What is Cytosol in a cell? Functions and Structure

What is Cytosol?

The cytosol is an aqueous solution which is a portion of the cytoplasm in cells that organelles, proteins, and other cell structures float in. This is one of the liquids in the cells of organisms (intracellular fluid ICF). The cytosol is also known as a cytoplasmic matrix or goes by the other name called groundplasm. However, do not confuse the cytosol for the cytoplasm as they are not the same. In the cell, the cytosol is a component of the cytoplasm while the cytoplasm is a component of the cell that is surrounded by the plasma (cell) membrane.

Virtually all the organelles in eukaryotic cells like the endoplasmic reticulum, nucleus, and mitochondria are located in the cytoplasm of the cells. Then, the portion of this cytoplasm that is not contained in the organelles is referred to as the cytosol. The cytosol is the matrix that surrounds these organelles in a eukaryotic cell. This matrix is a complex mixture of organic molecules and substances dissolved in water. Therefore, water is the largest component of the cytosol.

This complex solution contains proteins, mRNA, amino acids, ribosomes, ions, sugars, messenger molecules, etc. In the cytosol, the concentration of sodium ions and potassium ions is different compared to the ions in the extracellular fluid. These differences in the ion levels play a role in cellular activities such as cell signaling, osmoregulation, and the generation of action potentials in excitable cells like the nerve, endocrine, and muscle cells. Its properties and composition allow the functions of life to occur. This matrix also contains a large number of macromolecules and how the molecules behave can be altered through macromolecular crowding.

There are membrane-bound organelles that float in the cytosol even though the interior of these organelles is not considered as part of the cytosol. For instance, membrane-bound organelles such as nuclei, chloroplasts, mitochondria, and others within the cells possess their own internal fluid that is separate and different from the cytosol. The cytosol is said to have structure and organization as membranes separate it into compartments. It has multiple levels of organization which include:

• Concentration gradients of small molecules like calcium
• Large complexes of enzymes that work together and function in metabolic pathways
• Protein complexes like proteasomes and carboxysomes enclose and separate parts of the cytosol.

In a eukaryotic cell, the cell membrane surrounds the cytosol and makes it part of the cytoplasm. Hence, it is the liquid matrix around the mitochondria, plastids, and other organelles surrounded by the cell membrane. The majority of the chemical metabolic reactions in prokaryotes occur in the cytosol whereas a few of these reactions take place in the periplasmic space or membranes. As for eukaryotes, many metabolic pathways take place in the cytosol while others occur within organelles.

Functions of Cytosol

• The main function of the cytosol is that it serves as a medium for intracellular processes.
• It contains the proteins, ions, and other components for cytosolic activities.
• The cytosol function in signal transduction: Signal transduction starts from the plasma membrane to sites within the cell such as the nucleus and organelles. During the process of signal transduction, messenger molecules may diffuse through the cytosol to alter the functioning of organelles, enzymes, or DNA transcription. These messengers may be from one part of the cell to another part or from outside the cell.
• This matrix facilitates the transportation of metabolites from place to place in the cell. It transports the metabolites from their site of production to where they are needed.
• Through the cytosol, water-soluble molecules like amino acids diffuse freely while large hydrophobic molecules such as fatty acids and sterols are transported through the cytosol by specific binding proteins. These proteins shuttle these hydrophobic molecules between cell membranes.
• Through vesicles in the cytosol, some molecules that are subjected to endocytosis are transported.
• Another major function of the cytosol is that it plays a role in prokaryotic metabolism. Also, almost all life functions of the prokaryotic cells as well as glycolysis, DNA transcription, and replication take place in the cytosol.
• In eukaryotic cells, a large proportion of metabolism takes place in the cytosol. In the cells of mammals, about half of the proteins are localized to the cytosol. Even in yeast, the majority of its metabolic processes and metabolites take place in this matrix.
• In the animal cell, it is in this matrix that the major metabolic pathways take place. These metabolic pathways include glycolysis, protein biosynthesis, gluconeogenesis, and the pentose phosphate pathway.
• The cytosol function in enzyme activities: In order for enzymes to work properly, they need certain salt concentrations, pH levels, and other environmental conditions and in the cytosol, there are concentrations of some ions that give enzymes a favorable environment to function.
• In mitosis, after the breakdown of the nuclear membrane, the cytosol functions as a site for many cytokinesis processes.
• The cytosol function in the cell to give it and the organelles structural support. Thereby, the majority of cells depend on the volume of cytosol in order to create their shape as well as space for chemicals to move within the cell.

Properties and composition

In organisms, the proportion of the cell volume that comprises the cytosol varies among them. For instance, in a bacterial cell, the cytosol forms the bulk of the cell structure whereas, in plant cells, it is not the cytosol that forms the bulk of the cell structure but the large central vacuole. However, the composition of the cytosol is mostly of dissolved ions, water, and molecules. These molecules include small molecules and large water-soluble molecules like proteins.

