The cytosol is the cytoplasm, excluding organelles and the nucleus, whereas the cytoplasm is the cytosol plus organelles, but not the nucleus. The cytosol is an aqueous semifluid solution that surrounds organelles and the nucleus, that can be up to more than half of the cell volume of the animal cells, while in mature plant cells the majority of the cellular volume is occupied by a vacuole.
Cytosol mostly consists of water, which contains a large amount of organic molecules and ions. The concentration of molecules and ions can be so high that it makes the cytosol a viscous, gel-like solution. Unlike the extracellular space, the cytosol has a higher concentration of potassium and a lower concentration of sodium and calcium. It is buffered, about pH 7 to 7.4.
An intense molecular activity takes place in the cytosol: many metabolic reactions like glycolysis, translation of mRNA to proteins by free ribosomes, signaling cascades for cell communication and for communication between cellular compartments, etcetera. Ions, second messengers, and large molecules diffuse through the cytosol. Vesicles also move in the cytosol from the source compartments toward the target compartments.
The cytoskeleton is also found in the cytosol. Cytoskeleton is a set of proteins organized in filaments that work as the skeleton and the muscles of the cell. These filaments are really versatile to accomplish the cell needs. In the cytosol, there are also carbohydrates stored as glycogen.
Although eukaryote cells contain many and diverse membrane-bond comparments, there are non-membrane-bound compartments in the cytosol that behave distinctly as if they were organelles. They are known as MLO (membraneless organelles). These "organelles" are formed by liquid-liquid phase separation; that is, two sets of proteins are spatially segregated in an aqueous environment. In the cytosol, as well as in the nucleoplasma, there are proteins, lipids and nucleic acids that associate to form a liquid phase more dense than the surroundings. MLOs have been found in eukaryotes and prokaryotes. For instance, the nucleolus is a MLO within the nucleus. In the cytosol, stress granules, ARN transporter granules and P bodies are MLO. The functions of MLOs are diverse. ARN transporter granules and stress granules store ARN, preventing their translation. Other MLOs acelerate biochemical reactions by bringing components together. Mechanisms of MLO modulation have been observed, such as phosphorilation, methylation, acethylation, and addition of poly-riboses.
The cytosol of many animal cells is not shared with other surrounding cells. However, direct cytosol-cytosol connections are observed between some cells. These connections are known as gap junctions, which are made up of proteins organized as channels that create passages that connect the cytoplasm of two adjoining cells. Water, ions, and low-weight proteins can freely cross through gap junctions. On the other hand, it is a common feature for plant cells to share their cytosol through plasmodesmata. They are channels through the cell wall that directly connect the cytosols of two cells. The walls of plasmodesmata are actually the cell membrane, which is continuous between the two cells. There is a third way of cytosol-cytosol communication: shedding vesicles and exosomes, which are vesicles released to the extracellular space with cytosolic content. These vesicles fuse with other cells and release their content into the cytosol of the target cell.
Lipid droplets