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3237 Words Essay on Cell Organelles

Fungi lack chloroplasts and are, therefore, heterotrophic. In prokaryotic cells, like blue green algae and bacteria, organised chlorplasts are absent. Diameter of chloroplasts varies from 4-8 mm. It is about 2 mm thick.

Structure:

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Chloroplast is surrounded by two unit membranes. There is a space between two membranes, called periplastidial space. The internal structure of the plastid shows two distinct parts— (i) colourless ground substance called stroma and (ii) closed, flat, stack-like membrane system called grana.

Stroma is watery and proteinaceous ground substance. It contains starch grains, lipid droplets, DNA and free ribosomes. The dark reaction of photosynthesis takes place in the stroma.

Gram are densely packed stacks of membrane layers called thylakoids. Each thylakoid is bounded by a single membrane but because of the flatness of these structures they appear as double membrane layers or lamellae. Large number of grana are formed at frequent intervals by packed stacks of thylakoids. This is the place where light reaction of photosynthesis takes place. Two adjacent grana are joined with one another by lamellae called intergranal lamellae or stroma lamellae or fret channel. Each chloroplast contains about 40-60 grana.

Each lamella is made of a bimolecular layer of lipo-pigment sandwiched between two protein layers. The lipo-pigment layer contains chlorophyll and carotenoid pigments alongwith phospholipids. The chlorophyll molecules are so arranged in this layer that their hydrophilic porphyrin head is close to the outer layer of proteins. Amongst these lamellae, distinct groups, each consisting of about 230-300 chlorophyll molecules called quantasomes, are present. Quantasomes are the smallest morphologically distinct and photosynthetically functional units.

Chloroplasts also have 70S ribosomes which are either free or attached to intergranal lamellae. Pyrenoid is present in the chloroplasts of some algae and other plants. It consists of a protein core surrounded by peripheral starch plates.

Functions:

The following are major functions of chloroplasts.

1. Absorption of light energy.

2. Production of NADPH2 and evolution of oxygen through the process of Hill reaction.

3. Transfer of carbon dioxide obtained from the air to 5 carbon sugar (ribulose 1, 5 diphosphate).

4. Breaking of 6-carbon atom compound into two molecules of phosphoglyceric acid.

5. Hydrogenation of phosphoglyceric acid to form phosphoglyceraldehyde.

Mitochondria:

Mitochondria are membrane bound cell organelles that contain the enzymes responsible for aerobic respiration of eukaryotic cell. Distribution

Mitochondria are present in the living eukaryotic cells and absent from prokaryotic cells like bacteria and blue green algae. Mitochondria are lost secondarily in some highly specialised eukaryotic cells like mammalian RBC.

Structure:

Mitochondria can be stained in vivo with Janus Green B. It reveals that mitochondria has two parts — an outer envelope and a central cavity filled with matrix. The envelope that surrounds the mitochondria is made of two unit membranes. It is called perimitochondrial space. The outer membrane is smooth but the inner membrane has many infoldings which extend into the matrix. These infoldings are called cristae.

Golgi appartus consists of a set memberane bounded, fluid-filled vesicles, vacuoles, and flattenened cisterinae. The single memberane structure is called cisternae.

The cristae provide an increased surface area within the mitochondrion for enzymatic activity. The inner surface of the inner membrane (i.e., the one facing the matrix) is covered by small tennis racket-like particles with a head and a stalk. These particles have been variously named as inner membrane sub-units, elementary particles or oxysomes. Parsons (1963) called them electron transport particles (ETP).

Each particle consists of a head called F1 sub­unit, approximately 100 A in diameter and is attached to a base piece called F1 sub-unit. It is 35-50 A in length F1 sub-unit projects into the matrix. It is an integral protein of the inner membrane.

The inner membrane has all the enzymes required for electron transport. The F1 – F0 combination has special ATPase (ATP synthetase) for oxidative phosphorylation. ATP is called as the currency of the cells.

