Cytosol: Definition, Nature, Properties, and Function.

cytosol

What is Cytosol?

The cytosol is a semi-fluid or gel-like liquid matrix of cells that exist outside the organelles and inside the cell membrane. It is also known as cytoplasmic matrix, intracellular fluid (ICF), and Groundplasm. Together the cytosol and all organelles, except the nucleus make up the cytoplasm.

It occurs in both prokaryotic and eukaryotic cells. In eukaryote, it includes the liquid enclosed within the cell membrane but not the cell nucleus, organelles, or fluid contained within organelles. In contrast, all of the liquid within a prokaryotic cell is the cytoplasm, since prokaryotic cells lack organelles or a nucleus.

Cytosol is separated into compartment by membranes just like the mitochondrion matrix separates the mitochondria into many compartments. In prokaryotes, most of the chemical reactions of metabolism take place in the cytosol, while a few take places in membranes or in the periplasmic space. In eukaryotes, while many metabolic pathways still occur in the cytosol, others take place within organelles.

Cytosol was once thought to be simple solution of molecules but it has multiple level of organization. These include concentration gradient of small molecules such as calcium, large complex of enzymes that act together and take actively part in the metabolic pathways and protein complexes such as proteasomes and carboxysomes. That enclose the various part of the cytosol.

The main component of cytosol is water it also contains dissolved ions, small molecules and proteins. The cytoplasm serves the several function it is the site of most metabolite processes, transports metabolites and is involved in signal transduction within cell.

Organization and structure of the Cytosol

Physical structure of cytoplasmic matrix.

The cytosol (cytoplasmic matrix) is a colorless or greyish, translucent, viscid, gelatinous or jelly-like colloidal substance. It is heavier than water and capable of flowing. In past, there has been a lot of controversy about the physical nature of the matrix. Different workers advanced different theories about the physical characteristics of the matrix.

Their theories can be represented as follows:

  1. Reticular theory suggests that the matrix is composed of reticulum of fibers or particles in the ground substances.
  2. Alveolar theory was proposed by Butschili in 1892 and according to it, the matrix consists of many suspended droplets or alveoli or minute bubbles resembling the foams of emulsion
  3. Granular theory was propounded by Altmann in 1893. This theory supports the view that the matrix contains many granules of smaller and larger size arranged differently. These granules were known as bioplasts.
  4. Fibrillar theory was proposed by Fleming and it holds that the matrix is fibrillar in nature.
  5. Colloidal theory has been forwarded very recently after the electron microscopical observations of the matrix. According to the recent concept, the matrix is partly a true solution, partly a colloidalsystem.

The solution part of the matrix consists of water as solvent in which various solutes of biological importance such as glucose, amino acids, fatty acids, electrolytes, minerals, vitamins, hormones and enzymes remain dissolved.

A colloidal system can be defined as a system which contains a liquid medium in which the particles ranging from about 1/1,000,000 to 1/10,000 millimeter in diameter, remain dispersed. Thus, the colloidal state is a condition in which one substance, such as protein or other macromolecule, is dispersed in another substance to form many small phases suspended in one continuous phase.

In this way every colloidal system consists of two phases: a discontinuous or dispersed phase and a continuous or dispersion phase. Whole of the protoplasm (cytoplasm + nucleoplasm) is a colloidal solution, because the main molecular components of protoplasm— proteins—show all characteristics of the colloidal state. Proteins form stable colloids because, firstly, they are charged ions in solution that repel each other, and, secondly each protein molecule attracts water molecules around it in definite layers.

Phase Reversal

Cytosol (cytoplasmic matrix) like many colloidal systems, shows the property of phase reversal. For example, gelatin particles (discontinuous phase) are dispersed through water (continuous phase) in a thin consistency that is freely shakable.

Such a condition is called a sol. When the solution cools, gelatin now becomes the continuous phase and the water is in the discontinuous phase. Moreover, now the solution has stiffened and becomes semisolid and is called a gel.

In gel state the molecules of colloidal substance remain held together by various types of chemical bonds or bond between H—H, C—H or C—N. The stability of gel depends on the nature and strength of chemical bonds.

Heating the gel solution will cause it to become sol again, and the phases are reversed. Under the natural conditions, the phase reversal of the cytosol (cytoplasmic matrix) depends on various physiological, mechanical and biochemical activities of the cell.

Chemical organization of cytoplasmic matrix.

Chemically, the cytoplasmic matrix is composed of many chemical elements in the form of atoms, ions and molecules.

