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Unit 2.3 Cell organelles

Nucleus Nucleus is the largest cell organelle. It was first observed by Leeuwenhoek in red blood corpuscles of fish, and was first studied in orchid root cells by Robert Brown in 1831. Presence of hereditary information in the nucleus was proved by the work of Joachim Hammerling (1953) on single celled alga Acetabularia Definition of Nucleus: Nucleus (L. nucleus- kernel) is a specialized double membrane bound protoplasmic body which contains all the genetic information for controlling cellular metabolism and transmission to the posterity. Occurance: A nucleus is present in all living eukaryotic cells with the exception of mature sieve cells of vascular plants and red blood corpuscles of mammals. Even here a nucleus is present during the early stages of their development. Number of Nucleus: Commonly cells are uninucleate, that is, they possess a single nucleus, some has two nuclei (bi- nucleate) Multinucleate or polynucleate condition. Multinucleate cells are called syncytial cells (e.g., epidermis of Ascaris) while in plants and fungi they are called coenocytic cells (e.g., Rhizopus, Vaucheria). Position of Nucleus: Nucleus is usually found in the region of maximum metabolic activity in the cytoplasm. Commonly it is situated in the geometric centre of the cell. In plant cells it is pushed to peripheral position on one side due to the development of a large central vacuole. Nucleus is peripheral in fat-storing cells or adipocytes, and basal in glandular cells. It is suspended in central vacuole by cytoplasmic strands in Spirogyra. Shape of Nucleus: The nuclei are generally rounded in outline. They appear oval or elliptical in plant cells having large central vacuoles, Disc-shaped nuclei occur in the cells of squamous epithelium, lobed in white blood corpuscles and irregularly branched in silk spinning cells of insects. Biochemical Analysis of Nucleus:

o DNA- 9-12%. o RNA- 5%. o Lipids- 3%. o Basic Proteins- 15%. o Acid proteins, neutral proteins and enzymes- 65%. o Traces of minerals like Calcium, Magnesium, Potassium and Sodium

Structure: A typical interphase nucleus is 5-25 pm in diameter. It is differentiated into five parts—

i. Nuclear envelope ii. Nucleoplasm

iii. Nuclear matrix iv. Nucleolus and v. Chromatin

i. Nuclear Envelope (= Karyotheca): It is made up of two lipoprotein and trilaminar membranes, each of which is 60-90Å thick. The inner membrane is smooth. The outer membrane may be smooth or its cytoplasmic surface may bear ribosomes like the rough endoplasmic reticulum. The two membranes of the nuclear envelope are separated by an electron transparent perinuclear space. The space is 100—500Å in width. The outer membrane is often connected to endoplasmic

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reticulum. Nuclear envelope contains a large number of pores or perforations called nuclear pores. In some cases 10% of the envelope is occupied by pores. 40-145/sqμm

Nuclear pores have complex structure. The nuclear pores are octagonal in shape. Their diameter varies from 400-1000 A, and they are separated from each other by a space of 1500 A. The diameter of nuclear pore is about 100nm. The pores are enclosed by circular and cylindrical structures called annuli. The annulus is an electron dense material. The pores and annuli are collectively called the nuclear pore complex. The central granule may be an inactive ribosome or a newly made ribosome attached to the periphery of the pore complex or other particles caught in transit. Eight radial spokes also extend from plug to rings. The nuclear pores control the passage of substances to the inside or outside of the nucleus, e.g., RNAs, ribosomes, proteins.

Nuclear membrane protein Two proteins have been associated to the nuclear envelope.One is the integral membrane protein- a glycoprotein of 120,000 Dalton which binds the annuli to the lipid bilayer. The second protein is a 63,000 Dalton protein which binds on the cytoplasmic side of the nuclear envelope. These proteins may be involved in the transport of materials through the nuclear pore Functions of nuclear envelope:

• The nuclear envelope acts as a shield and protects the inner contents of the nuclear compartment. • It helps to prevent the entrance of active ribosomes and other cytoplasmic components. • The nuclear membrane is very selective for the exchange of materials between nucleus and cytoplasm

through nuclear pore complexes. • The nuclear envelope is very conservative and it does not allow to enter any large cytoplasmic protein or

component ii. Nucleoplasm (Nuclear Sap, Karyolymph) The nucleus contains a transparent, semi-solid, granular and homogeneous matrix during interphase called as nuclear sap or karyolymph. The nucleus sap contains, Nucleic acids which includes DNA, RNA various Enzymes involved in replication, Proteins which are of two kinds, histone and non-histone, Chromatin, Some lipids, various organic phosphorus compounds and various inorganic compounds, mostly salts

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iii. Nuclear Matrix or nuclear scaffold or nuclear skeleton It is a network of fine fibrils of acid proteins that function as scaffold for chromatin. On the periphery, below the nuclear envelope, nuclear matrix forms a dense fibrous layer called nuclear lamina. It is 30-100 nm thick. It connects the inner membrane with chromatin. Nuclear matrix consists of two types of intermediate filaments, lamin A and lamin B.