Functions:

Mitochondria are known as ‘power house of the cell’. This is because of the formation and storage of energy rich compound ATP (adenosine triphosphate) as a result of oxidation. ATP is a readily available source of energy for numerous cellular reactions.

Mitochondria are concerned with oxidation and phosphorylation reactions of Krebs cycle during aerobic respiration. Mitochondria are absent in cells which do not respire aerobically. Besides oxidation of carbohydrates, mitochondria are also involved in the oxidation of proteins and fats.

Endoplasmic reticulum (ER):

The term ER was first used by Keith Porter (1953) to identify a fine reticulum (network) in endoplasm of the cell, ER (endoplastic reticulum) is a series of interconnected membranous tubules in the cytoplasm.

Distribution:

All cells do not possess ER. It is generally absent in egg and embryonic cells. It is well developed in differentiated cells. In spermatocytes, though present, it is poorly developed. Generally ER is fully developed in cells actively engaged in protein and hormone synthesis. The size and shape of the ER changes with the type of cells in which it occurs.

Structure:

ER is composed of three different types of structures. These are — cisternae, vesicles and tubules.

1. Cisternae:

These are long, flat and unbranched plates or lamellae arranged in parallel rows.

2. Vesicles:

These are usually round or ovoid sacs. They often occur isolated in the cytoplasm.

3. Tubules:

They are irregularly branched tube-like structures. These structures are surrounded by thin unit membrane of 50- 60 A thickness and their lumen is filled with the secretory products of the cell.

Types of ER:

Two types of endoplasmic reticulum have been recognized. These may be present in the same or different types of cells.

1. Agranular or smooth endoplasmic reticulum (SER):

The surface of this type of ER is smooth, ribosomes being not attached. Smooth ER is present in cells which are actively engaged in steroid synthesis (e.g.. cholesterol, progesterone, testosteron, etc.), carbohydrate metabolism, pigment production, etc.

2. Granular or rough endoplasmic reticulum (RER):

The rough endoplasmic reticulum have ribosomes attached throughout the surface. This type of ER is present in cells which show active protein synthesis.

Functions:

Some lipids and proteins made by RER and SER functions as enzymes and hormones. ER is involved in following major functions.

1. Functions common to both smooth and rough ER:

(i) Forms skeletal framework.

(ii) Active transport ER helps in the transfer of materials from cytoplasm to cytoplasm or b/w nucleus to cytoplasm.

(iii) Metabolic activities due to enzymes and hormones formed by some lipids and pro­teins.

(iv) Provides increased surface area.

(v) Formation of new nuclear membrane during cell division.

(vi) ER produces lipids and proteins which help in the formation of plasma mem brane by process called memberame biogenese

2. Functions of smooth ER:

(i) Lipid synthesis.

(ii) Glycogen synthesis.

(iii) Steroid synthesis like cholesterol, progesterone, testosterone, etc.

3. Functions of rough ER:

(i) It provides site for protein synthesis.

(ii) It helps in transport of proteins.

Golgi Complex:

Distribution:

Golgi apparatus or complex as organelle present in all the eukaryotic cells but it is absent in prokaryotic cells. Among eukaryotes, Golgi complex is not found in the cells of fungi, male gametes of bryophytes and pteridophytes, mature sieve tubes and mature sperms and red blood cells of animals. It was first identified in 1898 by Italain physician Camillo Golgi.

Structure:

It consists of three main parts—cisternae, tubules and vesicles. Cisternae, which form the central flattened or disc-shaped part of Golgi complex, are stacks of parallel double membranes or lamellae. In a plant cell 2 to 7 such cisternae constitute one Golgi apparatus.

Tubules arise from the periphery of the cisternae and form a highly branched anastomosing network.

Vesicles are of two types — (i) rough vesicles present at the ends of the cisternae and (ii) smooth vesicles usually present within the network or sometimes near the centre of the stack of cisternae.

