What Is Cytosol? How Is It Different From Cytoplasm?

Cytosol is the fluid found inside living cells. More specifically, it’s the water-based fluid in which organelles, proteins and other structures of the cell live. Also known as the cytoplasmic matrix, it constitutes most of the intracellular fluid (ICF). Cytosol is often confused with cytoplasm, however, which is an entirely different entity within a cell.

Cytosol is often confused with cytoplasm. They both start with ‘cyto’ and seem to refer to the same thing in most cellular biology textbooks. However, though they appear to be interchangeable, they are two separate terms and their usage can provide different information.

While cytoplasm consists of all the contents found inside a cell (excluding the nucleus), cytosol is just the liquid or aqueous part of the cytoplasm. In other words, cytoplasm is the area of space outside the nucleus that consists of cytosol and other organelles.

Cytosol And Cytoplasm Definitions

Cytosol is the fluid found inside living cells. More specifically, it’s the water-based fluid in which organelles, proteins and other structures of the cell live. Also known as the cytoplasmic matrix, it constitutes most of the intracellular fluid (ICF).

The cytoplasm, on the other hand, is everything between the boundaries of the cell, excluding the nucleus. Cell biologists refer to the cytoplasm as everything between the cell membrane and the nucleus. The mitochondria floats in the cytoplasm.

The cytosol is the jelly-like fluid that forms the cytoplasmic medium. The mitochondria and its contents aren’t a part of the cytosol, though cytosol is a part of the cytoplasm.

The cytosol also contains dissolved molecules, free ribosomes (the molecular machines that make proteins), and the cytoskeleton (proteins that help the cell maintain its shape).

As you may already know, the cell is the basic unit of life. Every living being in the world is made of billions upon billions of cells, which are responsible for a number of essential life processes.

The boundary of the cell is called the cell membrane, enclosing all the cellular components. It is like the skin of the cell. The insides of the cell are made of various components, including the cellular organelles like the mitochondria, nucleus, lysosomes, molecules and ions. All of this is floating in a primarily water-based fluid.

This water-based fluid, with dissolved ions like calcium and sodium, as well as larger dissolved molecules, is the cytosol. Another element of the cytosol are insoluble proteins like the ribosomes (the giant protein machine that makes proteins), and the cytoskeleton (the proteins that help the cell hold its shape, just like our bony skeleton).

Cytosol Composition

The majority of cytosol is water, which makes up nearly 70% of the total volume of a cell. While the pH of the intracellular fluid is typically around 7.4, human cytosolic pH lies between 7.0-7.4, and is typically higher when a cell is growing. In addition to water, cytosol also consists of small molecules, dissolved ions and large water-soluble molecules (e.g., proteins).

Note that the concentrations of the other ions present in the cytosol are different from those of the ions present in the extracellular fluid. In addition to that, higher amounts of charged macromolecules (e.g., proteins) and nucleic acids are present inside the cytosol than outside the cell structure.

Different cells will have a different cytosolic composition, meaning different concentrations of certain ions, amino acids (molecules that form proteins), sugars, and proteins. These differences in composition allow the myocardial cells (cardiac muscle cells) to keep the heart beating, and the neurons (brain cells) sending information throughout the body.

Cytosol Functions

The functions of the cytosol and the cytoplasm overlap, as the cytosol is a component of the cytoplasm. For this section, we will refer to the functions of the cytosol.

The cytosol does not have a single, well-defined function like other cellular components. Rather, it serves as a site for a number of intracellular processes.

One of these is signal transduction, a cellular mechanism that converts a stimulus into a response within the cell, from the cell membrane to sites present within the cell. Signal transduction is important since it passes on information regarding changes happening outside the cell, and sends info from within the cell to other parts of the cell. In this way, the cell can respond appropriately to these changes.

A number of enzymatic activities require cytosol, as enzymes often require certain pH levels, salt concentrations and other environmental conditions that are appropriately fulfilled by cytosol. In addition to that, cytosol provides structural support for organelles. In fact, most cells depend on the volume of cytosol to make space for chemicals to move within the cell.

The pH level and ionic concentration of the cytosol are crucial factors that provide the right environment for these organelles to function. It also dictates how enzymes found in the cytosol (cytosolic enzymes) function, since enzymes need very specific conditions to operate properly.

