An A-Z of keywords and phrases, all of which are relevant to the post-16 biology curriculum.


Saturated fat

Fat molecules that contain no double bonds between their carbon atoms. Saturated fat is found in large amounts in foods like butter and lard. A diet high in saturated fats can increase the risk of cardiovascular disease.

Second messenger

A molecule that is created intracellularly in response to the binding of a first messenger to a cell membrane receptor. The second messenger is responsible for initiating intracellular responses.

Secondary metabolites

Organic compounds produced by an organism that are not directly involved in its growth, development or reproduction.

Secondary structure

The shape that a chain of amino acids takes. It includes ‘elements’ such as helices and beta-pleated sheets, depending on the type of protein.


How an organism’s own cells are described. Proteins called major histocompatibility complex (MHC) proteins are found on the surface of cells to indicate that they are ‘self’ and should be left alone by the immune response.


Refers to a material or membrane’s ability to allow some substances through but prevent the movement of others.


Made in the central nervous system, but mostly found in the gut, this neurotransmitter is involved in controlling body temperature, mood, appetite and sleep. Drugs to increase serotonin levels are used to treat depression.

Sexual reproduction

When an offspring’s genes come from two parents.

Signal transduction cascade

A series of intracellular events that starts with the binding of a first messenger, causing an increase in levels of the second messenger that causes multiple downstream effects. It is called a ‘cascade’ because the binding of one first messenger can create multiple molecules of the second messenger and bring about multiple effects on the cell.

Single nucleotide polymorphism (SNP)

A genetic variation based on a single ‘letter’ difference in the genetic code.

Soluble fibre

Found in foods including fruit, vegetables and oats, this fibre absorbs water, turning into a gel-like substance that slows digestion.


Any substance(s) dissolved in a solvent.

Somatic cells

All cells of the body – including cells of the organs, skin, bones and blood – that are not gametes (sex cells) are somatic cells.

Somatic nervous system

Part of the peripheral nervous system, the somatic nervous system is made up of nerves that allow the central nervous system to control the voluntary movement of skeletal muscles.


Structural adaptions to a cell that allow it to perform a specific function.


A group of living organisms that are able to reproduce and create fertile offspring.

Specific immune system (also adaptive immune system)

In mammals, this provides long-lasting protection against specific foreign substances. Helper T cells stimulate plasma B cells to produce antibodies. Memory B cells maintain a ‘memory’ of previous infections the organism has fought.

Spinal cord

A large bundle of millions of nerve fibres, which carries information back and forth between the brain and the body.


A class of organic chemicals that occur naturally in plants. Plant stanols are known to block the absorption of cholesterol.


An incandescent ball of hydrogen and helium that generates its own light and is held together by its own gravity. The gravity of some stars is so great that it can pull planets into orbit around them (such as the Sun with the Earth).

Stem cell

An undifferentiated cell capable of producing more stem cells, which can give rise to other cell types through differentiation.


A chemical substance that occurs naturally in the body and can also be made. Naturally occurring steroids include the steroid hormones testosterone, oestrogen and aldosterone.


A class of organic chemicals that occur naturally in plants, animals and fungi. The most well-known is the animal sterol cholesterol. Plant sterols are known to block the absorption of cholesterol.


Pores in plant stems and leaves that can be opened and closed to control gas exchange and water loss.


The layer above the troposphere. Ozone in the stratosphere is protective, as it absorbs ultraviolet radiation from the Sun.


A condition where blood flow to an area of the brain is cut off, causing brain cells to die as they are deprived of oxygen and glucose.


A term used to describe the African regions south of the Sahara desert.

Substantia nigra

Located in the brain, the ‘black substance’ contains cells that produce the neurotransmitter dopamine and the pigment melatonin, giving it a black appearance.

Substrate-level phosphorylation

A process that makes ATP by donating a phosphate group from an intermediate directly to ADP, for example during glycolysis.


A disaccharide sugar composed of two monosaccharide sugars: glucose and fructose.


Regions of relatively low temperature on the surface of the Sun. Sunspots rise and fall in cycles lasting about 11 years; their numbers also vary over long timeframes.

Sunspots seem to affect the Earth’s climate – not because less solar energy hits the Earth (sunspots affect less than 0.5 per cent of the Sun’s output), but possibly through their impact on cosmic rays and cloud cover. Sunspot activity is factored into models of climate change, and its effects are thought to be small compared with those of greenhouse gases.


Close interaction between two or more different species, which includes both mutualism and parasitism.


A neurological condition that can be described as a ‘union of the senses’. Synaesthetes might associate particular colours with letters or taste specific flavours when they hear certain words.


The junction between neurons. When a nerve signal travelling along an axon reaches a synapse, it triggers the release of a neurotransmitter that diffuses across the synaptic gap and binds to receptor proteins on the surface of the receiving neuron. This binding causes an influx of ions, changing the membrane voltage and initiating an electrical signal in the second neuron.

Synaptic cleft

The microscopic space in the synapse, into which neurotransmitters are released.

Systematic random sampling

A technique used to select a sample to represent a population. Selection is often done using a sampling frame (a list of all those within a population that can be sampled) and a random number table.