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



A family of cell adhesion molecules that are involved in cell–cell anchoring.


The build-up of calcium salts in body tissue. It is a normal part of bone formation, but it can also occur in the blood vessels, causing cardiovascular disease.

Calorie (kcal)

A unit (technically a kilocalorie) used to measure energy – particularly in food. One kcal is the amount of heat required to raise the temperature of one kilogram of water by one degree Celsius, and is equal to 4.184 kilojoules (kJ), or 4,184 joules.

cAMP (cyclic adenosine monophosphate)

A common second messenger, essential in many signal transduction cascades.


Compounds that are chemically similar to cannabis – including endocannabinoids, which are made in the human body.

Carbon cycle

A set of interlinking biological and geological processes in which carbon moves through the Earth’s atmosphere, oceans and rocks as well as living things.

Carbon fixation

The first stage of the Calvin cycle. The enzyme RuBisCO catalyses a reaction between ribulose bisphosphate (RuBP) and carbon dioxide, producing glycerate phosphate (GP), thus fixing inorganic carbon into an organic compound.


A substance or chemical that can cause cancer in living tissue.

Cardiovascular disease

A general term that refers to conditions involving constricted or blocked blood vessels. Examples include various forms of heart disease as well as stroke.


Pigments found in the chloroplasts of plants (and some other photosynthetic organisms). They help chlorophyll absorb light energy and protect it from overexposure to light.

Carrying capacity

The maximum population that the environment and all of its resources can support.

Case-control studies

Studies that use people who already have a disease or other condition to see whether there are characteristics of these individuals that differ from matched partners who don’t have the disease.


The process by which large molecules are broken down into smaller molecules. Catabolism is the opposite of anabolism and part of metabolism. Catabolic reactions release energy, some of which is used to synthesise ATP. Glycolysis – the stepwise conversion of glucose to pyruvate – is an example of a catabolic reaction.


A substance that increases the rate of a chemical reaction without itself undergoing any permanent change.


Relating to the posterior part of the body.

Cell body (soma)

The part of the neuron that contains many components typically found in other types of cell. This includes DNA (in the nucleus), which holds instructions for producing the proteins that determine the shape and function of the cell.

Cell membrane

A film of fatty molecules (phospholipids) that controls substances entering and leaving the cell.

Cell wall

A structural layer in the cells of bacteria, fungi and plants outside of the cell membrane. In plants, the cell wall is mostly made of cellulose.

Cell-based immune response

The part of the specific (adaptive) immune response that involves B cells and antibodies.

Central nervous system (CNS)

Made up of the brain and spinal cord, the CNS sends and receives information to and from the body (via the peripheral nervous system), controlling its actions.


A pair of organelles that organise microtubules into spindles on which chromosomes are separated when cells divide.


The ‘little brain’ that controls balance and coordinates movements. It’s normally required for learning motor skills, such as riding a bike, and is involved in thought processes.

Cerebral hemispheres

The two halves of the brain, each of which controls and receives information from the opposite side of the body.

Chaperone proteins

Proteins found in the cytoplasm of eukaryotic and prokaryotic cells that bind to newly created polypeptide (protein) chains and ensure they fold correctly.


The process used by bacteria and some other organisms to form nutrients from carbon dioxide or methane, using energy produced by reactions involving inorganic chemicals such as sulfur.


A polysaccharide found in the cell walls of fungi and the exoskeletons of insects and crustaceans.


A family of green pigments found in and around the photosystems of chloroplasts.


The pigment-rich organelles where photosynthesis takes place in plant cells.


A type of lipid that is made by animals cells and is needed for maintaining the stability of cell membranes, for the insulation of nerve fibres and for the production of steroid hormones, including testosterone and oestrogen. Most cholesterol is made in the body (including in the liver), though some we get from our diet. Cholesterol is transported in the blood in complexes called lipoproteins.


A large package of DNA found in cells. It contains a set of genes and other DNA elements. Humans have 46 (23 pairs).


Small hair-like projections on the surface of cells. In animal cells, they are used to move small particles; in some prokaryotes, they are used for movement.


A molecular arrangement in which two particular atoms or groups are found on the same side of a double bond. For example, in a particular unsaturated fatty acid, the hydrogens can be on the same side of the carbon–carbon double bond (cis) or on opposite sides (trans).


