4 Main Components of the Cytoplasm (With Diagram)

The following points highlight the four main components of the Cytoplasm. The components are: 1. Groundplasm or Cytoplasmic Matrix 2. Organoids or Organelles 3. Inclusions or Ergastic Substances 4. Vacuoles.

Cytoplasm: Component # 1. Groundplasm or Cytoplasmic Matrix:

In light microscopy the term groundplasm refers to the liquid fraction of cytoplasm in which other components of the protoplast are suspended.

In ultrastructure ground substance or cytoplasmic matrix is defined as the viscous, homogeneous, clear and transparent liquid part of the cytoplasm. It has unusual property of being capable of both viscous flow like a liquid and elastic deformations like a solid.

The cytoplasmic matrix near the outer membrane tends to be dense like solid and is generally referred to as ectoplasm whereas the cytoplasmic matrix in the interior of the cell, generally in fluid state, is referred to as endoplasm.

The groundplasm contains a great variety of inorganic salts and ions, as well as carbohydrates, proteins, fats and many other organic substances which are beyond the resolution limit of electron microscope. This homogeneous ground substance still remains a challenge because the structural world of the atoms of cytoplasm still remains unknown and needs exploration.

Cytoplasm: Component # 2. The Organoids or Organelles:

A variety of living bodies of definite structures and functions are seen suspended in the cytoplasm which are known as organoids or organelles. These organelles are the main sites for the various cytoplasmic activities. The organelles are of two types: some concerned with the chemical works or metabolism of the cytoplasm and the others concerned with the mechanical works.

The organelles concerned with the chemical works of the cytoplasm are as under:

(i) Plastids (only in the plant cells),

(ii) Mitochondria,

(iii) Golgi complex,

(iv) Ribosomes,

(v) Endoplasmic reticulum and

(vi) Lysosomes, Microbodies and Peroxysomes.

The other kinds of organelles concerned with the mechanical works of cytoplasm are:

(i) Microtubules,

(ii) Centrosomes (not seen in the cells of higher plants),

(iii) Flagella and Cilia.

a. The Plastids:

These are coloured or colourless cytoplasmic bodies present in all the plant cells except fungi and prokaryotes. It is only on account of the presence of these plastids the plant cells are variously coloured.

On colour basis, these plastids may be grouped into the following types:

(i) Leucoplasts or leukoplasts (colourless),

(ii) Chloroplasts (green),

(iii) Chromoplasts (coloured other than green).

The Chromoplasts are responsible for yellow, pink, red colours of the flowers, fruits and young leaves of plants. Leucoplasts are sometimes called amyloplasts if they store starch. Amyloplasts are found chiefly in the cells of storage organs, as in potato tubers.

The plastids are generally coloured owing to the presence of several kinds of colouring substances called pigments.

Some important pigments and their respective colours are listed below:

These pigments are found in the plastids. However, there are certain pigments, such as anthocyanins which are not found in the plastids but are dissolved in the ground substance of cytoplasm. The blue, red and pink colours of some flowers and young foliage leaves appear due to the presence of anthocyanins.

Chloroplast is the most important among all the plastids and is the chief site of photosynthesis in plant cells. The chloroplasts are of various shapes and sizes. In higher plants they are generally shaped like a biconvex lens and are 4 to 6 µ in diameter and 2 to 3 µ in thickness.

Their number in a cell varies from plant to plant and tissue to tissue. Juvenile cells usually lack chloroplasts but contain sub-microscopic bodies called proplastids from which chloroplasts develop as the cells mature. The chloroplast develops only in the cells exposed to light.

b. Mitochondria:

There are several synonymous terms for mitochondria such as chondriochonds, chondriomes, mitosomes, chondriosomes and so on. It was Kolliker who observed granule-like structures in the muscle cells of insects in the year 1880. Flemming (1882) named them fila and later on Altmann (1890) named them bioplasts.

