8 Chief Metabolically Active Cell Organelles of Cytoplasm

The following points highlight the eight chief metabolically active cell organelles of cytoplasm. The organelles are: 1. Plastids 2. Mitochondria 3. Endoplasmic Reticulum 4. Golgi Bodies 5. Ribosomes 6. Centrosome 7. Lomasomes 8. Microtubules.

Organelle # 1. Plastids:

The term ‘plastid’ was firstly used by Haekel (1866). These are the largest bodies or largest cytoplasmic organelles of a plant cell. These may be pigmented or non-pigmented (colourless) one. These are absent in fungi, few algae, myxomycetes. There are 3 kind of plastids which may convert from one form to another form in the cell under proper conditions. These are also concerned with the metabolic activities of the plant.

i. Leucoplastid (= Leucoplast):

These are colourless plastids and are found in embryonic, sex or meristematic cells including other plant organs like underground stem and roots which are devoid of light. These are asymmetrical, rod shaped, oval or spherical shaped. It lacks stroma lamellae.

Leucoplasts are of 3 types:

(i) Amyloplasts (Amyl= starch). These are starch containing plastids, get converted into chloroplastid in presence of light.

(ii) Elaioplasts. They are found in seeds and store fats.

(iii) Proteinoplasts. They are also found in seeds and store proteins.

ii. Chloroplastid:

(Chlore = green). The credit of discovery of chloroplast goes to Von Sachs and the term was given by Schimper (1883). These are green plastids. It contains chlorophyll of green colour due to which some parts of the plants specially leaves appear green. Chloroplast are the most important plastids as they carry on the process of photosynthesis. Size, shape and distribution of chloroplasts vary in different species.

Generally these are disc shaped or discoid, 4-6 µm long or in diameter and 2-3 µm thick. Maximum variability in shape is exhibited in algae cells e.g., in algae plant named selaginella, each cell consists of single chloroplast. In contrast several chloroplasts are visible in Bryophytes.

In lower plants chloroplast shape is most variables e.g., spiral (Spirogyra), collar (Ulothrix), star like or stellate (Zygnema), kidney, heart and arrow shaped etc. In mosses, ferns and seed plants the shape is generally fixed i.e., flat, elliptical, circular etc.

Ultra-Structure:

The electron microscope has revealed that the chloroplast has a covering of a double layered membrane which is made up of lipo­protein. Each individual membrane is 35-50 Å thick. The space enclosed by the two membranes is referred to as stroma which is filled with colourless, transparent, proteinaceous matrix or ground substance.

Inside the stroma various membranes are found which are visible and lying- parallel to each other in total length of chloroplast, is called as lamellae. Clearly speaking, each lamellae is made up of two membranes.

In stroma at the interval of small distances 10-100 lamellae are seen arranged like stacks of coins made up of a spherical, flattened, double unit membranes known as thylakoids. This term was given by Manke (1961) and is also called baggy trousers. A group of thylakoids is known as granum.

Each cell contains 40-60 grana composed of protein and lipids which are different in number and size in different plants, well connected with each other by simple or reticulate intergrana membranes, are said as stroma lamellae or frets.

On the inner surface of each thylakoid or just below the membranous surface of a stack of grana is a layer of elements-quantsomes, which measure about 200 Å in diameter and 100 A thick. They absorb radiant energy available from sunlight and are supposed to be structural and functional unit of chloroplasts.

The quantsome term was proposed by Park and Biggins (1964) as small spherical structure situated 011 the inner surface of the lamella membrane. According to them, quantasomes are photosynthetic units of chloroplast comprising 230 molecules of chlorophyll.

Each molecule of chloroplast exhibits the presence of hydrophilic or water loving head and hydrophobic or water hating tail. The head and tail are made up of pyrol and phytol respectively.

