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The cytosol (cf. cytoplasm, which also includes the organelles) is the internal fluid of the cell, and a portion of cell metabolism occurs here. Proteins within the cytosol play an important role in signal transduction pathways and glycolysis. They also act as intracellular receptors and form part of the ribosomes, enabling protein synthesis.
In prokaryotes, all chemical reactions take place in the cytosol. In eukaryotes, the cytosol contains the cell organelles; this is collectively called cytoplasm. In plants, the amount of cytosol can be reduced due to the large tonoplast (central vacuole) that takes up most of the cell interior volume.
The cytosol also surrounds the cytoskeleton, which is made of fibrous proteins (e.g. microfilaments, microtubules, and intermediate filaments). In many organisms, the cytoskeleton maintains the shape of the cell, anchors organelles, and controls internal movement of structures (e.g. transport vesicles).
The cytosol is not a "soup" with free-floating particles, but is highly organized on the molecular level. As the concentration of soluble molecules increases within the cytosol, an osmotic gradient builds up toward the outside of the cell. Water flows into the cell, making the cell larger. To prevent the cell from bursting apart, molecular pumps in the plasma membrane, the cytoskeleton, the tonoplast or the cell wall (if present), are used to counteract the osmotic pressure.
Cytosol mostly consists of water, dissolved ions, small molecules, and large water-soluble molecules (such as protein). It contains about 20% to 30% protein.
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