlorophyll. It is the site of photosynthesis, where light energy is absorbed and used to make glucose from carbon dioxide and water.
Chromosome
A thread-like structure made of DNA and protein, found in the nucleus of a cell. Chromosomes carry genes in a specific order along their length. Humans have 46 chromosomes (23 pairs), including one pair of sex chromosomes (XX in females, XY in males).
Cirrhosis
A disease of the liver caused by long-term excessive alcohol consumption, in which liver cells are damaged and replaced by scar tissue. Cirrhosis reduces the liver's ability to carry out its functions, including deamination, detoxification and bile production.
Cloning
The process of producing genetically identical copies of an organism, cell or gene. In plants, cloning can occur naturally through asexual reproduction or artificially through tissue culture (using small pieces of plant tissue grown on nutrient agar with hormones).
Co-dominance
A pattern of inheritance in which both alleles of a gene are expressed equally in the phenotype of a heterozygote, rather than one being dominant over the other. For example, in sickle cell disease, a heterozygote (HbA HbS) produces both normal and sickle-shaped red blood cells.
Collecting duct
The final part of the nephron where the permeability of the walls is controlled by ADH (antidiuretic hormone). When the body is dehydrated, ADH increases the permeability of the collecting duct walls, allowing more water to be reabsorbed back into the blood, producing concentrated urine.
Community
All the populations of different species living and interacting in the same area at the same time. A community includes all the plants, animals, fungi, bacteria and other organisms in a defined habitat.
Concentration gradient
The difference in concentration of a substance between two regions. Diffusion occurs down a concentration gradient (from higher to lower concentration), while active transport moves substances against the concentration gradient (from lower to higher concentration).
Conservation
The management and protection of ecosystems, habitats and species to maintain biodiversity and ensure sustainable use of natural resources. Conservation strategies include establishing protected areas, breeding programmes for endangered species, seed banks and legislation against poaching.
Consumer
An organism that obtains energy by feeding on other organisms. Primary consumers eat producers, secondary consumers eat primary consumers, and tertiary consumers eat secondary consumers. Consumers are heterotrophs.
Continuous variation
Variation that shows a complete range of phenotypes between two extremes, with no distinct categories. Examples include height, mass and foot length. Continuous variation is typically controlled by many genes (polygenic) and influenced by the environment.
Contraception
Methods used to prevent pregnancy. These include barrier methods (condoms, which also prevent STI transmission), hormonal methods (the contraceptive pill, containing oestrogen and/or progesterone, which inhibit ovulation), intrauterine devices and natural methods such as abstinence.
Cornea
The transparent front layer of the eye that refracts (bends) light entering the eye. The cornea provides most of the eye's optical power, bending light rays so they begin to focus on the retina.
Coronary artery
A blood vessel that branches from the aorta and supplies the heart muscle with oxygenated blood and glucose for respiration. Blockage of a coronary artery by a blood clot or fatty deposits (atherosclerosis) can lead to a heart attack.
Cystic fibrosis
A genetic disorder caused by a recessive allele that results in the production of thick, sticky mucus in the lungs, pancreas and other organs. The thick mucus blocks airways, making breathing difficult, and blocks pancreatic ducts, reducing enzyme secretion for digestion.
Cytoplasm
The jelly-like material that fills the cell between the cell membrane and the nucleus. Most chemical reactions of metabolism take place here, including many enzyme-controlled reactions.
D
Deamination
The removal of the nitrogen-containing part of amino acids to form urea, which takes place in the liver.
Decomposer
An organism that gets its energy from dead or waste organic material, breaking it down and returning nutrients to the environment.
Deforestation
The large-scale removal of forests by cutting down or burning trees, often to clear land for agriculture, cattle ranching or urban development. Deforestation leads to habitat loss, reduced biodiversity, soil erosion, flooding and increased atmospheric carbon dioxide.
Denaturation
A permanent change in the shape of an enzyme's active site, caused by high temperature or unsuitable pH, so the substrate no longer fits and the enzyme stops working.
Denitrification
The conversion of nitrates in the soil back into nitrogen gas (N2), which is released into the atmosphere. This process is carried out by denitrifying bacteria, which are anaerobic and thrive in waterlogged, oxygen-poor soils.
Diaphragm
A sheet of muscle beneath the lungs that plays a key role in ventilation. During inhalation, the diaphragm contracts and flattens, increasing the volume of the thorax and decreasing pressure so air is drawn into the lungs. During exhalation it relaxes and domes upwards.
Dichotomous key
A branching identification tool that presents a series of paired statements or questions, each with two choices. At each step the user selects the statement that matches the organism, eventually arriving at the correct identification.
Diffusion
The net movement of particles from a region of their higher concentration to a region of their lower concentration, down a concentration gradient, as a result of their random movement.
Diploid
A cell or organism that has two complete sets of chromosomes, one set inherited from each parent. In humans, diploid cells contain 46 chromosomes (23 pairs). All body cells are diploid, while gametes are haploid.
