Biology key terms

• Achondroplasia


  Genetic condition caused by a dominant allele. Someone who is heterozygous for this condition has very restricted growth.
• Acinar cell


  A cell from the pancreas. It produces enzymes that are used in digestion.
• Acrosome, Acrosome reaction


  An organelle in the head of a sperm that contains digestive enzymes. When fertilisation takes place, the membrane surrounding these enzymes bursts. The enzymes digest the follicle cells; they also digest the jelly-like layer surrounding the egg cell allowing the sperm to fuse with the egg membrane. This process is known as the acrosome reaction.
• Actin


  A protein found in many cells. Actin plays an important part in muscle contraction. It is also thought to be involved in cell cleavage.
• Activation energy


  Before a chemical reaction can take place, bonds must be broken. This requires energy. This activation energy is normally provided by heating the substances involved in the reaction. Enzymes reduce the amount of activation energy necessary, so reactions in living organisms can take place at relatively low temperatures.
• Active site


  The part of an enzyme molecule into which a substrate molecule fits during a chemical reaction. It is like a pocket on the surface of the enzyme and it has a specific shape. Only a substrate molecule with the complementary shape will be able to fit into this active site.
• Active transport


  A process that involves the movement of substances from where they are in a low concentration to where they are in a higher concentration; in other words, it involves the movement of substances against a concentration gradient. Active transport involves the use of specific carrier proteins in cell membranes. It also requires energy in the form of ATP.
• Adaptation


  Any features of an organism that make it well suited to survive in its environment. Adaptations can be anatomical, physiological or behavioural.
• Adaptive radiation


  The way in which a common ancestor may give rise to different species, each occupying a different ecological niche. For example, it is thought that all molluscs evolved from a worm-like ancestor. There are now many different species which are adapted to a wide range of niches. Present-day molluscs include slow-moving, land-dwelling herbivores such as slugs and snails, as well as rapid swimming carnivores like squid.
• Adenine


  One of the nucleotide bases found in nucleic acids. When the two strands of nucleotides which make up a molecule of DNA come together, adenine always pairs with thymine. The atoms of the two bases are arranged in such a way that two hydrogen bonds form between them.
• Adhesion


  A force resulting from attraction between molecules of different substances. In xylem, water molecules adhere to the molecules of lignin and to other substances which make up the walls of the vessel.
• ADP, Adenosine diphosphate


  Adenosine diphosphate (ADP) is produced when a phosphate group is removed from ATP. Energy can be transferred from this reaction to do useful work in the cell.
• Adrenaline


  A hormone produced by the adrenal glands at times of stress such as when we are angry or frightened. Scientists have shown that there are links between stress, adrenaline and an increased risk of heart disease.
• Aerobic


  Aerobic means requiring the presence of oxygen. Aerobic bacteria are bacteria that can only live and multiply in the presence of oxygen. Aerobic respiration is respiration that requires the presence of oxygen.
• Albinism


  Genetic condition caused by a recessive allele that results in the non-production of pigment.
• Alga, Algal, Algae


  A group of plant-like organisms that include the one-celled organisms that form phytoplankton as well as the seaweeds. Although algae photosynthesise and their cells are surrounded by a cell wall, they are not plants. They belong to a group of organisms called the Protoctista.
• Allele


  One of the different forms of a particular gene. In humans, a gene codes for the protein that transports chloride ions across the cell surface membrane. We all have this gene, the CF gene. The CF gene has two alternative forms or alleles. The normal allele, F, produces a functioning protein. The other allele, f, produces a protein which is very slightly different and does not function. If a person has two copies of the f allele, he or she will have cystic fibrosis.
• Allergen


  A substance that produces an allergic reaction in a sensitive person. Many different substances can act as allergens. One concern over the widespread use of genetically modified foods is that they may make allergens more widespread.
• Alveolus, Alveoli


  One of the tiny air sacs in the lungs where gas exchange takes place. The alveoli are lined by a layer of cells known as squamous epithelium. These cells are very thin and this helps to ensure efficient diffusion of oxygen from the alveoli into the blood, and of carbon dioxide from the blood into the alveoli. The total surface area of all the alveoli in the lungs is very large. This large surface area also helps to ensure efficient diffusion.
• Amino acid


  The basic unit or monomer from which proteins are formed. There are twenty different amino acids which may be linked by condensation to form proteins. All of these amino acids have the same basic chemical structure.
• Amniocentesis


  A way of obtaining cells from a developing fetus. A developing fetus is wrapped in a membrane called the amnion. The space between the amnion and the fetus is filled with a fluid called amniotic fluid. Amniotic fluid contains some fetal cells. These cells can be examined and their chromosomes observed or the DNA that they contain may be investigated.
• Amniotic fluid


