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I've generally seen cytosol defined as the solution inside cells minus the organelles, cytoskeleton, etc and cytoplasm as the cytosol plus the organelles, cytoskeleton, etc. This naturally leads to the impression that cytosol is the cytoplasm minus all the solids. The problem here is that there are all sorts of other large molecules in the cells which could be thought of as solid. Are they also part of the cytosol or are they suspended in it? (I.e. are they part of the cytosol or are they non-cytosol components of the cytoplasm?)

Basically, I'm asking if the precise definition of cytosol is just anything in the cell that's not behind an endomembrane (save the exoskeleton) or if the dividing line is something else.

Subquestion: things can get even more terminologically confused because the cytosol is sometimes called the matrix. What the heck is the preferred terminology with this stuff?

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  • $\begingroup$ 'The problem here is that there are all sorts of other large molecules in the cells which could be thought of as solid.' Can you give some examples so that we understand how you are using the term 'solid'? $\endgroup$
    – Alan Boyd
    Commented May 7, 2017 at 10:39

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IMO, the definitive answer to this question is given in a paper by J. S Clegg. He traced the origin of the term cytosol to a book chapter by H. A. Lardy, and confirmed by email that Lardy had indeed coined the term. Their definition of cytosol is as follows:

... that portion of the cell which is found in the supernatant fraction after centrifuging the homogenate at 105 000 x g for 1 hour.

Thus cytosol is a centrifugation term, (ie a fraction obtained by centrifugation) and ' should not be applied, in any context, to the intact cell' (Clegg, 1983).

The cytosol may roughly be though of as the 'soluble' portion of the cellular extract obtained after everything else, including the microsomes, have been sedimented. (For those not familiar with subcellular frationation, centrifugation at 100 000 g for 1 hr is the 'standard' method of sedimenting microsomes. If your enzyme is found in the 100 000 g supernatant fraction, you can be pretty sure it is soluble).

When we are referring to the cellular aqueous fluid that 'holds' the organelles and other components we should use the term cytoplasm.

At the bottom of Clegg's article, the editors of TIBS have supplied the following quote, attributed to F.H.C Crick & A. F. W Hughes (1950):

If we were compelled to suggest a model for the structure of cytoplasm we would propose Mother's Work Basket -a jumble of threads, beads and buttons of all shapes and sizes, with pins and threads for good measure, all jostling about and held together by "colloidal forces".'

In being a centrifugation fraction, the term 'cytosol' is very like microsome: the intact cell contains neither cytosol or microsome.

Ref

Clegg, J. S. (1983) What is the cytosol? Trends in Biochemical Sciences 8, (Dec, issue 12), pp 436-437

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Per my bio textbook, they have cytoplasm as "the region contained within the plasma membrane" and cytosol as described as "site of many metabolic pathways" and "the region of a eukaryotic cell that is inside the plasma membrane and outside the organelles."

Ref

Brooker, R. J.; Widmaier, E. P.; Graham, L. E.; Stiling, P. D. Biology; McGraw-Hill Education: New York, NY, 2017.

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