Chemically chloroplast contains 40-45% protein, 20-30% phospholipids, 3-10% chlorophyll, 4-5% carotenoids, 2-3% RNA, small amount of DNA and carbohydrate, ribosome particles, minerals like Fe, Cu, Mn, Zn, Mg etc. in small amount, enzymes, co-enzymes etc. The presence of RNA and DNA in chloroplast indicates that chloroplast is in-fact a semiautonomous cell organelle which has power of self-reproduction. That is why, chloroplast has been said “cell within a cell”.

Types of Chlorophyll found in Plants:

(i) Chlorophyll ‘a’:

Having a CH₃ group, found in all photosynthetic organisms except photosynthetic bacteria. (C₅₅ H₇₂ O₅ N₄ Mg)—75% (Blue coloured).

(ii) Chlorophyll ‘b’:

Having a CHO group found in all higher plants and in green algae (C₅₅ H₇₀ O₆ N₄ Mg)—25%- (Green- black).

(iii) Chlorophyll ‘c’:

Found in brown algae and diatoms (family-Phacophyceae and Bacillariphyceae) which do not contain chlorophyll ‘b’.

(iv) Chlorophyll ‘d’:

Found in red algae (family- Rhodophyceae) which do not contain chlorophyll ‘b’.

(v) Chlorophyll ‘e’:

Found in yellow-green algae which do not contain chlorophyll ‘b’.

(vi) Carotenes:

C₄₀ H₅₆ (orange coloured)

(vii) Xanthophyll:

C₄₀ H₅₆ O₂ (yellow coloured). Carotene and Xanthophyll are collectively termed as carotenoides. Both are hydrocarbons.

Pyrenoid:

In green algae and some other plants like Anthoceros, Selaginella, Isoetes, a special proteinaceous body is also found in the stroma of chloroplast said as pyrenoid. Starch is stored around the pyrenoid.

Function:

It is evident that chloroplast is among one of the most important biochemical machines. These produce energy at molecular level. These have a wonderful capacity to absorb radiant energy from sun light and under the process of photophosphorylation (synthesis of ATP in presence of light) convert it in form of chemical energy (ATP).

The products formed in light reaction NADPH₂ and ATP are said as assimilatory powers which by the help of CO₂ synthesize carbohydrate or sugar in dark reaction. Light and dark reaction of photosynthesis is performed in granum and stroma respectively. Fundamentally chloroplast is the centre place of photosynthesis.

iii. Chromo plastid:

These are coloured plastids. Red pigments are found in place of blue and green pigments. These coloured pigments already produce red, yellow, orange, violet colours in plant parts. They are mostly found in petals of flowers and fruits.

Purple colour of flowers and beet root is due to presence of anthocyanin pigment which are found dissolved in cell sap. The best example is tomato which after ripening is converted in to red due to presence of red pigment, lycopene.

The yellow and orange-red colour is due to presence of xanthophyll and carotene respectively. Violet or purple colour is due to presence of anthocyanin pigment which are dissolved in vacuolar sap.

Due to presence of anthocyanin pigment, the roots of many plants like Beta vulgaris, stem of Balsum and embryonic leaves of mango tree appear red coloured. Chromo plastids show major differences in their shapes and size i.e., spherical, elongated or angular type. In chromo plastid, however, stroma is not found.

These are of 3 types:

(i) Phaeoplasts:

These plastids are found in brown algae, Diatoms and Dinoflagellates. Fucoxanthin is the main brown coloured pigment along with chlorophyll ‘a’ and chlorophyll ‘c’ in place of chlorophyll ‘b’.

(ii) Rhodoplasts:

These plastids are present in red algae. R-phycoerythrin and R-phycocyanin are red pigments along with chlorophyll ‘a’ and chl. ‘d’ in place of chl. ‘b’.

(iii) Chromatophores of blue-green algae:

They have Phycocyanin, C-phycoerytherin and chlorophyll and are found in blue-green algae.

(iv) Chromatophores of photosynthetic bacteria:

Purple and non-purple photosynthetic bacteria contain purple-red carotenoid pigment. Bacteriochlorophyll and Bacteriovirdin are also found.