In 1897, Benda demonstrated similar objects in cells and assigned the name mitochondria to them. Lewis and Lewis (1914) demonstrated the possibility that mitochondria are concerned with some metabolic activity of the cell and Hogeboom (1948) has shown that the mitochondria are the chief sites for the cellular respiration. The presence of mitochondria in the plant cell was first detected by F. Meves in 1904 in Nymphaea.

The mitochondria are virtually present in all the aerobic cells. They are absent in bacteria, other prokaryotic cells and mature red blood cells of the multicellular organisms.

The mitochondria are bounded by two layers of unit membranes. The inner layer is invaginated to form finger-like, plate-like or sac-like plates in the lumen of mitochondrion. These folds or plates are called cristae. The space between the outer and inner membranes and the central space is filled with viscous mitochondrical matrix which contains oxidative enzymes and coenzymes.

The mitochondria are colourless bodies widely distributed in the ground substance of the cytoplasm. These are easily differentiated from other cytoplasmic components by staining process. They are selectively stained by a special stain Janus green.

The mitochondria are of different shapes. They may be fibrillar, spherical, rod shaped and oval and they may change from one form to another depending upon the physiological conditions of the cells.

Mitochondria measure usually from 0.5 µ to 1.0 µ in diam and reach a length up to 40 µ. They contain numerous enzymes which take part in the oxidative steps of Krebs’ cycle in respiratory process. The high energy phosphate compounds such as Adenosine diphosphate (ADP) and Adenosine triphosphate (ATP) are also synthesized and stored in the mitochondria.

These phosphate compounds after breakdown liberate tremendous amount of energy which is required in the completion of many chemical processes of the cytoplasm. This is why mitochondria are regarded as “power house” of the cell.

They are the principal but not the only sites of oxidation since the oxidation of some compounds also takes place in the ground substance of cytoplasm with the help of enzymes present therein (Fig. 1.19).

c. Golgi Complex:

This cytoplasmic organelle is named after its discoverer Golgi. The structure was discovered in 1898. The golgi bodies are also called lipochondria. For several years there was considerable disagreement about the existence of that organelle. Most of the early biologists believed that it was an artifact of fixation or staining procedures.

Studies with phase contrast microscope in the early 1940s also indicated the existence of golgi bodies. The study of electron micrographs of thin sections of cells in 1950s finally proved beyond doubt the existence of golgi bodies in all the cells of eukaryotes. The golgi apparatus does not exist in the prokaryotes.

The electron microscopic studies have revealed that this organelle consists of a series of compactly grouped smooth contoured membrane limited vesicles of variable shapes and dimensions and variable number of small vacuoles [Fig. 1.19 (b)].

The membranes of golgi complex are of lipoproteins. The functions of golgi complex are storage of proteins and enzymes and secretion of many important materials including cell wall materials.

d. Endoplasmic Reticulum:

The term “endoplasmic reticulum” was introduced by Porter and Kallman (1952). By the use of ultrathin sections and improved fixation techniques developed by Palade and Porter (1954), it was finally recognized that the endoplasmic reticulum presented cavities of a great variety of shapes and dimensions surrounded by membranes.

The endoplasmic reticulum is found almost universally in eukaryotic cells. It is lacking in the bacterial and myxophycean cells. It is a system or network of interconnected membrane bound fine tubules called canaliculae (Fig. 1.20). Obviously, endoplasmic reticulum (ER) is a hollow system.

Sometimes, it appears as a continuous system, connected on one side to plasma membrane and on the other side to the nuclear envelop. This continuity is not recognizable in the thin sections of the cell. The membranes of endoplasmic reticulum are studded with single layer of opaque particles of ribonucleoprotein called the ribosomes.

The endoplasmic reticulum in certain region of the cell may be devoid of ribosome particles.

e. Function:

The functions of endoplasmic reticulum are still not perfectly understood. The canaliculae of endoplasmic reticulum probably serve as system for conveying raw materials from cytoplasmic environment to the enzymatic machinery located in mitochondria and elsewhere in the cell and they also provide pathways for the diffusion of metabolites throughout the cell.

The canaliculae act as channels for transportation of secretory products. Palade (1956) has observed secretory granules in the cavity of the endoplasmic reticulum.

