The following points highlight the ten main types of cell organelles present in the cell. The types are: 1. Nucleus 2. Plastids 3. Mitochondria 4. Endoplasmic Reticulum 5. Ribosomes 6. Lysosomes 7. Golgi Bodies 8. Centrioles 9. Cell wall 10. Plasma Membrane.
Cell Organelle: Type # 1. Nucleus:
The nucleus was first discovered by Robert Brown in 1833. Since nucleus contains chromosomes and genes, it is called as controlling centre of the cell. There is, usually, a single nucleus per cell, but multinucleate condition is also observed in some protozoa and fungi which results due to repeated nuclear division without division of cytoplasm.
The nucleus is, usually, of spherical or oval shape. However, flattened, irregular, branched and lanceolate shapes are also found in some cells. The nucleus is generally larger in the active cells than in resting cells. The nucleus, usually, occupies a central position, but it occupies peripheral position in some cells. A nucleus consists of three main parts, viz., nuclear envelope, nucleolus and chromatin (Fig. 2.1).
It constitutes outer boundary of the nucleus and appears as a double layer membrane under electron microscope. The space between outer and inner membranes is called perinuclear space, which is about 200 Å. Nuclear envelope has many small apertures known as nuclear pores.
These pores vary from 300 to 1000 Å in diameter. These pores provide direct connection between nucleus and cytoplasm. The outer and inner unit membranes together are known as nuclear envelope. Each membrane is about 75 Å in diameter and is composed of lipoprotein.
It has several functions, viz:
(1) Protects chromosomes from cytoplasmic effects,
(2) Permits transport of electrons and exchange of material between nucleus and cytoplasm, and
(3) Gives rise to some cell organelles.
A spherical body found in the nucleus is called nucleolus. It is found in higher organisms and is attached with specific region of a particular chromosome. It disappears during prophase of mitosis and meiosis and reappears during telophase.
There is, usually, one nucleolus per cell, but polyploids have more nucleoli per cell. These nucleoli may sometimes fuse and form one large nucleolus. Nucleolus consists of three parts, viz., granules, fibrils and matrix. Granular region is 150-200 Å in diameter and contains proteins and RNA. The matrix contains scattered granules and fibrils.
The important functions of nucleolus are:
(1) formation of ribosomes and
(2) synthesis of RNA.
It produces 70 to 90 per cent of cellular RNA in many cells.
Chromatin refers to partly clumped and tangled mass of nuclear chromosomes. The chromatin fibre is about 230 Å in diameter and contains about 55 per cent proteins, 40 per cent DNA and 4-5 per cent RNA. Chromatin is a basic unit of chromosomes, contains genes and thus plays an important role in the inheritance of characters from the parents to their offspring.
Cell Organelle: Type # 2. Plastids:
Plastids are self-replicating cytoplasmic organelles found in plant cells. Plastids are absent in bacteria, certain fungi and animals. Plastids are of three types, viz., leucoplast, chromoplast and chloroplast. Leucoplasts are colourless and are associated with storage of starch, protein and fat.
Chromoplasts have colour but other than green, viz. plucoxanthin, phycocyanin, etc. Their function is not yet known. Chloroplasts are green and are associated with photosynthesis. These three types of plastids originate from pro-plastid and are interchangeable (Fig. 2.2).
Chloroplasts contain green pigment chlorophyll and are sites of photosynthesis in green plants. They have usually spherical, oval or disc shapes. They are 4 to 6 µ in diameter and are about 1 µ thick. They are larger in shade plants than in sun plants. Moreover, they are larger in polyploids than in diploid cells.
Their number varies from 20 to 40 per cell in higher plants. However, in some algae there is only one chloroplast per cell. In higher plants, chloroplasts contain about 35-55 per cent proteins, 25- 30 per cent lipids, 5-9 per cent chlorophyll, 4-5 per cent Carotenoids and 2-3 per cent nucleic acids.
