Do Both Eukaryotes and Prokaryotes Have Cytoplasm?

Every living cell on Earth, whether it belongs to a bacterium, an amoeba, or a human, contains cytoplasm. This gel-like substance fills the interior of the cell and serves as the medium in which most of life’s chemistry takes place. The real question is not whether both groups have cytoplasm but how differently that cytoplasm is organized and what it can do. Prokaryotic cytoplasm was long dismissed as a simple soup of molecules, but research over the past few decades has revealed a far more structured and dynamic interior than anyone expected.

What Cytoplasm Actually Is

Cytoplasm is everything inside the cell membrane except, in eukaryotes, the contents of the nucleus. It includes water, dissolved ions, small molecules like sugars and amino acids, proteins, RNA, and in many cells, various larger structures. In eukaryotic cells, the cytoplasm also houses membrane-bound organelles like mitochondria, the endoplasmic reticulum, and the Golgi apparatus. In prokaryotic cells, those membrane-bound compartments are absent, so the cytoplasm is essentially the entire interior of the cell.

Both types of cytoplasm share a common job: they provide the physical and chemical environment where enzymes work, where genetic information gets translated into proteins, and where energy-generating reactions occur. The differences lie in how all of that is organized spatially.

The Old “Bag of Enzymes” View and Why It Was Wrong

For most of the twentieth century, prokaryotic cells were described as little more than membrane-enclosed bags of enzymes floating in water. Eukaryotic cells, with their visible organelles and internal membranes, were considered the complex ones; bacteria and archaea were their simpler, less interesting relatives. Fluorescence-based imaging techniques over the past three decades have demolished that picture. Real-time imaging of living bacteria revealed that their interiors are highly organized, with proteins, RNA, and other molecules localized to specific regions of the cytoplasm rather than drifting randomly.¹

One striking example comes from studies of mRNA localization in bacteria. Certain mRNA molecules cluster along the cell membrane, others accumulate near the edges of the chromosome region, and still others concentrate at the cell poles. In some cases, cells expressing specific proteins contain dense polar mRNA clusters confined to one end of the cell.² That kind of spatial precision is hard to reconcile with the “bag of enzymes” label.

How Eukaryotic Cytoplasm Is Compartmentalized

The hallmark of eukaryotic cytoplasm is its membrane-bound organelles. Mitochondria generate energy, the endoplasmic reticulum folds and ships proteins, lysosomes digest waste, and so on. Each organelle creates a distinct chemical environment inside its membrane, allowing reactions that would interfere with each other to proceed in isolation. This internal compartmentalization is enormously diverse across the eukaryotic tree of life, with many compartments being specific to particular lineages or tissue types.³

In bacteria and archaea, membrane-bound internal compartments are generally absent. Yet both groups still need to control where specific proteins end up inside the cell, spatially separate incompatible biochemical reactions, and actively transport large structures like chromosomes.⁴ So they have evolved other strategies to get the job done without wrapping everything in its own lipid bilayer.

Prokaryotic Microcompartments

One of those strategies is the bacterial microcompartment: a protein-shelled structure that encloses enzymes inside a polyhedral cage. These microcompartments function like tiny organelles, but their walls are made of protein rather than lipid membrane. The shells are assembled from just two types of structural building blocks that snap together to form the vertices and flat faces of the polyhedron.⁵ Inside, sequentially acting enzymes are arranged so that the product of one reaction feeds directly into the next, protecting key metabolic pathways and boosting their efficiency.⁶

The best-known example is the carboxysome, used by cyanobacteria and some other microbes to concentrate carbon dioxide around the enzyme that fixes it into organic molecules. Other microcompartments handle potentially toxic intermediates from metabolizing compounds like propanediol or ethanolamine. These structures blur the old boundary between “simple” prokaryotes and “complex” eukaryotes. They are not membrane-bound organelles, but they perform the same conceptual function: segregating chemistry that needs to be kept separate from the rest of the cytoplasm.