Various secretory granules of granular endoplasmic reticulum are transported to other organelles as follows:

Granular ER → agranular ER →cistemae of Golgibody → secretory granules

The endoplarmic reticulum acts as ultrastructural skeletal framework in the cell. The endoplasmic reticulum provides increased surface for the various enzymatic activities. It contains many enzymes which perform various metabolic reactions.

f. Microsomes:

The microsomes are special type of organelles in the cytoplasm which are described as small vesicles bounded by thin surface membranes of lipoproteins impregnated with small ribosome particles. These are supposed to have developed when canaliculae of the ergastoplasm are broken into small spheres.

The microsomes lie scattered in the cytoplasm. These RNA rich bodies are the chief sites for protein synthesis and their membranes are involved in the steroid synthesis.

g. Lysosomes, Microbodies and Peroxysomes:

In the cytoplasmic matrix of animal cells are found variously shaped bodies usually bounded by a single surface membrane and containing hydrolytic enzymes. These are called lysosomes (Fig. 1.19). Lysosomes were first reported by de Duve in 1955.

In 1964, P.Matile demonstrated the occurrence of lysosomes in the fungus Neurospora. The lysosomes originate from golgi complex. The lysosomes are lytic in nature and are involved in the digestion of intracellular substances.

The function of lysosome membrane is to separate the hydrolytic enzymes from other part of the cell thus protecting the cell from self-digestion. When the cells become dead the lysosomes release their enzymes which rapidly digest the cell. The lysosomes of plant cells are membrane bound storage granules containing a variety of hydrolytic enzymes and they comprise of sphaerosomes, aleurone grains and vacuoles.

h. Microbodies:

In the groundplasm of many cells are found spherical or ovoid membrane bound bodies of variable size, 0.2-1.5 mµ in diameter which are called microbodies. These bodies are surrounded by single unit membrane and contain dense or crystalline materials, the matrix. Recently they have been found to contain enzymes particularly catalases, oxidases and enzymes for hydrogen peroxide metabolism.

i. Ribosomes:

In the cytoplasm of cell there occur particles composed of ribonucleic acid (RNA) and proteins. These are called Ribosomes. These particles occur freely in the cytoplasm and also remain attached with the membrane of endoplasmic reticulum.

The sizes of the ribosomes vary somewhat, being approximately 150 Å in bacteria, chloroplasts and mitochondria and 140-200 Å in the cytoplasm of the eukaryotic plant and animal cells. Each ribosome is composed of two sub-units; a smaller sub-unit, called 40s sub-unit and a larger sub-unit called 60s sub-unit.

The ribosomes remain attached with the membrane of endoplasmic reticulum by the larger sub-unit. The smaller subunit remains attached to the larger sub-unit forming a cap like structure. The ribosomes are the chief sites 3f protein synthesis.

j. Microtubules:

Microtubules and cytoplasmic filaments have been discovered recently in the cytoplasm of a wide variety of both plant and animal cells and so they can now be considered as the universal component of eukaryotic cells. Cytoplasmic filaments are rods of indefinite length and 40-50 Å in thickness. Such filaments are observed in most of the undifferentiated plant cells.

The filamentous system presumably forms a part of mechanical machinery converting chemical energy into work and thus bringing about the rapid cytoplasmic streaming and provides strength. They are straight and indefinite in length and have a hollow appearance. Each microtubule is an aggregate of some 10 to 14 longitudinal microfilaments (Fig. 1.21).

It is also possible that the sub-units responsible for the formation of the cytoplasmic filaments can aggregate in such a manner as to form microtubules directly without first forming microfilaments. Electron microscopic study has thrown much light on the structure of these cytoplasmic tubules.

They are different from other tubular structures, such as endoplasmic reticulum, golgi bodies. Microtubules are about 200 Å to 270 Å in external diameter having an electron dense wall of some 50 to 70 Å thick. They are found in rotating condition. They are present in nuclear spindle, the kinetosomes and the cilia.