Chloroplasts contain circular or ring shape DNA. Ultrastructure of chloroplast consists of three parts, viz., membrane, stroma and grana (Fig. 2.3).
Each chloroplast is enclosed by two concentric unit membranes, the outer and the inner membranes. The outer membrane is permeable to small particles, whereas the inner membrane is impermeable.
The space inside the inner membrane is known as stroma. It contains enzymes related to dark reaction of photosynthesis.
In higher plants, the stroma contains small cylindrical structures called grana. Number of grana varies from 40-80 per stroma. In spinach, each chloroplast contains 40-60 grana. Each granum consists of 5-25 flat cisternae (thylakoids) placed one above the other.
The grana are inter connected by fine tubules known as inter-grana lamellae or stroma lamellae. A group of units which constitute stroma and grana lamellae are known as quantasomes. These are associated with electron transport and photo-phosphorylation.
There are two views about the origin of chloroplasts. Either they originate from proplastids, which are small vesicles enclosed with double membranes or may develop from fission of preexisting chloroplasts. The prime function of chloroplast is to carry out the process of photosynthesis.
The light reaction takes place in grana and dark reaction in stroma. Chloroplasts absorb light energy in the form of photons and use the same for conversion of ADP to ATP. Chloroplasts contain some amount of DNA and thus also play an important role in cytoplasmic inheritance.
Cell Organelle: Type # 3. Mitochondria:
Mitochondrion is a rod-like cytoplasmic organelle which is the main site of cellular respiration. They are sources of energy and are often called as the power house of the cell. They have average length of 3-4 µ and diameter of 0.5-1 µ. Under light microscope, they appear as rod shaped, filamentous or granular structures in majority of the cells.
Their average number varies from 200- 800 per cell. In some protozoa, the number has been recorded up to 500,000 per cell. Mitochondria, contain about 65-70 per cent proteins, 25-30 per cent lipids, 1 per cent RNA and less than 1 per cent DNA.
Mitochondria consists of three main parts, viz:
(2) Christae, and
(3) Matrix (Fig. 2.4).
Each mitochondrion is enclosed by two concentric unit membranes, the outer membrane and the inner membrane. Each of the two unit membranes is 60 Å thick. The space between these two membranes is 40-47 Å. The outer membrane contains about 50 per cent proteins and 50 per cent lipids, whereas inner membrane has 75 per cent proteins and 25 per cent lipids.
The inner membrane has a series of inside folds known as cristae. Cristae project into the inner chamber. The space between two membranes is known as outer chamber.
The sace between cristae into the inner chamber is called matrix. Each mitochondrion has several copies of ring or circular DNA molecule. The DNA is either present in the matrix or is attached to the inner membrane.
The enzymes associated with Kreb’s cycle are also present in the matrix. Since mitochondria have their own DNA, tRNA, RNA polymerase, ribosomes, amino acids, activating enzymes, etc. they are considered as semi-autonomous.
There are two main views about the origin of mitochondria. According to one view, mitochondria have self-replication capacity, and the new mitochondria are formed from the fission of pre-existing mitochondria. According to second view, they originate from nuclear envelope.
Mitochondria have two important functions. First, they are the sites of cell respiration. The oxidation of carbohydrates, lipids and proteins occurs in the mitochondria. They supply energy to various processes of cell in the form of ATP. Second, mitochondria contain some amount of DNA and thus are associated with cytoplasmic inheritance.
Cell Organelle: Type # 4. Endoplasmic Reticulum:
The term endoplasmic reticulum (ER) was first used by Porter in 1948 to describe a fine reticulum in the endoplasmic cells. It refers to a vast network of membrane enclosed tubules, vesicles and sacs in the cytoplasm.
ER is found in differentiated cells. It is absent in prokaryotic cells and undifferentiated cells. Under electron microscope, ER appears as a double membrane structure with variable spaces. ER forms a continuous system. It is attached to nuclear envelope on one side and cell membrane on the other.
