Cell Cycle

Cell cycle

CELL CYCLE

Phases of cell phases- a somatic cell exist in two main states : a long undividing stage called interphase and a short dividing state termed mitotic or M phase.

      I.            Interphase:- in the interphase, the cell grows by synthesizing biological molecules such as carbohydrates, lipids, protein and nucleic acids. Also the cellular molecules needed for cell decision are stocked. The interphase lasts for 10 – 30 hours. It is further decided into three periods : first gap or G₁ phase, synthetic or S phase, and second gap or G₂ phase. The duration of these phases varies in different organisms.

1.   G₁ phase: this phase represents the gap between previous mitosis and beginning or DNA synthesis. It is a stage of initial growth of a newly formed cell. The cell carries on normal metabolism in preparation for DNA replication. Carbohydrates, lipids, protein (including some non-histons) and RNAs are synthesized in this phase. No change occurs in the DNA content of the cell. The duration of G₁ phase is most variable. It may last from hours to months or years. Many undividing cells, such as muscle cells, nerve cells and fibroblasts, suspend cell cycle after mitosis just prior to S phase. Such cells are said to be in G₀ phase to distinguish them from G₁ cells which will soon enter S phase. The muscles and nerve cells remain in the G₀ phase for the life of the animal and never decide again. However, the fibroblasts that help in healing of wounds grow and divide on demand.

2.   S phase:  it is a phase of duplication of each chromosome by replication of a new DNA molecule on the template of the existing DNA. Each chromosome now consists of two identical sister chromotids held together at the region of primary constriction’s kinetochores, and carries a duplicate set of genes. A diploid cell (2n) thus, becomes tetraploid (4n) at the end of S phase. Synthesis of histone proteins and their mRNAs, and formation of new nucleosomes also occur only in the S phase. On completion of replication, the histone mRNAs are selectively destroyed. Some non-histone proteins are also formed in S phase. By the time the S phase in complete, the duplicated chromosomes have a full complement of histone and nonhistone proteins. Each chromosome has one old and one new DNA chain, and also has old and histones in about equal quantities. The S phase in most eukaryotes lasts for 6 to 8 hours.

Once the S phase begins, G₂ and mitosis follow without delay.

3.   G₂ phase: this phase marks the gap between DNA synthesis and nuclear division. It is a stage of further growth of the cell and preparation for its division. During this phase, RNA transcription and protein synthesis continue. The cytoplasmic organelles such as centrioles, mitochondria, and golgi apparatus, are doubled, protein for spindle and asters are synthesized and active metabolism stores energy for the next mitosis. The G₂ phase in most cells lasts for 2 to 5 hours. Repair of damaged DNA sequences also takes place in the interphase.

A resting cell.  An interphase cell is sometimes described as a ‘’resting cell’’. This is incorrect, as the cell does not rest even when it is not dividing. Thought it does not show the changes on the above vital activities. In fact, interphases are the most active period of the cell. Some workers have suggested the term energy phase for it.

   II.            Mitotic phase: - mitotic or M phase follows the interphase. It is aimed at orderly distribution of the already duplicated chromosomes to the daughter cells. The latte is diploid, and contains exactly the same hereditary information as the parent cell. Other cell components (organelles and molecules) are divided approximately equally between the daughter cells, although not wish the same precision as the DNA. After the completion of mitosis, the daughter cells enter the G₁ phase of the next cell cycle.

Many structural and physiological changes occur in the cell during mitosis. Chromatin of the nucleus is packed into visible chromosomes, which are set free by breakdown of nuclear envelope. There is an extensive reorganization of the membranous components and cytoskeletal elements. Endoplasmic reticulum and golgi apparatus breakdown into small vesicles. This stops protein movement. Microtubules dissociate into tubulin dimmers, which are assembled into the spindle. The latter occupies most of the cell and assists in the distribution of chromosomes into the daughter cells. Actinfilaments are recognized to form a contractile ring for the cytoplasmic division.

As a result of the above changes, the former activities of the cell, namely, gene expression, protein synthesis, secretion and cell motility stop. Cell’s entire attention is devoted to the process of division.

The purpose of these changes is to decide the components of the parent cell equally between the two daughter cells. The mode of distribution depends on the nature of the components. The organelles which occur in many copies are divided by their sheer distribution in the cytoplasm. Approximately half of the cytoplasm is received by each daughter cell. The single nucleus breaks down so that two nuclei are reconstructed from its components. The breakdown of sheet-like ER into vesicles allows the reformation of ER in the daughter cells from the vesicles that pass into them during division.

