The cytosol is the liquid portion of the cell that is outside the nucleus and the structures covered with membranes, known as organelles. It is a gel in which most of the metabolism of the cell takes place. About 70% of the volume of an animal cell is composed of the cytosol, so it is also known as intracellular fluid.
The intracellular fluid was originally known as protoplasm before much was known about the content of cells. The cytoplasm is a more specific term for the contents of a cell within the cell membrane, excluding the nucleus. The cytoplasmic matrix is another term for the cytosol.
Much of this cellular matrix is comprised of water, but there are many things dissolved in it. There are ions, small organic molecules, and larger molecules, such as water-soluble proteins. The filaments that comprise the cytoskeleton are also found in the cytoplasmic matrix.
The concentration of ions inside the cytosol can differ greatly from those outside of the cell. This can have physiological implications for the organism. For instance, the concentration of positive potassium ions is much higher, while the negative sodium ion concentration is much lower. The cell pumps negative sodium and chloride ions out of the cell to prevent them from taking up excessive amounts of water.
There is very little calcium in the cytosol. Calcium is a common secondary messenger, relaying signals from outside of the cell to intracellular targets that themselves relay signals to molecules, to initiate or terminate reactions. The binding of a hormone to a cell membrane receptor is one way in which calcium signaling can be activated.
Important cellular metabolism takes place within cell membranes, in organelles such as the mitochondria or the endoplasmic reticulum (ER). Studies with yeast, however, show that most of the cell’s metabolism takes place in the cytosol. In it, small molecules are degraded or synthesized to provide the building blocks for larger molecules, known as macromolecules. A protein is an example of a macromolecule, and protein synthesis occurs in the cytoplasmic matrix.
It appears that the concentration of molecules is very high within the cytosol, leading to a phenomenon known as macromolecular crowding. This means that an individual molecule has less room to carry out its reaction, thus increasing the relative concentration of other molecules. Estimates of the rates of protein binding, and chemical reactions carried out in experimental biochemical assays, may not apply to what actually happens in a cell because of this crowding effect.
There are some large complexes in the cytoplasmic matrix. For instance, there are proteasomes. These are large assemblies of protein complexes that degrade the proteins found in the cytosol. Also, other proteins with similar functions join together in complexes, so they can pass their product directly from one enzyme to the next. This makes the overall reaction more efficient, and is known as channeling.