Cytoplasm is the material contained within a cell between the plasma membrane and, in eukaryotic cells, the nuclear envelope. It consists broadly of the cytosol together with the structures, molecules and inclusions suspended within it, although terminology varies between textbooks and disciplines. The cytosol is the aqueous phase containing dissolved ions, metabolites, proteins, nucleotides, carbohydrates and other molecules, while organelles such as mitochondria and endoplasmic reticulum occupy the cytoplasmic space. The cytoplasm is therefore not merely an inert filling material.
It is a highly organised chemical environment in which numerous metabolic reactions, signalling processes, transport events, structural interactions and regulatory mechanisms occur. In prokaryotic cells, which lack a membrane bound nucleus, the cytoplasm includes the region containing the chromosome and ribosomes, whereas in eukaryotic cells the nuclear contents are separated from the surrounding cytoplasm by the nuclear envelope.
The physical properties of cytoplasm are important to cellular function. It contains a large proportion of water, but its dissolved macromolecules and ions give it properties that differ substantially from pure water. Proteins, nucleic acids, sugars, lipids and other macromolecules occupy space and can produce molecular crowding, which affects reaction rates, diffusion and interactions between cellular components. Cytoplasm can behave as a complex viscoelastic material rather than as a simple homogeneous liquid.
Cytoplasmic organisation is also influenced by the cytoskeleton, organelle membranes, phase separated molecular assemblies and local concentration gradients. Diffusion is an important means of intracellular transport for small molecules, but larger structures commonly require directed transport systems. Cytoplasmic pH, ionic composition, redox state, osmotic conditions and concentrations of metabolites are actively regulated because relatively small changes can alter enzyme activity, membrane function and protein stability.
A major function of cytoplasm is to provide the environment for metabolism. Glycolysis, for example, occurs in the cytosol and converts glucose through a series of enzyme catalysed reactions into pyruvate while producing ATP and reduced NADH. Many stages of amino acid metabolism, nucleotide metabolism, lipid synthesis and carbohydrate metabolism also involve cytoplasmic enzymes or intermediates. Cytoplasm also contains messenger RNA, transfer RNA, ribosomes and numerous regulatory proteins involved in protein synthesis and gene expression.
Translation occurs on ribosomes located in the cytosol or associated with the cytoplasmic surface of the rough endoplasmic reticulum. Metabolic pathways do not operate independently. Substrates and products move between cytoplasmic reactions and organelles, allowing the cell to coordinate energy production, biosynthesis, degradation and signalling according to its physiological requirements.
Cytoplasmic organisation differs substantially between cell types. In animal cells, the cytoplasm may contain mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, vesicles and extensive cytoskeletal networks. Plant cells contain these structures as well as chloroplasts and a large central vacuole, which can occupy much of the cellular volume and displace cytoplasm toward the cell periphery. Some specialised cells contain particularly abundant cytoplasmic structures because of their functions.
Muscle cells contain extensive contractile machinery, secretory cells contain abundant rough endoplasmic reticulum and Golgi apparatus, and cells with high energetic requirements commonly contain many mitochondria. The cytoplasm can therefore be considered a dynamic compartment whose composition reflects the specialised activities, developmental state and physiological demands of each cell.
Cytoplasmic movement is another important biological phenomenon. In many plant cells, cytoplasmic streaming can move organelles and dissolved materials through the cell and can increase the effective distribution of resources. Cytoplasmic flows can also occur during cell division, development and changes in cell shape. Motor proteins associated with the cytoskeleton transport vesicles, organelles and molecular complexes through the cytoplasm along microtubules or actin filaments. These mechanisms allow intracellular distribution to be spatially organised rather than dependent exclusively on random diffusion. Cytoplasmic compartmentalisation can also arise without conventional membranes through biomolecular condensates produced by selective molecular interactions and phase separation. Such condensates can concentrate particular proteins and nucleic acids and influence processes including RNA metabolism, signalling and stress responses.
The cytoplasm participates directly in cellular signalling and homeostasis. Changes in calcium concentration, for example, can act as intracellular signals that influence muscle contraction, secretion, metabolism, gene regulation and other processes. ATP concentration provides information about cellular energetic status, while reactive oxygen species can function both as signalling molecules and as damaging oxidants when their levels become excessive. Cytoplasmic kinases, phosphatases, GTPases and other regulatory proteins transmit information from receptors and internal sensors to downstream targets. Cytoplasmic quality control systems identify damaged proteins and organelles and coordinate repair, recycling or degradation. The cytoplasm therefore acts as an integrated biochemical environment in which information about extracellular conditions, nutrient availability, energy status and cellular damage can influence cellular behaviour.
The cytoplasm should be distinguished from cytosol, cytoplasmic matrix and protoplasm. Cytosol generally refers specifically to the aqueous, soluble portion of the cytoplasm excluding membrane bound organelles, whereas cytoplasm is commonly used more broadly for the material inside the plasma membrane outside the nucleus. Protoplasm is an older and broader term referring to the living material of a cell, potentially including both cytoplasm and nucleoplasm.
These distinctions can vary according to context, and scientific writing sometimes uses cytoplasm in a less technically restrictive manner. Cytoplasmic inclusions may include stored glycogen, lipid droplets, pigments, crystals or other accumulated materials, although these are not necessarily membrane bound organelles. Understanding these terminological distinctions prevents the common misconception that cytoplasm means only the liquid component of a cell.
Cytoplasmic dysfunction is associated with many forms of cellular injury and disease because so many essential reactions depend on its chemical and structural organisation. Disturbed ion concentrations can disrupt enzyme activity and osmotic balance, oxidative stress can damage proteins, lipids and nucleic acids, and defects in intracellular trafficking can cause accumulation of cellular components. Protein aggregation can interfere with normal cytoplasmic organisation, while mutations affecting cytoskeletal proteins, metabolic enzymes or organelle maintenance systems can alter cytoplasmic function.
Cytoplasmic inheritance is also relevant in some organisms because organelles such as mitochondria and chloroplasts contain their own genomes that are maintained within the cytoplasmic compartment. Overall, cytoplasm is best understood as a dynamic, regulated and spatially organised cellular environment rather than simply the material surrounding organelles.