Organelle # 2. Mitochondria (Mitos = thread, chondrion = granule):

There are various filamentous, minute, mostly rod shaped, spherical structures known as mitochondria. The credit of discovery of mitochondria goes to Richard Altman (1894) who termed it ‘bio blasts’ and later on named as ‘mitochondria’ by C. Benda (1897).

This is also called as chondriosomes or plastochondria. F. Meves (1902) firstly saw the presence of this structure in plant cell of Nymphaea (waterlily). Bensley et al (1934) isolated mitochondria from the liver cells of Guinea pig.

These are smaller than plastids. The average length and diameter ranges from 0.5 µm to 7 µm and 0.2 µm to 2.0 µm respectively. Mitochondria are equally distributed in cytoplasm but, however, situated maximum near nucleus. Their number varies in cells of different plants and other even in the cells of the same plant. It is found in limited numbers in green plants because chloroplasts also synthesise ATP which is absent in animals.

The number of mitochondria is found 200-400 per cell. The minimum number is one per cell (example-Alga-Microsterias, unicellular organism). The maximum number as many as 500,000 is found in some protozoa (Giant Amoeba, Zoological name: Chaos chaos).

The average composition of mitochondria is as follows:

Protein — 70%

Lipids — 25-30%

RNA — approximately 1%

DNA — less than 1%

Ultra-Structure:

The mitochondria possess a complex, sub-microscopic organisation which is only visible through an electron microscope. Each mitochondrion is surrounded by two units or double layered membranes known as outer and inner membrane which is nearly 7.5 pm thick and made up of phospholipids and proteins.

The outer membrane is smooth whereas the inner membrane forms finger like infoldings or sometimes several small plates by inserting inside the matrix of fluid part at different places and giving rise to cristae. The space between both membrane is almost 10 pm and termed as peri-mitochondrial space. Cristae increases the surface area of inner membrane. It was discovered by Palade.

Many small particles, by thousands are found over cristae (in-folds of inner membrane) which are said as oxysomes or F₁ particles (factor of Rocker) or elementary particles. Elementary particle was discovered by H.F. Moran (1964). These are knob like or resemble bat of tennis. The size of the oxysome is approximately 8.5 pm. Each oxysome is differentiated into base, stalk and head.

It is also found that oxysomes are present even on outer membrane of the mitochondria but these are without base and stalk. Oxysomes contain the enzyme, ATP synthetase which is helpful in the process of oxidation and phosphorylation. The term oxysome was proposed by Britton Chance.

The inner membrane which runs into the cavity of mitochondria, divides it into two chambers (i) outer chamber or compartment and inner chamber or compartment. Both chambers are filled by homogenous, proteinaceous, semifluid, jelly like substance which is called as mitochondrial matrix. The matrix of both the compartments are not completely isolated from each other because the inside membrane is incomplete at certain places.

Avers and King (1960), Wilson and Bonner (1971) found two types of mitochondrion in higher plants and gave different views. Likewise, J.S. Fletcher (1972) also classified mitochondria in two types on basis of his research models.

According to them one view is related mainly with respiration and other with the supply of carbon skeleton for biosynthesis or other chemical functions. It is clear from chemical composition that these are composed of mainly protein and phospholipids. Ribosomes (70 S, discovered by Kislevetal, 1965), RNA, Cytochrome, DNA (Hollenburg, 1969), Co- enzymes and electron carriers are also present.

Mitochondria is the power house or energy transformer of the cell. The major part of the aerobic respiration is completed in it.

Inside the mitochondria carbohydrates, fats and proteins are broken down or oxidised in to simpler substances by means of various enzymes resulting in the production of energy. This energy is not allowed to escape but is stored in the form of ATP (adenosine triphosphate) and utilized by the cell in various vital activities of the plant.

Total Kreb’s cycle is completed in mitochondria and resulting the formation of O₂ and energy. The presence of ribosomes, RNA and DNA demonstrates that these are also autonomous cell organelles like chloroplast which have the capacity of self-production and well linked with protein synthesis. According to some scientists, they are believed to play an important role in gametogenesis.