Definition of cell cycle – the regular sequences of G₁, S, G2 and M phase phases is called the cell cycle.

Duration of cell cycle: under optimum conditions of nutrition’s and temperature, the duration of cell cycle for a particular kind of cell is constant. Under less favorable conditions it may become longer, but it is not possible to speed up the cell cycle and make cells grow faster. This shows that the duration of cell cycle is the time required for carrying out some precise programmer that has been built into each cell. This programmer seems to include replication of chromosome and doubling of all other constituents of the cell meant for growth.The interphase between the two meiotic divisions in unique in lacking a DNA synthesizing S phase.

Variation in cell cycle phases: cell cycle of mammalian cells in culture takes 10-30 hours. In an adult human cell, G₁ lasts for 8 hours, DNA is synthesized for 6 hours in S phase, G₂ Continues for 4-5- hours, and mitosis is completed within 1 hour. The maximum variation on the cell of similar cells affects the duration of G₁. The duration of S and G₂ shows the least variation in response to external conditions.

Cell cycle of most growing animal and plants cells takes 10-30 hours. M phase lasts for more than 1 hour in plant cells.

Embryonic cells show a lot of variation in their cell cycle. Cells of many animal embryos undergo rapid divisions, forming progressively small cells. The rate of DNA synthesis is about 100 times faster in these non growing embryonic cells than it is in the adult cells of the same species. These are usually no G₁ phase, and DNA synthesis starts during or immediately after the M phase. The embryonic cells of Xenopus (a toad) have 25 minutes cell cycle. There is no G1 phase. S phase lasts for less than 15 minutes and starts during telophase of mitosis, G₂ is 20

Many protozoans, fungi and other lower organisms have a relatively brief cell cycle devoid of G₁ phase. The G₁ phase appears to be correlated with the amount of growth and biosynthesis taking place in the cell. If growth is minimum, as in animal embryos, or very rapid, as in lower organisms grown under optimum conditions, the G₁ phase is short or lacking. The G₁ phase can be easily done away with because S phase may begin before M phase is completed. The S and M phases are essential and usually there Is G₂ phase in between.

Mitosis occurs after DNA synthesis has taken place. If DNA synthesis stops, the cell does not undergo mitosis. The cell also does not divide if protein synthesis is stopped during G₂ phase. Both structural and enzymatic proteins are needed by the cell to undergo mitosis.

The cell cycle is prokaryotes consist of DNA replication followed immediately by cell division. Escherichia coli can divide in 30 minutes and only a minute or so of this time is used in DNA replication.

Potential of cells for growth and division: The cells of an organism vary in their capacity to grow and divide. Three categories of cells are recognized regarding their potential for growth and division –

                              i.            Permanently or terminally differentiated cells: these cells undergo extreme structure specialization and lose their ability to divide forever. Examples: muscle cells, nerve cells, red blood corpuscles.

                           ii.            Temporarily differentiated (quiescent) cells: These cells are differentiated and normally do not divide. However, they can be induced to begin DNA synthesis and divide by appropriate stimuli. E.g. liver cells can be induced to start proliferation by surgical removal of a part of liver. Lymphocytes can be induced to divide by interaction with suitable antigen. Fibroblasts are stimulated to proliferation on injury to a tissue for healing.

                        iii.            Undifferentiated cells: these cells retain the ability to divide. Their descendants replace the lost cells in certain tissues, such as epithelia and blood, under normal physiological conditions. These reserve cells are often called stem cells.

The quiescent cells usually have duplicated DNA and are said to be in G₀ state. Some epithelial cells may rest in G₂ phase.

Control of cell cycle: the mechanism that controls the cell cycle is at present the subject of an intensive research in cell biology. This mechanism can reveal the ways in which cells work, and more importantly why cells grow and divide in an uncontrollable manner in cancer.

