Each cell is surrounded by watery fluid called extracellular or interstitial fluid. This is separated from the cell contents by a cell membrane. Within the membrane is cytoplasm, containing a nucleus, cytosol and cell organelles. The cell membrane separates the cell contents from the interstitial fluid. It regulates the exchange of substances with the extracellular environment.
Cytosol refers to the fluid contained by the cell membrane.
It has more potassium ions and less sodium ions compared with the interstitial fluid. It also contains a high concentration of proteins, many involved in biochemical reactions within the cell. The cell contains two kinds of organelles, membranous, bounded by a lipid membrane, and non-membranous. These include the cytoskeleton of the cell, with microfilaments, thick and thin tubules and microtubules. There are also microvilli, short finger shaped projections of the cell membrane, and cilia which also project from the cell surface but are longer and contain microtubules.