Thus, like filaments, microtubules seem to be involved with the machinery of motion. These tubules also play important role in maintaining the shape of the cell. Thus like filaments, microtubules form the part of cytoskeleton in the cytoplasmic matrix. The other functions of these tubules are yet to be discovered.

k. Centrosome:

In the cells of some lower plants and all animals there occurs radiating structure in the cytoplasm very close to the nucleus. That is termed centrosome. The term centrosome was coined by T. Boveri in 1888. It consists of radiating system, the aster or astral rays, and a pair of granules, the centrioles or diplosomes (Fig. 1.19).

The centrioles are independent self-duplicating bodies, 300—500 mµ in length and 150 mµ to 160 mµ, in diameter. They form 90° angle with each other. The ultrastructure of a cell reveals that the centriole is a small barrel shaped cylinder made up of evenly spaced nine longitudinally oriented triplets of rodlets or fibrils embedded in dense amorphous matrix.

One end of centriole appears to be closed while the other end is open.

The innermost unit of each triplet is designated subfibril A and the other two subfibril B and C. All the three subfibrils have microtubules a dense rim and a less dense centre giving them hollow appearance. In fact, it has not been established whether they are tubules or solid fibres with a dense cortex and light interior.

Subfibrils A of the nine triplets are uniformly spaced on the circumference of a circle about 150 mμ in diameter and each triplet is inclined so that a line through the centres of its sub-units make an angle of about 30° with a tangent to this circle at the midpoint of subfibril A.

The arrangement of fibrils in the wall of centriole thus resembles the set of the propellants of a pyrotechnic pinwheel. Subfibril A is provided with two short diverging arms. One of the two arms, directed inward along a radius, has free end pointing toward the centre of the centriole and the other arm, directed outward connects A with subfibril C of the next triplet.

The successive triplets are thus linked together, subfibril A Fig C, around the circumference of centriole by a series of slender linear densities (Fig. 1.22).

The interior of the centriole is usually filled with a homogeneous cytoplasm of low density but may contain one or more small dense granules. Sometimes fibrous arms or microtubules also radiate from them. In some cell types two or more pericentriolar structures or satellites are deployed around the centriole. These take various forms.

Recently, the presence of DNA and RNA has been noticed in centrioles. Probably the information for the synthesis of centrioles is obtained from a tiny DNA containing unit with weight of about 2 X 10^-16 gm.

l. Function:

Centrosome performs mechanical function in the cell. Before the onset of nuclear division the centrosome divides into two. Two centrosomes migrate to the two opposite poles of the nucleus where they are involved in the formation of mitotic apparatus and direct the separation of chromosomes during nuclear division.

m. Flagella and Cilia:

The cilia and flagella are rapidly beating and contractile filamentous processes emerging out from the cytoplasm. Recently they have been examined with electron microscope and it has been found that cilia and flagella throughout the plant and animal kingdoms show identical structures.

Cilia have been defined as long cylindrical processes tapered at the tip and composed of an axial filament complex embedded in a matrix and enclosed in a ciliary membrane which is continuous at the base with cell membrane proper.

Though there is no fundamental structural difference between cilia and flagella yet both can be distinguished from each other by the following features:

(i) The flagella are few in number in each cell but the cilia are numerous in number per cell.

(ii) The flagella are long in proportion to the size of the cell whereas the cilia are small. The shaft of the cilium is 0.2-0.25 µ in diameter and 5 to 10 µ. long. Flagella range from this length upward to 150 µ or more.

(iii) The flagella beat independently and exhibit undulatory motion while the cilia tend to beat in coordinated rhythms and move in a sweeping or pendular stroke.

The flagella and cilia have ‘axial-fibre-complex’ consisting of a constant number of internal fibrils or microtubules, that is 11, out of which two are in the centre and nine evenly spaced micro-fibrils remain arranged around the two central fibrils (9 + 2). These eleven fibrils remain embedded in the matrix of liquid consistency and enclosed in a unit membrane of 90 Å thickness (Fig. 1.23).