Ions

In the cytosol, the concentration of ions is different from the ones in extracellular fluids. It contains a much higher amount of charged macromolecules such as nucleic acids and proteins than outside of the cell.

The cytosol in contrast with extracellular fluids has a  low concentration of sodium ions and a high concentration of potassium ions. For, osmoregulation, this difference in ion concentration is crucial because if the ion levels were the same outside and inside the cell, water would enter constantly by osmosis. However, since the levels of macromolecules are higher inside the cell than their level outside, sodium ions are expelled instead and potassium ions are taken up by the Na⁺/K⁺-ATPase.

Then, through potassium-selection ion channels, the potassium ions flow down their concentration gradients. Negative membrane potential is then created due to the loss of the positive charge. Through selective chloride channels, the negative chloride ions also exit the cell in order to balance this potential difference. However, this loss of chloride and sodium ion makes up for the osmotic effect of the higher concentration of organic molecules in the cell.

Furthermore, by accumulating osmoprotectants in their cytosol, cells can handle even larger osmotic changes. Typical examples of such osmoprotectants are trehalose and betaines. These molecules can allow the cell to survive being dried out completely and allow the organism to enter a state of suspended animation referred to as cryptobiosis. Thereby, the cytosol and osmoprotectants in such a case become a glass-like solid that stabilizes the plasma membrane and proteins from the damage effects that come with desiccation.

Calcium ions function as a second messenger in calcium signaling due to their low concentration in the cytosol.  In this case, a signal like an action potential or a hormone opens the calcium channel for the calcium to flood into the cytosol. Therefore, other signaling molecules like protein kinase and calmodulin are activated due to the sudden increase in the cytosolic calcium. However, there is an ambiguous hypothesis that other ions such as potassium and chloride may have signaling functions too.

Water

The major content of the cytosol is water, making about 70% of the total cell volume. This intracellular fluid has a pH of 7.4 whereas the human cytosolic pH ranges between 7.0-7.4. However if the cell is growing, the human cytosolic pH range is usually higher. The viscosity of the cytoplasm and pure water is roughly the same. Nevertheless, the diffusion of small molecules through the cytoplasm is about fourfold slower than in pure water. This slow rate of diffusion is a result of the collisions with the large numbers of macromolecules in the cytosol.

Studies have shown that water affects cell functions. In a cell, a reduction in the amount of water can inhibit metabolism, and as metabolism decreases the cell dries out. Hence, all metabolic activity can halt when the water level reaches 70% below normal. The structural view of the water in cells is that about 5% of it as water of solvation is bound strongly by macromolecules or solutes. Then, the majority of this water in cells has the same structure as pure water.

It is said that the water in cells behaves differently from the water in dilute solutions. This idea proposes that cells have zones of low and high-density water that could have an extensive effect on the functions and structures of the other cell parts. However, the use of advanced nuclear magnetic resonance methods for the direct measurement of water mobility in cells suggests that 85% of the water in cells acts like pure water. Whereas, the remaining water percentage in cells is less mobile and bound probably to macromolecules.

Macromolecules

There are protein molecules dissolved in the cytosol which amount is very high in cells. These protein molecules do not bind to the cytoskeleton or cell membranes and about 20-30% of the cytosol volume is composed of protein. Some proteins tend to be weakly associated with organelles or membranes in cells and upon cell lysis are released into the cytosol.

There is a structure known as nucleoid in prokaryotic cells. The cytosol contains the genome of the cell within this very nucleoid structure. This irregular mass of DNA and associated proteins are in charge of the transcription and replication of the bacterial chromosome and plasmids. The genome, however, in eukaryotic cells is held within the nucleus of the cell that is separated by nuclear pores from the cytosol. These nuclear pores hinder the free diffusion of larger molecules that are more than 10 nanometers in diameter.

An effect called macromolecular crowding occurs as a result of the high concentration of macromolecules in the cytosol. This crowding effect is an increase in the effective concentration of other macromolecules that have less volume to move in. It can produce huge changes in the position and rates of the chemical equilibrium of reactions in the cytosol.  This effect has the ability to alter dissociation constants by favoring the association of macromolecules. A typical example is seen when multiple proteins come together to form protein complexes or in the genome when DNA-binding proteins bind to their targets.

Cytosol Structure

The components of the cytosol may not be separated or partitioned by the cell membrane into regions. However, its components do not mix randomly. Therefore, this matrix has several levels of organization and structure where specific molecules are localized to defined sites within it.

