Cytoplasm

The term was introduced by Rudolf von Kölliker in 1863, originally as a synonym for protoplasm, but later it has come to mean the cell substance and organelles outside the nucleus.{{Cite book |last=von Kölliker |first=Rudolf |title=Handbuch der Gewebelehre des Menschen |date=1863 |publisher=Wilhelm Engelmann |location=Leipzig |chapter=4. Auflage |chapter-url=https://books.google.com/books?id=5mtARc4NAi0C |name-list-style=vanc}}{{Cite book |url=https://books.google.com/books?id=Ian_AwAAQBAJ |title=Dictionary of the history of science |vauthors=Bynum WF, Browne EJ, Porter R |date=1981 |isbn=9781400853410 |location=Princeton University Press}}

There has been certain disagreement on the definition of cytoplasm, as some authors prefer to exclude from it some organelles, especially the vacuoles{{Cite book |title=Water deficits and plant growth, Vol. III. Plant responses and control of water balance. |vauthors=Parker J |date=1972 |publisher=Academic Press |isbn=9780323153010 |veditors=Kozlowski TT |location=New York |pages=125–176 |chapter=Protoplasmic resistance to water deficits |chapter-url=https://books.google.com/books?id=gOEr2alLRUYC}} and sometimes the plastids.{{Cite journal |vauthors=Strasburger E |year=1882 |title=Ueber den Theilungsvorgang der Zellkerne und das Verhältnis der Kernteilung zur Zellteilung |url=https://www.biodiversitylibrary.org/item/49525#page/536/mode/1up |url-status=live |journal=Arch Mikr Anat |volume=21 |pages=476–590 |doi=10.1007/BF02952628 |s2cid=85233009 |archive-url=https://web.archive.org/web/20170827124018/http://www.biodiversitylibrary.org/item/49525#page/536/mode/1up |archive-date=27 August 2017 |hdl-access=free |hdl=2027/hvd.32044106199177}}

It remains uncertain how the various components of the cytoplasm interact to allow movement of organelles while maintaining the cell's structure. The flow of cytoplasmic components plays an important role in many cellular functions which are dependent on the permeability of the cytoplasm.{{Cite book |title=Computational Methods in Cell Biology |vauthors=Cowan AE, Moraru II, Schaff JC, Slepchenko BM, Loew LM |year=2012 |isbn=9780123884039 |volume=110 |pages=195–221 |chapter=Spatial Modeling of Cell Signaling Networks |doi=10.1016/B978-0-12-388403-9.00008-4 |pmc=3519356 |pmid=22482950}} An example of such function is cell signalling, a process which is dependent on the manner in which signaling molecules are allowed to diffuse across the cell.{{Cite journal |vauthors=Holcman D, Korenbrot JI |date=April 2004 |title=Longitudinal diffusion in retinal rod and cone outer segment cytoplasm: the consequence of cell structure |journal=Biophysical Journal |volume=86 |issue=4 |pages=2566–82 |bibcode=2004BpJ....86.2566H |doi=10.1016/S0006-3495(04)74312-X |pmc=1304104 |pmid=15041693}} While small signaling molecules like calcium ions are able to diffuse with ease, larger molecules and subcellular structures often require aid in moving through the cytoplasm.{{Cite journal |vauthors=Parry BR, Surovtsev IV, Cabeen MT, O'Hern CS, Dufresne ER, Jacobs-Wagner C |date=January 2014 |title=The bacterial cytoplasm has glass-like properties and is fluidized by metabolic activity |journal=Cell |volume=156 |issue=1–2 |pages=183–94 |bibcode=2014APS..MARJ16002P |doi=10.1016/j.cell.2013.11.028 |pmc=3956598 |pmid=24361104}} 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 a sol-gel.{{Cite journal |vauthors=Taylor CV |year=1923 |title=The contractile vacuole in Euplotes: An example of the sol-gel reversibility of cytoplasm |journal=Journal of Experimental Zoology |volume=37 |issue=3 |pages=259–289 |bibcode=1923JEZ....37..259T |doi=10.1002/jez.1400370302}} It is thought that the component molecules and structures of the cytoplasm behave at times like a disordered colloidal 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 at length scale smaller than 100 nm, the cytoplasm acts like a liquid, while in a larger length scale, it acts like a gel.{{Cite journal |last=Kwapiszewska |first=Karina |last2=Szczepański |first2=Krzysztof |display-authors=1 |date=31 July 2020 |title=Nanoscale Viscosity of Cytoplasm Is Conserved in Human Cell Lines |journal=The Journal of Physical Chemistry Letters |volume=11 |issue=16 |pages=6914–6920 |doi=10.1021/acs.jpclett.0c01748 |pmc=7450658 |pmid=32787203 |doi-access=free}}

It has been proposed that the cytoplasm behaves like a glass-forming liquid approaching the glass transition. 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). 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 from dormancy.

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 by motor proteins explain the non-Brownian motion of cytoplasmic constituents.{{Cite journal |vauthors=Guo M, Ehrlicher AJ, Jensen MH, Renz M, Moore JR, Goldman RD, Lippincott-Schwartz J, Mackintosh FC, Weitz DA |date=August 2014 |title=Probing the stochastic, motor-driven properties of the cytoplasm using force spectrum microscopy |journal=Cell |volume=158 |issue=4 |pages=822–832 |doi=10.1016/j.cell.2014.06.051 |pmc=4183065 |pmid=25126787}}

The three major elements of the cytoplasm are the cytosol, organelles and inclusions.