The origin of mitochondria is fairly controversial and several views have been given on this problem. There is no evidence that these develop as denovo. The most acceptable hypothesis is that the mitochondria rise from micro bodies which have been recently observed by electron microscope. A micro body is a fine granule ranging in size from 0.1-0.5 µm in diameter having a single outer membrane, a dense matrix and a few cristae.

At the time of cell division, mitochondria are almost equally distributed between daughter cells. Thus, new mitochondria may develop few cristae. At the time of cell division, mitochondria are almost equally distributed between daughter cells. Thus, new mitochondria may develop either from micro bodies or from pre-existing mitochondria.

Similarities between Mitochondria and Chloroplast:

(i) Cytoplasmic organelles

(ii) Double membrane envelope

(iii) Lipoprotein complex

(iv) Division (Multiplication)

(v) Enzymes

(vi) Nucleic acid (DNA and RNA)

(vii) Ribosomes 70 S

(viii) Semi-autonomous structure

(ix) Ability to synthesise ATP molecules.

Organelle # 3. Endoplasmic Reticulum (ER):

The endoplasmic reticulum is a net work of membrane in the cytoplasm of a cell. Porter (1945) called this net work “Endoplasmic reticulum” as it was found to be more concentrated in the endoplasm (or tonoplast) of the cell than in the ectoplasm or plasma membrane.

However, it has been shown that this net work is not restricted to the endoplasm only but extends up to the plasma-membrane, with which it becomes connected at many points. In the deeper region of the cell, it is continuous with the nuclear membrane. Endoplasmic reticulum is well developed in meristematic cells.

The net membranes are made up of double layers and forms innumerable or numberless tubules. This is also said as ergastoplasm. It is found in eukaryotic cell but absent in prokaryotes e.g.; bacteria etc. Endoplasmic reticulum in general constitutes the transport system inside the cell and also serves as cell- skeleton.

Kinds of Endoplasmic reticulum:

(i) Rough ER = RER (= Granular ER). With ribosomes (100-150 A in diameter) which take part in protein metabolism and are composed of 50% RNA and 50% protein.

(i) Smooth ER = SER (= Agranular ER).Without ribosome which does not take part in protein metabolism.

Structure of ER.:

ER may occur in three different forms:

(i) Cisternae (= Saccules = Lamellae):

These are long, flattened, non-branched sacs, 40-50µm thick and lie parallel to each other.

(ii) Vesicles:

Which are generally round or ovoidal sacs. 25-500µm in diameter.

(iii) Tubules:

These are fine, small, branched, variously shaped sacs which are 50-100µm in diameter and form a net structure. All above forms of the reticulum may be seen in a cell at different or at the same time during the cell cycle. Within a cell if active protein synthesis is going on as in liver cell, the reticulum assumes the form of cisternae.

The unit membrane of the ER resembles closely with the plasma membrane and the nuclear membrane both structurally and chemically. These three membranes are continuous with one another and thus form an elaborate system for communication, intra as well as intercellular.

It has also been found that rough ER may be converted in to smooth ER and vice-versa, depending upon the metabolic requirement of the cell. The term ‘micro some’ is often used. Really these are small fragments of ER in the form of small vesicles.

Function:

(i) Intracellular circulation of cytoplasmic substance.

(ii) Mechanical support.

(iii) Protein synthesis by Rough ER. Synthetically active cells contain highly developed endoplasmic reticulum and numerous ribosomes are attached to it. This association is often referred to as ergastoplasm.

(iv) Lipid or fat synthesis. Smooth ER is related to it.

(v) Glycogen synthesis. Smooth ER is related to it.

(vi)Synthesis of cholesterol and steroid hormone. Smooth ER is related to it.

(vii) Membrane flow. Exchange of ion uptake.

(viii) Detoxification of several endogenous and exogenous compounds, smooth ER is related to it.

(ix)It may also serve as a source of formation of nuclear membrane during cell division at the end of telophase stage.

(x) It provides a large surface area within the cytoplasm of the cell for various metabolic activities.

(xi)Site or place for ATP synthesis in the cell.