Concentration gradients

Concentration gradients are still produced within the cytosol even though small molecules diffuse rapidly within it. For example, in the region around an open calcium channel, the calcium sparks are produced for a short period which are about 2 micrometers in diameter. They last only for a few milliseconds even though several of these calcium sparks can merge to form larger gradients known as calcium waves. Furthermore, in cells, there may be a production of concentration gradients of other small molecules like the adenosine triphosphate and oxygen around clusters of mitochondria.

Protein complexes

When proteins associate together, they form protein complexes which usually contain a set of proteins with similar functions. For example, are enzymes that carry out many processes in the same metabolic pathway. This association can permit substrate channeling which involves the product of one enzyme being passed directly to the next enzyme in a pathway without being released into solution.

Channeling tends to make a pathway more efficient and rapid compared to how it would have been if the enzymes were distributed randomly in the cytosol. It also prevents unstable reaction intermediates from getting released.

Enzymes that are tightly bound to each other are involved in a wide variety of metabolic pathways. However, other metabolic pathways may involve loosely associated complexes that are very difficult to study outside the cell.

Protein compartments

There are some protein complexes that possess a large central cavity that is isolated from the remainder of the cytosol. The proteasome is an example of such an enclosed compartment. In the proteasome, a set of subunits form a hollow barrel that contains proteases that degrade cytosolic proteins. Therefore, there will be damage if they mix with the remainder of the cytosol. Hence, in regard to that, a set of regulatory proteins caps the barrel. These regulatory proteins recognize proteins with a signal that directs them for degradation and feed them into the proteolytic cavity.

The bacterial microcompartments are another large class of protein compartments. They are made of a protein shell that encapsulates several enzymes. These compartments are made of interlocking proteins and are about 100-200 nanometers across. The carboxysome is a typical example that contains enzymes involved in carbon fixation.

Biomolecular condensates

There can be the formation of biomolecular condensates by non-membrane-bound organelles. Biomolecular condensates arise by oligomerization, clustering, or polymerization of macromolecules in order to drive colloidal phase separation of the nucleus or cytoplasm.

Cytoskeletal sieving

The cytoskeleton (network of filaments) may not be part of the cytosol but its presence restricts the diffusion of large particles in the cell. For instance, tracer particles that are larger than about 25 nanometres were seen in several studies to be excluded from parts of the cytosol around the edges of the cell, next to the nucleus.

These excluding compartments tend to have a much denser meshwork of actin fibers than the remainder of the cytosol. Thus, the distribution of large structures like the ribosome and organelles within the cytosol can be influenced by these microdomains. They achieve this by excluding them from some areas and concentrating them in other areas.

Cytosol vs cytoplasm

The cytosol as explained earlier should not be confused for the cytoplasm as they are not the same. In the cell, the cytosol is a component of the cytoplasm while the cytoplasm is a component of the cell that is surrounded by the plasma (cell) membrane.

Virtually all the organelles in eukaryotic cells like the endoplasmic reticulum, nucleus, and mitochondria are located in the cytoplasm of the cells. Then, the portion of this cytoplasm that is not contained in the organelles is referred to as the cytosol. The cytosol is the matrix that surrounds these organelles in a eukaryotic cell.

Therefore, the cytosol and cytoplasm are both intracellular fluids and constituents of the cell. The cytosol is the intracellular fluid of the cell whereas, the cytoplasm contains all the components of the cell within the cell membrane except the nucleus. Thereby, the main constituents of the cytoplasm include the cytosol, organelles, and cytoplasmic inclusions whereas the cytosol is made up mainly of water, ions, and molecules.

These two fluids have different organizational levels in the cell. The cytosol is composed of concentration gradients, protein complexes, protein compartments, and cytoskeletal sieving. Whereas, the cytoplasm is divided into endoplasm and ectoplasm. The endoplasm is the granular mass in the cytoplasm while the ectoplasm is described as the surrounding lucid layer.

It is seen that even though the cytosol and cytoplasm are both fluids in the cell, they have their different specific function as well as common function in the cell. The cytosol concentrates its dissolved molecules into the correct positions for efficient metabolism. While it concentrates molecules in the correct portions of the cytoplasm, the cytoplasm freezes organelles in place, ensuring efficient metabolism. However, they both play a role in the transportation of molecules, signal transduction, cytokinesis, and nuclear division.

In a cell, the cytosol is located within the cytoplasm where it surrounds all organelles that are embedded or suspended in the cytoplasm. In prokaryotic cells, the cytoplasm occupies the entire cell environment that is within the cell membrane while in eukaryotic cells, it is located between the nuclear membrane and the cell membrane. Therefore, the cytosol and the cytoplasm both form the dynamic solution in the cell. Hence, the diversity of both the soluble and insoluble particles are high in the cytoplasm since the cytosol is a portion of the cytoplasm.