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A form of cell division which occurs during formation of the gametes: eggs and sperm. It differs from mitosis in that the chromosomal number is halved from 46 to 23. This is achieved by having a sequence of two cell divisions after replicating (copying) the chromosomes only once.
The process by which previous events can be recalled. This term is usually used when discussing brain activity, but memory can be a feature of other systems too. Thus, you will encounter discussions about immunological memory - the ability of the immune system to 'remember' previous infections - and of course computer memory.
A piece of optical equipment that allows visual examination of very small objects. The light microscope has glass lenses and gives a maximum magnification of about 1000 times actual size. The electron microscope uses a beam of electrons in place of a light source and has magnetic lenses to focus the beam. It can magnify up to about 1 million times actual size, at which level the larger molecules become visible. The scanning tunnelling microscope measures tiny electrical changes that are produced when a needle-tip is moved across the surface of a material, and can produce sufficient magnification to distinguish individual atoms.
Tiny finger-like projections of the outer surface of some cells. They increase surface area for improved absorption into the cells.
The mucus-secreting linings of the digestive tract and airways. It consists of an epithelium plus the underlying connective tissue. Different types of epithelium occur depending on the functional requirements - stratified squamous epithelium lines the oesophagus to protect against wear and tear, while a single layer of ciliated cells lines the airways to help in conditioning the air before it reaches the lungs.
This is an insulating coat wrapped around the axons of neurons. The myelin sheath, which is made predominantly of concentric layers of a lipid material comparable to that forming the cell membrane, increases the speed of conduction of nerve impulses along the axon. Myelin sheaths are formed by the oligodendroglia in the central nervous system, and by schwann cells in the peripheral nervous system. Not all axons are myelinated, however. Unmyelinated axons are a feature of much of the peripheral autonomic nervous system, for example, and pain fibres.