Golgi apparatus forms an extensive inter­communicating membrane system in association with cell membrane, endoplasmic reticulum, lysosome and nuclear membrane. It is also called endomembrane system.

Golgi apparatus is very rich in phospholipids, proteins, various enzymes, carotenoids, fatty acids and vitamin C.

Functions:

The following are some of the major functions of Golgi apparatus.

1. Synthesis of polysaccharides.

2. Formation of glycoproteins by combining carbohydrates with proteins.

3. Synthesis of pectins and other carbohydrates necessary for cell wall formation. Secretory vesicles of Golgi apparatus fuse and get incorporated into cell membrane. Similarly, in the plant cells, Golgi apparatus deposits pectic substances and cellulose microfibrils to form a cell plate during cell division.

4. It also secretes gum and mucilage.

5. It is associated with storage, condensa­tion, packaging and transportation of var­ious substances.

6. It is also active in the transformation of one type of membrane into another.

7. Secretory vesicles and lysosomes originate from Golgi apparatus.

Ribosomes:

Ribosomes are small particles composed of rRNA and proteins.

Distribution:

Ribosomes are present in both prokaryotic and eukaryotic cells. In eukaryotic cells, they are free as well as attached to the endoplasmic reticulum and nuclear membrane. Ribosomes are also present in mitochondria and chloroplasts. There may be as many as 5, 00,000 ribosomes in an ordinary plant cell. In prokaryotic cells ribosomes are free.

Structure:

These dense and rounded granules are perhaps the smallest cell organelles and can be seen only with the electron microscope.

Ribosomes are isolated by differential centrifugation, and its sedimentation coefficient is measured in Svedberg Units or S units. On the basis of sedimentation coefficient two types of ribosomes have been recognised.

1. 70 S ribosome:

This type of ribosome is smaller of the two. 70S type ribosomes are found in all prokaryotic cells like bacteria and blue green algae and also in chloroplasts and mitochondria of eukaryotic cells. These consist of two sub-units— 50S and 30 S, 50 S sub-unit is larger and the smaller 30 S sub-unit is placed over it like a cap.

2. 80 S ribosome:

This type of ribosome is larger in size with a sedimentation coefficient of 80 S. It has a molecular weight of 40 x 106 daltons. These ribosomes are found in the cells of animals and plants (eukaryotic cells). 80 S ribosome consists of two sub-units — 60 Sand 40 S. 60 S sub-unit is larger and the smaller 30 S sub-unit is placed over it like a cap.

Each sub-unit of ribosome is further composed of smaller sub-units (Fig. 2). For example, 50 S sub-unit of 70 S ribosome consists of core particles 40 S and split proteins — SP 50. Similarly 80 S ribosomes are also made of smaller sub-units. Core particles are further composed of RNA and core particles. The amount of RNA present in the ribosomes is about 70-75% of the total amount of RNA present in the cell.

The association of ribosomal sub-units is dependent upon the concentration of Mg++ ions. This association requires very low concentration of Mg++ (0-001 M). If the concentration is lesser, both the ribosomal sub-units remain separate. If the concentration of Mg++ ions is increased ten folds, two ribosomes get associated to form dimer. In bacterial cells, the two subunits occur freely in the cytoplasm.

During protein synthesis many ribosomes occur in a chain on common m-RNA strand and are called polyribosomes or polysomes.

Ribosomes (80S) remain attached to endoplasmic reticulum with their 60 S end by electrostatic reactions and also be the nascent polypeptide chain that grows from the ribosome and penetrates across the membrane. These could also be bound with ER by two transmembrane glycoproteins – ribophorin I and ribophorin II.

Functions:

Ribosomes are the centres of protein synthesis.

Lysosomes:

These were first reported by Belgian biochemist. Christian de Duve (1955). These are small spherical vesicles measuring about 500 nm and are involved in intracellular digestion. Distribution

Lysosomes are common in animal cells but are also found in plant cells. These can be observed in secretory cells like pancreatic cells, spleen cells, leucocytes, liver cells, meristematic plant cells, etc.