Discontinuous variation
Variation that falls into distinct, separate categories with no intermediates. Examples include blood group (A, B, AB, O), tongue rolling (can or cannot) and eye colour. Discontinuous variation is usually controlled by a single gene and is not influenced by the environment.
DNA
Deoxyribonucleic acid, a double-stranded molecule in the shape of a double helix found in chromosomes. DNA carries the genetic code as a sequence of bases (A, T, C, G) that determines the order of amino acids in proteins and therefore controls cell activities.
Dominant allele
An allele that is expressed in the phenotype when only one copy is present (in the heterozygous condition).
Double circulation
A circulatory system in which blood passes through the heart twice for each complete circuit of the body. Once in the pulmonary circuit (to the lungs) and once in the systemic circuit (to the rest of the body).
Drug dependence
The condition where a person's body has adapted to the presence of a drug and cannot function normally without it. If the drug is withdrawn, the person experiences unpleasant withdrawal symptoms such as headaches, nausea, anxiety or tremors.
Drug tolerance
The condition where a person needs increasing doses of a drug to achieve the same effect, because the body adapts to the presence of the drug. Tolerance develops over time with repeated use and can lead to increased consumption and greater health risks.
E
Ecosystem
A unit containing the community of organisms and their environment, interacting together. For example, a decomposing log, a pond, or a rainforest.
Effector
A muscle or gland that carries out a response when stimulated by a nerve impulse from a motor neurone. Muscles respond by contracting (producing movement) and glands respond by secreting substances such as hormones or enzymes.
Egestion
The passing out of undigested food (faeces) through the anus.
Endocrine gland
A ductless gland that produces hormones and secretes them directly into the blood. Examples include the pancreas (insulin, glucagon), adrenal glands (adrenaline), pituitary gland (ADH, growth hormone, FSH, LH), ovaries (oestrogen, progesterone) and testes (testosterone).
Enzyme
A protein that functions as a biological catalyst. It speeds up a chemical reaction without being changed or used up by the reaction.
Ethanol
One of the products of anaerobic respiration in yeast and some plant cells. The equation for anaerobic respiration in yeast is: glucose produces ethanol plus carbon dioxide. This process is used in brewing and baking.
Eukaryote
An organism whose cells contain a true nucleus enclosed by a nuclear envelope, and membrane-bound organelles such as mitochondria and, in plants, chloroplasts. Animals, plants, fungi and protoctists are all eukaryotes.
Eutrophication
The over-enrichment of water with nutrients (from fertiliser run-off or sewage), causing an algal bloom that blocks light, the death and decomposition of plants and algae, oxygen depletion by decomposer respiration, and the death of aquatic animals.
Evolution
The gradual change in the inherited characteristics of a population over many generations, which may result in the formation of new species. Evolution occurs through natural selection, where organisms with favourable characteristics are more likely to survive and reproduce.
Excretion
The removal from organisms of the waste products of metabolism (chemical reactions in cells including respiration), toxic materials, and substances in excess of requirements.
F
Fatty acid
One of the two components of a lipid (fat or oil) molecule. Fatty acids are long hydrocarbon chains with a carboxyl group at one end. Three fatty acid molecules combine with one glycerol molecule to form a lipid.
Fermenter
A large container (bioreactor) used for growing microorganisms on a large scale under controlled conditions. Fermenters have inputs for nutrients, air (if aerobic) and monitoring systems for temperature and pH. They are used to produce antibiotics, enzymes, single-cell protein and other biotechnology products.
Fertilisation
The fusion of the nucleus of a male gamete (sperm) with the nucleus of a female gamete (egg cell) to form a zygote. In humans, fertilisation usually occurs in the oviduct (fallopian tube). The zygote is diploid, having received one set of chromosomes from each parent.
Fibre
Indigestible plant material (mainly cellulose) in the diet that adds bulk to food in the intestine, stimulating peristalsis and helping move food through the digestive system. Also called roughage or dietary fibre.
Flaccid
The state of a plant cell that has lost some water so that the cytoplasm no longer pushes firmly against the cell wall. The cell is soft and limp but not plasmolysed. Flaccid cells cause wilting in plants.
Food chain
A diagram showing the transfer of energy from one organism to the next, beginning with a producer. For example: grass → grasshopper → frog → snake.
Food web
A diagram showing the interconnected food chains within an ecosystem. A food web is more realistic than a single food chain because most organisms feed on more than one species and are eaten by more than one predator.
G
Gamete
A sex cell (reproductive cell) that is haploid, containing half the number of chromosomes of a normal body cell. In humans, the male gamete is the sperm cell and the female gamete is the egg cell (ovum). Gametes are produced by meiosis.
Gene
A length of DNA that codes for a protein (the unit of inheritance passed from parent to offspring).
Genetic modification
The process of altering the genetic material of an organism by inserting a gene from a different organism, often from a different species, to give the organism a desired characteristic. Also called genetic engineering. Examples include producing insulin in bacteria and making crops resistant to herbicides or pests.
Genotype
The genetic make-up of an organism. The combination of alleles it carries for a characteristic, written as letters (e.g. RR, Rr or rr).
