BGCSE Human & Social Biology key terms

The net movement of particles from a region where they are more concentrated to a region where they are less concentrated, caused by their random motion. It needs no energy input from the cell and stops once concentrations are equal.

The movement of water molecules through a partially permeable membrane from a dilute solution to a more concentrated solution. It is a special case of diffusion and is important in water uptake by roots and in keeping cells firm.

The movement of dissolved substances across a cell membrane against their concentration gradient, using energy released by respiration. Carrier proteins in the membrane pick up the substance and release it on the other side.

A protein that speeds up a chemical reaction in living things without being used up itself. Each enzyme has an active site with a shape that fits only particular substrate molecules, so it works best at a specific temperature and pH.

The firm, swollen state of a plant cell when water has entered its vacuole and pressed the cytoplasm against the cell wall. Turgor keeps soft plant parts upright; loss of it causes wilting.

The process in which green plants use light energy trapped by chlorophyll to combine carbon dioxide and water into glucose, releasing oxygen as a by-product. It stores energy in chemical form and is the starting point of most food chains.

The green pigment found in chloroplasts that absorbs light energy for photosynthesis. It absorbs mainly red and blue light and reflects green light, which is why most leaves look green.

The condition that is in shortest supply and therefore holds back the rate of a process such as photosynthesis or growth. Increasing light, carbon dioxide or temperature can raise the rate only until another factor becomes limiting.

A diet containing all the nutrients the body needs in the correct proportions: carbohydrates, fats, proteins, vitamins, minerals, water and fibre. It supplies enough energy without excess and helps prevent deficiency diseases and obesity.

An organic nutrient needed in very small amounts for normal health and growth, because the body cannot make enough of it. Lack of a particular vitamin causes a specific deficiency disease, such as scurvy when vitamin C is missing.

A green fluid made in the liver, stored in the gall bladder and released into the small intestine. It breaks large fat drops into smaller ones, giving a larger surface area for enzymes, and it neutralises acidic food from the stomach.

One of many tiny finger-like projections covering the inner wall of the small intestine. Villi greatly increase the surface area for absorbing digested food, and each contains blood capillaries and a lacteal.

Wave-like muscular contractions of the gut wall that push food along the alimentary canal. The same action moves food down the oesophagus and moves faeces through the large intestine.

The chemical breakdown of food molecules, usually glucose, inside cells to release energy for life processes. Aerobic respiration uses oxygen and gives carbon dioxide and water; anaerobic respiration releases less energy and forms lactic acid or ethanol.

One of millions of tiny air sacs at the ends of the bronchioles in the lungs. Their thin, moist, richly supplied walls give a very large surface area for gas exchange by diffusion.

Release of energy from glucose without oxygen. In human muscle it produces lactic acid and causes oxygen debt; in yeast it produces ethanol and carbon dioxide, a reaction used in brewing and bread making.

The pale yellow liquid part of blood, mostly water, which carries dissolved substances. It transports glucose, amino acids, hormones, urea, carbon dioxide and heat, and also carries red and white cells and platelets.

The red iron-containing pigment in red blood cells that combines with oxygen in the lungs and releases it in tissues. It gives blood its colour and allows blood to carry far more oxygen than plasma alone could.

A small cell fragment in the blood that helps it to clot at a wound. Platelets release substances that start a chain of reactions converting fibrinogen into fibrin threads, which trap blood cells and seal the break.

A blood vessel that carries blood away from the heart. Its wall is thick, muscular and elastic to withstand high pressure, and its narrow lumen keeps blood flowing quickly; most arteries carry oxygenated blood.

A very narrow blood vessel with walls only one cell thick, linking arteries to veins. Its thin walls and huge total surface area allow rapid exchange of gases, food and waste between blood and body cells.

The microscopic filtering unit of the kidney; each kidney contains thousands. Fluid is filtered from the blood into a tubule, useful substances are reabsorbed, and the remaining urine passes to the bladder.

A nitrogen-containing waste made in the liver from excess amino acids and carried in the blood to the kidneys. It is removed from the blood by the nephrons and leaves the body dissolved in urine.

The control systems that keep the internal conditions of the body steady, such as temperature, blood glucose and water content. Receptors detect changes and effectors such as muscles or glands bring about correcting responses.

A hormone made by the pancreas that lowers blood glucose. It causes liver and muscle cells to take up glucose and store it as glycogen, and its failure or shortage leads to diabetes.

A rapid, automatic response to a stimulus that does not involve conscious thought, protecting the body from harm. The message passes from receptor along a sensory neurone, through a relay neurone in the spinal cord, to a motor neurone.

The release of a mature egg from an ovary, usually about once every 28 days in the human menstrual cycle. It is triggered by hormones from the pituitary gland and the egg then travels along the oviduct.

The organ formed in the uterus during pregnancy through which the embryo obtains food and oxygen from the mother's blood and passes out waste. It also produces hormones that maintain the pregnancy.

One of the alternative forms of a gene that can occupy the same position on a chromosome. For example, the gene for eye colour may have a brown allele and a blue allele, which may be dominant or recessive.

The combination of alleles an organism carries for a particular characteristic, written as letters such as AA, Aa or aa. The visible result of this combination is called the phenotype.

A random change in the base sequence of DNA or in the structure of a chromosome. Most mutations have no effect or are harmful, but occasionally one gives an advantage and can be passed on to offspring.

The process by which individuals best suited to their environment are more likely to survive and breed, so their useful features become more common in later generations. Over long periods it can produce new species.

A protein made by white blood cells in response to a specific antigen on a germ or toxin. It locks onto that antigen and helps destroy the invader; memory cells make the response faster on a second infection.

A type of white blood cell that engulfs and digests bacteria and other foreign particles. It is part of the body's general defence and acts quickly, before specific antibodies have been produced.

Introducing a weakened or harmless form of a pathogen, or its antigens, into the body so that white blood cells make antibodies and memory cells. The person then resists the real disease without suffering its symptoms.

A sequence showing how energy passes from one organism to another by feeding, beginning with a producer and ending with a top consumer. Arrows point in the direction in which energy flows.

An organism such as a bacterium or fungus that breaks down dead plants, animals and waste, releasing minerals back into the soil. Decomposers are essential for recycling nutrients through an ecosystem.

The over-enrichment of water with nitrate or phosphate, often from fertiliser run-off, which causes rapid growth of algae. When these die and decay, oxygen is used up and fish and other animals may suffocate.

The trapping of heat in the atmosphere by gases such as carbon dioxide and methane, which absorb outgoing radiation. It keeps Earth warm enough for life, but extra gases from burning fuels are causing global warming.