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Plant cells have strong walls, and inside the wall is a cell membrane, then a thin layer of cytoplasm with the nucleus and other organelles in it. The cytoplasm is pressed against the cell wall by the very large vacuole. In this picture the cell nuclei are dark-coloured.

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.