Germination
The process by which a seed develops into a new plant. Germination requires water (to activate enzymes and soften the seed coat), oxygen (for aerobic respiration to release energy for growth) and a suitable temperature (for enzyme activity). Light is not always required initially.
Glomerulus
A tight knot of blood capillaries inside the Bowman's capsule of a nephron in the kidney. Blood enters under high pressure through the afferent arteriole. Small molecules (water, glucose, urea, ions) are forced out of the blood into the Bowman's capsule by ultrafiltration.
Glucagon
A hormone produced by the alpha cells of the islets of Langerhans in the pancreas when blood glucose concentration is too low. Glucagon stimulates the liver to convert stored glycogen back into glucose and release it into the blood, raising blood glucose levels.
Glucose
A simple sugar (monosaccharide) with the formula C6H12O6 that serves as the primary substrate for respiration in cells. It is produced during photosynthesis and digestion of carbohydrates, and is transported in the blood to supply energy.
Glycerol
A small organic molecule with three hydroxyl groups that forms the backbone of lipid molecules. One glycerol molecule combines with three fatty acid molecules to form a fat or oil.
Glycogen
A complex carbohydrate (polysaccharide) used as the main energy storage molecule in animals and fungi. It is stored in the liver and muscles and can be rapidly broken down into glucose when energy is needed.
Greenhouse effect
The natural process by which certain gases in the Earth's atmosphere (greenhouse gases such as carbon dioxide, methane and water vapour) trap heat energy radiated from the Earth's surface, keeping the planet warm enough to support life. The enhanced greenhouse effect caused by increased greenhouse gas emissions leads to global warming.
Guard cell
A specialised bean-shaped cell found in pairs either side of a stoma. Guard cells control the opening and closing of stomata. When they absorb water and become turgid, the stoma opens; when they lose water and become flaccid, the stoma closes.
H
Habitat
The specific place or environment in which an organism lives, providing the conditions and resources it needs to survive. Examples include a pond, a woodland floor or a coral reef.
Haemoglobin
A red iron-containing protein found in red blood cells that reversibly binds with oxygen to form oxyhaemoglobin in the lungs. In tissues where oxygen concentration is low, oxyhaemoglobin releases its oxygen for use in aerobic respiration.
Haploid
A cell that has one complete set of chromosomes, half the diploid number. In humans, haploid cells (gametes) contain 23 chromosomes. Haploid cells are produced by meiosis and restore the diploid number when they fuse at fertilisation.
Heart
A muscular organ that pumps blood around the body through a double circulatory system. The right side pumps deoxygenated blood to the lungs (pulmonary circulation) and the left side pumps oxygenated blood to the rest of the body (systemic circulation). It has four chambers: two atria and two ventricles.
Herbivore
An animal that feeds only on plants or other producers. Herbivores are primary consumers and occupy the second trophic level in a food chain. Examples include rabbits, caterpillars and cows.
Heterozygous
Having two different alleles of a particular gene.
Homeostasis
The maintenance of a constant internal environment in the body. For example, keeping core temperature and blood glucose concentration within narrow limits.
Homozygous
Having two identical alleles of a particular gene.
Hormones
Chemical substances produced by endocrine glands and transported in the blood to target organs where they produce a specific effect. Hormones are slower-acting but longer-lasting than nerve impulses. Examples include insulin, adrenaline, oestrogen and testosterone.
I
Immune response
The body's reaction to the presence of a pathogen or foreign substance. The non-specific response involves phagocytes engulfing pathogens. The specific response involves lymphocytes recognising antigens and producing complementary antibodies.
Implantation
The process by which a developing embryo embeds itself into the thick, blood-rich lining of the uterus, approximately six to seven days after fertilisation. Once implanted, the embryo receives nutrients and oxygen from the mother's blood supply through the developing placenta.
Ingestion
The taking of food into the body through the mouth. It is the first stage of the digestive process, before digestion, absorption, assimilation and egestion.
Insulin
A hormone produced by the beta cells of the islets of Langerhans in the pancreas when blood glucose concentration is too high. Insulin stimulates liver and muscle cells to take up glucose and convert it to glycogen for storage, lowering blood glucose levels.
Intercostal muscle
Muscles found between the ribs that work together with the diaphragm to ventilate the lungs. External intercostal muscles contract during inhalation, pulling the ribs up and out. Internal intercostal muscles contract during forced exhalation, pulling the ribs down and in.
Invertebrate
An animal without a backbone. Invertebrates make up over 95 percent of all animal species and include arthropods (insects, arachnids, crustaceans), molluscs, and annelids.
Iris
The coloured circular structure in the eye that controls the size of the pupil, regulating how much light enters the eye. In bright light, circular muscles in the iris contract and radial muscles relax, making the pupil smaller. In dim light, radial muscles contract and circular muscles relax, making the pupil larger.
K
L
Lacteal
A small lymph vessel found in the centre of each villus in the small intestine. Lacteals absorb fatty acids and glycerol (the products of fat digestion), which are then transported in the lymphatic system before entering the blood.
Lactic acid
The waste product of anaerobic respiration in animals. When muscles do not receive enough oxygen during intense exercise, glucose is partially broken down to lactic acid, which accumulates in muscles causing fatigue and cramp.
