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Cytosol may be defined as the part of cell that remains after cytoskeletal components, membrane and organelles have been removed. Cytosol is often confounded with cytoplasm, which is the fluid that fills the space between nucleus and plasma membrane. Cytosol is a part of cytoplasm and it is not the cytoplasm itself. Therefore, the cytosol does not include organelles. This differentiation is often ignored, but it is very important for the understanding of the functions of cytosol.
The main component of cytosol is water that assists chemical reactions within the cell. Cytosol dissolves proteins and macromolecules that are not used, because most of macromolecules, excepting lipids, are polar, thus they may dissolve in the water.
Bidirectional traffic happens constantly between the nucleus and cytosol. The proteins that function in the nucleus, such as gene regulatory proteins, DNA and RNA polymerases, histones and RNA processing proteins, are imported from cytosol, where they are produced, into nuclear compartment. The transport process may be complex. For instance, ribosomal proteins produced in cytosol are imported into nucleus, where they congregate with new ribosomal RNA into particles, and they are exported back to the cytosol as fraction of ribosomal subunit.
Cytosol contains enzymes that break down the larger molecules. The large glucose molecules in cytosol cannot be used by mitochondria, therefore, the enzymes dissolved in cytosol break down glucose molecules into smaller pyruvate molecules, that are then sent back to mitochondria and used for fuel.
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