Organelle # 4. Ribosome:

Ribosomes are smallest, non-membranous, knob shaped cell organelles seen in electron microscope. They were discovered by G.E. Palade (1955). Ribosomes are found in all the living cells except the red blood cells (R.B.C’s) and sperm cells of animals. The diameter of ribosome ranges from 150A-250A. In prokaryotic cells it is found in cytoplasm and in eukaryotic cell mitochondria, chloroplast and rough endoplasmic reticulum.

Ribosomes are actually synthesised in nucleolus, within the nucleus from where they migrate in to cytoplasm from pore of nuclear membrane. The ribosomes which are formed in mitochondria and chloroplast, are said as manosomes. Mostly the ribosomes are associated with rough membrane of endoplasmic reticulum & are visible in clusters.

The ribosomes situated in clusters are called polyribosomes or polysomes or ergosomes. Warner (1962) and Rich (1963) have shown that functional units of protein synthesis are the clusters of ribosomes rather than single ribosome.

Some ribosomes are present in hyaloplasm or ground-plasm which are of three types:

(a) Reserve materials like carbohydrate, protein, fat and oils

(b) Secretory products

(c) Excretory products, unattached to any membrane.

Composition:

These are made up of nucleoprotein which includes nucleic acid and protein. Nucleic acid then gives rise to ribosomal RNA (rRNA). Now rRNA unites with protein and synthesise ‘Ribonucleoprotein’. It has been observed that RNA and ribonucleoprotein are found in equal amount.

Structure:

On the basis of size and sedimentation coefficient (the sedimentation rate of a ribosome is expressed as S (sedimentation coefficient). Ribosomes may be classified in two groups.

1- 70S. Sedimentation coefficient is 70 S. These are small in size.

2- 80S. Sedimentation coefficient is 80 S. These are big in size.

70 S ribosomes are found in mitochondria, plastids and prokaryotic cells. In which the large unit is 50 S and small unit 30 S. Where as in 80 S the big unit is 60 S and small unit is 40 S. Both big and small units are associated with each other by Mg++ ion.

In the low condition of Mg++ concentration, each plant ribosome (80 S) dissociates or get separated in to larger subunit of 60 S and a smaller subunit of 40S. If concentration of Mg++ ion, however, is increased, two ribosomes become attached and form a dimer. Ribosome is highly porous and hydrated structure.

Ribosomes are essential for protein synthesis as mRNA can support protein synthesis only when they are attached to ribosomes. Several ribosomes, 5-8 in number, may bind to a single mRNA during synthesis of protein. Such ribosomes together with mRNA constitute a polysome.

(The size or weight of ribosome, a macromolecule is expressed in S unit or Svedburg units. For study of weight of ribosomes an instrument is used which is said as ultracentrifuge. It contains some special type of tubules or test tubes in which we may keep the ribosomes in a special type of solution.

When this instrument is centrifuged or rotated at a definite high speeds of 50,000 rpm (revolution per minute) or more, a special type of ribosomes settle down on the surface as a sediment.

On other hand, when the speed of the instrument is changed, another type of ribosomes settle down on the surface as a sediment. This definite speed is said as sedimentation coefficient which is measured as Svedberg unit (S). For example, these ribosomes which sit down on 70 S and 80 S, are termed as 70 S and 80 S ribosomes respectively).

It has been stated above that ribosome is composed of two parts which may be dissociated into two sub units by decreasing the concentration of Mg⁺⁺ ion. If the mixture of both the parts are kept again in the instrument and is centrifuged, the one part of 80 S ribosomes settle down at 60 S and other part at 40 S which are termed as 60 S and 40 S ribosomes respectively. These numbers should not be seen from mathematical point of view i.e., 60 S + 40 S = 100 S.

Organelle # 5. Golgi Bodies (= Dictyosome):

The credit of discovery of Golgi body goes to Italian physician Camillo Golgi (1898). He observed it in animal (owl and cat) nerve cells. It is also termed as lipocondria. These are found in all the eukaryotic cell except red blood cells and absent in prokaryotes, It can be seen after treatment with silver nitrate.