Lens
A transparent, flexible biconvex structure in the eye that fine-focuses light onto the retina by changing shape. For near objects, the ciliary muscles contract, suspensory ligaments slacken and the lens becomes fatter (more convex). For distant objects, ciliary muscles relax, ligaments pull tight and the lens becomes thinner.
Ligase
An enzyme that joins together two pieces of DNA by forming bonds between the sugar-phosphate backbones. In genetic engineering, DNA ligase is used to seal the desired gene into the plasmid vector after it has been cut by a restriction enzyme.
Limiting factor
Something present in the environment in such short supply that it restricts life processes, such as the rate of photosynthesis.
Lipase
An enzyme that catalyses the breakdown of lipids (fats and oils) into fatty acids and glycerol. Lipase is produced in the pancreas and small intestine, and works best in alkaline conditions created by bile.
Lock-and-key hypothesis
A model explaining enzyme specificity: the enzyme's active site (the lock) has a shape complementary to the substrate (the key). Only the correct substrate can fit into the active site, so each enzyme catalyses only one type of reaction.
Lymphocyte
A type of white blood cell that produces antibodies as part of the specific immune response. Each lymphocyte produces one type of antibody that is specific to a particular antigen on a pathogen.
M
Magnification
The number of times larger an image is compared with the actual (real) size of the object.
Meiosis
Reduction division in which the chromosome number is halved, producing four genetically different gametes from one parent cell.
Memory cell
A long-lived lymphocyte that remains in the blood after an initial immune response. If the same pathogen re-enters the body, memory cells recognise its antigens and rapidly produce large quantities of the specific antibody, mounting a faster and stronger secondary immune response that prevents the disease from developing.
Menstrual cycle
A repeating cycle of approximately 28 days in which the uterus lining is prepared for possible pregnancy. It involves menstruation (shedding of the lining), repair and thickening of the lining (controlled by oestrogen), ovulation around day 14, and maintenance of the lining (controlled by progesterone).
Mineral ion
An inorganic nutrient absorbed by plant roots from the soil, essential for healthy growth. Key mineral ions include nitrate (for amino acid and protein synthesis) and magnesium (for chlorophyll synthesis).
Mitochondrion
An organelle with a folded inner membrane where aerobic respiration takes place, releasing energy from glucose. Cells with high energy demands, such as muscle cells and sperm cells, contain large numbers of mitochondria.
Mitosis
Nuclear division that produces two genetically identical daughter cells with the same number of chromosomes as the parent cell. Used for growth, repair of damaged tissue, replacement of cells, and asexual reproduction.
Motor neurone
A neurone that carries nerve impulses from the central nervous system (brain or spinal cord) to an effector, which is a muscle or gland. The motor neurone has a long axon that extends from the CNS to the effector.
Mutation
A random genetic change, which may produce a new allele or change a gene or chromosome.
N
Natural selection
The process by which individuals with variation better suited to the environment are more likely to survive and reproduce, passing on their alleles, so the favourable feature becomes more common over generations.
Negative feedback
A control mechanism in which a change in a condition triggers a response that reverses the change, returning the condition to its set point. The basis of homeostasis.
Nephron
The functional unit of the kidney responsible for filtering blood and forming urine. Each nephron consists of a Bowman's capsule containing a glomerulus, followed by the proximal convoluted tubule, loop of Henle, distal convoluted tubule and collecting duct.
Net movement
The overall direction in which particles move when diffusion or osmosis occurs. Individual molecules move randomly in all directions, but there is a net movement from the region of higher concentration to the region of lower concentration because more molecules move in that direction.
Neurone
A specialised nerve cell that transmits electrical impulses rapidly around the body. Neurones have a cell body containing the nucleus, dendrites that receive impulses, and a long axon that carries the impulse away from the cell body.
Niche
The role of an organism within its ecosystem, including what it feeds on, what feeds on it, and how it interacts with both the living and non-living parts of its environment. No two species can occupy exactly the same niche in the same habitat.
Nicotine
An addictive chemical in tobacco smoke that stimulates the nervous system and causes the release of adrenaline, increasing heart rate and blood pressure. Nicotine is the substance responsible for tobacco addiction and makes it difficult to stop smoking.
Nitrate ion
A mineral ion (NO3-) absorbed by plant roots that provides nitrogen for the synthesis of amino acids and proteins. Without sufficient nitrate, plants show stunted growth because they cannot make the proteins needed for cell growth.
Nitrification
The conversion of ammonium compounds in the soil into nitrites and then into nitrates by nitrifying bacteria. This is an aerobic process and is important because plants can absorb nitrogen in the form of nitrate ions through their roots.
Nitrogen cycle
The biogeochemical cycle in which nitrogen is converted between various chemical forms as it circulates through the atmosphere, soil and organisms. Key processes include nitrogen fixation, nitrification, denitrification and decomposition.
