CytoplasmEdit
Cytoplasm is the jelly-like substance inside cells, occupying the space between the plasma membrane and the nucleus in most eukaryotic cells. It comprises the cytosol—the gel-like fluid filled with water, ions, and dissolved molecules—and the organelles and other structures that float within it. In prokaryotic cells, the cytoplasm is the entire content enclosed by the cell membrane, since there is no nucleus. The cytoplasm is not a passive backdrop; it is a bustling milieu where much of a cell’s metabolism, growth, and response to the environment take place. Its properties—such as macromolecular crowding, viscosity, and spatial organization—shape how chemical reactions proceed and how signals are transmitted inside the cell. The arena of the cytoplasm supports both energy production and biosynthesis, and it serves as the staging ground for the cell’s movement, division, and adaptation.
Because it contains a wide array of enzymes, substrates, and structural networks, the cytoplasm is organized yet dynamic. It houses the organelles that carry out specialized tasks, such as mitochondria, chloroplasts in plants, peroxisomes, lysosomes, and other membrane-bound compartments, all suspended within the cytosol. The boundaries of the cytoplasm are defined by the plasma membrane, but the nuclear envelope marks a distinct interface with the nucleus. The cytoplasm also includes the cytoskeleton, a network of filaments that supports shape, anchors organelles, and provides tracks for motor proteins to move cargo. These components work together to coordinate metabolism, growth, and response to stress, while maintaining the homeostasis essential for cellular life.
Structure and composition
The cytosol
The cytosol is a viscous, water-based solution in which most soluble enzymes operate. It contains a complex mixture of ions, metabolites, nucleotides, and macromolecules, all at high concentration relative to dilute solutions used in experiments. Within this milieu, a variety of pathways—such as glycolysis and parts of nucleotide metabolism—occur in the cytosol. The crowded interior of the cytosol influences reaction rates and the diffusion of molecules, making spatial organization a key factor in cellular efficiency. The cytosol is also the region where many ribosomes float, translating mRNA into polypeptides. cytosol; glycolysis; metabolism; ribosome
Organelles and inclusions
Suspended in the cytosol are membrane-bound organelles, each with specialized functions. Mitochondria generate most of a cell’s ATP through oxidative phosphorylation, while chloroplasts (in plants and some algae) carry out photosynthesis. Other organelles, such as the endoplasmic reticulum (rough ER and smooth ER), the Golgi apparatus, peroxisomes, and lysosomes, contribute to protein synthesis, processing, and waste management. The organization of these compartments within the cytoplasm enables coordinated metabolism and trafficking of biomolecules. mitochondrion; chloroplast; endoplasmic reticulum; Golgi apparatus; peroxisome; lysosome; organelle
Boundaries and interface with membranes
The cytoplasm is enclosed by the plasma membrane, which defines the cell’s boundary and regulates exchange with the external environment. The nucleus is separated from the cytoplasm by the nuclear envelope, creating a distinct compartment for genetic material while still allowing communication through nuclear pores. Within the cytoplasm, vesicular transport and membrane-bound organelles interact with the cytosolic environment to distribute proteins and lipids where they are needed. plasma membrane; nucleus; nuclear envelope; vesicle; organelle
The cytoskeleton
A dynamic scaffold composed of microfilaments (actin), intermediate filaments, and microtubules gives structural support and provides pathways for intracellular transport. Motor proteins move along these filaments to deliver cargo, reposition organelles, and contribute to cell movement and division. The cytoskeleton’s organization affects enzyme access, signaling efficiency, and mechanical resilience. cytoskeleton; actin; microtubule; kinesin; dynein
Functions
Metabolism and biosynthesis
The cytoplasm hosts many core metabolic processes, including glycolysis, which converts glucose to pyruvate in the cytosol and yields a portion of the cell’s usable energy. In conjunction with organelles, the cytoplasm supplies precursors for macromolecule synthesis, supporting growth and maintenance. The distribution and concentration of enzymes in the cytoplasm influence reaction pathways and flux through metabolic networks. glycolysis; metabolism
Protein synthesis and folding
While ribosomes can be attached to organelles such as the rough endoplasmic reticulum, a large fraction of ribosomes reside freely in the cytosol, translating mRNA into polypeptides that then fold into functional proteins. The cytoplasm also contains quality-control systems that monitor folding and misfolding, directing defective proteins to degradation pathways when necessary. ribosome; translation; protein folding
Transport and signaling
Vesicles and protein complexes move through the cytoplasm along cytoskeletal tracks, delivering cargo to specific destinations. The cytoplasmic environment supports signaling cascades, in which ions and second messengers propagate information from the cell surface to internal targets, coordinating responses to environmental cues. vesicle; cytoskeleton; signal transduction
Response to stress and maintenance of homeostasis
The cytoplasm responds to changes in temperature, osmotic conditions, and nutrient availability by adjusting the activity and localization of enzymes and structural components. This adaptability helps cells survive fluctuations in their environment while maintaining essential functions. osmotic stress; homeostasis
Cytoplasmic dynamics
Movement and crowding
The cytoplasm is not a uniform liquid; it is a crowded, gel-like medium where macromolecules occupy a substantial fraction of the volume. This crowding influences diffusion rates and reaction thermodynamics, and it can give rise to phase-like behavior where certain components gather into functional clusters. macromolecular crowding; phase separation
Cytoplasmic streaming and transport
In some plant and algal cells, cytoplasmic streaming distributes nutrients and signaling molecules through the cell, a process powered by motor proteins along cytoskeletal filaments. This dynamic distribution supports growth and responses to light and other environmental factors. cytoplasmic streaming; motor protein; cytoskeleton
Cytoplasm across organisms
Prokaryotes
In bacteria and archaea, the cytoplasm fills the interior of the cell and contains the nucleoid region where DNA resides, as well as ribosomes and metabolic enzymes. There are no membrane-bound organelles, so many processes occur in a more homogeneous cytosolic space, yet spatial organization still influences efficiency and regulation. prokaryote; nucleoid; ribosome
Eukaryotes
In eukaryotic cells, the cytoplasm sits between the plasma membrane and the nucleus, housing a suite of organelles that compartmentalize metabolism and trafficking. The relative abundance and arrangement of organelles reflect the cell type and its functional priorities, from energy production in mitochondria to macromolecule processing in the ER and Golgi apparatus. eukaryote; organelles
Historical notes and terminology
The term cytoplasm has long been used to describe the contents of the cell interior, but definitions can vary by tradition: some accounts emphasize the soluble cytosol, while others include organelles as part of the cytoplasmic space. Advances in cell biology have refined our understanding of how the cytoplasm supports metabolism, protein synthesis, and transport in a spatially organized context. cytosol; cell biology