Thecytoplasm describes all the material within aeukaryotic orprokaryoticcell, enclosed by thecell membrane, including theorganelles[1] and excluding thenucleus in eukaryotic cells. The material inside the nucleus of a eukaryotic cell and contained within thenuclear membrane is termed thenucleoplasm. The main components of the cytoplasm are thecytosol (a gel-like substance), the cell's internal sub-structures, and variouscytoplasmic inclusions. In eukaryotes the cytoplasm also includes the nucleus, and othermembrane-bound organelles.The cytoplasm is about 80% water and is usually colorless.[2]
The submicroscopic ground cell substance, or cytoplasmic matrix, that remains after the exclusion of the cellorganelles and particles isgroundplasm. It is thehyaloplasm of light microscopy, a highly complex, polyphasic system in which all resolvable cytoplasmic elements are suspended, including the larger organelles such as theribosomes,mitochondria, plantplastids,lipid droplets, andvacuoles.
The term was introduced byRudolf von Kölliker in 1863, originally as a synonym forprotoplasm, but later it has come to mean the cell substance and organelles outside the nucleus.[4][5]
There has been certain disagreement on the definition of cytoplasm, as some authors prefer to exclude from it some organelles, especially thevacuoles[6] and sometimes the plastids.[7]
It remains uncertain how the various components of the cytoplasm interact to allow movement oforganelles while maintaining the cell's structure. The flow of cytoplasmic components plays an important role in many cellular functions which are dependent on thepermeability of the cytoplasm.[8] An example of such function iscell signalling, a process which is dependent on the manner in which signaling molecules are allowed todiffuse across the cell.[9] While small signaling molecules likecalcium ions are able to diffuse with ease, larger molecules and subcellular structures often require aid in moving through the cytoplasm.[10] The irregular dynamics of such particles have given rise to various theories on the nature of the cytoplasm.
There has long been evidence that the cytoplasm behaves like asol-gel.[11] It is thought that the component molecules and structures of the cytoplasm behave at times like a disorderedcolloidal solution (sol) and at other times like an integrated network, forming a solid mass (gel). This theory thus proposes that the cytoplasm exists in distinct fluid and solid phases depending on the level of interaction between cytoplasmic components, which may explain the differential dynamics of different particles observed moving through the cytoplasm. A papers suggested that atlength scale smaller than 100 nm, the cytoplasm acts like a liquid, while in a larger length scale, it acts like a gel.[12]
It has been proposed that the cytoplasm behaves like aglass-forming liquid approaching theglass transition.[10] In this theory, the greater the concentration of cytoplasmic components, the less the cytoplasm behaves like a liquid and the more it behaves as a solid glass, freezing more significant cytoplasmic components in place (it is thought that the cell's metabolic activity can fluidize the cytoplasm to allow the movement of such more significant cytoplasmic components).[10] A cell's ability to vitrify in the absence of metabolic activity, as in dormant periods, may be beneficial as a defense strategy. A solid glass cytoplasm would freeze subcellular structures in place, preventing damage, while allowing the transmission of tiny proteins and metabolites, helping to kickstart growth upon the cell's revival fromdormancy.[10]
Research has examined the motion of cytoplasmic particles independent of the nature of the cytoplasm. In such an alternative approach, the aggregate random forces within the cell caused bymotor proteins explain the non-Brownian motion of cytoplasmic constituents.[13]
The cytosol is the portion of the cytoplasm not contained within membrane-bound organelles. Cytosol makes up about 70% of the cell volume and is a complex mixture ofcytoskeleton filaments, dissolved molecules, and water. The cytosol's filaments include theprotein filaments such asactin filaments andmicrotubules that make up the cytoskeleton, as well as solubleproteins and small structures such asribosomes,proteasomes, and the mysteriousvault complexes.[14] The inner, granular and more fluid portion of the cytoplasm is referred to as endoplasm.
Due to this network of fibres and high concentrations of dissolvedmacromolecules, such asproteins, an effect calledmacromolecular crowding occurs and the cytosol does not act as anideal solution. This crowding effect alters how the components of the cytosol interact with each other.
The inclusions are small particles of insoluble substances suspended in the cytosol. A huge range of inclusions exist in different cell types, and range from crystals ofcalcium oxalate orsilicon dioxide in plants,[15][16] to granules of energy-storage materials such asstarch,[17]glycogen,[18] orpolyhydroxybutyrate.[19] A particularly widespread example arelipid droplets, which are spherical droplets composed of lipids and proteins that are used in both prokaryotes and eukaryotes as a way of storing lipids such asfatty acids andsterols.[20] Lipid droplets make up much of the volume ofadipocytes, which are specialized lipid-storage cells, but they are also found in a range of other cell types.
The cytoplasm, mitochondria, and most organelles are contributions to the cell from the maternal gamete. Contrary to the older information that disregards any notion of the cytoplasm being active, new research has shown it to be in control of movement and flow of nutrients in and out of the cell byviscoplastic behavior and a measure of the reciprocal rate of bond breakage within the cytoplasmic network.[21]
The material properties of the cytoplasm remain an ongoing investigation. A method of determining the mechanical behaviour of living cell mammalian cytoplasm with the aid ofoptical tweezers has been described.[22]
^Hogan CM (2010)."Calcium". In Jorgensen A, Cleveland C (eds.).Encyclopedia of Earth. National Council for Science and the Environment. Archived fromthe original on 12 June 2012.
^von Kölliker R (1863)."4. Auflage".Handbuch der Gewebelehre des Menschen. Leipzig: Wilhelm Engelmann.
^Taylor CV (1923). "The contractile vacuole in Euplotes: An example of the sol-gel reversibility of cytoplasm".Journal of Experimental Zoology.37 (3):259–289.Bibcode:1923JEZ....37..259T.doi:10.1002/jez.1400370302.
^Shearer J, Graham TE (April 2002). "New perspectives on the storage and organization of muscle glycogen".Canadian Journal of Applied Physiology.27 (2):179–203.doi:10.1139/h02-012.PMID12179957.
^Murphy DJ (September 2001). "The biogenesis and functions of lipid bodies in animals, plants and microorganisms".Progress in Lipid Research.40 (5):325–438.doi:10.1016/S0163-7827(01)00013-3.PMID11470496.
^Feneberg W, Westphal M, Sackmann E (August 2001). "Dictyostelium cells' cytoplasm as an active viscoplastic body".European Biophysics Journal.30 (4):284–94.doi:10.1007/s002490100135.PMID11548131.S2CID9782043.