Nitrogen fixation
The conversion of atmospheric nitrogen gas (N2) into ammonium compounds or nitrates that can be used by plants. This is carried out by nitrogen-fixing bacteria, some of which live freely in the soil and others in root nodules of leguminous plants. Lightning can also fix nitrogen.
Nucleus
A large organelle enclosed by a nuclear envelope that contains the cell's chromosomes, made of DNA. It controls the activities of the cell by determining which proteins are made.
O
Oestrogen
A hormone produced by the ovaries that controls the development of female secondary sexual characteristics during puberty and plays a key role in the menstrual cycle, where it stimulates the repair and thickening of the uterus lining after menstruation.
Omnivore
An animal that feeds on both plants and animals. Omnivores can occupy more than one trophic level in a food web. Examples include humans, bears and pigs.
Optic nerve
A bundle of sensory neurone fibres that carries nerve impulses from the light receptors in the retina to the brain, where the impulses are interpreted as visual images.
Optimum pH
The pH at which an enzyme works at its maximum rate. Different enzymes have different optimum pH values: pepsin works best at pH 2 (acidic, in the stomach), while trypsin works best at pH 8 (alkaline, in the small intestine).
Optimum temperature
The temperature at which an enzyme works at its fastest rate, producing the maximum amount of product per unit time. For most human enzymes, this is approximately 37 degrees Celsius. Above this temperature, the enzyme begins to denature.
Organism
A living thing that carries out all seven life processes: movement, respiration, sensitivity, growth, reproduction, excretion and nutrition (MRS GREN). Organisms can be unicellular (single-celled) or multicellular.
Osmosis
The net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane.
Ovary
The female reproductive organ that produces egg cells (ova) and secretes the hormones oestrogen and progesterone. In plants, the ovary is the part of the flower that contains ovules, which develop into seeds after fertilisation, while the ovary wall becomes the fruit.
Ovulation
The release of a mature egg cell (ovum) from an ovary, which usually occurs around day 14 of the menstrual cycle. The egg then travels along the oviduct towards the uterus, and can be fertilised by a sperm cell during this journey.
Oxygen debt
The amount of extra oxygen needed after vigorous exercise to break down the lactic acid that accumulated during anaerobic respiration. This is why breathing rate and heart rate remain elevated after exercise stops.
P
Palisade mesophyll
A layer of tightly packed, elongated cells near the upper surface of a leaf that are the main site of photosynthesis. These cells contain many chloroplasts and are positioned close to the upper epidermis to receive maximum light.
Partially permeable membrane
A membrane that allows some molecules to pass through but not others, depending on their size or other properties. The cell membrane is partially permeable, allowing small molecules like water to pass through while blocking larger solute molecules.
Passive immunity
Short-term defence against a pathogen by antibodies acquired from another individual, for example from mother to infant through the placenta or breast milk.
Pasteurisation
A process used to reduce the number of harmful microorganisms in food or drink (especially milk) by heating it to a specific temperature (usually 72 degrees Celsius for 15 seconds) and then rapidly cooling it. This kills most pathogenic bacteria without significantly altering taste or nutritional value.
Pathogen
A disease-causing organism. For example, bacteria, viruses, fungi or protoctists that cause transmissible diseases.
Peristalsis
Waves of muscular contraction and relaxation that push food along the digestive tract. Circular muscles behind the food bolus contract while those ahead relax, squeezing the food forward through the oesophagus, stomach and intestines.
Phagocyte
A type of white blood cell that destroys pathogens by engulfing and digesting them in a process called phagocytosis. Phagocytes are non-specific: they attack any foreign particle and do not produce antibodies.
Phenotype
The observable features of an organism that result from its genotype and its environment. For example, round seeds or brown eyes.
Phloem
A plant transport tissue that carries dissolved sugars (mainly sucrose) and amino acids from sources (such as leaves where they are made) to sinks (such as roots, fruits and growing tips where they are used or stored). This transport is called translocation.
Photosynthesis
The process by which plants synthesise carbohydrates from carbon dioxide and water, using energy from light trapped by chlorophyll. Word equation: carbon dioxide + water → glucose + oxygen, in the presence of light and chlorophyll.
Placenta
An organ that develops in the uterus during pregnancy, providing the interface between the mother's blood and the fetus's blood. Nutrients and oxygen diffuse from the mother's blood to the fetus, while carbon dioxide and urea diffuse from the fetus to the mother. The two blood supplies do not mix.
Plasma
The pale yellow liquid component of blood in which blood cells are suspended. Plasma transports dissolved substances including glucose, amino acids, urea, carbon dioxide, hormones, antibodies and heat energy around the body.
Plasmid
A small circular ring of DNA found in bacterial cells, separate from the main bacterial chromosome. In genetic engineering, plasmids are used as vectors: they are cut open with restriction enzymes, the desired gene is inserted, and the recombinant plasmid is returned to a bacterium.
Plasmolysis
The shrinkage of a plant cell's cytoplasm away from the cell wall when the cell is placed in a concentrated solution. Water leaves the vacuole and cytoplasm by osmosis, causing the cell membrane to pull away from the cell wall.
