What Is Cytosol?

The interior of eukaryotic cells can be divided into two main parts: the nucleus and the cytoplasm. The nucleus contains genetic information in the form of DNA and is closed off from the rest of the cell via the nuclear envelope. The cytoplasm consists of the rest of the interior of the cell and contains its various organelles and structures. The cytoplasm is further enclosed by the cell membrane and, in the case of plants, the cell wall.

The cytosol is the name of the liquid substance that composes the bulk of the cytoplasm. Cytosol is sometimes called intracellular fluid. The cytosol exists as a liquid matrix that surrounds and suspends the nucleus and other organelles. Cytosol is an aqueous solution of ions and soluble macromolecules. Both eukaryote and prokaryote cells contain cytoplasm. In prokaryotes, the majority of metabolic reactions occur directly in the cytosol. In eukaryotes, the cytosol contains substances that are implicated in cell processes like osmosis, cellular transport, and cell signaling. The cytosol also helps create the electric potentials in excitable cells, like neurons.

Cytosol Composition And Structure

Although the main component of cytosol is water, it contains complex mixture of ions, molecules, and proteins. The exact proportion of the cytoplasm composed of cytosol differs depending on the cell. In bacteria, cytosol is the bulk of the intracellular space while in plant cells, the bulk of the cytoplasm is taken up by large central vacuoles. Cytosol has roughly the same viscosity as water but particles diffuse through it slower, on account of the many dissolved substances. The amount of water in cytosol affects cell functioning. Reducing the amount of water in cytosol by even 30% can cause cell functioning to stop. It is thought that water in cytoplasm takes on a complex structure due to the interaction of water and macromolecules. Water-macromolecule interactions could cause regions of higher and lower density water.

Compared to extracellular fluid, the cytosol contains a different concentration of ions and more macromolecules like proteins and nucleic acids. The pH of cytosol is around 7.0-7.4, higher if the cell is growing. Cytosol has a much lower concentration of sodium ions than the extracellular fluid and a higher concentration of potassium ions. The differing concentrations of ions are important for osmoregulation. Concentrations of sodium are kept low via the activity of the sodium-potassium pump that actively transports sodium ions out and potassium ions in the cell.

Concentrations of ions are important because they allow for electrochemical reactions that occur in cells. Having a difference in ion concentrations over a space creates an electric potential which can be exploited to do work.

In prokaryotes, the cytosol contains the organisms DNA, localized in a region called the nucleoid. Prokaryotes lack membrane-bound nuclei, so their genetic material floats freely in the cytoplasm. The region where DNA is located is called the nucleoid. The transcription and translation factors in bacteria also freely reside in the cytosol.

Cytosol is not a homogenous mixture. The components of cytosol do not mix randomly and there is some organization present. Differences in concentration in the cytosol will create temporary concentration gradients, particularly around organelles where there is metabolic activity. For example, regions of the cytosol near the mitochondria have higher concentrations of ATP and oxygen than the rest of the fluid.  Dissolved proteins will also conglomerate to form protein complexes. Sometimes, the proteins fold to create cytosol-free pockets in the cell, which protect fragile protein structure. The internal organization and structure of cytosol is not static and is constantly changing.

Cytosol Function

Cytosol does not perform any one specific function but is involved in a number of processes. Primarily, cytosol is the site of many important reactions in the cell.

The cytosol is the main site of cell mitosis and meiosis. During the later stages of interphase, a cell will begin making copies of its DNA. Once complete, the nuclear envelope dissolves, releasing the original and replicant set of chromosomes directly into the cytosol. The chromosomes are pulled to the opposite poles of the cell, and the cell divides in two. The cell membrane and the nuclear envelope reform, creating two cells. Structures in the cytosol like the microtubule spindle apparatus and motor proteins assist in this process.

