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Biology Dictionary for GCSE Biology and A Level Biology


Capillary: Narrow blood vessel with single celled endothelium (wall) to allow exchange of materials. Walls are permeable, which allows the formation of tissue fluid.


Cardiovascular Centre: centre in the medulla (lower part of brain) which can modify the rate of heartbeat. 


Carotid Body: patch of chemoreceptors located in wall of carotid artery. Senses change in blood CO2 and H+ levels. Sends sensory impulses to the medulla. See also; Aortic body. 


Casparian Strip: In plant roots, a waxy waterproof strip, made from suberin, which surrounds endodermal cells. Its function is to block the apoplast pathway, so that all water and ions enter by the symplast route and so the plant has some control over the water/ions entering the xylem. 


Centriole: Bundles of protein fibres, found near the nucleus. Centrioles migrate to opposite poles of the cell during mitosis and form the spindle.


Centromere: The point of attachment of two chromatids on a double chromosome.


Chemoreceptor: cells sensitive to chemical change. e.g. in the aortic and carotid bodies there are chemoreceptors which are sensitive to CO2/pH levels. 


Chiasmata: The point at which crossing over occurs. i.e. sites where homologous chromosomes break / rejoin.


Chromatid: One half of a double chromosome (present at the start of cell division)


Chromatin: ‘Spread out’ DNA in the nucleus of a non-dividing cell i.e. not coiled up into chromosomes


Chromosome: Condensed mass of DNA which appears just before cell division. Each chromosome is one super-coiled DNA molecule containing 1000s of genes.


Clone: A identical genetic copy. You can clone DNA strands, cells or whole organisms


Codon: A sequence of three bases coding for a particular amino acid e.g GUG = valine.


Cohesion Tension: The force that draws water up the xylem of a plant.  Water evaporating from the upper surfaces of a leaf creates a negative pressure in the xylem which draws water up through the plant from roots to leaves. Water is cohesive (sticky) so it can form long, continuous unbroken columns all the way up a plant.


Companion cells: a cell in the phloem that is connected to a sieve-tube member by numerous plasmodesmata (strands of cytoplasm). They control the activity of the sieve tubes.

Community: The living (Biotic) part of an ecosystem. i.e. All the living organisms of all the different species living in one habitat.


Continuous variation: Variation which can be any value within a range. e.g. height, shoe size. Tends to be controlled by many alleles ( polygenic) which combine to give phenotype.


Coronary Arteries: vital arteries supplying blood to the heart muscle itself. A narrowing of these arteries leads to coronary heart disease. 


Cortex: root tissue between the vascular bundle (phloem and xylem) and epidermis. In plants, it’s a general word for ‘packing tissue’


Crop rotation: Planting different crops in one area in successive years.


Crossover [crossing over]: In meiosis, the process which swaps block of genes between homologous chromosomes, thereby increasing variation.


Cystic fibrosis: Genetic disease due to faulty allele. Sticky mucus blocks lungs and pancreas.

Diastole: Relaxation of heart muscles.

As I write the biology dictionary and make the videos to help you learn the definitions by listening in addition to reading them, you'll get the added benefit of hearing and learning the correct pronunciation of biological terminology - but there's more! I'm also making some interactive revision flashcards too! So you can revisit the key words when revising and really embed those a level biology definitions in your head! - The flashcards will be like this (See image below and click to download).

A-Level Biology is a whole lot easier when you know the definitions of commonly used words. This biology dictionary is here to help you learn those A level biology definitions. Remember, a good command of biological terminology (at GCSE but especially at A level) can mean the difference between a poor grade and an excellent one! Since of course you get marks awarded for using the correct biological definitions in the right context when answering exam questions and writing up lab work, coursework and assignments.

The biology dictionary is mainly aimed at a level biology students since at a level, the amount of new technical language expect is quite a lot!


Make learning biology definitions a regular part of your studies and keep checking in as more keywords and gcse biology definitions and a level biology definitions are added.

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Helping you to Learn, Revise, Practice and Succeed in your GCSE and A-Level Biology.

*Note this A-Level biology Dictionary / Glossary of key terms is not comprehensive and will be updated and added to regularly, so keep checking in and make learning and revising a level biology definitions a consistent part of your biology studies

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