As the beginning of the S phase eventually leads to cell division, it is important to find out what triggers DNA replication. Many views have been expressed:-

                     i.            Nucleo-cytoplasmic ratio: Hertwig in 1910 proposed that cell division starts when the ratio between the volume of the nucleus and the volume of the cytoplasm is upset. Growth of a cell involves the synthesis of protein, nucleic acids, lipids and other cellular components. The synthesis requires the movements of materials back and forth through the nuclear and cell membranes. As a cell grows, its volume increases more than the surface of the nucleus and the cell. At a critical point, the surface of the nucleus becomes inadequate for the exchange of materials between the nucleus and the cytoplasm required for further growth. At this stage the cell divides and regains the nucleocytoplasmic ratio that allows growth.Although cell division usually takes place after a cell has grown to a certain size, there are important exceptions to this pattern. During embryonic development of many animals, cell divisions occur without a net increase in the size of the embryo. The eggs of many animals grow very large before division.

                  ii.            Surface-volume ratio: usually it is held that the surface-volume ratio of a cell plays an important role. As a cell grows in size, its volume increases more than its surface. Since a cell draws all materials for maintenance and growth through its surface, a stage will reach when the surface area is insufficient to supply the large volume. It has been suggested that these is a critical point in the surface-volume ratio at which the division starts. The division   of the cell greatly increases the surface without increasing the volume.

This theory too fails in some cases. If the cells are started, they may divide without doubling their size and smaller daughter cells.

               iii.            Nucleolus: nucleolus has also been assigned a trigger function because damage to it at a certain critical time (telophase to midprophase) stops cell division.

                iv.            Cyclic nucleotide: it has been  found that two cyclic nucleotides, cAMP , and cGMP , influence cell division. Concentrations of these nucleotides vary regularly during cell cycle. In many cells, the concentration of cAMP is high during G₁ phase, significantly drops as the cells enter S phase and mitosis, and rises again in the next G₁ phase. The concentration of cGMP often varies in the reverse pattern. It is low in G₁ phase , becomes high in the beginning of S phase and fails again by the end of M phase. Addition or removal of either of these nucleotides can start of stop entry of many cells into S phase and the subsequent M phase.

                   v.            In many cells, the concentration of these cyclic                   nucleotides remains constant throughout cell cycle. Plant cells do not have cycle nucleotides. In view of these facts, cyclic AMP and GMP are no longer through to regulate cell cycle.

                vi.            Phosphorylation: the number of phosphate groups added to the histone group, particularly to H1, varies during the cell cycle. Typically phosphate groups are added to H1 as the cells enter S phase, increase during M phase, and are removed on the completion of mitosis before G₁ starts. Addition and removal of phosphate groups to the nonhistone proteins have also been noted during the cell cycle. These changes in the histone and nonhistone proteins occur in organisms as diverse as fungi, plants and animal. It seems likely that changes in the histones and nonhistones may have a role in the control of cell cycle because these proteins have been found to regulate the activity of genes of genes in RNA transcription during interphase.

                         vii.            Cyclin: mitosis appears to be controlled by the concentration of a protein builds up in the cell during interphase and is degraded during mitosis. Uncontrolled mitosis leads to cancer. Certain radiations, smoking, toxic chemicals, some viruses,etc., cause cancer.

Soil profile

Soil profile

Definition of soil profile:-

The vertical section of the soil showing the various layers from the surface to the unaffected parent material is known as a soil profile.

The various layers are known as horizons. A soil profile contains three main horizons. They are named as horizon A, horizon B and horizon C.

Ø The surface soil or that layer of soil at the top which is liable to leaching and from which some soil constituents have been removed is known as horizon A or the horizon of eluviations.

Ø The intermediate layer in which the materials leached from horizon A have been redepositing is known as horizon B or the horizon of illuviation.

Ø The parent material from which the soil is formed is known as horizon C.

A study of soil profile is important as it is historic record of all the soil forming processes and it forms the basis for the study in pedagogical investigations. Soil profile is the key for the soil classification and also forms the basis for the practical utility of soils.

A hypothetical minerals soil profile will include O, A, B, C and R master horizon and all the possible sub-horizon,

Master horizon and sub-horizon.

O horizon -  it is called as organic horizon. It is formed in the upper part of the mineral soil, dominated by fresh or partly decomposed organic materials.

Ø This horizon contain more than 30% organic matter if minerals fraction has more than 50% clay (or) more than 20% organic matter if mineral fraction has less clay.

Ø The organic horizons are commonly seen in forest areas and generally absent in grassland, cultivated soils.

Ø O1 – organic horizon in which the original forms of the plant and animal residues can be recognized through naked eye.

Ø O2 – organic horizon in which the original plant and animal matter cannot be recognized through naked eye.

