Structural Biochemistry/Cell Organelles/Cytosol

The intracellular fluid inside a cell is called the cytosol. It is separate from certain cell organelles such as the nucleus and the mitochondria. In eukaryotes, the cytoplasm is the content within a cell membrane minus the content in the cell nucleus. It is where many metabolic reactions occur, there are still others within organelles. In prokaryotes, cytosol is where most metabolic chemical reactions occur. These reactions also occur in the membranes or in the periplasmic space but to a lesser extent.

A complex mixture of substances dissolved in water forms the cytosol even though water is the large majority of the mixture. Sodium and potassium concentrations are different in the cytosol compared with the concentrations in the extracellular fluid. The difference in ion levels are important for osmoregulation and cell signaling. Besides ions, the cytosol also has macromolecules.

Cytoplasm vs. Cytosol

There is often much confusion between the cytoplasm and the cytosol. The cytoplasm is the fluid contained within the cell that holds and surround the cell's organelles in a liquid environment which is necessary for many of the cell's vital functions to occur. Some of the organelles that are held within the cytoplasm include the mitochondria, the golgi apparatus, the endoplasmic reticulum, and other organelles. One thing to note is that the nucleus isn't considered to be part of the cytoplasm because it contains its own type of fluid-like material that is referred to as the nucleoplasm. The materials that are found within the cytoplasm are also typically found within the cell membrane. Typically, the cytoplasm contains materials that are known as cytoplasmic inclusions. These inclusions are typically starch granules, mineral crystals, or lipid droplets that are floating around within the cytoplasm.

When looking at the two, the cytosol is often confused as being the cytoplasm itself, and many individuals view the two as being synonymous. However, they cytosol is in fact just a part of the cytoplasm. The gel-like translucent fluid is what the cytosol really is. It is typically not enclosed within the organelles found within the cell. The cytosol is responsible for suspending the other elements contained within the cytoplasm, like the cytoplasmic inclusions and the cell organelles. Consisting of around 75% of the cell's total volume, the cytosol is composed of many different cellular components. Concentration gradients, cytoskeletal sieves, protein complexes, protein compartments, water, salts, organic compounds, and dissolved ions all consist of the contents of cytosol. Structurally, the cytosol consists mostly of water. However, it does house cytoskeleton filaments, which are necessary for maintaining the overall structure of the cell. In regards to function, the cytosol is also the location where many of the cell's chemical reactions occur. Transportation of metabolite and cell communication are among some of the important functions that occur within the cytosol.

Cytosol and Importing tRNA into Mitochondria

It has been observed that the mitochondria compensate for its lack of mitochondrial tRNA genes by taking in tRNA contained within the cytosol of the cell. The organelle does this by co-importing tRNA along with mitochondrial precursor proteins that intercepts the organelles pathway for traditional protein importing. This method also illustrates the complexity of mitochondrial protein importing, because specific modifications and mitochondrial precursor proteins are required to carry out imports for certain proteins; and with the variety of proteins that the organelle imports, this method possesses a unique set of problems. Another, more direct method, of obtaining more tRNA can be directly importing the tRNA without the help of mitochondrial precursors; a method that is seen in even earlier protozoa.

Macromolecules

Commonly seen, protein molecules that do not bind to cytoskeleton or cell membranes are simply because they dissolve in the cytosol. Since the amount of protein in cells approaches up to 200 mg/ml, it is known to be very high. Protein also occupies approximately 20-30% of the volume of cytosol. However, even with this many protein occupying cytosol, some proteins with the membranes or organelles in cells are known to be very weak and are eventually released into solution upon cell lysis.

If given ATP and amino acids, cells are able to synthesize proteins. This shows that many of the enzymes in cytosol are bound to the cytoskeleton in the cell. In cells such as prokaryotes cell, the cytosol within nucleoid contains the cell's genome. This is known to be an unusual mass of DNA and associated proteins that control the replication and transcription of the bacterial chromosomes and plasmids.

On the other hand, cells such as eukaryotes cell the cytosol containing the cell's genome is held within the cell nucleus, which then separates from the cytosol by nuclear pores that blocks the free diffusion of any kind of molecule that is larger than 10 nm in diameter.

The effect of macromolecular crowding is cause because of too high concentration of macromolecules in the cytosol. In other words, this happens when the effective concentration of other macromolecules is increased, since they have less volume to move in. This squishy effect can cause the position and rate of chemical equilibrium of the cytosol's reactions. Sometimes in the genome forming protein complexes or DNA binding proteins occurs to the favor the association of marcromolecules.

References

Schneider A., “Mitochondrial tRNA import and its consequences for mitochondrial translation.” Annu Rev Biochem. 2011 Jun 7;80:1033-53. Print http://en.wikipedia.org/wiki/Cytosol