In cell biology
, the cytoplasm
is the material within a living cell, excluding the cell nucleus
. It comprises cytosol
(the gel-like substance enclosed within the cell membrane) and the organelle
s – the cell
's internal sub-structures. All of the contents of the cells of prokaryotic
organisms (such as bacteria
, which lack a cell nucleus) are contained within the cytoplasm. Within the cells of eukaryotic
organisms the contents of the cell nucleus are separated from the cytoplasm, and are then called the nucleoplasm
. The cytoplasm is about 80% water and usually colorless.
The submicroscopic ground cell substance or cytoplasmatic matrix which remains after exclusion the cell organelle
s and particles is groundplasm
. It is the hyaloplasm
of light microscopy, and high complex, polyphasic system in which all of resolvable cytoplasmic elements of are suspended, including the larger organelles such as the ribosome
, the plant plastid
droplets, and vacuole
It is within the cytoplasm that most cellular activities occur, such as many metabolic pathway
s including glycolysis
, and processes such as cell division
. The concentrated inner area is called the endoplasm
and the outer layer is called the cell cortex
or the ectoplasm
Movement of calcium ion
s in and out of the cytoplasm is a signaling
activity for metabolic
processes.Hogan, C. Michael (2010). "Calcium"
in Encyclopedia of Earth
. A. Jorgensen, C. Cleveland (eds.). National Council for Science and the Environment.
s, movement of the cytoplasm around vacuoles is known as cytoplasmic streaming
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.Kölliker, R. A. v. (1863). Handbuch der Gewebelehre des Menschen.
4. Auflage. Leipzig: Wilhelm Engelmann.Bynum, W. F., Browne, E. J. and Porter, Ray (1981). Dictionary of the history of science
. 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 vacuole
sParker, J. (1972). "Protoplasmic resistance to water deficits", pp. 125–176 in Kozlowski, T. T. (ed.), Water deficits and plant growth
. Vol. III. Plant responses and control of water balance. Academic Press, New York, p. 144, link
. and sometimes the plastid
The physical properties of the cytoplasm have been contested in recent years. It remains uncertain how the varied components of the cytoplasm interact to allow movement of particles and organelle
s 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. 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. 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. The irregular dynamics of such particles have given rise to various theories on the nature of the cytoplasm.
As a sol-gel
There has long been evidence that the cytoplasm behaves like a sol-gel
. 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.
As a glass
Recently 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 larger cytoplasmic components in place (it is thought that the cell's metabolic activity is able to fluidize the cytoplasm to allow the movement of such larger cytoplasmic components). A cell's ability to vitrify in the absence of metabolic activity, as in dormant periods, may be beneficial as a defence strategy. A solid glass cytoplasm would freeze subcellular structures in place, preventing damage, while allowing the transmission of very small proteins and metabolites, helping to kickstart growth upon the cell's revival from dormancy
There has been research examining 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.
The three major elements of the cytoplasm are the cytosol
s and inclusions
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 filament
s such as actin filament
s and microtubule
s that make up the cytoskeleton, as well as soluble protein
s and small structures such as ribosome
s, and the mysterious vault complexes
. 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 dissolved macromolecule
s, such as protein
s, 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.
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
s, and in plant cells, chloroplast
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, to granules of energy-storage materials such as starch
, or polyhydroxybutyrate
. A particularly widespread example are lipid droplet
s, 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 acid
s and sterol
s. Lipid droplets make up much of the volume of adipocyte
s, which are specialized lipid-storage cells, but they are also found in a range of other cell types.
Controversy and research
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.
The material properties of the cytoplasm remain an ongoing investigation. Recent measurements using force spectrum microscopy
reveal that the cytoplasm can be likened to an elastic solid, rather than a viscoelastic fluid.