Difference Between Cytosol and Cytoplasm

Main Difference – Cytosol vs Cytoplasm

Cytosol and the cytoplasm are two constituents of the cell. Cytosol is a part of the cytoplasm. It is the intracellular fluid in the cell. Most metabolic reactions take place in the cytosol. Water is the most abundant element in both cytosol and cytoplasm. The main difference between cytosol and cytoplasm is that cytosol is a component of the cytoplasm of a cell whereas cytoplasm is a component of the cell which is surrounded by the cell membrane.

This article explores,

1. What is Cytosol
      – Composition, Properties, Function, Organization
2. What is Cytoplasm
      – Composition, Properties, Function, Organization
3. What is the difference between Cytosol and Cytoplasm

What is the Cytosol

The cytosol is a liquid which is considered as the matrix of the cytoplasm. This liquid is composed of intracellular fluid and is compartmentalized by the cell membrane into mitochondrial matrix, chloroplast stroma-like structures. In eukaryotes, cytosol is a component of the cytoplasm. It surrounds the organelles in the cytoplasm. In prokaryotes, metabolic reactions occur in the cytosol. Most eukaryotic metabolic reactions occur inside the organelles rather than in the cytosol.

Composition and Properties of Cytosol

The cytosol is mainly composed of water, small and large soluble molecules and dissolved ions. It dissolves non-protein molecules which are less than 300 Da in size. In the plant cell cytoplasm, around 200,000 different small molecules can be dissolved in the cytoplasm. Water makes up of about 70% of the cytosol’s total volume. Thus, the pH of the cytoplasm pH ranges from 7.0-7.4. The viscosity is also similar to water. But, the diffusion through the cytosol can be fourfold slow for the small molecules. Water constantly enters the cytosol by osmosis. The concentration of the calcium ions in the cytosol is as low as <0.0002 mM, allowing the calcium ions to perform as a second messenger in signaling pathways.     

The cytosol contains a comparatively large amount of charged macromolecules like proteins and nucleic acids. The amount of proteins dissolved in the cytosol is around 200 mg/mL. A complex cytoskeleton filaments mixture, which is composed of microtubules and actin filaments can be found in the cytosol. These filaments form a network of cytoskeleton. The filament network and higher concentration of macromolecules contribute to macromolecular crowding effect inside the cytosol. Due to this effect, cytosol alters its properties from an ideal solution. The crowded solution of cytosol by different types of molecules is shown in figure 1.

Figure 1: Macromolecular Crowding

Organization

The cytosol consists of a concentration gradient of some molecules, though most of the small molecules are equally distributed. For example, calcium ion gradient is created by the opening of the calcium channels which only lasts for milliseconds. Several calcium gradients spark to form a large calcium gradient, known as calcium waves. Moreover, protein complexes form in the cytosol, allowing the substrate channeling where one product is directly passed to its next step. Some of these complexes also consist of a large, isolated, central cavity like proteosome. These protein compartments contain proteases degrading cytosolic proteins. Another example of a protein compartment is microcompartments in bacteria, which are 100 to 200 nm in diameter. Carboxysome is a type of microcompartment which is involved in carbon fixation. The cytoskeleton sieving concentrates ribosome like organelles in a particular area with the aid of excluding compartments. These excluding compartments are made up of denser actin fibres.

Function of Cytosol

The cytosol contributes to the signal transduction starting from cell membrane to the effective site, most of the times, the nucleus. Place to place transportation of metabolites is facilitated by the cytosol. Amino acids like small soluble molecules freely diffuse from the cytosol. Large hydrophobic molecules like sterols and fatty acids are transported via binding to specific proteins. The molecules subjected to endocytosis are transported via vesicles in the cytosol. Prokaryotic metabolism also occurs in the cytosol. In animals, translation, glycolysis, pentose phosphate pathway and gluconeogenesis occur in the cytoplasm. 

What is the Cytoplasm

The cytoplasm is the component of a cell which is surrounded by the cell membrane. The cytosol is a component of the cytoplasm. Other than the cytosol, cytoplasm contains organelles. In prokaryotes, all the cellular structures are embedded in the cytoplasm. The organelles suspended in the cytoplasm is shown in figure 2.

Figure 2: Cytoplasm with organelles: 1.Nucleolus 2.Nucleus 3.Ribosome 3.Vesicle 5.Rough endoplasmic reticulum 6.Golgi apparatus  7.Cytoskeleton 8.Smooth endoplasmic reticulum 9. Mitochondrion 10.Vacuole 11.Cytosol 12.Lysosome 13.Centriole

Composition and Properties of Cytoplasm

Cell signaling through cytoplasm depends on the permeability of the cytoplasm. It depends on the diffusion of the signaling molecule through the cytoplasm. Small signaling molecules like Calcium ions diffuse through the cytoplasm. The cytoplasm also acts as a sol-gel, sometimes as a liquid (sol) and other times as a solid mass (gel). The motor proteins in the cytoplasm lead to the non-Brownian motion of the particles in the cytoplasm. 