Structure:

It is composed of a stack of parallel, flattened cisternae or saccules, a tubular net work and several small vesicles which are “pinched off’ (cut off or cut in to pieces) from the tubules, grouped together around the ends and outer surfaces of the cisternae including clear and large vacuoles which are associated with the inner cisternae especially in glandular cells.

Golgi body is 1-3 µm in length, 0.5 µm in height. The number of Golgi bodies in a cell depends on the synthetic activity of the cell. In a synthetically active cell well differentiated and developed Golgi bodies are present while synthetically inactive cells have few poorly developed Golgi bodies.

Golgi bodies originate from endoplasmic reticulum. It is believed that materials synthesised in association with ER, proteins, lipids, phospholipids etc. are transported to Golgi bodies, where they are packed in to vesicles cut off from them. The Golgi body is composed of 60% protein and about 40% lipids and polysaccharides. The function of Golgi body is still not clear.

According to scientists, the following synthesis are concerned with this structure:

1. Secretion of cytoplasmic substance:

(i) Mucous

(ii) Polysaccharides

(iii) Mucopolysaccharides

(iv) Glycoproteins

(v) Various enzymes-Phosphatase

-Thiamine pyrophosphatase (TPP)

-Nucleosides

-Peroxidase

2. Secretion of cellulose, hemicellulose and pectic compounds in plants cells during cell division.

3. Formation of cell plate during cell division in plant cells. Vesicles of Golgi bodies make continuous movements towards metaphase plate. Thus help in the cell plate formation.

4. Formation of primary lysosome-Help in the cell plate formation.

5. Formation of enamel.

6. Pigment synthesis in animals-Melanin of Melanocytes, retinal pigment.

7. Formation of acrosome during gametogenesis. In animals when the sperms attain maturity, Golgi body forms acrosome in them.

Lysosome:

The credit of discovery of lysosome goes to Christian de Duve (1955) in rat liver cells and for the same he was awarded Nobel prize in 1974.

It is made up of two Greek words:

(i) Lyso-digestive

(ii) Soma means body.

This is said as digestive body.

These are dense, bag like structures being rich in hydrolytic enzymes. They are bounded by single layered membrane composed of lipoprotein and inner fluid mass. The diameter is almost 0.4-0.8 µm. The presence of lysosome is clear in animal cells like liver cells, kidney cells, leucocytes, pancreatic cell etc. but in plants rarely or no more. Matile (1964) found its presence in Neurospora fungus.

Many digestive or hydrolytic enzymes (nearly 50) for example, sulphates, ribonuclease, deoxyribonucleic, acid phosphatase, cathepins, α-glucosidase, collagenase, β- galactosidase, β-glucuronidase and others are found in lysosome. Besides these, occur bulk of cellular ferritin and phospholipids but the oxidative enzymes are never found.

All of the digestive enzymes are acid hydrolase (with acidic pH). Whenever these digestive enzymes come out of lysosome by any means, they degenerate or cause death to carbohydrates, proteins, RNA, DNA including even cell also. Various cell inclusions like the mitochondria, endoplasmic reticulum etc. are also broken down and digested by the enzymes of the lysosome. This is called cellular autophagy.

During starvation, autophagy is a means of energy production in the cell, making the use of its own constituents as a source. Therefore, the lysosome are well said “Suicide bag” or “Suicide packet”. This is also termed as “Autolysis” which means self death or cell death.

The lysosomes also play an important role in removing the dead or degenerating cells. The lysosomes of such cells rupture and liberate enzymes in to the cell so that it may be digested away. Tissue degeneration may be largely due to the lysosomal activity. These have role in metamorphosis and fertilization too.

On basis of form and function, lysosome may be classified in to the following classes:

(i) Primary lysosome (= Strorge granules):

A small ridge developed by cisternae of Golgi bodies or from ER (Novikoff, 1965) containing inactive or some portion of enzyme acid hydrolase are said as primary lysosome. They only contain hydrolytic enzymes.

