cytoplasm and cytosol
/ SY-toh-plaz-um and SY-toh-sol /
Imagine the inside of a cell as a busy warehouse. Almost everything that makes life happen sits not in the warehouse walls but in the crowded space inside. That whole interior — the watery jelly plus all the structures floating in it — is the cytoplasm. The thin liquid by itself, with the big structures taken out, is the cytosol. Roughly: cytoplasm is the soup plus the dumplings; cytosol is just the broth.
More precisely, the cytoplasm is everything enclosed by the cell's outer membrane except the nucleus. It includes the cytosol (a water-based gel packed with salts, sugars, amino acids, and thousands of proteins) plus all the suspended organelles such as mitochondria, the endoplasmic reticulum, and ribosomes. The cytosol is the fluid portion only — the medium in which chemical reactions occur and through which materials diffuse. The cytosol is far from empty: it is so crowded with molecules that it behaves more like a thick gel than like plain water.
The cytosol is where a great deal of basic metabolism happens, including the first stage of sugar breakdown. The cytoplasm as a whole is also held together and organized by a protein scaffold called the cytoskeleton, which keeps organelles in place and lets the cell move things around. A common point of confusion is the boundary between the two words; the simplest rule is that cytoplasm = cytosol + organelles, while cytosol = the liquid alone.
When a yeast cell ferments sugar into alcohol, the first ten steps — glycolysis — take place in the cytosol, before any product is handed off to an organelle.
The cytosol is itself a chemical reactor, not just empty filler.
Cytoplasm includes the organelles; cytosol does not. People often use "cytoplasm" loosely to mean the fluid, but strictly the fluid alone is the cytosol.