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Biology Dictionary for GCSE Biology and A Level Biology




SAN (Sino-Atrial Node): Cells found in wall of right atrium that generate impulses for atrial contraction. The SAN is the pacemaker, and can speed up or slow the heart according to motor impulses from the cardiovascular centre.


Secondary Cell Wall: innermost layer of a cell wall deposited after cell enlargement has ceased, contains lignin. Important in maintaining cell shape


Semi-conservative: The mechanism of DNA replication. In each new strand, half is original (has been conserved) and half is new. This was shown by Meselsohn and Stahl


Semi-lunar Valve: Found in veins, and between ventricles and aorta/pulmonary artery to prevent backflow. 


Sense strand: The side of the DNA molecule (in a gene) which codes for making the protein


Sex chromosome: Chromosomes that decide the sex of an individual. Humans have one pair of sex chromosomes. XX = female, XY  = male


Sickle cell anaemia: Genetic disease caused by a recessive allele. Haemoglobin polymerises, distorting the red cells into sickle shapes, causing clots and other problems


Sieve element: a conducting cell in the phloem. Has no nucleus but does have some organelles around the outside.


Sieve plate: the end wall of a sieve-tube element that is perforated by sieve plate pores.

Species: A group of organisms that can successfully interbreed to produce fertile offspring.


Speciation: How new species develop. Usually has three basic stages; 1 Part of a population becomes isolated; it cannot interbreed with the rest. 2 Natural selection acts differently on the two sub-populations 3 In time, genetic differences accumulate, -----> different species


Specific name: Name of the species; second part of Latin name. No capital; e.g. sapiens


Spindle: Cradle of protein fibres which organise/move chromosomes during cell division


Sticky ends: Staggered cuts in DNA resulting from cutting by restriction enzymes


Stoma (Stomata - plural): Pores (holes) in the lower epidermis of a plant leaf, surrounded by guard cells, allowing for gaseous exchange and transpiration. Generally, stomata are open in the day to allow gas exchange (the rapidly photosynthesising palisade cells need CO2) and closed at night to reduce water loss. 


Stretch Receptors: Nerve cells which are sensitive to tension changes. Found in the lungs where they provide feedback about the degree of inflation of the lungs, so we know when to stop breathing in and start breathing out. 


Symplast Pathway: Water and ions transport route through the plant cell cytoplasm. This is a route through living tissue with membranes, so the plant has some control over what passes into the xylem. See apoplast pathway. 


Systole: Contraction of the 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.

Thanks for using
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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