Platelet
A small cell fragment in the blood that plays a key role in blood clotting. When a blood vessel is damaged, platelets accumulate at the wound site and release chemicals that trigger the conversion of fibrinogen to fibrin, forming a mesh that traps blood cells and creates a clot.
Pollen
A fine powder produced by the anthers of a flower, containing the male gametes. Pollen grains are transferred from the anther to the stigma of a flower (pollination) by wind or insects. Once on the stigma, a pollen tube grows down the style to deliver the male nucleus to the ovule for fertilisation.
Pollination
The transfer of pollen grains from the anther of a flower to the stigma. Self-pollination occurs when pollen is transferred within the same flower or between flowers on the same plant. Cross-pollination involves transfer between flowers on different plants of the same species.
Pollution
The introduction of harmful substances or excessive energy into the environment, causing damage to living organisms and ecosystems. Pollution can affect air (e.g. Sulfur dioxide, carbon monoxide), water (e.g. Sewage, fertiliser runoff) or land (e.g. Pesticides, landfill waste).
Population
A group of organisms of the same species living in the same area at the same time and able to interbreed. Population size is affected by birth rate, death rate, immigration and emigration.
Potometer
A piece of apparatus used to measure the rate of water uptake by a cut shoot, which is used as an estimate of the rate of transpiration. It works by tracking the movement of an air bubble along a graduated capillary tube as the shoot takes up water.
Producer
An organism that makes its own organic nutrients from simple inorganic molecules, usually by photosynthesis. Producers are autotrophs and form the first trophic level of a food chain. Green plants and algae are the main producers in most ecosystems.
Progesterone
A hormone produced by the corpus luteum (in the ovary after ovulation) that maintains the thick, blood-rich lining of the uterus during the second half of the menstrual cycle. If an embryo implants, progesterone levels remain high to support pregnancy. If not, progesterone drops and menstruation begins.
Prokaryote
An organism whose cells lack a true nucleus and membrane-bound organelles. The DNA lies free in the cytoplasm as a single circular chromosome. Bacteria are prokaryotes. They also have a cell wall, but it is not made of cellulose.
Protease
An enzyme that catalyses the breakdown of proteins into amino acids. Proteases include pepsin (which works in acidic conditions in the stomach) and trypsin (which works in alkaline conditions in the small intestine).
Protein synthesis
The process by which the sequence of bases in DNA is used to determine the sequence of amino acids in a protein. DNA is transcribed into mRNA in the nucleus, and the mRNA moves to ribosomes in the cytoplasm where it is translated into a chain of amino acids.
Punnett square
A diagram used to predict the possible genotypes and phenotypes of offspring from a genetic cross. The alleles of one parent are placed along the top and the other parent's alleles along the side, and the boxes show all possible offspring combinations.
Pyramid of biomass
A diagram showing the total dry mass of organisms at each trophic level in a food chain at a given time. Pyramids of biomass are almost always pyramid-shaped, with the greatest biomass at the producer level and decreasing biomass at each successive level.
Pyramid of energy
A diagram showing the total amount of energy flowing through each trophic level in a food chain over a set period of time, measured in kilojoules per square metre per year. Pyramids of energy are always true pyramids because energy is always lost between trophic levels.
Q
R
Receptor
A specialised cell or group of cells that detects a stimulus (a change in the environment) and converts it into a nerve impulse. Examples include light receptors in the retina, temperature receptors in the skin and chemical receptors on the tongue.
Recessive allele
An allele that is only expressed in the phenotype when two copies are present (in the homozygous condition).
Red blood cell
A biconcave disc-shaped blood cell that contains haemoglobin and transports oxygen from the lungs to body tissues. Red blood cells have no nucleus, which maximises space for haemoglobin, and their biconcave shape increases surface area for oxygen absorption.
Reflex arc
The nerve pathway of a reflex action: receptor → sensory neurone → relay neurone (in the spinal cord) → motor neurone → effector, producing a fast, automatic response that protects the body.
Relay neurone
A short neurone found within the central nervous system (brain or spinal cord) that connects sensory neurones to motor neurones. In reflex arcs, the relay neurone transfers the impulse between the incoming sensory signal and the outgoing motor signal.
Restriction enzyme
An enzyme that cuts DNA at specific base sequences, producing fragments with either blunt or sticky ends. In genetic engineering, restriction enzymes are used to cut out a desired gene from donor DNA and to cut open a plasmid vector at a corresponding site.
Retina
The light-sensitive layer at the back of the eye containing photoreceptor cells (rods and cones) that detect light and convert it into nerve impulses. These impulses are sent to the brain via the optic nerve for interpretation as images.
Ribosome
A tiny organelle found in the cytoplasm where protein synthesis occurs. Ribosomes read the instructions from mRNA and assemble amino acids into polypeptide chains.
Root hair cell
A specialised cell on the surface of a plant root that has a long, thin extension (the root hair) projecting into the soil. This greatly increases the surface area for absorption of water by osmosis and mineral ions by active transport.
S
Sebum
An oily substance produced by sebaceous glands in the skin that helps to waterproof the skin surface and inhibit the growth of some bacteria and fungi.
