What Is Cytosol? Definition and Functions

Cytosol is the liquid matrix found inside cells. It occurs in both eukaryotic (plant and animal) and prokaryotic (bacteria) cells. In eukaryotic cells, it includes the liquid enclosed within the cell membrane, but not the cell nucleus, organelles (e.g., chloroplasts, mitochondria, vacuoles), or fluid contained within organelles. In contrast, all of the liquid within a prokaryotic cell is cytoplasm, since prokaryotic cells lack organelles or a nucleus. The cytosol is also known as groundplasm, intracellular fluid (ICF), or cytoplasmic matrix.

Difference Between Cytosol and Cytoplasm

Cytosol and cytoplasm are related, but the two terms are not usually interchangeable. The cytosol is a component of cytoplasm. The cytoplasm encompasses all of the material in the cell membrane, including the organelles, but excluding the nucleus. So, the liquid within mitochondria, chloroplasts, and vacuoles is part of the cytoplasm, but is not a component of the cytosol. In prokaryotic cells, the cytoplasm and the cytosol are the same.

Cytosol Composition

The cytosol consists of a variety of ions, small molecules, and macromolecules in water, however, this fluid is not a homogeneous solution. About 70% of the cytosol is water. In humans, its pH ranges between 7.0 and 7.4. The pH is higher when the cell is growing. Ions dissolved in the cytosol include K⁺, Na⁺, C^l-, Mg²⁺, Ca²⁺, and bicarbonate. It also contains amino acids, proteins, and molecules that regulate osmolarity, such as protein kinase C and calmodulin.

Organization and Structure

The concentration of substances in the cytosol is affected by gravity, channels in the cell membrane and around organelles that affect calcium, oxygen, and ATP concentration, and channels formed by protein complexes. Some proteins also contain central cavities filled with cytosol having a different composition from the outside fluid. While the cytoskeleton is not considered to be part of the cytosol, its filaments control diffusion throughout the cell and restrict movement of large particles from one part of the cytosol to another.

Cytosol Functions

The cytosol serves several functions within a cell. It is involved in signal transduction between the cell membrane and the nucleus and organelles. It transports metabolites from their production site to other parts of the cell. It is important for cytokinesis, when the cell divides in mitosis. The cytosol plays a role in eukaryote metabolism. In animals, this includes glycolysis, gluconeogenesis, protein biosynthesis, and the pentose phosphate pathway. However, in plants, fatty acid synthesis occurs within chloroplasts, which are not part of the cytoplasm. Nearly all of a prokaryote's metabolism occurs in the cytosol.

History

When the term "cytosol" was coined by H. A. Lardy in 1965, it referred to the liquid produced when cells broke apart during centrifugation and the solid components were removed. However, the fluid is more accurately called the cytoplasmic fraction. Other terms sometimes used to refer to cytoplasm include hyaloplasm and protoplasm.

In modern usage, cytosol refers to the liquid portion of the cytoplasm in an intact cell or to extracts of this liquid from cells. Because the properties of this liquid depend on whether or not the cell is alive, some scientists refer to the liquid contents of living cells as aqueous cytoplasm.

Sources

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