  A developing fetus is wrapped in a membrane called the amnion. The space between the amnion and the fetus is filled with a fluid called amniotic fluid. This cushions and supports the delicate tissues of the developing fetus. Amniotic fluid contains some fetal cells. These cells can be examined and their chromosomes observed, or the DNA that they contain may be investigated.
• Amphibian


  A member of the class of animals that includes frogs and toads. Amphibians have moist skin that acts as an additional gas exchange surface. They breed in water and produce aquatic larvae called tadpoles, which change into terrestrial adults.
• Amylase


  An enzyme that digests starch into soluble sugars. This reaction involves the addition of water molecules so it is an example of hydrolysis. Amylases are important digestive enzymes in animals but they are also found in many microorganisms and plant tissues.
• Amylose


  Amylose is a polysaccharide formed from a long straight chain of glucose molecules. This chain winds into a spiral shape. Starch consists of a mixture of amylose and amylopectin.
• Amylopectin


  Amylopectin is a polysaccharide formed of branched chains of glucose molecules. Starch consists of a mixture of amylopectin and amylose.
• Amyloplast


  An organelle found in plant cells which stores starch. Each amyloplast consists of a tiny grain of starch surrounded by a double-layered cell membrane.
• Anabolic


  An anabolic reaction is a chemical reaction in which smaller molecules combine together to produce larger ones. Examples of anabolic reactions include photosynthesis and protein synthesis.
• Anaemia


  A condition in which there is a reduced amount of haemoglobin, the oxygen-carrying pigment found in red blood cells. People suffering from anaemia tire easily and soon get out of breath if they exert themselves. There are several causes of anaemia. One of them involves a shortage of iron in the diet.
• Anaerobic, Anaerobically


  Anaerobic means the absence of oxygen. Anaerobic bacteria are bacteria that only live and multiply in the absence of oxygen. Anaerobic respiration is respiration that takes place in the absence of oxygen.
• Anaphase


  A stage in cell division in which chromosomes are pulled apart by the spindle fibres. One chromosome from each pair goes to each of the 'poles' of the cell.
• Aneurysm


  A weakening of the wall of an artery, which results in a balloon-like swelling. The condition is obviously very serious but in some cases it is possible to repair the damaged artery surgically.
• Angina


  Pain in the chest that occurs when the heart muscle does not receive enough oxygen-rich blood. During exercise heart muscle requires more oxygen because its rate of respiration is higher. If the coronary arteries cannot supply enough oxygenated blood, the heart muscle respires anaerobically. Lactic acid is produced and this causes the pain of angina.
• Angiosperm


  An angiosperm is another name for a flowering plant. The angiosperms include a variety of plants such as daffodils, oak trees and grasses like wheat and rice. Most economically important plants are angiosperms. Angiosperms have reproductive organs, which are found in flowers. Angiosperms produce seeds, which are formed as a result of fertilisation. These seeds are enclosed within fruits.
• Animalia


  The kingdom containing animals. Animals share the following characteristics. 1) They are multicellular eukaryotic organisms. They have different sorts of cells which are specialised to form different organs. 2) Their cells do not have cell walls or large vacuoles. 3) Their nutrition is heterotrophic and they are unable to photosynthesise. 4) Most animals can move from one place to another and these movements are coordinated by a nervous system.
• Antenna, Antennae


  A pair of long, jointed structures found on the head of an animal such as an insect, a woodlouse or a lobster. Antennae have a sensory function and have sense organs that are sensitive to touch or enable the animal to detect the presence of particular substances.
• Anterior


  The front or head end of an animal.
• Antibiotic


  A term originally used to describe a substance produced by one type of microorganism that kills or stops the growth of another. Substances which have an antibiotic effect have now been found in a number of other organisms ranging from toads to snowdrops. Antibiotics are often modified chemically to make them more effective. These substances are not only important in medicine but are also used in helping to identify genetically modified organisms with a marker gene.
• Antibody, Antibodies


  There are several different types of white blood cell. One of these is the lymphocyte. An antibody is a molecule produced by a lymphocyte when it encounters a particular antigen. For example, the virus that causes measles has antigens on its surface. If a person gets measles, these antigens cause the lymphocytes to produce measles antibodies. The antibodies help the person to overcome the infection and recover.
• Anticodon


  A sequence of three nucleotide bases on a transfer RNA molecule, which is complementary to the corresponding messenger RNA codon. For example, CCA is one of the messenger RNA codons for the amino acid proline. The sequence of nucleotide bases from the corresponding anticodon is GGU.