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Transmission electron micrograph of a salivary gland from a mosquito infected by a virus, which has been colorized red; magnified 83,900x

Protoplasm is an old term, which means the living substance that makes up a cell. It is no longer much used. Biologists prefer to talk about the cytoplasm and the cell nucleus.

In plant cells, it is surrounded by a cell wall. In animal cells, the whole cell is made of protoplasm, surrounded by a cell membrane. Protoplasm in living beings is made up of about 75–80% water. However, this is rather misleading because the cytoplasm is full of structures called organelles, which do various tasks. The endoplasmic reticulum is the largest of these structures; there are many other organelles.

The word "protoplasm" was first used in 1846 by Hugo von Mohl to describe the substance in plant cells, apart from the cell wall, the cell nucleus and the vacuole. After the invention of the electron microscope it was clear that a living cell is much more complicated than von Mohl thought.