The cytosol is also where protein translation occurs. mRNA transcribed from DNA is shuttled out of the cell into the cytosol. There the mRNA is fed into ribosomes, which construct proteins according to the sequence of nucleotide bases. In eukaryotes, most ribosomes exist freely in the cytosol while some exist bonded to the endoplasmic reticulum.

In prokaryotes, the cytosol is the location of the majority of metabolic reactions. Prokaryotes have developed a diverse range of metabolisms, from sulfur-eating bacteria to nitrogen eating archaea. As such prokaryote cytosol contains a very different mixture of chemicals than eukaryotes.  In prokaryotes, the cytosol is the location of DNA. Unlike eukaryotes, prokaryote DNA is not encased in a nucleus but floats freely in the intracellular cavity. As such, RNA transcription and protein translation take place directly in the cytosol. In many cases, transcription and translation occur in tandem, with ribosomes translating one end of an mRNA strand while the other end is still being transcribed.

In eukaryotes, a large proportion of metabolic reactions occur in the cytosol, but not as much as prokaryotes. One major reaction that occurs in the cytosol in animal cells is glycolysis, the first step in cellular respiration. During glycolysis, a single glucose molecule is broken down into pyruvate and carbon dioxide. In eukaryotes, glycolysis depends on the presence of oxygen, which can freely diffuse through the cytosol.

The ion concentration of cytosol is also important for functions. One particular mechanism that depends on ion concentrations in cytosol is action potentials in cells that conduct electricity, like nerve cells and neurons. When neurons are in an equilibrium state, the body constantly pumps sodium ions out of the cell and brings potassium ions in. The result is that there is a larger concentration of potassium ions inside the cell and a larger concentration of sodium ions outside the cell.  The difference in ion concentration across the cell membrane creates an electrochemical potential across the membrane.

When given the correct stimulus, sodium ion channels in the cell membrane open up and sodium ions diffuse into the cell. The rapid influx of sodium ions increases the voltage across the membrane, causing the membrane to rapidly depolarize, creating an action potential event. One the neuron fires, potassium channels in the membrane open, allowing potassium ions to flow out to repolarize the membrane. Afterward, the sodium-potassium pump starts taking sodium ion out of the cell and potassium ion into the cell, returning it to its equilibrium state.

The correct firing of the action potential relies on there being the right concentration of ions in cytosol and extracellular fluid. If the sodium potassium-pump does not function correctly then the concentration of ions cannot be maintained. sodium ion, proteins, and excess water will rush into the cell, which can cell lysis (rupture).

Cytosol ion concentration are also important for osmotic regulation in cells. Cells must keep ion concentrations inside the cytosol different than outside the cell; otherwise, water would flow into the cell on account of the greater concentration of large macromolecules inside. Channeling ions out of the cell keeps the relative difference in ion concentration, which regulates the amount of water in cells.

Lastly, the cytosol is a major pathway for cell signaling and intracellular transport. The cytosol transports metabolites from where they are produced to where they are needed. Most of the time this is simple, the molecules dissolve in the cytosol and diffuse to their location. Other hydrophobic particles, like lipids, bind to special proteins that shuttle them through. The majority of signaling pathways go right through the cytosol as well.

To sum up, cytosol is the name given to the liquid component of the cell cytoplasm. The cytosol surrounds the nucleus and other organelles, suspending them in a liquid matrix. Cytosol is an aqueous solution with a complex mixture of ions, proteins, and other chemicals. The concentration of solute in cytosol is very different than the concentration of solutes outside the cell in the extracellular fluid. Mechanisms like the sodium-potassium pump work to keep these concentrations constant. In prokaryotes, the cytosol makes up the bulk of the intracellular cavity.

Cytosol in the main site for many cellular processes, including metabolic reactions, cell division, electrical-chemical processes, and signaling pathways. The cytosol is the main site for mitosis and meiosis, it is the site of the first major step in cellular respiration, and cytosol solute concentration plays a role in regulating the amount of water inside cells. Cytosol ion concentrations are important for electrochemical mechanisms, like the action potential in neurons.