Cytoplasm is everything inside a cell between the outer membrane and the nucleus. It consists of three main components: a watery fluid called cytosol, the organelles suspended within it, and a network of protein filaments called the cytoskeleton that gives the cell its shape. Far from being a simple jelly, the cytoplasm is a densely packed, dynamic environment where most of a cell’s chemical reactions take place.
Cytosol: The Fluid Base
The cytosol is the liquid portion of the cytoplasm. It’s mostly water, dissolved salts, and a high concentration of proteins and other large molecules. That concentration is strikingly dense: macromolecules occupy anywhere from 5% to 40% of the total cellular volume, reaching concentrations around 200 to 350 milligrams per milliliter. Scientists call this “macromolecular crowding,” and it has real consequences for how the cell works.
In a dilute solution like a test tube of salt water, molecules bounce around freely. Inside the cytosol, all that crowding slows things down. Small proteins still diffuse relatively freely, but larger molecular complexes move in irregular, restricted patterns because there’s simply less open space for them to navigate. Metabolic activity helps keep things moving. In dormant bacteria, even small particles start to get stuck, suggesting that the cell actively maintains its fluid state through energy expenditure.
The cytosol maintains a pH of roughly 7.2 to 7.4 in mammalian cells, which is close to neutral. This matters because even modest shifts in acidity can change the physical state of the cytoplasm. Research in yeast cells has shown that when pH drops, proteins begin assembling into larger structures throughout the cytoplasm, transforming it from a fluid into something closer to a solid. This transition helps cells enter a dormant, protected state when conditions become harsh.
How the Cytoplasm Behaves Physically
The cytoplasm doesn’t fit neatly into the category of liquid or solid. Physically, it behaves as a viscoelastic material, meaning it has properties of both. Push something through it quickly and it resists like a gel. Give it time and it flows like a thick fluid. This dual nature comes from the web of proteins, filaments, and organelles packed inside it.
Under normal conditions, the cytoplasm stays closer to the fluid end of the spectrum. Metabolic energy keeps proteins from clumping together and maintains the internal environment in a state scientists have described as a liquid near the transition point to a glass-like state. When cells lose energy or become acidified, the cytoplasm stiffens measurably. Experiments tracking tiny particles inside cells found that their movement shifted from fluid-like to increasingly constrained, with restoring forces pushing displaced particles back into place, much like a spring. The analysis showed that cells transitioned from a compliant, more viscous material to a stiffer, more elastic one as pH dropped.
The Cytoskeleton
Running through the cytoplasm is a scaffold of protein filaments collectively called the cytoskeleton. It serves as the cell’s structural framework, its internal highway system, and its engine for movement. Three types of filaments make up this network, each with a different size and job.
Microfilaments
These are the thinnest filaments, just 4 to 6 nanometers in diameter. They’re made of a protein called actin, arranged like two strings of pearls twisted around each other. Microfilaments concentrate near the cell’s outer edge, where they help the cell change shape, crawl across surfaces, and divide. They also play a role in moving materials out of the cell during secretion.
Intermediate Filaments
At 8 to 12 nanometers in diameter, these rope-like fibers provide mechanical strength. They act like the cell’s internal cables, resisting stretching and compression. In nerve cells, intermediate filaments called neurofilaments help determine the thickness of the long axon that transmits signals. Different cell types use different versions of these filaments, but their structural role is consistent: they keep the cell from being torn apart by physical stress.
Microtubules
The largest cytoskeletal filaments are hollow tubes about 25 nanometers across. Microtubules serve as tracks along which motor proteins transport organelles, vesicles, and other cargo to specific destinations within the cell. They also form the machinery that pulls chromosomes apart during cell division. Unlike intermediate filaments, microtubules are highly dynamic, constantly growing and shrinking as the cell’s needs change.
Organelles Suspended in the Cytoplasm
Scattered throughout the cytoplasm of animal and plant cells are membrane-bound compartments called organelles, each handling a different set of tasks. Major organelles include mitochondria (which generate energy), the endoplasmic reticulum (which folds proteins and builds lipids), the Golgi apparatus (which packages and ships molecular cargo), and lysosomes (which break down waste). Plant cells also contain chloroplasts for photosynthesis. Each organelle is enclosed by its own membrane, creating a separate internal environment with its own pH and chemical conditions. Lysosomes, for example, maintain a highly acidic interior around pH 4.7, while mitochondria are slightly alkaline at about pH 8.0.
These organelles aren’t fixed in place. They move constantly through the cytoplasm, sometimes traveling to specific locations, sometimes merging with one another, sometimes growing or shrinking. The cytoskeleton provides the tracks and motor systems that make this movement possible.
Cytoplasmic Streaming
In many cells, especially large plant cells, the entire fluid contents of the cytoplasm circulate in an organized flow called cytoplasmic streaming, or cyclosis. The driving mechanism relies on motor proteins called myosins that travel along actin filament tracks anchored at the cell’s periphery. As these motor proteins move, they drag organelles and fluid along with them, creating large-scale circulation patterns.
This streaming solves a practical problem. In a small cell, molecules can reach their destinations by simple diffusion in a fraction of a second. In a large plant cell, diffusion alone would be far too slow. Streaming physically transports nutrients, chloroplasts, and signaling molecules where they need to go. Aquatic plant cells like those in Elodea leaves use streaming to reposition chloroplasts in response to light. Pollen tubes use fountain-like flows to deliver growth material to their extending tips. Even animal cells use versions of this process: fruit fly egg cells use disorderly streaming patterns to distribute proteins without creating counterproductive backflows.
Cytoplasm in Prokaryotic Cells
Bacterial and archaeal cells have cytoplasm too, but it’s organized differently. Prokaryotic cells generally lack the membrane-bound organelles found in animal and plant cells. Their DNA floats directly in the cytoplasm rather than being enclosed in a nucleus, concentrated in a region called the nucleoid. Ribosomes, the molecular machines that build proteins, are scattered freely throughout.
That said, the old view of prokaryotic cytoplasm as a featureless bag of enzymes has been overturned. Bacteria have their own cytoskeletal proteins, and mounting evidence shows they possess a more organized subcellular architecture than previously thought. Some bacteria even have simple internal compartments, though these are far less elaborate than eukaryotic organelles.
Key Chemical Reactions in the Cytoplasm
The cytoplasm is where many of the cell’s most important metabolic reactions happen. Glycolysis, the ten-step process that breaks down glucose to extract energy, occurs entirely in the cytosol. This pathway is one of the oldest and most universal energy-harvesting processes in biology, used by nearly every living cell. When oxygen is available, the products of glycolysis move into the mitochondria for further energy extraction. When oxygen is scarce, the remaining steps also happen in the cytoplasm, converting pyruvate into lactate through fermentation.
Protein synthesis also begins in the cytoplasm, where ribosomes read messenger RNA and assemble amino acids into new proteins. Some of these ribosomes are free-floating in the cytosol, while others are attached to the surface of the endoplasmic reticulum. The location determines where the finished protein ends up: free ribosomes generally produce proteins that stay in the cytoplasm, while those on the endoplasmic reticulum make proteins destined for export or insertion into membranes.