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



Antigen-presenting cells (APCs) that destroy foreign substances by phagocytosis (engulfing them) and activate other immune cells. A type of white blood cell.

Magnetic resonance imaging (MRI)

A medical imaging technique that uses powerful magnets to visualise the inside of the body. It can be used to study the structure and function of body parts, including the brain.


The ability to use a microscope to make an object look bigger.

Mast cells

Cells involved in allergic responses, releasing histamine and other inflammatory molecules. Mast cells sit within skin and mucosal tissues. A type of white blood cell.

MC4R (melanocortin 4 receptor)

A protein found on neurons in the hypothalamus. Activation of MC4R leads to a drop in appetite. Mutations in MC4R account for some cases of severe obesity.

Medial temporal lobe

An area of the brain responsible for long-term memory, containing the hippocampus.

Medicines and Healthcare Products Regulatory Agency (MHRA)

The body that licenses new drugs in the UK.

Medulla oblongata

A part of the brainstem; controls vital involuntary functions such as breathing and heart rate.


A form of cell division that results in four non-identical daughter haploid cells.

Memory B cells

Long-lived B cells that ‘remember’ past infections by recognising antigens to provide a secondary immune response. A type of lymphocyte.

Memory T cells

Long-lived T cells that ‘remember’ past infections to provide a secondary immune response. A type of lymphocyte.

Metabolic pathway

A series of reactions that transform one chemical into another.


The sum of the chemical reactions that occur within the body. Metabolism can be divided into anabolism and catabolism. Chemical reactions are usually grouped into metabolic pathways, with enzymes used to control the rate at which they occur.


The chemicals produced by the reactions in a metabolic pathway.


Mass sequencing of genomes directly from environmental or other samples, without a culturing step. Provides insight into the communities of organisms living in the sampled environment.


A compound composed of one carbon atom and four hydrogen atoms (CH4) . It is a naturally occurring gas found in the ground, under the sea and in smaller amounts in the atmosphere. Methane is sometimes considered a biosignature, particularly when it’s present with another gas like oxygen.

MHC (major histocompatibility complex)

A series of proteins found on the surface of our cells. MHC proteins indicate that your cells are ‘self’. Apart from identical twins, we all have our own unique set of MHC proteins.


A living organism broadly defined as one that is usually too small to be seen by the human eye. Includes all bacteria, some fungi and algae, and even some animals. Viruses are sometimes included, but they are not living organisms.


The total number and diversity of microbes found in, and on, the human body. Different species inhabit different areas. Those found in the gut have several important roles in digestion.


Smaller than microtubules, these are made from repeating actin subunits. They are responsible for cell movement and changes in cell shape, and make muscle contraction possible.


The condition of perceived weightlessness in outer space. Sometimes called ‘zero gravity’ or 0 G, microgravity is actually equal to 1 x 10-6 G.


Small, tubular assemblies of protein, made from repeating tubulin subunits, which help maintain the cell’s internal structure and move organelles and cytoplasm using molecular motors.


The practice of attempting to reduce the scale or severity of a particular outcome. In the context of climate change this could involve improving energy efficiency or switching to renewable resources in order to stabilise and reduce greenhouse gas levels.

Mitochondria (singular: mitochondrion)

Rod-shaped bodies in the cytoplasm that supply chemical energy to the rest of the cell.


A form of cell division that results in the creation of two identical daughter cells.

Monoclonal antibody

Monoclonal antibodies are produced from the progeny of a single plasma B cell and so ones of the same type are identical to each other. They are created in the laboratory and are made to be specific to a given antigen. They are increasingly being investigated for the treatment of cancer and other diseases, and are used in pregnancy tests.

mRNA (messenger RNA)

RNA molecules that transfer the gene sequence for a protein from the DNA in the nucleus to the ribosome, where it is ‘read’ to make the protein.


An organism that is made up of more than one cell (usually many cells).


The ability to give rise to many (but not all) but limited cell types. For example, a haematopoietic (blood) stem cell can give rise to many blood cell types but can’t give rise to liver cells.


A change in the DNA sequence of an organism. The simplest mutation is the loss or alteration of a single base pair. Mutations can have a positive, negative or neutral effect. We use the term ‘altered’ to describe an allele that has been mutated from a ‘normal’ version. You might see ‘mutated’ or ‘faulty’ used elsewhere rather than ‘altered’.


A type of symbiosis involving close cooperation between species, in which both benefit.


Symbiotic relationships between plants and fungi that inhabit their roots.


The white fatty substance that makes up the myelin sheath. This is a multi-layered fatty cell membrane that wraps around an axon and acts as an electrical insulator. The sheath is interrupted at regular intervals (the nodes of Ranvier), where the channels that generate the electrical signal are located.

Myelin sheath

Many neurons are insulated by myelin a multi-layered fatty cell membrane that wraps around the axon. The sheath is interrupted at regular intervals (the ‘nodes of Ranvier’), where the channels that generate the electrical signal are located.


Made up of many subunits called sarcomeres, cylindrical myofibrils bundle together to form muscle fibres.


A protein which carries and stores oxygen in muscle cells.


One of the main proteins involved in muscle contraction. Myosin binds with actin, and then pulls the actin to cause the muscle to contract.