Functions:

The following are the main functions of lysosomes.

1. Digestion of extracellular particles. Lysosomes, pinocytes and phagocytes digest the external food particles. Lysosomes of leucocytes digest proteins, bacteria and viruses.

2. Digestion of intracellular substances. Lysosomes digest stored materials like proteins, fats and glycogen to provide energy to the starved cells. Lysosomes in plant cells hydrolyse stored material in germinating seeds.

3. Autolysis. Under certain pathological conditions lysosomes digest cell organelles. This is also called autophagy and results in the death of the cell. Hence, lysosomes are also known as suicidal bags.

4. Extracellular digestion. Sometimes enzymes from lysosomes are liberated outside the cell, e.g., the lysosomes of sperm release enzymes which dissolve the protective coating of ovum.

5. Mitotic division. Lysosomes are said to initiate the process of mitosis.

Micro bodies:

In protozoans, fungi, some plants, liver and kidney, some small membrane bound organelles are found associated with ER, mitochondria and chloroplasts. These are called microbodies. Following are some of the common types of microbodies.

Peroxisomes:

The term peroxisome was introduced by Beaufaytt and Bert her in 1963. These are found in many plant and animal cells. Peroxisomes mainly occur in photosynthesizing cells of higher plants, mature pear fruit, algae, fungi, liverworts, mosses and cells of ferns.

It consists of a anular matrix enclosed by a unit membrane. A nucleoid that consists of parallel tubules or twisted strands, occupies the centre. The enzymatic contents of animal and plant peroxisomes differ. However, they contain some peroxodie producing enzymes like urate oxidase.

The exact function of peroxisomes is not known. However, glycolate metabolism is known to occur in them. Thus these are associated with photorespiration in plants (C3 plants) and lipid metabolism in animal cells.

Sphaerosomes:

They originate from endoplasmic reticulum by budding. Sphaerosomes are bound by a single membrane enclosing enzymatic proteins which synthesize fats and oils. Like lysosomes, sphaerosomes also contain phosphatases, esterases, ribonucleases and hydrolases. Thus, sphaerosomes do not appear to be different from lysosomes, except for their specific lipidic nature. The main function of spherosomes is to synthesize fats and their transportation.

Glyoxysomes:

These occur in the cells of yeast, Neurospora and oil seeds of many higher plants. They have enzymes for fatty acid metabolism and gluconeogenesis (conversion of lipids of germinating seeds into carbohydrates).

Centrosome:

Distribution:

Centrioles are found in algae, fungi, bryophytes, ferns, gymnosperms, protozoans and most of the animals. However, these are absent from the cells of prokaryotes, diatoms, yeast and most of the higher plants (e.g., conifiers and angiosperms).

Functions:

Centrioles take part in the formation of basal bodies and mitotic spindle, and facilitate the movement of separating chromosomes during nuclear division.

Microtubules:

Distribution:

Microtubules are found in the cytoplasm of plant and animal cells, flagella, centriole, basal body, mitotic apparatus and meristematic plant cells, etc. Functions

Following are the main functions of microtubules:

1. These are related with the movement of flagella and cilia.

2. During cell division they form spindle fibres which control the movement and alignment of chromosomes.

Nucleus:

History:

The credit for the discovery of nucleus goes to Robert Brown (1831). He observed the nucleus in the cells of an orchid. J. Hammerling (l 953) conducted experiments on a green algae Acetabularia and proved hereditary role of nucleus.

Distribution:

Nucleus is present in all the eukaryotic cells. Some cells contain nucleus when they are young but it degenerates when cells mature, e.g., sieve tubes in plants and RBC in mammals. In prokaryotic cells ‘true nucleus’ is absent.