All these small functions in conjunction make it possible for the cell to continue to perform a myriad of other essential functions. One can say that the main function of the cytoplasm, which includes the cytosol, is to support all the components within it in so that they can perform their respective functions.

Where Is The Cytosol Located In A Cell?

If you could shrink down and float inside a cell, the cytosol is the watery medium you would find yourself swimming in. It fills the space inside the cell membrane and surrounds every organelle, from the mitochondria to the lysosomes. In an animal or plant cell (a eukaryotic cell), the cytosol sits between the cell membrane and the nucleus, bathing the organelles that hang within it.

Importantly, the cytosol is only the fluid outside the organelles. The liquid sealed inside the mitochondria or the nucleus is not counted as cytosol, even though it lies within the cell. In bacteria and other prokaryotic cells, which have no nucleus or membrane-bound organelles, the cytosol simply fills the entire space inside the cell membrane, and that is where most of a prokaryote's chemical reactions take place.

Is Cytosol Found In Both Plant And Animal Cells?

Both plant and animal cells contain cytosol, and both contain the wider cytoplasm that holds it. The cytosol is one of the features these two cell types share, along with the cell membrane, nucleus, mitochondria and ribosomes. Wherever there are organelles suspended inside a cell, there is cytosol filling the gaps between them.

What differs between the two is not the cytosol itself, but the structures floating in it. Plant cells have a few components that animal cells lack: a rigid cell wall outside the membrane, one large central vacuole, and chloroplasts (the green organelles where photosynthesis happens). Animal cells have none of these. In both cases, though, the cytosol remains the aqueous background in which all of this machinery is suspended and in which countless reactions take place. You can read more about these distinctions in our guide to the difference between plant and animal cells.

How Are The Cytosol And Mitochondria Connected?

The mitochondria are often described as floating in the cytosol, and that picture is broadly correct: the cytosol is the fluid that surrounds them. However, the space inside a mitochondrion, called the mitochondrial matrix, is sealed off by the organelle's own membranes and is not part of the cytosol. The two compartments are kept chemically separate for a good reason.

That separation matters because energy production is split between them. The first stage of breaking down glucose, called glycolysis, takes place entirely in the cytosol. There, a glucose molecule is split into two molecules of pyruvate, releasing a small amount of energy. The pyruvate is then shuttled into the mitochondria, where the later stages of cellular respiration (the Krebs cycle and the electron transport chain) extract far more energy from it. In this way, the cytosol and the mitochondria act as a relay team: the cytosol makes the opening move, and the mitochondria finish the job.

What Color Is Cytosol And Cytoplasm?

Despite how cells are often drawn in textbooks (in shades of pink, blue or purple), the cytosol and the surrounding cytoplasm have no real color of their own. Both are naturally clear and transparent, a property that comes from their very high water content. This is actually a nuisance for biologists, because a colorless cell is extremely hard to see against the bright background of a microscope slide.

To get around this, cells are usually treated with dyes before being examined. The most common pair, used in pathology laboratories every day, is hematoxylin and eosin, or H&E. Hematoxylin stains the nucleus dark blue or purple, while eosin stains the cytoplasm pink. So the pink you see filling a cell in a stained microscope image is not the true color of the cytosol at all; it is simply the eosin dye clinging to the proteins dissolved within it.

Cytosol Vs Cytoplasm: Why Differentiate Between Them?

It is important to note that cytosol is a critical element of the cytoplasm. In a prokaryotic cell, cytosol is the host of almost all chemical reactions and metabolic processes that take place within the cell. Also, cytosol is the site for cell communication, while cytoplasm plays host to certain large-scale processes, such as cell division. Glycolysis, for instance, is a metabolic process that occurs specifically in the cytosol.

It is important to note that the cytosol excludes the components within the organelles. This distinction helps scientists clearly define the region where a molecule might be found. Saying a protein is a cytosolic protein indicates that it isn’t a part of any organelle, and it's floating in the aqueous fluid in between.

These distinctions matter. They prevent unnecessary confusion based on foggy nomenclature and words that mean too many different things.

These might sound confusing to someone at the start of their journey into learning about cell biology, but the more one encounters the words and their usage in literature, the clearer the meanings of the words will become.