Selective breeding
The process by which humans choose organisms with desirable characteristics and breed them together over many generations to enhance those traits. Also called artificial selection. Examples include breeding cows for high milk yield or wheat for disease resistance.
Selective reabsorption
The process in the kidney tubules where useful substances from the filtrate (all glucose, most water, some ions and amino acids) are reabsorbed back into the blood. Glucose and amino acids are reabsorbed by active transport in the proximal convoluted tubule, while water is reabsorbed by osmosis.
Sensory neurone
A neurone that carries nerve impulses from a receptor (such as a pain receptor in the skin) to the central nervous system (brain or spinal cord). It has a long dendron connecting the receptor to the cell body, and a short axon entering the CNS.
Sex linkage
A pattern of inheritance where a gene is located on the X chromosome and therefore the trait is more commonly expressed in males. Males have only one X chromosome (XY), so a single recessive allele on the X chromosome will be expressed since there is no second X to carry a dominant allele.
Sexual reproduction
A process involving the fusion of the nuclei of two gametes (sex cells) to form a zygote and the production of offspring that are genetically different from each other.
Sickle cell anaemia
A genetic blood disorder caused by a mutation in the gene for haemoglobin, resulting in abnormal sickle-shaped red blood cells. These rigid, crescent-shaped cells can block capillaries, reducing oxygen delivery to tissues and causing pain and fatigue.
Species
A group of organisms that can reproduce to produce fertile offspring.
Spongy mesophyll
A layer of loosely arranged, irregularly shaped cells in the lower part of a leaf with large air spaces between them. These air spaces allow gases (carbon dioxide and oxygen) to diffuse easily between the stomata and the photosynthesising cells.
Starch
A complex carbohydrate (polysaccharide) used as the main energy storage molecule in plants. It is made of long chains of alpha-glucose molecules and is insoluble in water, making it ideal for storage because it does not affect the water potential of cells.
Stomata
Small pores found mainly on the lower surface of leaves, each surrounded by a pair of guard cells that control opening and closing. When open, stomata allow carbon dioxide in and oxygen and water vapour out. The singular form is stoma.
Substrate
The molecule on which an enzyme acts. The substrate binds to the enzyme's active site, where it is converted into products. Each enzyme is specific to its substrate.
Sustainability
Using natural resources in a way that meets present needs without compromising the ability of future generations to meet their own needs. Sustainable practices include replanting forests, regulating fishing quotas and using renewable energy sources.
Sweating
The secretion of sweat (a dilute salt solution) onto the skin surface by sweat glands in response to high body temperature. As sweat evaporates from the skin, it absorbs heat energy (latent heat of vaporisation), cooling the body down.
Synapse
A tiny gap between two neurones across which nerve impulses are transmitted by chemical transmitter substances (neurotransmitters). The neurotransmitter is released from the presynaptic neurone, diffuses across the gap, and binds to receptors on the postsynaptic neurone, triggering a new impulse.
T
Tar
A sticky brown substance in tobacco smoke that accumulates in the lungs, damaging cilia on airway cells and increasing mucus production. Tar also contains carcinogens that can cause mutations in lung cells, leading to lung cancer.
Testis
The male reproductive organ that produces sperm cells and secretes the hormone testosterone. The testes are located outside the body in the scrotum, where the temperature is slightly lower than body temperature, which is necessary for sperm production.
Testosterone
A hormone produced by the testes that controls the development of male secondary sexual characteristics during puberty, including deepening of the voice, growth of facial and body hair, increased muscle mass and sperm production.
Thermoregulation
The maintenance of a constant core body temperature despite changes in the external environment. In humans, this involves responses controlled by the hypothalamus in the brain, including vasoconstriction, vasodilation, sweating and shivering.
Trachea
The main airway (windpipe) leading from the throat to the bronchi. The trachea is supported by C-shaped rings of cartilage that prevent it from collapsing, and is lined with ciliated epithelial cells and goblet cells that produce mucus to trap dust and pathogens.
Transect
A line placed across a habitat along which organisms are sampled at regular intervals, usually with quadrats. A transect is used to study how the distribution of organisms changes across an environmental gradient, such as from a shoreline to inland or from light to shade.
Translocation
The movement of sucrose and amino acids in the phloem, from regions of production (sources, e.g. Leaves) to regions of storage or use (sinks, e.g. Roots and fruits).
Transmissible disease
A disease caused by a pathogen that can be passed from one host to another. Transmission routes include direct contact, droplet infection (coughing/sneezing), contaminated food or water, and vectors such as mosquitoes.
Transpiration
The loss of water vapour from the leaves of a plant, mainly through the stomata, as water evaporates from the surfaces of mesophyll cells.
Transpiration rate
The speed at which water is lost from the leaves of a plant by evaporation through the stomata. Transpiration rate is increased by high temperature, low humidity, wind and high light intensity, and is decreased by the opposite conditions.
Trophic level
The position of an organism in a food chain, food web or pyramid. Producer, primary consumer, secondary consumer, and so on.
Tropism
A growth response of a plant in which the direction of growth is determined by the direction of the stimulus. Phototropism responds to light, gravitropism to gravity.