The Prokaryotic Cytoskeleton

Another surprise from the past few decades is that prokaryotes have cytoskeletons. Eukaryotic cytoskeletons, built from actin filaments, microtubules, and intermediate filaments, are essential for cell shape, division, and internal transport. These proteins were long assumed to be absent in prokaryotes, but researchers discovered that bacteria and some archaea possess structural and functional counterparts of tubulin and actin. The tubulin relative FtsZ plays a central role in bacterial cell division, while the actin relative MreB helps determine cell shape and assists with DNA segregation.⁷

The bacterial versions are not simple copies of their eukaryotic counterparts. They can vary widely in sequence and structure from one species to another, and a single cytoskeletal protein type can take on multiple jobs, while the same job can be performed by different protein types depending on the organism.⁸ But the core insight stands: the cytoplasm of a bacterial cell is physically scaffolded, not an unstructured fluid.

Transcription-Translation Coupling

One important consequence of the simpler prokaryotic cytoplasm is the way genes are expressed. In eukaryotic cells, the DNA sits inside the membrane-bound nucleus, and the mRNA must be exported to the cytoplasm before ribosomes can translate it into protein. These two processes, transcription and translation, are physically separated. In bacteria, everything happens in the same compartment. A ribosome can latch onto an mRNA molecule and start building a protein while the RNA polymerase is still reading the gene and extending the message. This coupled transcription-translation is a defining feature of bacterial gene expression and is possible only because there is no nuclear envelope dividing the cytoplasm from the genetic material.⁹

This coupling has real consequences for how quickly bacteria can respond to environmental changes. A new gene can go from being read to producing a functional protein in seconds, without the delay of nuclear export and mRNA processing that eukaryotic cells face. It also means that regulatory mechanisms in bacteria often work differently, because translation can begin before transcription is even finished.

Glass-Like Properties of Bacterial Cytoplasm

The physical behavior of cytoplasm is stranger than most people expect. Research tracking individual particles inside living bacteria found that the cytoplasm behaves like a glass-forming liquid: it can shift between a relatively fluid state and a more solid-like state depending on the size of the molecules moving through it and the metabolic activity of the cell.¹⁰

When a bacterial cell is metabolically active, its cytoplasm is fluid enough for large components like storage granules and plasmids to move around freely. But when metabolism drops, as it does when cells enter a dormant state, the cytoplasm stiffens. Large particles become trapped in their local environment, unable to diffuse. The cell essentially turns its interior into a gel by running out of chemical energy. This has profound implications for how bacteria survive periods of starvation or stress: the cytoplasm itself becomes a kind of suspended-animation medium, locking molecules in place until conditions improve and metabolic energy fluidizes everything again.

Macromolecular Crowding and Reaction Speed

Both prokaryotic and eukaryotic cytoplasm is extraordinarily crowded with macromolecules. Proteins, ribosomes, nucleic acids, and metabolites are packed together at concentrations far higher than the dilute solutions typically used in laboratory experiments. This crowding is not just a background nuisance; it fundamentally changes how chemical reactions behave inside a living cell. The solutions traditionally used by experimenters do not simulate what is actually going on in cytoplasm, where crowding and preferential interactions among molecules dominate critical processes.¹¹

The overall trend is that increased crowding slows down reactions because molecules have a harder time diffusing through the packed environment to find their reaction partners.¹² But the picture is more complicated than just “everything gets slower.” Simulations have shown that crowding can actually speed up reactions at early time points by pushing molecules closer together, only to slow the same reaction later by preventing them from diffusing to new partners.¹³ The effect is time-dependent and can go in either direction, which means the tidy reaction rates measured in test tubes may not apply to what happens inside a real cell.

Cytoplasmic Streaming in Eukaryotes

One phenomenon found in eukaryotic cytoplasm but not in prokaryotes is cytoplasmic streaming: a continuous, organized flow of the cell’s contents driven by molecular motors. This is especially prominent in large plant cells and algae, where simple diffusion would be far too slow to transport nutrients and waste products across the cell. In the giant green alga Chara, for example, molecular motors pull organelles along actin filament bundles fixed at the cell periphery, and the moving organelles drag the surrounding cytoplasm with them.¹⁴ The flow speeds can reach impressive rates, with some systems clocking in at up to 100 micrometers per second.¹⁵

The mechanism in algal cells involves ATP-dependent sliding between actin cables and myosin molecules attached to organelles. Interestingly, the speed of this sliding is many times faster than actin-myosin sliding in animal muscle, despite using a fundamentally similar molecular setup.¹⁶ Streaming is found across a broad range of eukaryotic organisms, from plants and algae to amoebae, nematodes, and fruit flies. Prokaryotes, lacking the actin-myosin motor system in this form, do not exhibit anything like it. Their cells are small enough that diffusion (supplemented by the metabolic fluidization described above) handles most internal transport needs.

