Biology year 2 A-D

• Abiotic

  An ecological factor that is part of the non-living or physical environment of an organism. Abiotic factors include climatic features (such as rainfall and temperature), solar energy input and edaphic factors. Edaphic factors relate to the soil.
• Acetyl coenzyme A, Acetyl CoA

  An intermediate product formed during respiration. Acetyl coenzyme A is produced by the link reaction when coenzyme A combines with a two-carbon fragment formed from pyruvate. Acetyl coenzyme A carries this fragment into the Krebs cycle.
• Acetylcholine

  A neurotransmitter involved in the transmission of impulses across synapses. When an impulse arrives at the presynaptic membrane, acetylcholine is released into the synaptic cleft. It diffuses across the cleft and attaches to receptors on the postsynaptic membrane, triggering an impulse in the postsynaptic neurone.
• Acetylcholinesterase

  An enzyme found in synapses that have acetylcholine as a neurotransmitter. Acetylcholinesterase breaks down acetylcholine once it has triggered an impulse in the postsynaptic neurone. This is important as unless acetylcholine is removed rapidly from a synapse there would be continuous transmission along the postsynaptic neurone.
• Achondroplasia


  A 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 that produces enzymes 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 and 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. In a muscle fibre, actin forms the thin filaments that slide between the thicker filaments of another protein, myosin, bringing about muscle contraction. It is also thought to be involved in cell cleavage.
• Action potential

  The changes which take place across the cell surface membrane of a nerve cell during the passage of an impulse. These changes are brought about by the opening and closing of voltage-dependent Na+ and K+ gated channels in the membrane.
• Activation energy

  The energy required to break bonds before a chemical reaction can take place. Enzymes reduce the amount of activation energy necessary, so reactions in living organisms can take place at relatively low temperatures.
• Active artificial immunity

  Immunity that develops following immunisation. Antigens in the vaccine trigger a specific immune response by the body's immune system.
• Active natural immunity

  Immunity that develops following an infection. The specific immune response to the foreign antigens helps to destroy the pathogens and produces a supply of antibodies and B memory and T memory cells that will respond quickly if re-infected with the same pathogen.
• Active site

  The part of an enzyme molecule into which a substrate molecule fits during a chemical reaction. It has a specific shape that only allows a substrate molecule with the complementary shape to fit.
• Active transport

  The movement of substances from where they are in a low concentration to where they are in a higher concentration, against a concentration gradient. It involves the use of specific carrier proteins in cell membranes and 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.
• Adenine

  One of the nucleotide bases found in nucleic acids. Adenine always pairs with thymine in DNA. It is also a constituent of ATP and ADP.
• Adhesion

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

  A substance found in all living cells where it is involved in the transfer of energy. ADP is produced when a phosphate group is removed from ATP, releasing energy.
• Adrenaline

  A hormone produced by the adrenal glands at times of stress. It has a range of effects on the body, including increasing the stroke volume and rate of the heart, dilating blood vessels supplying muscles, and stimulating the conversion of glycogen to glucose.
• Aerobic, Aerobe


  Requiring the presence of oxygen. Aerobic bacteria can only live and multiply in the presence of oxygen. Aerobic respiration requires oxygen.
• Aerobic capacity


  The ability to take in, transport and use oxygen. It determines our ability to undertake long periods of vigorous exercise.
• Agonist, Agonists


  A substance which mimics the action of a neurotransmitter at a synapse. Agonists have a similar shape to the neurotransmitter and can continue to stimulate the postsynaptic nerve cell.
• AIDS, Acquired Immune Deficiency Syndrome


  A collection of disease symptoms relating to the same cause, damage to the immune system caused by HIV.
• Albinism


  A 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.
• Algal bloom


  The rapid growth of a population of the microscopic freshwater or marine algae that make up the phytoplankton. Algal blooms often result from an increase in the concentration of nitrates or phosphates associated with pollution.
• Allele, Allele frequency


  One of the different forms of a particular gene. The relative frequency of alleles in a population of organisms changes over time with evolution.
• Allergen


  A substance that produces an allergic reaction in a sensitive person.
• Allergic reaction


  A reaction involving a particular substance, called an allergen, to which a person is sensitive. It results in the production of large amounts of histamine, which can have localised or widespread effects.
• Allopatric speciation


  The formation of a new species due to reduced gene flow between two populations as a result of geographical isolation.
• All-or-nothing


  An expression used to describe an action potential. When an action potential is produced in a nerve cell, it is always the same size, regardless of the strength of the initial stimulus.
• Alveolus, Alveoli


  The tiny air sacs in the lungs where gas exchange takes place. They are lined by a layer of very thin squamous epithelial cells, which helps 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.
• Amniocentesis


  A way of obtaining cells from a developing fetus by extracting amniotic fluid, which contains some fetal cells.
• Amniotic fluid


  The fluid that cushions and supports the delicate tissues of the developing fetus, contained within the amnion membrane.
• 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, and breed in water producing aquatic larvae.
• Amylase


  An enzyme that digests starch into soluble sugars, through a process of hydrolysis. Amylases are important digestive enzymes in animals, microorganisms and plants.
• Amylopectin


  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, consisting of a tiny grain of starch surrounded by a double-layered cell membrane.