Turgid
The state of a plant cell that has absorbed maximum water by osmosis so that the vacuole is swollen and the cytoplasm presses firmly against the cell wall. The cell wall prevents bursting. Turgid cells provide support for the plant.
Turgor
The state of a plant cell when it has absorbed water by osmosis and the cytoplasm pushes outwards against the cell wall. The cell wall prevents the cell from bursting and the cell is described as turgid. Turgor pressure helps support the plant and keeps stems and leaves rigid.
Type 1 diabetes
A condition in which the pancreas produces little or no insulin because the insulin-producing beta cells have been destroyed by the immune system. Without insulin, blood glucose levels remain dangerously high after eating. It is managed by regular insulin injections and monitoring blood glucose levels.
Type 2 diabetes
A condition in which the body's cells become resistant to insulin, or the pancreas does not produce enough insulin, so blood glucose levels are not properly regulated. It is often linked to obesity, a high-sugar diet and lack of exercise, and is managed primarily through diet control and exercise.
U
Ultrafiltration
The process in the kidney where blood is filtered under high pressure in the glomerulus. Small molecules (water, glucose, urea, amino acids, ions) are forced through the capillary walls and Bowman's capsule into the nephron, while large molecules (proteins) and blood cells are retained in the blood.
Umbilical cord
A flexible tube connecting the fetus to the placenta, containing two umbilical arteries (carrying deoxygenated blood from fetus to placenta) and one umbilical vein (carrying oxygenated blood from placenta to fetus).
Urea
A nitrogen-containing waste substance produced in the liver by the deamination of excess amino acids. Urea is transported in the blood plasma to the kidneys, where it is filtered out and excreted in urine.
V
Vaccination
Giving a weakened or dead pathogen (or its antigens) to stimulate lymphocytes to produce antibodies and memory cells, so a later infection by the real pathogen triggers a faster, stronger immune response.
Vacuole
A fluid-filled space enclosed by a membrane called the tonoplast. In plant cells, a large permanent vacuole contains cell sap (a solution of sugars, salts and pigments) and helps maintain turgor pressure. Animal cells may have small temporary vacuoles.
Variation
Differences between individuals of the same species. Continuous variation (e.g. Height) shows a complete range and is affected by many genes and environment; discontinuous variation (e.g. Blood group) has distinct categories and is usually controlled by genes alone.
Vasoconstriction
The narrowing of arterioles near the skin surface in response to cold. This reduces blood flow close to the skin surface, decreasing heat loss by radiation from the blood to the environment and helping to maintain core body temperature.
Vasodilation
The widening of arterioles near the skin surface in response to heat. This increases blood flow close to the skin surface, increasing heat loss by radiation from the blood to the cooler environment and helping to cool the body down.
Vector
In genetic engineering, a carrier molecule used to transfer a gene from one organism into another. The most common vector is a bacterial plasmid, but viruses can also be used. In disease biology, a vector is an organism (such as a mosquito) that transmits a pathogen from one host to another.
Vein
A blood vessel that carries blood towards the heart, usually deoxygenated (except the pulmonary vein). Veins have thinner walls than arteries, a wider lumen, and contain valves to prevent backflow of blood.
Ventilation
The mechanical process of breathing that moves air in and out of the lungs. Inhalation draws fresh air (rich in oxygen) into the alveoli, and exhalation expels stale air (rich in carbon dioxide). Ventilation maintains concentration gradients for gas exchange.
Vertebrate
An animal that has a backbone (vertebral column). The five classes of vertebrates are fish, amphibians, reptiles, birds and mammals, distinguished by features such as skin covering, reproduction method and thermoregulation.
Villus
A tiny finger-like projection on the inner lining of the small intestine that increases the surface area for absorption of digested food. Each villus has a thin epithelium, a dense network of blood capillaries and a central lacteal.
Vitamin C
A water-soluble vitamin found in citrus fruits and vegetables that is needed for the formation and maintenance of connective tissue, healthy skin and blood vessels. Deficiency of vitamin C causes scurvy.
Vitamin D
A fat-soluble vitamin needed for the absorption of calcium and phosphorus from the gut, essential for strong bones and teeth. Sources include oily fish, eggs and sunlight exposure on skin. Deficiency causes rickets in children.
W
Water potential
A measure of the tendency of water molecules to move from one region to another. Pure water has the highest water potential. Adding solute to water lowers its water potential. Water always moves by osmosis from higher water potential to lower water potential.
White blood cell
A blood cell that is part of the immune system. There are two main types: phagocytes, which engulf and digest pathogens by phagocytosis, and lymphocytes, which produce antibodies. White blood cells have a nucleus and can change shape.
Wilting
The drooping of a plant's leaves and stems that occurs when water loss by transpiration exceeds water absorption by the roots. Cells lose turgor pressure and become flaccid, so the plant can no longer support itself.
Withdrawal symptom
An unpleasant physical or psychological effect experienced when a person who is dependent on a drug suddenly stops taking it. Symptoms can include tremors, sweating, nausea, anxiety and pain, and vary depending on the drug.