(ii) Secondary lysosome (= Digestive vacuole):

It is also termed as heterophagosome. These contain complete enzymes of acid hydrolase. They are formed by the fusion of primary lysosomes with vacuoles containing engulfed material.

(iii) Residual bodies:

After incomplete digestion of food materials some food materials are left undigested. Therefore, they show their visibility in form of stored food granules are said as residual bodies.

(iv) Autophagic vacuole (= Autophagosome):

Various cell organelles are broken down and digested by the digestive enzymes of the lysosome which are found within the vacuole. Autophagy plays an important role in the metamorphosis of amphibians.

Here the digestion of material brought up by enzymes of lysosome is as follows:

Direct or indirect role of lysosome is intracellular digestion of solid food particles that enter the cell by phagocytosis.

Other functions brought by lysosome may be as under:

(i) It helps in heterotrophic nutrition.

(ii) By cellular digestion it provides nourishment in adverse time.

(iii) This may also be involved in the ageing of the cell by consuming the toxic substances or molecules which may harm cells.

(iv) These help in fertilization and differentiation.

(v) These degenerate old, dead or inactive cell organelles.

(vi) The prominent function of lysosome is to make digestion of carbohydrates, proteins and fats situated in cytoplasm in to soluble forms like monosaccharides, amino acids, fatty acids and glycerol. These soluble materials later come out of lysosome membrane and help in mitochondrion respiration.

(vii) Trigger of cell division:

According to some scientists, the lysosomes, which are located on periphery of the cell, when rupture, mitosis cell division starts.

Organelle # 6. Centrosome (= Cell Centre):

These are usually made up of one or two granules said as centriole and are found in the cytoplasm near the nucleus or in nucleolus. Centrosomes are not found in cells of higher plants and are confined to animal cells as well as lower plant cells of algae, fern, moss, fungi, etc. These are hollow cylinders approximately 1500 Å in diameter and 4000 Å long.

Centriole is well surrounded by clean surrounding said as centrosphere. Centriole and centrosphere collectively make centrosome. Around centrosphere are found rays radiating in all directions and called Aster or Astrosphere. Electron micrographs of transverse sections of centrioles reveal nine groups of three microtubules each arranged in a single plane so that each fibril has a flat topography and are embedded in a dense granular matrix.

Generally, new centriole develop in association with pre-existing centrioles. In some cases new centrioles do not develop from pre-existing centrioles but arise denovo or fresh. They play an important role in cell division. During mitosis, centrioles divide in two parts and move to the different poles.

In animal cells, centrioles are involved in the organisation of the spindle apparatus. In-fact, a pair of centrioles lie at each of the poles in a cell from which spindle fibres radiate toward the metaphase plate. In both plant and animal cells centrioles serve as the basal bodies of flagella.

Organelle # 7. Lomasomes:

These are found between the plasma lemma and cell wall in form of minute vesicular and membranous, bag like structures. The presence of lomasome has been found in both higher and lower plants, specially in fungi. They are believed to arise from Golgi bodies. Though, no definite role has been searched out but they are believed to have some role in cell wall extension.

Sphaerosomes:

The discovery of sphaerosome goes to Perner. These are minute, spherical or ovoid bodies surrounded by a single membrane. The diameter of sphaerosome is 5 to 1 µm. They contain five enzymes viz. esterase, protease, hydrolase, phosphatase and ribonuclease. Their functions are synthesis, storage and translocation of fatty substances and lipid metabolism. They appear to originate from ER.

Glyoxysomes:

These are also minute, spherical or ovoid particles or bodies which are found in cells of oil seeds like ground nut, castor etc. including fungal cells like yeast and Neurospora. Beevers (1961) discovered it in a crude mitochondrial preparation from castor bean endosperm. They have a single layered membrane enclosing a finely granular stroma.

These micro bodies are present in only those plant tissues where fat is likely to be converted into carbohydrates through glyoxylate cycle. Glyoxysomes contain many enzymes e.g., glycolate oxalate, malate dehydrogenase, catalase, isocitriclyase, aconitase, citrate synthetase, malate synthetase etc.