Ø A Horizon – horizon of organic matter accumulation adjacent to surface and that has lost clay, iron and aluminum.

Ø A1 - top most mineral horizon formed adjacent to the surface. There will be accumulation of humified organic matter associated with mineral fraction and darker in color than that of lower horizons due to organic matter.

Ø A2 – horizon of maximum elevation of clay, iron and aluminum oxide and organic matter. Loss of these constituents generally results in accumulation of quartz and other sand and silt size resistant minerals. Generally lighter in color than horizon above and below.

Ø A3 – a transitional later between A and B horizon with more dominated properties of A1 and A2 above than the underlying B horizon. This horizon is sometimes absent solum.

Ø B Horizon – horizon in which the dominant features are accumulation of clay, iron, aluminum or humus alone or in combination. Coating of sesquioxide will impart darker, stronger of red color than overlying or underlying horizons.

Ø B1 – a transitional layer between A and B. more like A than B.

Ø B2 – zone of maximum accumulation of clay, iron and aluminum oxide that may have moved down from upper horizons or may have formed in situ. The organic matter content is generally higher and color darker than that of A2 horizon above.

Ø B3 – transitional horizon between B and C and with properties more similar to that of overlying B2 than underlying C.

Ø C horizon – it is the horizon below the solum (A + B), relatively less affected by soil forming processes. It is outside the zone of major biological activity. It may contain accumulation of carbonates or sulphates, calcium and magnesium.

Ø  R horizon – underlying consolidated bed rock and it may not be like the parent rock from which the solum is formed.

Beside, lower case letters are used to indicate the special features of master horizons. This case letter follows the subdivisions of master horizons. E.g. Ap-ploughed layer e.g. B2t- illuvial clay. When two or more genetically unrelated (contrasting) materials are present in a profile as in the case of alluvial or colluvial soils then the phenomenon is known as lithological discontinuity. This is indicated by the use of Roman letter as prefixes to the master horizons.

Weathering

weathering

Weathering:- a process of disintegration and decomposition of rocks and minerals which are brought about by physical agents and chemical processes, leading to the formation of regolith(unconsolidated residues of the weathering rock on the weathering rock on the earth’s surface or above the solid rocks).

                                                          OR

The process by which the earth’s crust or lithosphere is broken down by the activities of the atmosphere, with the aid of the hydrosphere and biosphere and biosphere.

Parent material:- it is the regolith or at least it’s upper portion. May be defined as the unconsolidated and more or less chemically weathered mineral material from which soil is developed.

Weathering:-

Two basic processes

1). Physical/mechanical (disintegration)

2). Chemical (decomposition)

In addition, another process: biological and all these processes are work hand in hand. Depending up on the agents taking part in weathering processes, it is classified into three types.

Physical weathering:- the rocks are disintegrated and are broken down to comparatively smaller pieces, without producing any new substance.

1.    Physical condition of soil – the permeability of rocks is the most important factor. Coarse textured (porous) sand stone weather more readily than a fine textured (almost solid) basalt. Unconsolidated volcanic ash weather quickly as compared to unconsolidated coarse deposits such as gravels.

2.    Action of temperature – the variation in temperature exerts great influence on the disintegration of rocks.

Ø During day time, the rocks get heated up by the sun and expand. At night, the temperature falls and the rocks get cooled and contract.

Ø This alternate expansion and contraction weakens the surface of the rock and crumbiles it because the rocks do not conduct heat easily.

Ø The minerals within the rock also vary in their rate of expansion and contraction

·       The cubical expansion of quartz in twice as feldspar

·       Dark colored rocks as subjected to fast changes in temperature as compared to light colored rocks

Ø the differential expansion of minerals in a rock surface generates stress between the heated surface and cooled un expanded parts resulting in fragmentation of rocks.

Ø This process cause the surface layer to peel off from the parent mass and the rock ultimately disintegrates. This process is called exfoliation

3.    Action of water – water acts as a disintegrating, transporting and depositing agent

i.            Fragmentation: water beats over the surface of the rock when the rain occurs and starts flowing towards the ocean.

Ø Moving water has the great cutting and carrying force.

Ø It forms gullies and ravines and carries with the suspended soil material of variable sizes.

Ø Transportation power of water varies. It is estimated that the transporting power of stream varies as the sixth power of its velocity i.e. the greater the speed of water, more is the transporting power and carrying capacity.

