The cytoplasm is about 80% water and usually colorless.
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 ribosomes, mitochondria, the plant plastids, lipid droplets, and vacuoles.
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 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.
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
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.
Main article: 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.
The inner, granular and more fluid portion of the cytoplasm is referred to as endoplasm.
This crowding effect alters how the components of the cytosol interact with each other.
Main article: Organelle
Organelles (literally "little organs"), are usually membrane-bound structures inside the cell that have specific functions.
Main article: 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, to granules of energy-storage materials such as starch, glycogen, or polyhydroxybutyrate.
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.
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.
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.
A method of determining the mechanical behaviour of living cell mammalian cytoplasm with the aid of optical tweezers has been described.
Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Cytoplasm.