The inside of a cell may behave like liquid, jelly or glass
Cytoplasm looks like colourless goo, but physicists struggle to say what it is. It seems to flow like a liquid at scales under about 100 nanometres and act like a gel at larger ones. One theory even suggests dormant cells turn their interiors glassy, freezing delicate structures safely in place until revival.
Cytoplasm is everything inside a cell's outer membrane apart from the nucleus: the jelly-like cytosol, the organelles and assorted inclusions. It is roughly 80 per cent water. The cytosol alone fills about 70 per cent of the cell's volume and is thick with protein filaments, ribosomes, proteasomes and the enigmatic vault complexes. So many large molecules are packed in that they crowd one another, so the cytosol does not behave like an ideal solution.
A great deal of life happens here. Glycolysis, photosynthesis and cell division all take place in the cytoplasm, and calcium ions moving in and out act as signals for metabolism. In large plant and animal cells, amoebae and slime moulds, the cytoplasm circulates around organelles in a flow called cytoplasmic streaming. The Swiss anatomist Rudolf von Kolliker introduced the term in 1863, first as another word for protoplasm, and authors still disagree over whether vacuoles and plastids belong inside the definition.
How organelles move without the structure collapsing remains unclear, and competing models try to explain it. The sol-gel idea says the cytoplasm switches between a loose colloidal solution and a linked solid network. A poroelastic model treats it as an elastic, porous scaffold of cytoskeleton and organelles bathed in fluid, and one study found that this scaffold keeps its own distinct set of molecules, concentrating enzymes to speed up reactions.
The glass theory proposes that the more crowded the cytoplasm, the more solid it becomes, with metabolic activity keeping it fluid enough for large components to move. When metabolism stops, vitrification might protect the cell while still letting tiny proteins and metabolites circulate. Another approach sidesteps the question entirely, attributing the jittery, non-random motion of particles to the combined pushing of motor proteins.
Source: Cytoplasm
