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.