Speed/sec                                       carrying capacity

15 cm                                              fine sand

30 cm                                               gravel

1.2 cm                                              stones (1 kg)

9.0 cm                 boulders (several tons)

The disintegration is greater near the source of river than its mouth

ii.            Action of freezing: frost is much more effective than heat in producing physical weathering.

Ø During day summer/ dry weather – these clays shrink considerably forming deep cracks or wide cracks.

Ø On subsequent wetting, it swells.

Ø This alternate swelling and shrinking/ wetting or  drying of clay enriched rocks make them loose and eventually breaks

iii.            Action of glaciers :

Ø In cold regions, when snow falls, it accumulates and changes into a ice sheet.

Ø These big glaciers start moving owing to the change in temperature and /or gradient.

Ø On moving, these exert tremendous pressure over the rock on which they pass and carry the loose materials

Ø These materials get deposited on reaching the warmer regions, where its movement stops with the melting of ice

4.    Action of wind :-

Ø Wind has an erosive and transporting effect. Often when the wind is laden with fine material viz., fine sand, silt or clay particles, it has a serious abrasive effect and the sand laden winds itch the rocks and ultimately breaks down under its force.

Ø The dust storm any transport tons of material from one place to another. The shifting of soil cause serious wind erosion problem and may render cultivated land as degraded e.g Rajasthan deserts.

5.    Atmospheric electrical phenomenon :-

It is an important factor causing break down during rainy season and lighting breaks up rocks and or widens cracks.

CHEMICAL WEATHERING

     Decomposition of rocks and minerals by various chemical processes is called chemical weathering. It is the most important process for soil formation.

 Chemical weathering takes place mainly at the surface of rocks and minerals with disappearance of certain minerals and the formation of secondary product (new materials). This is called chemical transformation.

Feldspar + water → clay minerals + soluble cation and anion

Chemical weathering becomes more effective as the surface area of the rock increases.

Since the chemical reactions occur largely on the surface of the rocks, therefore the smaller the fragments, the greater the surface area per unit volume available for reaction. The effectiveness of chemical weathering is closely related to the mineral composition of rocks. E.g quartz responds far slowly to the chemical attack tan olivine or pyroxene.

Average mineralogical composition (%)

composition	granite	basalt	shale	s.stone	l.stone
Feldspar	52.4	46.2	30.0	11.5	-
Quartz	31.3	-	2.3	66.8	-
Pyrox-amphi	-	44.5	-	-	-
FeO mineral	2.0	9.3	10.5	2.0	-
Clay mineral	14.3	-	25.0	6.6	24.0
carbonates	-	-	5.7	11.1	76.0

Chemical processes of weathering:

1.    Hydration – chemical combination of water molecules with a particular substance or mineral leading to a change in structure. Soil forming minerals in rocks do not contain any water and they undergo hydration when exposed to humid conditions. Up on hydration there is swelling and increase in volume of minerals. The minerals lose their luster and become soft. It is one of the most common processes in nature and works with secondary minerals, such as aluminum oxide and iron oxide minerals and gypsum.

Example:

a)    2Fe₂O₃ + 3HOH →      2Fe₂O₃.3H₂O

(haematite) (red)                  (limonite) (yellow)

b)   Al₂O₃ + 3HOH               →      Al₂O₃.3H₂O

(bauxite)                           (hyd. Aluminum oxide)      

c)    CaSO₄ + 2H₂O              →      CaSO₄.2H₂O

(Anhydrite)                           (gypsum)

2.    Hydrolysis – most important process in chemical weathering. It is due to the dissociation of H₂O into H⁺ and OH^- ions which chemically combine with minerals and being about changes, such as exchange, decomposition of crystalline structure and formation of new compounds. Water acts as a weak acid on silicate minerals.

KAlSi₃O₈ + 8 HOH    →      HAlSi₃O₈ + KOH

(orthoclase)                      (acid silt clay)

HAlSi₃O₈ + 8HOH     →      Al₂O₃.3H₂O + 6H₂SiO₃

(Recombination)              (hyd. Alum.oxide) (silicic acid)

This reaction is important due to two reasons:

Ø Caly,bases and silicic acid – the substance formed in these reaction – are available to plants

Ø Water often containing CO₂ (absorbed from atmosphere), reacts with the minerals directly to produce insoluble clay minerals, positively charged metal ions (Ca⁺⁺,Mg⁺⁺,Na⁺,K⁺) and negatively charged ions (OH^-,HCO^-₃) and some soluble silica – all these ions are made available for plant growth.

