What Is Cytoplasm?

Cytoplasm is the material enclosed by a cell's plasma membrane, excluding the nucleus in eukaryotic cells. It consists of the cytosol, a concentrated aqueous solution of proteins, metabolites, salts, and nucleic acids, together with the membrane-bounded organelles and cytoskeletal filaments suspended in that solution. Most of a cell's metabolic chemistry happens there: glycolysis, protein synthesis on free ribosomes, fatty acid metabolism, and the signaling cascades that relay information from surface receptors to the nucleus.

The term entered biology in the nineteenth century, when light microscopists needed a name for the granular substance surrounding the nucleus. Modern usage is more precise. The cytosol is the fluid phase alone, while cytoplasm covers the cytosol plus everything suspended in it. For engineers working on intracellular delivery, biosensors, or cell-free reaction systems, cytoplasm is best understood as a crowded, structured, chemically active medium rather than a simple buffer.

The Cytosol and Macromolecular Crowding

The cytosol holds roughly 200 to 400 milligrams of macromolecules per milliliter, which occupies a substantial fraction of the available volume and leaves far less free space than a dilute test-tube solution. This condition, described in reviews of macromolecular crowding and cell physiology, shifts binding equilibria toward association, slows the diffusion of large complexes, and changes protein folding kinetics relative to in vitro measurements. Crowding is one reason that reaction rates measured in purified systems often fail to predict behavior inside a living cell.

Physical measurements have pushed the description further. Work reported in Cell showed that the bacterial cytoplasm has glass-like properties and is fluidized by metabolic activity, so that small proteins diffuse freely while large complexes become effectively trapped unless metabolism supplies energy. Cytoplasm therefore behaves less like water and more like a soft, active material whose mechanical state depends on the cell's energy status.

Organelles and Compartmentalization

In eukaryotes, much of the cytoplasmic volume is partitioned into membrane-bounded compartments. As described in the treatment of cellular compartmentalization in Molecular Biology of the Cell, each organelle maintains its own protein set, ionic conditions, and pH, and because the lipid bilayer blocks most hydrophilic molecules, every compartment depends on selective membrane transport proteins to exchange material with the cytosol.

The endoplasmic reticulum is the largest of these compartments. Roughly half the total membrane area of an animal cell encloses the labyrinthine spaces of the endoplasmic reticulum, whose cytosolic surface carries the ribosomes that synthesize secreted and membrane proteins. Mitochondria, the Golgi apparatus, peroxisomes, lysosomes, and transport vesicles occupy the remaining compartmentalized volume, each connected to the cytosol by defined import and export routes.

Structure and Intracellular Transport

Cytoplasm is not homogeneous. Actin filaments, microtubules, and intermediate filaments span it, giving the cell mechanical stiffness and defining tracks along which motor proteins such as kinesin, dynein, and myosin carry vesicles and organelles. Directed transport along microtubules moves cargo far faster than diffusion would over the length of a neuron's axon, which can exceed a meter. In many plant and algal cells, bulk cytoplasmic streaming driven by myosin motors along actin cables circulates the contents and distributes metabolites. Membraneless condensates formed by liquid-liquid phase separation, including stress granules and P bodies, add a further layer of organization without any bounding membrane.

Applications

Knowledge of cytoplasmic structure and behavior informs work in a range of technical fields, including:

• Cell-free synthetic biology, where cytoplasmic extracts drive transcription and translation outside a living cell
• Intracellular drug and gene delivery, where a carrier must cross the membrane and release its cargo into the cytosol
• Electroporation and microinjection systems used in cell engineering and transfection
• Whole-cell computational modeling, which represents crowding and diffusion explicitly
• Cryopreservation, where vitrification of the cytoplasm avoids damaging ice crystal formation
• Fluorescent and genetically encoded biosensors that report cytosolic calcium, pH, or ATP