ER is made up of three types of elements, viz., tubules, vesicles and cisterns. ER is of two types, namely, rough or granular and smooth or agranular (Fig. 2.5). In some regions, double membranes of ER carry granular structures on the external surface.
These structures are known as ribosomes. Some regions of ER are devoid of ribosomes. Thus, ribosome carrying region of ER has granular surface and is called rough ER, and the ribosome free region is known as smooth ER.
Rough ER is well developed in the cells which are actively engaged in protein synthesis. Smooth ER is found in the regions which are rich in glycogel. It consists of smooth membrane. BR is rich in lipid contents. The smooth ER contains more lipids than rough ER.
It is believed by some scientists that ER originates from the nuclear envelope.
ER has four main functions, viz:
(1) It is associated with the synthesis of proteins (rough ER), lipids and glycogen (rough and smooth ER),
(2) Acts as an inter-cellular transport system for various substances,
(3) Contains lot of enzymes, and
(4) Provides passage for mRNA from nucleus to the cytoplasm. It is believed that micro bodies originate from the ER.
Cell Organelle: Type # 5. Ribosomes:
Ribosomes are small cellular particles which are the sites of protein synthesis. Since they are rich in RNA contents, they are called ribosomes. They contain 40-60 per cent RNA and several kinds of protein. The position of ribosomes changes with the stage of cell.
In the young dividing cells, they are usually free in the cytoplasm, while in the mature cells, they are attached with ER. The portion where ribosomes are attached to ER becomes rough.
Ribosomes have two sub-units, viz., larger sub-unit and smaller subunit. The size or weight of ribosome molecule is expressed in S units on the basis of sedimentation rate. The complete unit, larger sub-unit and smaller subunit differ in lower and higher organisms (Table 2.1). Sometimes, ribosomes are observed in cluster. They are called poly-ribosomes. Such ribosomes play an active role in protein synthesis.
Sometimes, larger units dissociate (split) into smaller units. Such dissociation depends mainly on the concentration of Mg++ ions. When Mg++ ions are in low concentration, the ribosomes of 80s units dissociate into 60s and 36s sub-units in plants and into 60s and 45s sub-units in animals. The ribosomes of 70 units split into 50s and 30s sub-units. Addition of Mg++ promotes association of sub-units into complete ribosomes.
It is believed by most of the scientists that ribosomes are produced in the nucleolus within the nucleus. After synthesis, they migrate to cytoplasm and get attached to ER or remain free in the cytoplasm. The various units and sub-units of ribosomes are formed due to association and dissociation processes.
The main function of ribosomes is to carry out protein synthesis with the help of mRNA. A cluster of ribosomes called polyribosomes or polysomes is associated with protein synthesis.
Cell Organelle: Type # 6. Lysosomes:
Lysosomes are cellular particles which contain several digestive enzymes. The term ‘Lyosome’ was first used by Dave in 1955. Lysosomes are common in animal cells, but now they are also found in fungi like Neurospora. The identification of lysosomes is very difficult due to wide variation in their shape and size. Their size varies from 400 to 800 mm. They are identified only by the presence of digestive enzymes.
A lysosome consists of a membrane, a densely granulated stroma and a large vacuole. Several hydrolytic enzymes, viz., acid phosphatase, acid ribonuclease, acid de-oxy-ribonuclease, glycosidases, etc., are found in the lysosomes. There are two main types of lysosomes, viz., primary and secondary lysosomes.
Primary lysosomes originate from Golgi complex and have several digestive enzymes. Secondary lysosomes are believed to originate from primary lysosomes. They contain both enzymes as well as some food particles. They are also known as digestive vacuoles.
It is believed that lysosomes originate from Golgi bodies. First, primary lysosomes are formed and then secondary lysosomes arise from the primary ones. The main function of the lysosomes is digestion of intracellular substances and foreign particles. When a cell dies, lysosome releases its enzymes which digest the dead cell, resulting in cleaning of debris.
