Cytosol
Liquid matrix of the cytoplasm, site of many metabolic reactions.
The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cells (intracellular fluid). It is separated into compartments by membranes. In eukaryotic cells, the cytosol is surrounded by the cell membrane and is part of the cytoplasm, which also comprises organelles such as mitochondria and plastids; the cell nucleus is separate. In prokaryotes, most chemical reactions of metabolism take place in the cytosol.
- also_known_as
- cytoplasmic matrix, groundplasm, aqueous cytoplasm
- composition
- mostly water, dissolved ions, small molecules, and large water-soluble molecules
- typical_pH
- 7.4 (intracellular fluid); mouse cell cytosolic pH ranges 7.0–7.4
- water_content
- about 70% of total cell volume
- key_ion_difference
- high potassium, low sodium compared to extracellular fluid
Lore & Background
A. Lardy, initially referring to the liquid produced by breaking cells apart and pelleting insoluble components by ultracentrifugation. Such a soluble cell extract is not identical to the soluble part of the cell cytoplasm and is usually called a cytoplasmic fraction. The term cytosol is now used to refer to the liquid phase of the cytoplasm in an intact cell, excluding any part contained within organelles. Prior to this, other terms including hyaloplasm were used for the cell fluid, not always synonymously, as its nature was not well understood.
Reader's Guide
The cytosol is a complex mixture of substances dissolved in water, with water forming the large majority. Although it was once thought to be a simple solution of molecules, the cytosol has multiple levels of organization, including concentration gradients of small molecules such as calcium, large complexes of enzymes that act together, and protein complexes such as proteasomes and carboxysomes (in bacteria) that enclose and separate parts of the cytosol. The concentrations of ions such as sodium and potassium in the cytosol differ from those in extracellular fluid; these differences are important in osmoregulation, cell signaling, and the generation of action potentials in excitable cells. The cytosol also contains large amounts of macromolecules, which can alter how molecules behave through macromolecular crowding. The proportion of cell volume that is cytosol varies: for example, while this compartment forms the bulk of cell structure in bacteria, in plant cells the main compartment is the large central vacuole.
Did You Know?
- The cytosol makes up about 70% of the total volume of a typical cell.
- The viscosity of cytoplasm is roughly the same as pure water, but diffusion of small molecules is about fourfold slower due to collisions with macromolecules.
- The cytosol has a high concentration of potassium ions and a low concentration of sodium ions, critical for osmoregulation.
Frequently Asked Questions
Who is Cytosol?
Cytosol is the aqueous liquid matrix that fills the interior of a cell, sitting between the plasma membrane and the suspended organelles. It goes by several aliases in the canon—cytoplasmic matrix, groundplasm, or aqueous cytoplasm—and accounts for roughly 70% of a cell's total volume.
What are Cytosol's powers/role?
Cytosol is the primary stage where a vast array of metabolic reactions unfold, and in prokaryotic cells it hosts nearly all of the organism's chemistry. Its composition is mostly water laced with dissolved ions, small metabolites, and large water-soluble proteins, holding the internal pH steady around 7.4.
How does Cytosol's story end?
Because Cytosol is a persistent structural component rather than a character with a narrative arc, it has no traditional ending. It simply continues to fill the cell's interior for the organism's entire lifespan, maintaining the high-potassium, low-sodium ionic environment every downstream reaction depends on.
Why is Cytosol important?
Without Cytosol, enzymes, substrates, and signaling molecules would lack the shared medium in which they interact, effectively shutting down metabolism. It also provides the physical context that keeps organelles like mitochondria and plastids suspended and the nucleus distinct from the rest of the cytoplasm.
What's Cytosol's relationship with the Nucleus?
The Nucleus is a separate, double-membrane-bound compartment that sits *within* the cytoplasm but is not part of Cytosol itself. Cytosol surrounds the nucleus and every other organelle, serving as the shared aqueous environment while the nucleus keeps its own internal chemistry walled off.
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