Generally, there is one nucleus in each cell, but this number may vary in different types of ceils. Depending upon the number of nuclei, cells are called uni—, bi-ox multinucleate. Multinucleate condition may arise either by fusion of many cells (e.g., syncytium of coconut endosperm) or due to repeated nuclear divisions (e.g., coenocyte of Vaucheria).

Parts of the nucleus:

The nucleus can be easily distinguished into following four parts:

(1) Nuclear membrane,

(2) Nucleolus,

(3) Nucleoplasm,

(4) Chromatin

1. Nuclear membrane:

The nuclear membrane or karyotheca is the outer envelope of the nucleus. It occurs in all the eukaryotic nuclei except during later part of the cell division. In prokaryotic cells, nuclear membrane is always absent.

Nuclear membrane separates nucleus from the cytoplasm. Membrane is nuclear composed of two layers of membrane. The space between the two membranes called perinuclear space, is about 75 A. The outer unit membrane is continuous with endoplasmic reticulum. The nuclear mem­brane is interrupted by pores.

These pores are usually octagonal and their diameter varies from 300-1,000 A. These are called nuclear openings or pores. They maintain continuity between nucleo-cytoplasmic or regions. Nuclear pores, however, do not freely communicate with the cy­toplasm. They are plugged by a cylinder of pro­tein material called annulus. The passage of ions and small molecules through the pores is regu­lated by annulus.

The nuclear membrane breaks into small vesicles during prophase of cell division and gets dispersed in the ground substance. Later, during telophase, these vesicles either fuse once again to form a nuclear membrane or new vesicles are formed to produce a nuclear membrane.

Nuclear membrane allows a free exchange of ions. Nuclear pores permit mRNA to move out of the nucleus into the cytoplasm. Evagination of nuclear membrane results in the formation of organelle initials which may later transform into organelles like mitochondria and chloroplasts.

2. Nucleolus:

This organelle was first seen by Fontana (1781) and was termed nucleolus by Bowman (1840). Nucleolus is a spherical, dense, colloidal and acidophilic body that remains attached to a special type of chromosome—nucleolar organising chromosome at a specific place called nucleolar organising region or secondary constriction. Nucleolus disappears during prophase of mitosis and meiosis and reappears during telophase in mitosis and telophase II in meiosis.

A cell may have one or more nucleoli. The size of the nucleolus also varies depending upon its synthetic activity. Nucleolus consists of three parts as given:

(a) Granular region:

This is made of ribonucleoprotein granules.

(b) Fibrillar region:

This region, consisting of 50-80 A long proteinaceous fibrils, is called nucleonema.

(c) Amorphous matrix:

This, electrically less dense region, shows the presence of amorphous matrix and is called ‘pars amorpha’.

3. Nucleoplasm:

The nucleus is filled with a transparent, semi-solid, granular and acidophilic substance, known as nuclear sap, nucleoplasm or karyolymph. Nucleolus and chromatin fibres appear scattered in this sap.

Nucleoplasm is mainly made of nucleic acids, proteins, enzymes, lipids and minerals. The nucleic acids—DNA and RNA are found in the form of nucleoprotein. Proteins occur as basic proteins or histone proteins (e.g., nucleoprotamines and nucleohistones) and acidic proteins or non-histone proteins (e.g., phosphoprotein). Some of the important nuclear enzymes include DNA polymerase, RNA polymerase, nucleoside triphosphates, aldolases, enolases, etc. Lipids are found in small amounts. Minerals like phosphorus, potassium, sodium, calcium and magnesium are also present.

4. Chromatin:

When the cell is not reproducing the DNA and its associated protein appear as a threadlike mass called chromatin. They are thin thread like in tangled mass composed of DNA and proteins.

CHROMSOMES: are thread like structure which contain hereditary information in the form of genes. Genes are the hereditary units special functioning unit along with chromosomes, chromosome arrange into chromatin when cell is about to divide. Chromosomes and chromatin are really the same molecules, but they differ in structural arrangement.

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