If some bacterium enters the cell, lysosome releases the enzymes resulting in digestion of such bacterium. Lysosomes release digestive enzymes which get mixed with food or foreign particles in the cell resulting in their digestion.
Cell Organelle: Type # 7. Golgi Complex:
Golgi complex was first described by Camillo Golgi in 1822 in nerve cells of cat and owl. They are cellular organelles which exhibit wide variation in shape ranging from granular to dispersed filamentous reticulum. Their thickness varies from 300-500 A. They are located either near nuclear membrane or cell periphery.
Golgi complex is also known as Golgi bodies or Golgi apparatus. Golgi bodies consist of a series of concentric double membranes. Submicroscopic spaces or sacs are enclosed by double membranes which are known as cisternae (Fig. 2.7) Each Golgi complex has 3-12 interconnected cisternae which are composed of lipoprotein.
Golgi bodies arise from rough ER. The rough ER becomes smooth resulting in the formation of Golgi complex. The main function of Golgi complex is packaging of food materials such as proteins, lipids and phospholipids for transport to other cells. The transport of material between nucleus and cytoplasm has some relationship with Golgi bodies. Lysosomes are believed to be originated from Golgi bodies.
Cell Organelle: Type # 8. Centrioles:
Centrioles are found in animal cells only. In plant cells, structures similar to centrioles are found at the base of flagella. Centrioles are cylindrical cellular bodies and always found in pair. They are about 3000-5000 Å in diameter and 1500-1800 Å in length.
The wall of each centriole consists of nine triplet fibrils which are arranged around a central axis. Each triplet fibre or fibril consists of three secondary fibres or sub-fibrils. Each sub-fibril is about 200 Å in diameter. All these fibrils are interconnected and enclosed in a thick membrane. The cytoplasm at the poles of spindle is known as centrosphere. The centrioles and centrosphere together are known as centrosome.
The main function of centrioles is formation of spindle apparatus at the time of cell division. The spindle fibres help in orientation of chromosomes at equatorial plate during metaphase and also in the movement of chromosomes towards opposite poles during anaphase.
The new centrioles originate from the pre-existing centrioles. Each centriole gives rise to one new centriole. Their division starts when they separate during cell division and new centrioles are ready at interphase.
Microtubules have been reported in many plant cells. They have a diameter of about 240 Å and are found in the periphery of cell just near the cell membrane. They have also been observed in the spindle fibres.
Microtubules are considered to be associated with spindle fibres, movement of chromosomes during cell division and intracellular transport. They are also believed to cause orientation of newly synthesised cellulose micro fibrils in the cell.
Peroxisomes and Spherosomes:
Peroxisomes were first reported in plant cells by Talbert and his colleagues in 1968. They are 0.5-1 µ in diameter. Peroxisomes are enclosed by a single membrane.
They contain several enzymes, viz, glycolate oxidase glyoxylate, reductase, catalase, glutamate, glyoxilate transminase and malate dehydrogenase. These organelles are associated with glycolate metabolism in photosynthesis. They are also found in animal cells.
Spherosomes are found in plant cells only and are absent in prokaryotes and animals. They have 0.5-1 µ diameter and are enclosed by single membrane like per-oxysomes. They are considered to be associated with storage of lipids.
Vacuoles are also found in plant cells. Sometimes, two or more vacuoles fuse together to form a large vacuole. Each vacuole is surrounded by a Vacuolar membrane called tonoplast. Tonoplast has selective permeability and is composed of lipoprotein.
The fluid found inside the vacuole is called cell sap. Several substances, viz sugars, organic acids, inorganic salts, proteins and pigments are found in the cell sap in the form of true solutions.
Cell Organelle: Type # 9. Cell Wall:
The call wall is found only in the plants and is absent in animals. It is the outermost part of the cell and is always non-living, though produced and maintained by living protoplasm.