The cytoplasm is composed of cytosol, its organelles and cytoplasmic inclusions. Organelles in the cytoplasm include the nucleus, mitochondria, Golgi apparatus, endoplasmic reticulum, lysosomes and in plant cells, vacuoles and chloroplasts. Some insoluble particles suspended in the cytoplasm are called cytoplasmic inclusions. Particles like calcium oxalate, granules like starch and glycogen and lipid droplets are known as inclusions in the cytoplasm.

Organization

The inner area of the cytoplasm  is  concentrated and is called the endoplasm. The outer area of the cytoplasm is called the cell cortex or the ectoplasm.

Function of Cytoplasm

The cytoplasm is involved in large cellular activities like glycolysis and nuclear division. The solid glass structure of the cytoplasm freezes large organelles in place. The cytosol is also involved in the cytokinesis, which is the process of cytoplasm division followed by nuclear division. Other than that, the functions of the cytosol are also borne by the cytoplasm.

Difference Between Cytosol and Cytoplasm

Definition

Cytosol: Cytosol is the fluid present in the cell membrane.

Cytoplasm: Cytoplasm is the cell component inside the cell membrane.

Composition

Cytosol: The cytosol is composed of water, soluble ions, small and large water-soluble molecules and proteins.

Cytoplasm: The cytoplasm is composed of 80% of water, nucleic acids, enzymes, lipids, amino acids, carbohydrates and non-inorganic ions.

Diversity

Cytosol: The diversity of the cytosol is low.

Cytoplasm: The diversity of the components is high compared to the cytosol.

Components

Cytosol: The components of the cytosol are water, soluble small and large molecules.

Cytoplasm: The components of the cytoplasm are organelles, cytosol and cytoplasmic inclusions.

Metabolism

Cytosol: All the chemical reactions occur in the cytosol in prokaryotes.

Cytoplasm: The cytoplasm is involved in large cellular activities like glycolysis and cell division.

Functions

Cytosol: The cytosol concentrates its dissolved molecules into the correct positions for the efficient metabolism.

Cytoplasm: The cytoplasm freezes organelles in place, ensuring the efficient metabolism.

Extra Functions

Cytosol: Signal transduction and transportation of molecules take place in the cytosol.

Cytoplasm: Nuclear division, cytokinesis and signal transduction take place in the cytoplasm.

Conclusion

Both cytosol and cytoplasm collectively form the dynamic solution inside the cell. The cytoplasm, which is the transparent portion of both  prokaryotic and eukaryotic cells, is a semi-solid fluid. The cytoplasm makes the liquid part of the cytoplasm. Thus, the diversity of both soluble and insoluble particles are high in the cytoplasm. Components of the cytoplasm include organelles, cytosol and cytoplasmic inclusions. Organelles like nucleus, mitochondria, Golgi apparatus and particles like crystals, garnules and lipid droplets are suspended in the cytosol. Most metabolic pathways occur in the cytosol in prokaryotes and some of the reactions like glycolysis in eukaryotes occur in the cytosol. Cellular activities like cell division and cytokinesis take place in the cytoplasm. Molecules are concentrated in the correct portions of the cytoplasm by the cytosol and organelles are frozen in the correct places in the cell by cytoplasm. All these  features suggest that the main difference between cytosol and cytoplasm is their proportionality of size in the cell.

Reference:
1. “Cytosol.” Wikipedia. N.p.: Wikimedia Foundation, 26 Jan. 2017. Web. 6 Mar. 2017.
2. “Cytoplasm.” Wikipedia. N.p.: Wikimedia Foundation, 6 Mar. 2017. Web. 6 Mar. 2017.
3. “Structural Biochemistry/Cell Organelles/Cytosol.” Wikipedia. N.p.: Wikimedia Foundation, 23 Oct. 2017. Web. 6 Mar. 2017.

Image Courtesy:
1. “Crowded cytosol”By TimVickers – Own work by uploader, based upon simlar illustrations in Goodsell DS (June 1991). “Inside a living cell”. Trends Biochem. Sci. 16 (6): 203–6. DOI:10.1016/0968-0004(91)90083-8. PMID 1891800.(Public Domain) via Commons Wikimedia
2. “Biological cell”By MesserWoland and Szczepan1990 – Own work (Inkscape created) (CC BY-SA 3.0) via Commons Wikimedia