Biology A-Level Dictionary

Action potential A rapid change in the electrical potential across a plasma membrane that occurs when the cell has been activated by a stimulus. It is caused by a selective closing and opening of voltage-sensitive ion channels.
Adenosine triphosphate (ATP) The “energy currency” of the cell. The energy in ATP is consumed by various metabolic processes when ATP is hydrolyzed into ADP.
Amino acid One of 20 organic molecules serving as the monomers of proteins. The various amino acids differ only in their R group (also called the “side chain”).
Archaea Archaea is one of the two prokaryote domains, the other being Bacteria.
Autotroph An organism that can produce all of the organic molecules it requires from inorganic substrates such as carbon dioxide and water. Includes plants and photosynthetic bacteria.
B lymphocytes A type of lymphocyte that differentiates in the bone marrow in mammals. After contact with an antigen, B cells differentiate into antibody-secreting plasma cells.
Binomial A taxonomic name, composed of two separate words, usually of Greek or Latin origin. Used to indicate that the organism in question is treated as a species.
Biome A major type of ecological community (such as desert, savanna, or taiga).
Calvin cycle (also the dark reaction or the light-independent reaction) A series of biochemical reactions taking place in the stroma of the chloroplasts, resulting in carbon fixation. Requires ATP and NADPH produced in the light reactions.
Cholesterol The most important steroid in animals. It is a product of the liver and is essential to the functionality of the phospholipid bilayers of animal cell membranes.
Complementary sequence A single-stranded nucleotide sequence that can form base pairs with another single-stranded sequence. For example, the complementary sequence to CATG is GTAC.
Dihybrid cross A mating in which the parents differ with respect to two genetically inheritable traits.
Diploid A (somatic) cell that has two complete sets of chromosomes.
DNA replication The creation of a double-stranded DNA molecule from a single-stranded template by DNA polymerase. DNA replication is semi-conservative.
Ecological niche <color=”#000000″> The set of conditions, with respect to both the environment and associations with other organisms, to which a particular type of organism is suited.</color=”#000000″>
Endocrine glands Glands secreting into the bloodstream or lymphatic system (and not onto an epithelial surface, such as skin or into the digestive system).
Epistasis The masking of one gene by another. For example, a gene causing baldness would mask the effect of a hair-colour gene.
Eukaryote Organisms, both unicellular and multicellular, in which there is a membrane-bounded cell nucleus and other membrane-bounded organelles.
Gene therapy A treatment that attempts to replace, manipulate, or supplement misfunctioning genes with properly functioning ones.
Gene transfer The incorporation of foreign DNA into the cells of an organism, usually via a vector (such as a plasmid or a genetically engineered virus).
Glycolysis The production of pyruvate by the splitting of glucose. It is the single metabolic pathway occurring in all types of living cells. Pyruvate can be converted into acetyl-CoA, the main input for the Krebs cycle, or it can be metabolized into ethanol or lactic acid in anaerobic respiration (fermentation).
Glycoprotein A protein that has sugars attached to its side chains. Found in the cell membrane, they play important roles in cell-to-cell communication and recognition.
Haploid Having only a single complete set of chromosomes. Gametes are haploid.
Heterotroph An organism that obtains organic molecules by eating other organisms or substances produced by them.
Histocompatibility Having the same, or mostly the same, alleles of a set of genes called the major histocompatibility complex. These genes are expressed in most tissues as antigens, to which the immune system makes antibodies.
Imprinting Behaviour determined by exposure to stimuli at an early stage of development (for example, a duckling will follow the first living object it perceives to be its mother).
Independent assortment The random distribution into gametes of genes on different chromosomes during meiosis.
Karyotype The set of chromosomes present in a particular type of organism, arranged in terms of their characteristic size and structure.
Kreb’s cycle A series of reactions inside the matrix of mitochondria that consumes pyruvate, generating NADH and FADH2 for the electron transport chain.
Lichens Organisms composed of a fungus and an alga in a symbiotic relationship. Lichens can grow on bare rock, and slowly decomposes rocks to release nutrients essential for the growth of other living things.
Linkage The increased tendency of two alleles to segregate together because their loci are on the same chromosome.
Messenger RNA (mRNA) RNA that serves as a template for protein synthesis.
Mutualism A form of symbiosis in which both participants benefit. For example, the clown fish lives inside a sea anemone and is protected by it. In return, it lures larger fish into the anemone’s tentacles.
Nicotinamide adenine dinucleotide (NAD) A coenzyme electron carrier involved in respiration that occurs in two states, an oxidized form (NAD⁺) and a reduced one (NADH).
Nicotinamide adenine dinucleotide phosphate (NADP) A coenzyme electron carrier involved in photosynthesis that occurs in two states, an oxidized form (NADP⁺) and a reduced one (NADPH).
Oxidative phosphorylation A metabolic pathway that uses energy released by the oxidation of nutrients to produce adenosine triphosphate (ATP). Takes place in the mitochondria in humans.
Parasitism A form of symbiosis in which one participant benefits at the expense of the other.
Parthenogenesis The development of the embryo from an unfertilized egg.
Phosphorylation Adding a phosphate group to an organic molecule, usually using a phosphate group from ATP.
Phototropism A tropism (growth or movement) in response to light.
Phylogeny A hypothetical reconstruction of the evolutionary relationships of a group of organisms usually presented in the form of a tree.
Phytochrome A photoreceptor (a pigment) that plants use to detect light.
Plasmids Independently replicating extra-chromosomal circular DNA molecules, distinct from the normal bacterial chromosome and nonessential for cell survival under nonselective conditions. Some plasmids can integrate into the host chromosome. Artificial plasmids are often used as cloning vectors.
Ribonucleic acid (RNA) A class of single-stranded polynucleotides containing ribose sugar and the pyrimidine uracil, that replaces thymine.
Ribosomal RNA (rRNA) The most abundant type of RNA. With ribosomal proteins, rRNAs form the structure of ribosomes.
Signal transduction Any process in which a cell changes one type of stimulus into another. Most signal transduction pathways are mediated by enzymes and second messengers.
Single nucleotide polymorphism (SNP) DNA sequence variations that occur when a single nucleotide in the genome sequence is altered.
Sodium-potassium pump A pump in the cell membrane responsible for maintaining relatively high concentrations of potassium ions but low concentrations of sodium ions inside the cell. Requires ATP.
Taxonomy The classification of living things.
T lymphocyte A type of lymphocyte that differentiates in the thymus that is responsible for cell-mediated immunity.
Transcription The synthesis of a complementary RNA molecule through the use of a DNA template. Requires RNA polymerase.
Translation Synthesis of a protein on an mRNA template. Catalyzed by ribosomes.
Transfer RNA (tRNA) An RNA molecule that adds specific amino acids during translation.
Triglycerides The typical form of fat storage in cells. A triacylglyceride is formed by the esterification of one glycerol molecule and three fatty acids.
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