The main functions of cell wall are:
(1) To protect inner parts of cell,
(2) To give a definite shape to the cell and
(3) To provide mechanical support to the tissues.
In higher plants: cell wall is differentiated into three parts, viz., middle lemella, primary wall and secondary wall.
A brief description of these layers is presented as follows:
The middle lemella is common layer between adjacent cells. It is composed of calcium and magnesium pectate. Middle lemella remains un-lignified in soft tissues like pith and cambium, while in the woody tissues it is highly lignified. Cellulose is not found in the middle lemella. The adjacent cells are joined due to the presence of calcium and magnesium ions in the middle lemella.
Removal of these ions from the middle lemella results in the separation of adjacent cells. The fleshy fruits become soft after ripening due to dissolving of pectose substances of middle lemella by pectolytic enzymes. Middle lemella is formed due to fusion of vesicles and cisterns and deposition of pectin with these two structures during formation of cell plate, i.e., cytokinesis.
Primary Cell Wall:
Primary cell wall is thin and elastic and lies between middle lemella and secondary cell wall. It develops after the middle lemella by deposition of hemicellulose, cellulose and pectin substances. This is composed largely of cellulose and pectin substances during growth.
When growth is stopped, it becomes thick and lignified. Primary cell wall is mainly composed of cellulose, micro-fibrils, which form a network initially and become more organised later on.
The micro-fibrils are transversely oriented on the inner surface and longitudinally on the outer surface. Between two layers of microfibrills, one layer of pectin is found. The deposition does not take place near plasmodesmata resulting in the formation of pores. In primary cell wall, cellulose fibrils are arranged in dispersed manner (Fig. 2.8).
Secondary Cell Wall:
It is the innermost layer of the cell wall and lies between primary wall and plasma membrane. In the secondary cell wall, the cellulose micro-fibrils are arranged in a parallel manner [Fig. 2-8(b)]. Secondary cell wall is primarily composed of micro-fibrils of cellulose.
Each micro-fibril consists of several micro-fibrils of cellulose. In schleranchymatous tissues, the secondary wall is very thick consists of three sub-layers, namely, outer, middle and inner. Besides cellulose, in some tissues, lignin and suberin are also found in the secondary wall.
The cell wall has several functions, viz:
(i) It determines the shape and size of cell,
(ii) Provides protection to the inner parts of cell from the attack of pathogens,
(iii) Provides mechanical support to the plants,
(iv) Helps in transport of substances between two cells,
(v) withstands turgor pressure and
(vi) Contains certain enzymes.
Cell Organelle: Type # 10. Plasma Lemma:
The cytoplasm is surrounded on the outer side by a thin and flexible membrane known as plasma lemma or plasma membrane. It is composed of lipids and proteins. The lipid portion consists mainly of phosopholipids such as lecithin, cephalin, sphingomylin, etc. Cholesterols and triglycerides are also present in small proportion.
In the electron micrographs, three layers of this membrane are visible. These three layers are of two types, viz., dense and light. The dense layers are two in number and light layer is only one. Each of the two electron dense layers is 20 Å in thickness. The single light layer is 35 Å in width.
The cell membrane is about 75-100 Å thick; the outer protein layer is 25 Å thick, the lipid layer is 25-30 Å thick and the inner protein layer is 25-30 Å thick. This three layered protein-lipid-protein membrane is called unit membrane.
This type of membrane is also found in several cell organelles. The cell membrane consists of a single unit membrane, while the double membrane coverings of mitochondria, chloroplast, ER, nucleus and Golgi complex consist of two unit membranes in a paired manner.
The plasma lemma performs many important functions. It regulates the passage in and out of the cell. It acts as a selectively permeable membrane. It checks the entry of toxic elements from outside into the cytoplasm. Moreover, it permits only one way passage for molecules like minerals into the cell and restricts their outward movement.