The two central fibrils of the axial-fibre complex are in singlet state. They are approximately circular in cross- section, 240 A in diameter and about 360 A from centre to centre. They are enveloped by central sheath which shows spiral composition in longitudinal section (Fig. 1.24). They have tubular appearance having denser outer region and light central core.

The nine peripheral fibrils differ from the two central fibrils in being doublets (i.e., each composed of two subfibrils). They measure approximately 38 A by 260 A in cross- section. One subfibril of each doublet is designated as subfibril A and the other as subfibril B.

The subfibril A has a circular profile while the subfibril B is crescentric and shares a sector of the wall of subfibril A. The curving septum between the two is thus a part of the wall of subfibril A and is convex toward subfibril B. Subfibril B independent of subfibril Å would not be a complete tubule (Figs. 1.24 and 1.25).

The subfibril A bears 2 short arms that project toward subfibril B of the next doublet. Gibbons demonstrated that these arms are composed of proteins which he named dynein, that appears to be involved in the production of mechanical work. The interior of subfibril A in the cilia of some cell types appears darker than that of subfibril B.

The ciliary fibrils or tubules and the cytoplasmic microtubules are similar in appearance and dimension and like cytoplasmic microtubules ciliary microtubules are composed of some 10-14 cytoplasmic filaments. The microtubular complex in both cilia and flagella terminates at its base in a basal body or basal corpuscle which is a hollow cylinder with the same structure as a centriole.

The basal bodies of cilia and flagella are believed to arise by reduplication of centrioles. The manner of reduplication of centriole is not known at all, but like the reduplication manner of chromosomes, the centrioles move toward the surface membrane of the cell and form the basal bodies or kinetosomes which in turn give rise to cilia or flagella.

The basal granules situated at the base of cilia or flagella are also able to divide into two and each daughter granule is able to produce a new cilium or flagellum as the case may be.

The architecture of the cilium is intimately related to the ciliary beat. The direction of ciliary beat is perpendicular to a line joining the two central tubules.

Cytoplasm: Component # 3. Ergastic Substances or Cytoplasmic Inclusions:

In the ground substance of the cytoplasm are seen many non-living bodies called cytoplasmic inclusions. These substances are formed as a result of metabolism and are accumulated in the form of granules or crystals as for example, calcium carbonate, calcium oxalate, starch grains, proteins, pigments, tannin, resins and oil drops.

Cytoplasm: Component # 4. Vacuole:

In the ground substance of the cytoplasm may be seen one or more vacuoles in addition to the living components. It is a debatable matter whether a vacuole should be treated as a cell organelle. The vacuoles have vacuolar membranes or tonoplasts and are filled with vacuolar sap.

It is still an open question whether or not vacuoles are delimited by a membrane or a cytoplasmic interface. Some vacuoles seem to represent smooth walled vesicles formed by endoplasmic reticulum and have fibrillar extensions, i.e., the endoplasmic reticulum contributes to the membranes of vacuoles.

This view was questioned on the ground that vacuoles lack any continuity with either endoplasmic reticulum or nuclear membrane.

The second school of thought holds that the vacuoles are derived from golgi apparatus. The vacuolar development in the cells of barley shoot apex supports the second view. The vacuoles gain in volume by simple fusion of smaller ones.

Mollenhauer and his associates (1961) on the other hand suggest a different type of vacuolar development in the cells of root cap. The golgi apparatus of these cells has been found to produce small vesicles which serve to transport the material to the peripheri of the cell. The material of the vacuole after disposition goes into the composition of cell wall while the vacuolar membrane fuses with plasma membrane.

The vacuoles are of common occurrence in the cells of both plants and animals. In some primitive unicellular organisms the vacuoles contain food particles, hence they are called food vacuoles. These cavities have got special property of contraction and expansion.

They pump the excess amount of water and waste products out of cytoplasm and thus maintain the definite internal pressure in the cell. The liquid portion of the vacuole, the so called vacuolar sap, is never a living substance. It is a watery solution of soluble wastes including pigments, sugars, uric acid etc.