3. solution – some substance present in the rocks are directly soluble in water. The soluble substances are removed by the continuous action of water and the rock no longer remains solid and form holes, rill or rough surface and ultimately falls into pieces or decomposes. The action is considerably increased when the water is acidified by the dissolution of organic and inorganic acids. (e.g) halites, NaCl

NaCl + H₂O → Na^+’Cl^-,H₂O (Dissolved ions with water)

4.    Carbonation: carbon dioxide when dissolved in water it forms carbonic acid.

2H₂O + CO₂ → H₂CO₃

This carbonic acid attacks many rocks and mineral and being them into solution. The carbonated water ahs an etching effect up on some rocks especially lime stone. The removal of cement that holds sand particles together leads to their disintegration.

CaCO₃ + H₂CO₃ → Ca(HCO₃)₂

(Calcite)                        (ca bi carbonate)

              Slightly soluble                  readily soluble

5.    Oxidation – the process of addition and combination of oxygen to minerals. The absorption is usually from O₂ dissolved in soil water and that present in atmosphere. The oxidation is more active in the presence of moisture and results in hydrated oxide. E.g., minerals containing Fe and Mg.

4FeO(ferrous oxide) + O₂→2Fe₂O₃(ferric oxide)

4Fe₃O₄(magnetite) + O₂→6Fe₂O₃(haematite)

2Fe₂O₃(haematite)+H₂O→2Fe₂O₃.3H₂O(limonite)

6.    Reduction – the process of removal of oxygen and is the reverse of oxidation and is equally important in changing soil color to grey, blue or green as ferric iron is converted to ferrous iron compounds. Under the conditions of excess water or water logged condition (less or no oxygen), reduction takes palce.

2Fe₂O₃(haematite)-O₂ → 4FeO (ferrous oxide)-reduced form

                   In conclusion, during chemical weathering igneous and metamorphic rocks can be regarded as involving destruction of primary minerals and the production of secondary minerals.

In sedimentary rocks, which is made up of primary and secondary minerals, weathering acts initially to destroy any relatively weak bonding agents (FeO)and the particles are freed and can be individually subjected to weathering.

Biological weathering

Unlike physical and chemical weathering, the biological or living agents are responsible for both decomposition and disintegration of rocks and minerals. The biological life is mainly controlled largely by the prevailing environment.

1.    Man and animals

Ø  The action of man in disintegration of rocks is well known as he cuts rocks to build dams, channels and construct roads and buildings. All these activities result in increasing the surface area of the rocks for attack of chemical agents and accelerate the process of rocks decomposition.

Ø A large number of animals, birds, insects and worms, by their activities they make holes in them and thus aid for weathering.

Ø In tropical and sub tropical regions, ants and termites build galleries and passages and carry minerals from layer to upper surface and excrete acids. The oxygen and water with many dissolved substances, reach every part of the rock through the cracks, holes and galleries, and thus brings about speedy disintegration.

Ø Rabbits, by burrowing in to the ground, destroy soft rocks. Moles, ants and bodies of the dead animals, provide substances which react with minerals and aid in decaying process.

Ø The earthworms pass the soil through the alimentary canal and thus bring about physical and chemical changes in soil material.

2.    Higher plants and roots

         The roots of trees and other plants penetrate into the joints and crevices of the rocks. As they grew, they exert a great disruptive force and the hard rock may break apart. E.g. pipal tree growing on walls/rocks.

The grass roots from a sponge like mass, prevents erosion and conserve moisture and thus allowing moisture and air to enter in to the rock for further action.

Some roots penetrate deep into the soil and may open some sort of drainage channel. The roots running in crevices in lime stone and marble produces acids. These acids have a solvent action on carbonates.

The dead roots and plant residues decompose and produce carbon dioxide which is of great importance in weathering.

3.    Micro-organisms

          In early stages of mineral decomposition and soil formation, the lower forms of plants and animals like, mosses, bacteria and fungi and actinomycetes play an important role. They extract nutrients from the rock and N form air and live with a small quantity of water. In due course of time, the soil develops under the cluster of these micro-organisms.

These organisms closely associated with the decay of plant and animal remains and thus liberate nutrients for the use of next generation plants and also produce CO₂ and organic compounds which aid in mineral decomposition