Cytoplasm

The cytoplasm encompasses all material within a eukaryotic or prokaryotic cell that lies inside the cell membrane but outside the nucleus in eukaryotes. It forms the dynamic internal environment in which organelles are suspended, biochemical reactions proceed and intracellular movement takes place. Although largely aqueous and colourless, the cytoplasm is a complex, organised system composed of cytosol, subcellular structures, organelles and numerous inclusions. Together, these components support the cell’s structural integrity, metabolic activity and capacity for growth and division.

Structural components

The cytoplasm consists of three principal elements: cytosol, organelles and cytoplasmic inclusions. The cytosol is a gel-like solution containing water, ions, small molecules, proteins and an intricate network of cytoskeletal filaments. It constitutes roughly 70 per cent of total cell volume and provides the medium through which molecules, vesicles and organelles move. High concentrations of macromolecules within the cytosol give rise to macromolecular crowding, a phenomenon that alters biochemical reaction rates and the interactions of proteins and ribonucleoprotein complexes.
Cytoskeletal elements such as actin filaments and microtubules traverse the cytosol, providing mechanical support, enabling intracellular transport and facilitating processes including cell migration, cytokinesis and vesicle trafficking. Soluble ribosomes, proteasomes and small ribonucleoprotein particles are also suspended within this environment.
Organelles—membrane-bound compartments with specific metabolic or structural roles—occupy substantial regions of the cytoplasm. These include mitochondria, the endoplasmic reticulum, the Golgi apparatus, lysosomes, vacuoles and, in plant cells, chloroplasts. The nucleus in eukaryotes is surrounded by its own envelope and contains the nucleoplasm, which is functionally distinct from the cytoplasm. Surrounding the internal organelles is the groundplasm, the submicroscopic matrix visible as hyaloplasm under light microscopy.
Cytoplasmic inclusions represent non-membranous deposits such as starch granules, glycogen particles, lipid droplets and mineral crystals. In plants, inclusions may include calcium oxalate or silica crystals. In many animal cells, particularly adipocytes, lipid droplets form major storage depots for fatty acids and sterols.

Organisation and subregions

The cytoplasm is not uniform. The inner region, termed the endoplasm, is more granular and fluid, whereas the outer layer, known as the ectoplasm or cell cortex, is more viscous and enriched with cytoskeletal components. These differences support both structural stability and dynamic reorganisation. Movement of calcium ions within the cytoplasm serves as a signalling mechanism regulating numerous metabolic pathways, including processes related to secretion, contraction and gene regulation. In plant cells, the circulation of cytoplasm around large central vacuoles, known as cytoplasmic streaming, enhances the distribution of metabolites and organelles.

Historical development of the concept

The term cytoplasm was introduced by Rudolf von Kölliker in 1863. Initially used as a synonym for protoplasm, it later acquired a more precise definition referring specifically to all cell contents excluding the nucleus. Definitions varied among early cell biologists, with some excluding certain organelles such as plastids or vacuoles; however, the modern understanding encompasses all internal components outside the nuclear envelope in eukaryotes.

Physical nature and mechanical properties

Although predominantly composed of water, the cytoplasm exhibits complex viscoelastic behaviour. Subcellular components move through it in a manner that is not fully described by simple diffusion, especially in dense regions. Various theories have been proposed to account for the physical properties of the cytoplasm and the irregular movements of particles within it.
One classical interpretation views the cytoplasm as a sol–gel system, alternating between more fluid (sol) and more solid (gel) states depending on the strength of interactions among cytosolic constituents. At nanometre scales the cytoplasm may behave like a fluid, while at larger scales it may resemble a semi-solid network. This duality helps explain varying particle dynamics and the ability of cells to modulate internal rigidity according to functional requirements such as motility or division.
Another perspective suggests that the cytoplasm behaves analogously to a glass-forming liquid. As concentration of macromolecules increases, the cytoplasm can approach a glass-like state where molecular movements are restricted. The cell’s metabolic activity is thought to “fluidise” the cytoplasm, enabling structural rearrangements and transport processes. During dormancy, some cells may allow the cytoplasm to vitrify, stabilising internal structures and protecting them from damage while still permitting diffusion of very small metabolites essential for reactivation.
Alternative approaches focus on the active forces generated by motor proteins and cytoskeletal dynamics. Rather than attributing particle behaviour solely to the material properties of the cytoplasm, this view emphasises the contributions of random intracellular forces produced by molecular motors such as myosins, kinesins and dyneins. These forces give rise to non-Brownian motion and contribute to the robust mixing of cytoplasmic components.
Advances in biophysical research have enabled direct measurement of cytoplasmic mechanics. Techniques such as optical tweezers have been used to probe viscoelasticity in living mammalian cells, revealing the interplay between cytoskeletal tension, macromolecular crowding and the structural organisation of the cytoplasmic network.

Constituents and their roles

The cytosol is the site of numerous metabolic pathways, including glycolysis and, in photosynthetic organisms, parts of the photosynthetic process. It is also essential for the assembly of cytoskeletal elements and ribonucleoprotein complexes. Regulatory mechanisms control the diffusion and localisation of molecules; small ions such as calcium diffuse rapidly, whereas larger complexes often require active transport mediated by cytoskeletal motors.
Organelles within the cytoplasm perform specialised functions: mitochondria generate ATP via oxidative phosphorylation; the endoplasmic reticulum participates in protein and lipid synthesis; the Golgi apparatus modifies, sorts and packages macromolecules; and lysosomes and vacuoles carry out digestion and recycling. In plant cells, chloroplasts support photosynthesis and contribute to metabolic integration between the cytosol and plastidial compartments.
Cytoplasmic inclusions vary widely across cell types. Storage granules such as glycogen in hepatocytes, starch in plant cells and polyhydroxybutyrate in some bacteria serve as accessible energy reserves. Lipid droplets are ubiquitous in both prokaryotic and eukaryotic cells, providing lipid storage and playing roles in membrane synthesis, signalling and metabolic regulation.

Contemporary research and significance

Recent research has challenged earlier conceptions of the cytoplasm as merely a passive medium. It is now recognised as an active participant in the regulation of intracellular movement, nutrient distribution and mechanical response. Its viscoelastic nature, governed by the rate of bond formation and breakage within the cytoplasmic network, influences processes such as vesicle trafficking, cytokinesis and organelle positioning.
The cytoplasm also plays a role in heredity, as many organelles—including mitochondria and plastids—are inherited maternally in most species. This contributes to the transmission of non-nuclear genetic information and influences phenotypic variation.