The weather conditions that generally prevail in an area over a long period of time. These conditions include, but are not limited to, temperature, rainfall (precipitation) and humidity. Climate is a measure of patterns, and can be thought of as the average weather in one given location over 30 years or longer.

Climate change

A change in global or regional climate patterns beyond that of climate variability, especially change caused by increased levels of carbon dioxide in the atmosphere as a result of burning fossil fuels.

Climate variability

Changes within the normal range of weather defined by a climate – for example, an unusually cold winter, a wetter than average spring, a summer drought.

Clinical trial

A study of the effects of a drug in people.


A group of three bases in DNA or mRNA that codes for a specific amino acid. RNA codons are ‘read’ during protein synthesis.


Small non-protein organic molecules that bind to, and are required for the activity of, their associated enzyme. An example is coenzyme A (CoA), which combines with a two-carbon compound during the link reaction to form acetyl CoA.

Cohort study

In analytical epidemiology, a study in which a population exposed to a presumed cause of a disease is followed over a set period of time to monitor the appearance of the disease. A control group is also monitored to identify the cause and effect of infection. It is also known as a prospective study.


A protein that makes up connective tissues such as tendon, bone and cartilage.

Colony (or cell culture)

A group of identical cells, usually derived from a single parent cell, grown on a solid medium.

Combinatorial chemistry

Creates new compounds out of a limited number of ‘parts’ by putting together the parts in different ways. This makes vast compound libraries, which can then be tested as drug candidates.


The process of burning a substance in the presence of oxygen, which produces energy; in a biological context, this could refer to the burning of fossil fuels, which produces carbon dioxide.


A type of interaction between species (symbiosis) in which one species benefits without affecting the other.


Two or more populations living together and sharing a habitat.

Competitive exclusion principle

A principle that states that it is impossible for two species in the same ecosystem to coexist if they are competing for the same resources.


A set of around 30 proteins in the blood plasma that can be activated by the presence of microbes or antibody–antigen complexes. Complement can destroy pathogens and activate phagocytic cells.


Matching. Complementary bases are those that pair up in DNA and RNA, such as cytosine (C) with guanine (G).

Concentration gradient

The gradual difference in concentration of a substance. This is especially important for dictating diffusion – molecules will naturally diffuse down their concentration gradient.

Conformational change

An alteration in shape that is a result of binding a substrate molecule.


Something present from birth (such as a disease).

Control experiment

An experiment performed alongside a main experiment that is virtually identical, except that the control group does not receive the same treatment – or independent variable – as the experimental group. Performing a control experiment helps to increase the reliability of your experimental results.


The thin, folded structure on the outside of the brain.

Cranial nerve nuclei

Clusters of neurons in the brainstem. Their axons form the cranial nerves.


A waxy layer that sits on top of the epidermis of the leaves and stems of plants. It is a waterproof layer that helps limit water loss from transpiration and helps protect the plant from infection.

Cytochrome b6f

An enzyme in the thylakoid membrane that acts as an electron carrier.

Cytochrome c

A component of the electron transport chain in cellular respiration, in which it carries one electron. It differs from other cytochromes in that it is highly water-soluble.

Cytochrome oxidase

The terminal component of the electron transport chain in cellular respiration, which oxidises cytochrome c. This is where oxygen is actually consumed, by acting as the final electron acceptor and being converted to water.

Cytochrome P450

A group of enzymes that are found in the liver. They are responsible for processing drugs.


Special proteins that are responsible for generating ATP via electron transport.


Proteins that act as messengers between cells. Often released by immune cells, including helper T cells and others, to signal danger or damage.


A group of plant growth factors/hormones that promote cell division and the growth of side shoots and new roots. Cytokinins often work in opposition to auxins.


Everything in the cell outside the nucleus; a viscous fluid containing proteins, other organic and inorganic molecules, membranes and organelles.


The networks of protein filaments and tubules that give structure and aid intracellular transport. In eukaryotic cells, the main components of the cytoskeleton are microfilaments, microtubules and intermediate filaments.

Cytotoxic T cells

T cells that kill infected cells and cancer cells by releasing toxic chemicals. Also known as CD8+ cells, because of a protein that they express on their cell surface, and as killer T cells. A type of lymphocyte.