{{main|Cytosol}}

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 of cytoskeleton filaments, dissolved molecules, and water. The cytosol's filaments include the protein filaments such as actin filaments and microtubules that make up the cytoskeleton, as well as soluble proteins and small structures such as ribosomes, proteasomes, and the mysterious vault complexes.{{Cite journal |vauthors=van Zon A, Mossink MH, Scheper RJ, Sonneveld P, Wiemer EA |date=September 2003 |title=The vault complex |journal=Cellular and Molecular Life Sciences |volume=60 |issue=9 |pages=1828–37 |doi=10.1007/s00018-003-3030-y |pmc=11138885 |pmid=14523546 |s2cid=21196262}} The inner, granular and more fluid portion of the cytoplasm is referred to as endoplasm.File:Localisations02eng.jpgs and structures tagged with green fluorescent protein]]

Due to this network of fibres and high concentrations of dissolved macromolecules, such as proteins, an effect called macromolecular crowding occurs and the cytosol does not act as an ideal solution. This crowding effect alters how the components of the cytosol interact with each other.

{{main|Organelle}}

Organelles (literally "little organs") are usually membrane-bound structures inside the cell that have specific functions. Some major organelles that are suspended in the cytosol are the mitochondria, the endoplasmic reticulum, the Golgi apparatus, vacuoles, lysosomes, and in plant cells, chloroplasts.

{{main|Cytoplasmic inclusion}}

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 of calcium oxalate or silicon dioxide in plants,{{Cite journal |last=Prychid, Christina J. |last2=Rudall, Paula J. |year=1999 |title=Calcium Oxalate Crystals in Monocotyledons: A Review of their Structure and Systematics |url=https://academic.oup.com/aob/article-pdf/84/6/725/7983834/840725.pdf |journal=Annals of Botany |volume=84 |issue=6 |pages=725–739 |doi=10.1006/anbo.1999.0975 |doi-access=free}}{{Cite journal |last3=Gregory, M. |vauthors=Prychid CJ, Rudall PJ |year=2004 |title=Systematics and Biology of Silica Bodies in Monocotyledons |journal=The Botanical Review |volume=69 |issue=4 |pages=377–440 |doi=10.1663/0006-8101(2004)069[0377:SABOSB]2.0.CO;2 |jstor=4354467 |s2cid=24520433}} to granules of energy-storage materials such as starch,{{Cite journal |vauthors=Ball SG, Morell MK |year=2003 |title=From bacterial glycogen to starch: understanding the biogenesis of the plant starch granule |journal=Annual Review of Plant Biology |volume=54 |pages=207–233 |doi=10.1146/annurev.arplant.54.031902.134927 |pmid=14502990}} glycogen,{{Cite journal |vauthors=Shearer J, Graham TE |date=April 2002 |title=New perspectives on the storage and organization of muscle glycogen |journal=Canadian Journal of Applied Physiology |volume=27 |issue=2 |pages=179–203 |doi=10.1139/h02-012 |pmid=12179957}} or polyhydroxybutyrate.{{Cite journal |vauthors=Anderson AJ, Dawes EA |date=December 1990 |title=Occurrence, metabolism, metabolic role, and industrial uses of bacterial polyhydroxyalkanoates |journal=Microbiological Reviews |volume=54 |issue=4 |pages=450–472 |doi=10.1128/MMBR.54.4.450-472.1990 |pmc=372789 |pmid=2087222}} A particularly widespread example are lipid 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 as fatty acids and sterols.{{Cite journal |vauthors=Murphy DJ |date=September 2001 |title=The biogenesis and functions of lipid bodies in animals, plants and microorganisms |journal=Progress in Lipid Research |volume=40 |issue=5 |pages=325–438 |doi=10.1016/S0163-7827(01)00013-3 |pmid=11470496}} Lipid droplets make up much of the volume of adipocytes, 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 by viscoplastic behavior and a measure of the reciprocal rate of bond breakage within the cytoplasmic network.{{Cite journal |vauthors=Feneberg W, Westphal M, Sackmann E |date=August 2001 |title=Dictyostelium cells' cytoplasm as an active viscoplastic body |journal=European Biophysics Journal |volume=30 |issue=4 |pages=284–94 |doi=10.1007/s002490100135 |pmid=11548131 |s2cid=9782043}}

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 of optical tweezers has been described.{{Cite journal |vauthors=Hu J, Jafari S, Han Y, Grodzinsky AJ, Cai S, Guo M |date=September 2017 |title=Size- and speed-dependent mechanical behavior in living mammalian cytoplasm |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=114 |issue=36 |pages=9529–9534 |bibcode=2017PNAS..114.9529H |doi=10.1073/pnas.1702488114 |pmc=5594647 |pmid=28827333 |doi-access=free}}

• {{annotated link|Amoeboid movement}}
• {{annotated link|Cytoplasmic streaming}}
• {{annotated link|Protoplasm}}
• {{annotated link|Syncytium}}

{{Reflist|35em}}

• {{Cite book |last=Luby-Phelps K |url=http://www.rpgroup.caltech.edu/courses/aph161/Handouts/Luby-Phelps2000.pdf |title=Microcompartmentation and Phase Separation in Cytoplasm |work=Int Rev Cytol |year=2000 |isbn=9780123645968 |series=International Review of Cytology |volume=192 |pages=189–221 |chapter=Cytoarchitecture and physical properties of cytoplasm: volume, viscosity, diffusion, intracellular surface area |doi=10.1016/S0074-7696(08)60527-6 |pmid=10553280 |archive-url=https://web.archive.org/web/20080911153732/http://www.rpgroup.caltech.edu/courses/aph161/Handouts/Luby-Phelps2000.pdf |archive-date=11 September 2008 |url-status=dead}}

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Category:Cell anatomy