Cytoplasm is found in both prokaryotic and eukaryotic cells. It is one of the few cellular components shared by every living cell on Earth, from bacteria to human neurons. While the cytoplasm in both cell types serves as the main workspace where chemical reactions happen, what’s actually inside that cytoplasm differs significantly between the two.
What Cytoplasm Actually Is
Cytoplasm refers to everything inside a cell’s outer membrane except the nucleus. It includes the gel-like fluid (called cytosol), along with all the structures and molecules suspended in it. In a prokaryotic cell, which has no nucleus, the cytoplasm essentially fills the entire interior. In a eukaryotic cell, the cytoplasm is the space between the outer membrane and the nuclear membrane.
A related term worth knowing: cytosol is just the liquid portion of the cytoplasm. It excludes organelles and their contents. So cytosol is a subset of cytoplasm, not a synonym for it.
Cytoplasm in Prokaryotic Cells
Prokaryotes like bacteria and archaea have a relatively simple interior. Their cytoplasm contains ribosomes (for building proteins), a region called the nucleoid where their single circular chromosome sits, and a concentrated mix of organic molecules and salts. There are no membrane-enclosed compartments separating different functions. The DNA floats freely in the cytoplasm rather than being sealed inside a nucleus.
This open layout has a major consequence for how prokaryotes work. Because there’s no membrane barrier between DNA and the rest of the cell, transcription (reading genetic instructions) and translation (building proteins from those instructions) happen simultaneously. Ribosomes can latch onto a strand of messenger RNA and start assembling a protein before the cell has even finished copying the genetic message. This coupled process makes prokaryotes remarkably efficient at responding to environmental changes.
Scientists used to think prokaryotic cytoplasm was essentially a disorganized soup. That view has changed. Researchers have discovered that bacteria form membraneless organelles through a process called liquid-liquid phase separation, where certain molecules naturally cluster together into droplet-like compartments without needing a surrounding membrane. This gives prokaryotic cells more internal organization than their simple reputation suggests.
Cytoplasm in Eukaryotic Cells
Eukaryotic cells, found in animals, plants, fungi, and protists, pack their cytoplasm with membrane-bound organelles that divide labor the way rooms divide a house. Mitochondria generate energy from food molecules. The endoplasmic reticulum helps build membranes and transport proteins. The Golgi apparatus packages and ships molecules to their destinations. Lysosomes break down and recycle worn-out components. Plant cells add chloroplasts, which capture energy from sunlight.
Each of these organelles is separated from the surrounding cytosol by its own membrane, creating distinct chemical environments tailored to specific tasks. This compartmentalization is the defining feature of eukaryotic cytoplasm and the biggest difference from prokaryotic cytoplasm. In eukaryotes, transcription happens inside the nucleus while translation happens out in the cytoplasm, a physical separation that allows for additional layers of gene regulation that prokaryotes lack.
Shared Features of Cytoplasm in Both Cell Types
Despite these differences, the cytoplasm in prokaryotes and eukaryotes shares several core functions. Glycolysis, the ancient ten-step chemical pathway that breaks down sugar to produce energy, takes place in the cytoplasm of virtually all cells, prokaryotic and eukaryotic alike. This pathway generates a net gain of two ATP molecules (the cell’s energy currency) per sugar molecule and doesn’t require oxygen. Its universal presence in the cytoplasm of both cell types points to a shared evolutionary origin.
Both cell types also have ribosomes scattered throughout their cytoplasm. Though eukaryotic ribosomes are larger than prokaryotic ones, both perform the same fundamental job of reading genetic instructions and assembling proteins.
Even the cytoskeleton, a network of protein filaments that gives cells their shape, turns out to be shared. Scientists long believed prokaryotes lacked any internal scaffolding, but discoveries in the 1990s and early 2000s overturned that assumption. A protein called FtsZ, essential for bacterial cell division, was shown to be structurally similar to tubulin, the building block of the eukaryotic cytoskeleton. Another bacterial protein, MreB, which helps rod-shaped bacteria maintain their shape by forming spiral structures beneath the cell membrane, turned out to be a structural match for actin, one of the main cytoskeletal proteins in eukaryotic cells. The cytoskeleton didn’t arise in eukaryotes. It originated in prokaryotes.
Physical Properties of Cytoplasm
The cytoplasm isn’t uniform from species to species. Studies measuring the viscosity (thickness) of prokaryotic cytoplasm found surprising variation. Common lab bacteria like E. coli and Pseudomonas, growing between 20 and 40°C, have cytoplasm with a viscosity close to water. The same is true for the heat-loving archaeon Pyrococcus, which thrives near boiling temperatures. But other prokaryotes like Lactococcus and Geobacillus have cytoplasm far thicker than water, closer to the consistency of a concentrated glycol solution.
Eukaryotic cells generally show cytoplasmic viscosity only slightly above that of water or a dilute solution, though measurements have been more extensively studied in eukaryotes than prokaryotes. These physical differences affect how quickly molecules can move around inside cells and how efficiently chemical reactions proceed.
Why the Distinction Matters
Understanding that cytoplasm is universal but not identical across cell types helps clarify a lot of biology. Antibiotics, for example, can target prokaryotic ribosomes in the cytoplasm without affecting the larger eukaryotic ribosomes in your own cells. The absence of membrane-bound organelles in prokaryotic cytoplasm is the reason bacteria can couple transcription and translation for rapid protein production, something your cells cannot do. And the shared ancestry of cytoplasmic components like the cytoskeleton and glycolysis reflects billions of years of evolution from a common cellular ancestor that already had a functioning cytoplasm.