A2 Biology Glossary

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|α cells |Cells in the islets of Langerhans that release glucagon in response to low blood glucose levels. | |Acetylcholine |A neurotransmitter (transmitter substance) found in cholinergic synapses. | |Acetylcholinesterase |An enzyme in the synaptic cleft that breaks down the transmitter substance acetylcholine. | |Actin |A protein found in muscle cells. It is the main component of the thin filaments. | |Action potential |A brief reversal of the resting potential across the cell surface membrane of a neurone. All action | | |potentials have a value of +40 mV. | |Adenyl cyclase |The enzyme found inside cells, associated with hormone receptors, that converts ATP to cAMP. | |Afferent |Incoming or leading towards. | |Allele |An alternative version of a gene. | |All or nothing |Refers to the fact that a neurone either conducts an action potential or it does not. | |Allotransplantation |Transplantation of organs between individuals of the same species, for example transplantation of a | | |human heart into another human. | |Amplification (DNA) |The making of multiple copies of the same short section of DNA. The process of PCR is used in automatic| | |amplification of DNA sections. | |Anabolic steroids |Drugs that mimic the action of steroid hormones and increase muscle growth. | |Anabolism |Type of metabolism: biochemical reactions that synthesise large molecules from smaller molecules. This | | |requires energy/ATP. | |Antagonistic |Working against each other in a pair. | |Annealing |The term used to describe hydrogen-bond formation between complementary base pairs when sections of | | |single-stranded DNA or RNA join together. Annealing is seen when complementary sticky ends join and | | |where DNA probes attach to a complementary DNA section. |

|Antidiuretic hormone (ADH) |The hormone made in the hypothalamus and released from the pituitary gland that acts on the collecting | | |ducts in the kidneys to increase the reabsorption of water into the blood. | |Apical dominance |The growing apical bud at the tip of the shoot inhibits growth of lateral buds further down the shoot. | |Apoptosis |Programmed cell death. An orderly process by which cells die after they have undergone the maximum | | |number of divisions. | |Ascending limb |The limb of the loop of Henle that carries fluid...
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