Chemical Elements

 Out of the 92 naturally occurring elements, perhaps 46 are found in the cytosol (cytoplasmic matrix). Twenty-four of these are considered essential for life (called essential elements), while others are present in cytosol only because they exist in the environment with which the organism interacts. Out of that the 24 essential elements, six play especially important roles in living systems. These major elements are carbon (C, 20 per cent), hydrogen (H, 10 per cent), nitrogen (N, 3 per cent), oxygen (O, 62 per cent), phosphorus (P, 1.14 per cent) and Sulphur (S, 0.14 per cent).

Most organic molecules are built with these six elements. Another five essential elements found in less abundance in living systems are calcium (Ca, 2.5 per cent), potassium (K, 0.11 per cent), sodium (Na, 0.10 per cent), chlorine (Cl, 0.16 per cent) and magnesium (Mg, 0.07 per cent). Several other elements, called trace elements, are also found in minute amounts in animals and plants, but are nevertheless essential for life

Ions

The cytoplasmic matrix consists of various kinds of ions. The ions are important in maintaining osmotic pressure and acid-base balance in the cells. Retention of ions in the matrix produces an increase in osmotic pressure and, thus, the entrance of water in the cell.

The concentration of various ions in the intracellular fluid (matrix) differs from that in the interstitial fluid. For example, in the cell K+ and Mg++ can be high, and Na+ and Clhigh outside the cell. In muscle and nerve cells a high order of difference exists between intracellular K+ and extracellular Na+.

Free calcium ions (Ca++) may occur in cells or circulating blood. Silicon ions occur in the epithelium cells of grasses. The free ions of phosphate (primary, H2PO4 and secondary, HPO4 occur in the matrix and blood. These ions act as a buffering system and tend to stabilize pH of blood and cellular fluids.

The ions of different cells also include sulphate (SO4), carbonate (CO3), bicarbonate (HCO3), magnesium (Mg++) and amino acids.

Electrolytes and Non-electrolytes

The matrix consists of both electrolytes and non-electrolytes.

(i) Electrolytes. The electrolytes play a vital role in the maintenance of osmotic pressure and acid-base equilibrium in the matrix. Mg2+ ions, phosphate, etc., are good examples of the electrolytes.

(ii) Non-electrolytes. Some of the minerals occur in the matrix in a non-ionizing state. The non-electrolytes of the matrix are Na, K, Ca, Mg, Cu, I, Fe, Mn, Fl, Mo, Cl, Zn, Co, Ni, etc. The iron (Fe) occurs in the hemoglobin, ferritin, cytochromes, and some enzymes like catalase and cytochrome oxidase. The calcium (Ca) occurs in the blood, matrix, and the bones. The copper (Cu), manganese (Mn), molybdenum (Mo), zinc (Zn) are useful as cofactors for enzymatic actions. The iodine and fluorine are essential for the thyroid and enamel metabolism, respectively.

Types of Compound in Cytosol

Chemical compounds are conventionally divided into two groups: organic and inorganic. Organic compounds form 30% of a typical cell, rest are the inorganic substances such as water and other substances.

Inorganic Compound

The inorganic compounds are those compounds which normally found in the bulk of the physical, non-living universe, such as elements, metals, non-metals, and their compounds such as water, salts, and a variety of electrolytes and non-electrolytes. In the previous section, we have discussed a lot about the inorganic substances except the water which will be discussed in the following paragraph.

Water

The most abundant inorganic component of the cytosol is the water (the notable exceptions are seeds, bone, and enamel). Water constitutes about 65 to 80% of the matrix. In the matrix, the water occurs in two forms, viz., free water and bound water.

The 95%of the total cellular water is used by the matrix as the solvent for various inorganic substances and organic compounds and is known as free water. The remaining 5%of the total cellular water remains loosely linked with protein molecules by hydrogen bonds or other forces and is known as bound water.

The water contents of the cellular matrix of an organism depend directly on the age, habitat, and metabolic activities. For instance, the cells of the embryo have 90 to 95%water which decreases progressively in the cells of the adult organism.

The cells of lower aquatic animals contain a comparatively high percentage of the water than the cells of higher terrestrial animals. Further, the percentage of water in the matrix also varies from cell to cell according to the rate of the metabolism.

Organic Compound

The chemical substances which contain carbon (C) in combination with one or more other elements as hydrogen (H), nitrogen (N), Sulphur (S), etc., are called organic compounds. The main organic compounds of the matrix are the carbohydrates, lipids, proteins, vitamins, hormones, and nucleotides.