Peroxisomes:

The credit of discovery of this micro body goes to Tolbert (1969) who firstly isolated it from a ‘broken chloroplast’. The term ‘peroxisome’ was first used by Beaufay and Berther (1963). It is also seen in protozoans and kidney cells of rat. These are spherical or ovoid bodies surrounded by a single membrane and contain a dense and opaque (non-transparent) stroma.

The nucleoid may consist of parallel tubules or twisted strands. The average diameter of a per-oxysome is 0.6-0.7 µm and their number per cell may range from 70-100. Per-oxysomes contain four enzymes e.g., D-amino acid oxidase, peroxidase, catalase and glycolate or urate oxidase. These enzymes are for decomposition of H₂O₂.

Peroxisomes are originated by endoplasmic reticulum and their enzymes are synthesised by ribosomes found on rough endoplasmic reticulum (E.R.).

In C₃ green plants peroxisome helps in photo-respiration whereas in C₄ plants these are absent. In animal cells they help in lipid metabolism. Under photorespiration the glycolic acid found in peroxisome is oxidised firstly in to glyoxylic acid and then finally into amino acid and glycine which is 2-carbon compound.

Organelle # 8. Microtubules:

De Robertis and Franchi (1953) for the first time observed in nerve fibre in shape of hollow cylindrical rod. Microtubules or microfilaments are observed in many plant cells.

They have been found mostly in the external boundary of cytoplasm immediately within the cell membrane or forming part of centrioles, cilia and flagella. They enclose a central space of 10 microns. They are several microns in length and about 200- 250 A in diameter.

Their wall is about 45-70 A in thickness. In plants the wall of microtubules are composed of 13 fibres made up of protein and lipid molecules which provide stability to the structure of microtubules walls. Microtubules have also been primarily found in form of spindle fib) s within mitosis cells and are responsible for chromosome movement during cell division. They are made up of protein and tubulin.

Functions:

(i) Maintenance of cell shape.

(ii) Motility of flagella and cilia.

(iii) Formation of spindle fibre during cell division. Tubulin has main role in formation of spindle fibres.

(iv) They are also believed to direct the movement of Golgi vesicles and bring them in contact with the cell membrane.

(v) They are also considered to be responsible for streaming of cytoplasm (cyclosis), intracellular transport and the general shrinking and shortening (= contractility) of the cytoplasm.

Vacuole:

A vacuole is a semi-fluid sac bounded by a membrane called tonoplast or vacuolar membrane which is differentially or selective permeable. The tonoplast is lipoprotein membrane. Sometimes the cytoplasm sub-divide the vacuole in to many small vacuoles. The vacuole is filled with a watery solution of many inorganic, organic substances, sugars, pigments and gases etc. which is termed as cell sap.

Cell sap maintains the osmotic pressure of the cell. Secondary products of the cell like tannins and alkaloids also accumulate in cell sap or vacuolar fluid. Tannins are specially found in vacuole of leaves, unripe fruits, wood, bark and seed coats.

Latex may also be stored in vacuoles. Colours of many of the flowers are mainly due to the presence of anthocyanin pigments dissolved in the cell sap. In some prokaryotes gas vacuoles are found which regulate buoyancy of the cell.

The various main compounds present in the vacuolar sap are in the form of true solutions, suspensions or in the form of droplets. Chiefly the sugar content of the vacuole is responsible for maintaining the turgidity of the plant cells.

Frey-Wyssling (1965) believed that vacuoles are produced because plants can not produce enough protein to fill their enlarging cells with protoplasm.

Nucleus:

The study of nucleus is known as Karyology. It was discovered by Robert Brown (1831). New nuclei arise by the division of pre-existing nucleus. This is said as karyokinesis.

On the dry weight basis the following chemical substance are found in nucleus:

(i) Proteins 70%

(ii) Phospholipids 3-5%

(iii) DNA 10%

(iv) RNA 2-3%.