Cytosol | Definition, Function & Structure

Cytosol includes water, large water-soluble molecules, dissolved ions, small molecules, and proteins. Cytoplasm comprises nearly 80% water, enzymes, lightweight molecular compounds, nucleic acids, lipids, amino acids, non-organic ions, and carbohydrates.

Metabolic chemical reactions in prokaryotes happen in the cytosol. Many cellular activities like glycolysis, cell division, and other metabolic reactions occur in the cytoplasm.

Cytosol Diagram

The above diagram is that of a typical animal cell. The labels in it are as follows-

1. Nucleolus, 2. Nucleus, 3. Ribosomes, 4. Vesicle, 5. The rough endoplasmic reticulum, 6. Golgi apparatus, 7. cytoskeleton, 8. smooth endoplasmic reticulum, 9. Mitochondrion, 10. Vacuole, 11. Cytosol, 12. Lysosome, 13. Centriole, 14. Cell membrane.

Where is cytosol found?

Cytosol extends below the cell membrane towards the center and spans the entire inner space of the cell, not permeating the cell organelle membranes and nuclear membrane.

Ions

It also consists of various ions, macromolecules, and small molecules in water. The pH of cytosol varies from 7.0 to 7.4. The pH tends to be higher in the cytosol of a growing cell. The ions that are usually found in the cytosol include Ca2+, K+, Na+, Cl-, Mg2+, and bicarbonate. It is found that the potassium concentration was more in the cytoplasm than in the extracellular fluid.

Macromolecules

The macromolecules present in the cytoplasm are protein molecules that exist separately from the cell membrane or cytoskeleton, amino acids, calmodulin, and protein kinase C, which regulate osmolarity. The cytosol is a homogenous solution with an entire single viscosity that supports free mobility of small molecules, solute diffusion through long distances, and consistency of bulk. The mobility of solutes in the complex and crowded mixture of the cell interior is based on the solute molecule size and the time or space gap where it was studied.

The movement of small molecules like organic solutes and ions will not be influenced by crowding, which indicates that the viscosity of intracellular water is the same as that of bulk water. For the movement of macromolecules to cover a longer distance and time, the possible impact of crowding on solute or macromolecule mobility must be considered.

Human Protein Atlas has identified 24% of all proteins in humans to be present in the cytosol. Experimental evidence showed that 1665 proteins are in the cytosol, and the Human Protein Atlas enhanced the number by 354 proteins. The proteins that have multiple locations in the cytosol are 3738 in number. Variation from cell to cell is seen in 676 proteins. Among these, 582 showed differences in intensity, and 105 showed variations in spatial distribution. The proteins of the cytosol often associate with mRNA degradation, protein modification, signal transduction, metabolic processes, and cell death.

The concentration of solutes in the cytosol is influenced by the cell membrane channels, gravity, channels around organelles that impact oxygen, calcium, ATP concentration, and protein molecules channels. A few proteins also consist of central cavities that are filled by cytosol having different compositions to that found in the outer fluid. The cytoskeletal filaments control the diffusion throughout the cell and stop the movement of larger particles from one part to another.

Function of cell cytoplasm - Cytoplasm acts like a medium for all the organelles to operate inside the cell. The cytoplasm is involved in cell growth, expansion, and replication.

Cytosol structure

In the case of eukaryotes, metabolic processes like glycolysis, the pentose phosphate pathway, and protein biosynthesis occur in the cytosol. Due to the lack of organelles, metabolic processes in prokaryotes occur in the cytosol. These processes include protein synthesis, the first stage of cellular respiration, and cell division through mitosis and meiosis. Cytosol also plays a significant role in gradient maintenance across the membranes which is vital for cell signaling, cellular excitability, and osmosis.

Activities like cell signaling occur in the cytosol. One of many such activities like activation of the cell calcium channels by the hormones to open and allow the influx of calcium ions results in a series of reactions. These reactions contain the activation of signaling molecules like protein kinase C and calmodulin.

Osmoregulation is the process of maintaining a balance of water and electrolytes within the cell which ensures the regulation of the suitable number of ions. Osmoregulation maintains constant optimal osmotic pressure in the cell, which brings back the optimal concentration of solutes and water in the body fluids. The cytosol is also found to have osmoprotectants like trehalose to help the cell to survive during adverse conditions. The cytosol with osmoprotectants will turn into a glass-like solid substance to maintain the cell membrane stability and protect the proteins from desiccation.

Many metabolic reactions occur in the cytosol. The cytosol transports metabolites and is associated with signal transduction within the cell. The difference between cytosol and cytoplasm is that first is the intracellular fluid within the cell and outside the organelles. On the other hand, the cytoplasm is the fluid present inside the entire cell membrane, including the organelles and excluding the nucleus. Osmoregulation operates in the cell to maintain a balance in the water and the electrolyte content of the cytosol.