Carbohydrates

The carbohydrates are the compounds of carbon, hydrogen, and oxygen. They form the main source of the energy of all living beings. Only the green part of plants and certain microbes have the power of synthesizing the carbohydrates from the water and CO2 in the presence of sunlight and chlorophyll by the process of photosynthesis. All the animals, non-green parts of the plants (viz., stem, root, etc.), non-green plants (e.g., fungi), bacteria, and viruses depend on green parts of plants for the supply of carbohydrates.

Lipids (Fats)

The lipids are the organic compounds that are insoluble in the water but soluble in the non-polar organic solvents such as acetone, benzene, chloroform, and ether. The cause of this general property of lipids is the predominance of long chains of aliphatic hydrocarbons or benzene rings in their molecules.

The lipids are non-polar and hydrophobic. The common examples of lipids are cooking oil, butter, ghee, waxes, natural rubber, and cholesterol. Like the carbohydrates, lipids serve two major roles in cells and tissues:

  1. They occur as constituents of certain structural components of cells such as membranous organelles; plant pigments such as carotene found in carrots and lycopene that occurs in tomatoes; vitamins like A, E, and K; menthol and eucalyptus oil;
  2. They may be stored within cells as reserve energy sources. Like the starch and glycogen, fat is compact and insoluble and provides a convenient form in which energy-yielding molecules (the fatty acids) can be stored for use when the occasion arises.

Proteins

Out of all the macromolecules found in the cell, the proteins are chemically and physically more diverse. They are important constituents of the cell forming more than 50%of the cell’s dry weight.

Proteins serve as the chief structural material of protoplasm and play numerous other essential roles in living systems. They form enzymes globular proteins specialized to serve as catalysts in virtually all biochemical activities of the cells.

Other proteins are antibodies (immunoglobulins), transport proteins, storage proteins, contractile proteins, and some hormones. In every living organism, there are thousands of different proteins, each fitted to perform a specific functional or structural role.

Enzymes

The cytoplasmic matrix and many cellular organelles contain very important organic compounds known as the enzymes. The enzymes are the specialized proteins and they have the capacity to act as catalysts in a chemical reaction.

Like the other catalysts of the chemical world, the enzymes are the catalysts of the biological world and they influence the rate of a chemical reaction, while themselves remain quite unchanged at the end of the reaction. The substance on which the enzymes act is known as a substrate.

The enzymes play a vital role in various metabolic and biosynthetic activities of the cell such as synthesis (anabolism) of DNA, RNA, and protein molecules and catabolism of carbohydrates, lipids, fats, and other chemical substances.

Properties of cytosol

The matrix is a living substance and it has following physical and biological properties:

Physical Properties

The most of the physical properties of the matrix are due to its colloidal nature and these are as follows:

  • Tyndall’s effect:
    •  When a beam of strong light is passed through the colloidal system of the matrix at right angles in the dark room, the small colloidal particles which remain suspended in the colloidal system, reflect the light. The path of the light appears like a cone. This light cone is known as Tyndall’s cone because this phenomenon has been first of all reported by Tyndall (1820—1893) in colloids.
  • Brownian movement:
    • The suspended colloidal particles of the matrix always move in zig-zag fashion. This movement of molecules is caused by moving water molecules which strike with the colloidal molecules to provide motion to them.
    • This type of movement was first of all observed by Scottish botanist Robert Brown in 1827 in the colloidal solution. Therefore, such movements are known as the Brownian movement. The Brownian movement is the peculiarity of all colloidal solutions and depends on the size of the particles and temperature.
  • Cyclosis and amoeboid movement:
    • Due to the phase reversal property of the cytoplasmic matrix, the intracellular streaming or movement of the matrix takes place. This property of the intracellular movement of the matrix is known as the cyclosis.
    • The cyclosis usually occurs in the sol-phase of the matrix and is affected by the hydrostatic pressure, temperature, pH, viscosity, etc. The intracellular movements of the pinosomes, phagosomes, and various cytoplasmic organelles such as the lysosomes, mitochondria, chromosomes, centrioles, etc., occur only due to cyclosis of the matrix.
    • Cyclosis has been observed in most animal and plant cells. The amoeboid movement depends directly on the cyclosis. The amoeboid movement occurs in the protozoans, leucocytes, epithelial, mesenchymal, and other cells.
    • In the amoeboid movement, the cell changes its shape actively and gives out cytoplasmic projections known as pseudopodia. Due to cyclosis matrix moves these pseudopodia and this causes forward motion of the cell.
  • Surface tension:
    • The molecules in the interior of a homogeneous liquid are free to move and are attracted by surrounding molecules equally in all directions. At the surface of the liquid where it touches air or some other liquid, however, they are attracted downward and sideways or inward, more than upward; consequently, they are subjected to unequal stress and are held together to form a membrane.
    • The force by which the molecules are bound is called the surface tension of the liquid. The cytoplasmic matrix being a liquid possesses the property of surface tension. The proteins and lipids of the matrix have less surface tension, therefore, occur at the surface and form the membrane, while the chemical substances such as NaCl have high surface tension, therefore, occur in the deeper part of the matrix.
  • Adsorption:
    • The increase in the concentration of a substance at the surface of a solution is known as adsorption. The phenomenon of adsorption helps the matrix to form protein boundaries.
  • Other mechanical or physical properties of a matrix:
    • Besides surface tension and adsorption, the matrix possesses other mechanical properties, e.g., elasticity, contractility, rigidity, and viscosity which provide to the matrix many physiological utilities.
  • The polarity of the egg:
    • The colloidal system due to its stable phase determines the polarity of the cell-matrix which cannot be altered by centrifugation of other mechanical means.
  • Buffers and pH:
    • The matrix has a definite pH value and it does not tolerate significant variations in its pH balance. Yet various metabolic activities produce a small number of excess acids or bases. Therefore, to protect itself from such pH variation the matrix contains certain chemical compounds as a carbonate-bicarbonate system known as buffers which maintain a constant state of pH in the matrix.

Biological Properties

The matrix is a living substance and it has following biological properties :

  • Irritability:
    • The irritability is the fundamental and inherent property of the matrix. It possesses a sensitivity to stimulation, ability to the transmission of excitation, and the ability to react according to stimuli. The heat, light, chemical substances, and other factors stimulate the cytoplasmic matrix to contract.
  • Conductivity:
    • The conductivity is the process of conduction or transmission of excitation from the place of its origin to the region of its reaction. The matrix of nerve cells possesses the property of conductivity.
  • Movement:
    • The cytoplasmic matrix can perform movement due to cyclosis. The cyclosis depends on the age, water contents, heredity factors, and composition of the cells.
  • Metabolism:
    • The matrix is the seat of various chemical activities. These activities may be either constructive or destructive in nature. The constructive processes such as biosynthesis of proteins, lipids, carbohydrates, and nucleic acids are known as anabolic processes, while the destructive processes such as oxidation of foodstuffs, etc., are known as catabolic processes. The anabolic and catabolic processes are collectively known as the metabolic processes.
  • Growth:
    • Due to the secretory or anabolic activities (Gr., anabolism= a throwing up) of the cell, new protoplasm continuously increases in its volume. The increase in the volume of the matrix causes the growth of the cell which ultimately divides into daughter cells by the cell division.
  • Reproduction:
    • The cytoplasm has the property of asexual and sexual reproduction.

Function of Cytosol

The cytosol has no single function and is instead the site of multiple cellular processes. Examples of these processes include signal transduction from the cell membrane to sites within the cell, such as the cell nucleus, or organelles.

This compartment is also the site of many of the processes of cytokinesis, after the breakdown of the nuclear membrane in mitosis. Another major function of cytosol is to transport metabolites from their site of production to where they are used.

This is relatively simple for water-soluble molecules, such as amino acids, which can diffuse rapidly through the cytosol. However, hydrophobic molecules, such as fatty acids or sterols, can be transported through the cytosol by specific binding proteins, which shuttle these molecules between cell membranes.

Molecules taken into the cell by endocytosis or on their way to be secreted can also be transported through the cytosol inside vesicles, which are small spheres of lipids that are moved along the cytoskeleton by motor proteins.

The cytosol is the site of most metabolism in prokaryotes, and a large proportion of the metabolism of eukaryotes. For instance, in mammals about half of the proteins in the cell are localized to the cytosol. The most complete data are available in yeast, where metabolic reconstructions indicate that the majority of both metabolic processes and metabolites occur in the cytosol.

Major metabolic pathways that occur in the cytosol in animals are protein biosynthesis, the pentose phosphate pathway, glycolysis and gluconeogenesis. The localization of pathways can be different in other organisms, for instance fatty acid synthesis occurs in chloroplasts in plants and in Apicoplasts in Apicomplexa.

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