Storage Granules and Cytoplasmic Inclusions

Both eukaryotic and prokaryotic cells use their cytoplasm to stockpile reserves, but bacteria and archaea do it in particularly visible ways. Inclusions in prokaryotic cytoplasm can serve as metabolic reserves, help position the cell in its environment, or act as metabolic organelles in their own right. Common storage materials include glycogen, polyhydroxyalkanoate (PHA), polyphosphate granules, and sulfur globules, many of which accumulate when the cell has more carbon or energy available than it can immediately use.¹⁷

These granules can be massive relative to the cell. In one study of magnetotactic bacteria grown under oxygen-rich conditions, PHA and polyphosphate granules together occupied more than half of the cytoplasmic space.¹⁸ When conditions shifted to oxygen-free, the granules vanished. This responsiveness highlights how dynamic prokaryotic cytoplasm is: its composition can swing drastically depending on what the cell needs at any given moment.

How Cells Keep Their Cytoplasm Stable

Regardless of domain, cells must maintain their cytoplasm within a livable range of pH, ion concentration, and osmotic pressure. Bacteria are particularly impressive at this because they often live in environments with wild pH swings. They use a toolkit of ion exchangers, proton pumps, and metabolic pathways to keep their internal pH within a narrow range even when the outside environment is highly acidic or alkaline.¹⁹

Maintaining the right ionic strength inside the cytoplasm matters because it tunes how enzymes work and how membrane channels behave. When internal ion levels climb too high, bacteria like E. coli swap out potassium ions for electrically neutral molecules like trehalose or proline, preserving osmotic balance without the disruptive effects of excess charge.²⁰ Eukaryotic cells face the same challenges and use broadly similar principles, though the specific molecular players differ and the presence of organelles adds layers of complexity.

Stress Granules and Phase Separation

A relatively recent area of research in eukaryotic cell biology is the discovery that cytoplasm can spontaneously form droplet-like compartments through a process called phase separation. Think of it like oil droplets forming in water: certain proteins and RNA molecules, when concentrated enough under the right conditions, can coalesce into dense assemblies without any surrounding membrane. One well-studied example is the stress granule, a condensate of RNA and RNA-binding proteins that forms when a cell is under duress from heat, toxins, or infection.²¹

Stress granules temporarily sequester mRNA molecules that the cell does not urgently need, freeing up the translation machinery to focus on producing proteins that help the cell survive the crisis. When conditions improve, the granules dissolve and the stored mRNA is released. This kind of membraneless organization is being recognized across both eukaryotic and prokaryotic biology, as researchers find that bacteria also form condensate-like structures within their cytoplasm under certain conditions. Phase separation may represent an ancient organizational strategy, predating the evolution of membrane-bound organelles, that both domains still rely on.

Why the Distinction Still Matters in Practice

Knowing that prokaryotes and eukaryotes share cytoplasm but organize it differently has practical consequences that reach well beyond textbook classification. Antibiotics, for instance, often exploit the unique features of bacterial cytoplasm. Drugs that target the bacterial ribosome work precisely because bacterial ribosomes are structurally different from the eukaryotic ribosomes floating in your own cytoplasm. The fact that bacteria couple transcription and translation in a single cytoplasmic compartment also creates drug targets: interfere with that coupling and you can shut down bacterial protein production without affecting human cells, where the two processes are separated by the nuclear envelope.

Biotechnology takes advantage of prokaryotic cytoplasmic features too. Bacterial microcompartments are being engineered as nanoreactors for industrial enzymes, packaging catalytic pathways inside protein shells to improve yields and prevent side reactions. PHA granules, those storage inclusions that can dominate the bacterial cytoplasm under the right growth conditions, are harvested as biodegradable plastics. Understanding the physical rules of cytoplasmic crowding and phase separation is also reshaping how drug designers think about whether a compound that works beautifully in a dilute test tube will perform the same way inside the packed interior of a living cell.