Nuclear matrix and nuclear lamina form: Scaffold for chromatin, Attachment sites to telomeric parts, Mechanical strength to nuclear envelope, and Components of nuclear pore complex. Function of Nuclear Matrix: The nuclear matrix performs the following functions:

• It is responsible for the determination of nuclear shape. • It gives the mechanical support to resist disaggregation of nucleus in high ionic strength buffers. • The matrix may help to organize chromosomes, to localize genes and to regulate DNA transcription and

replication. iv. Nucleolus (plural-nucleoli): Dense, rounded, oval and acidophilic body. It was first discovered by Fontana in 1781, described by Wagner in 1840 and coined by Bowman in 1840. Nucleolus is a naked-a covering membrane is absent around nucleolus and is denser than the surrounding nucleoplasm and hence is distinctly visible. Size is variable Number: Commonly 1-4 nucleoli are found in a nucleus. Up to 1600 nucleoli are reported in the oocytes of Xenopus Position: round or slightly irregular structure which is attached to the chromatin at a specific region called nucleolar organizer region (NOR). Composition: RNA, proteins, Calcium seems to be essential for maintaining its configuration.

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Nucleolus has four components. They are amorphous matrix, granular part, fibrillar portion and chromatin (i) Amorphous Matrix (pars amorpha): It is the homogeneous ground substance of the nucleolus. Matrix is formed of protein. (ii) Granular Portion: It consists of granules of the size of 150-200Å which lie scattered in the amorphous matrix. The granules are formed of protein and RNA in the ratio of 2:1. Precursor of ribosome (iii) Fibrillar Portion (Nucleolonema): It is formed of a large number of small fibrils that are 50 A long. The fibrils are made up of both protein and RNA. Precursor of granules (iv) Chromatin portion: It is that part of chromatin which is associated with nucleolus. Depending upon its position nucleolar, chromatin is of two types. perinucleolar and intra-nucleolar. The perinucleolar chromatin lies around the periphery of the nucleolus. It gives rise to ingrowths or trabeculae which produce the intra-nucleolar chromatin

Functions of nucleous: • (i) Nucleolus is the principal site for the development of ribosomal RNAs. • (ii) It is the centre for the formation of ribosome components, • (iii) Nucleolus stores nucleoproteins. The same are synthesised in the cytoplasm (over the ribosomes) and

transferred to nucleolus, • (iv) It is essential for spindle formation during nuclear division

v. Chromatin:

It is hereditary DNA-protein fibrillar complex. It is named so because of its ability to get stained with certain basic dyes. Chromatin occurs in the form of fine overlapping and coiled fibres which appear to produce a network called chromatin reticulum. Chromatin fibres are distributed throughout the nucleoplasm. They are differentiated into two regions— euchromatin and heterochromatin, Heitz (1928). Euchromatin is narrow (10-30nm thick) lightly stained and diffused fibrous part which forms the bulk of chromatin. Heterochromatin is wider (100 nm thick), darkly stained and condensed granular part which is attached here and there on the euchromatin. The whole of chromatin is not functional. Generally only a portion of euchromatin which is associated with acid proteins takes part in transcription or formation of RNAs. During prophase of nuclear division, the chromatin fibres condense to form a definite number of thread-like structures called chromosomes

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Functions of Nucleus

i. Chromatin: Nucleus contains hereditary material called chromatin.

ii. Genetic Information: Chromatin part of nucleus possesses all the genetic information that is required for

growth and development of the organism, its reproduction, metabolism and behaviour.

iii. Cellular Activities: Nucleus controls cell metabolism and other activities through the formation of RNAs

(mRNA, rRNA, tRNA) which control synthesis of particular type of enzymes.

iv. Ribosomes: Ribosomes are formed in nucleolus part of the nucleus.

v. Variations: All variations are caused by changes in genetic material present in the nucleus.

vi. Cell Growth and Maintenance: With the help of RNAs, nucleus directs the synthesis of some structural

proteins and chemicals required for cell growth and maintenance.

vii. Cell Differentiation: It directs cell differentiation by allowing certain particular sets of genes to operate.

viii. Cell Replication: Replication of nucleus is essential for cell replication

Chloroplasts

It is a type of a plastids which is found only in algal and plant cells. It is the main site for photosynthesis. The word

chloroplast comes from the Greek words khloros, meaning “green”, and plastes, meaning “formed”. Andreas Franz

Wilhelm Schimper in 1883 called them Chloroplastids. In 1884, Eduard Strasburger coined the term "chloroplasts"

(Chloroplasten).

Shape: In plants, chloroplast may be filamentous, saucer-shaped, spheroid, ovoid, discoid or club shaped. In algae a

single huge chloroplast is seen that appears as a network, a spiral band or a stellate plate.

Size: its size varies from 2 to 3µ in thickness and it is 5 to 10µ long

Number: In higher plants there are usually 20 to 1000 per cell. However, in many algae there may be only one or a

few chloroplasts.

Structure:

Chloroplasts, are oval-shaped, trilaminar in structure. It has an outer membrane, which forms the external surface of

the chloroplast. The outer membrane is highly permeable. An inner membrane lies just beneath outer membrane

and it is selectively permeable. Each membrane is 40-60Å thick. Between the outer and inner membrane is a thin

intermembrane space or periplastidial space about 25-75Å. The space within the inner membrane is called

the stroma. Chloroplasts have many small disc-shaped sacs called thylakoids within their stroma called as Grana.

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Stroma: Plastids are filled with a watery, proteinaceous substance called the matrix or stroma. It contains about

50% of the chloroplast proteins and most of these are of the soluble type. It has ribosomes and also chloroplast DNA.

It also contains starch granules and osmiophilic droplets.

Grana: In the stroma are embedded disc-like flattened structures made of double membrane called thylakoids. The

outer surface of the thylakoid is in contact with the stroma and its inner surface encloses an intra-thylakoid space.

10-100 of Thylakoids may be stacked like a pile of coins, forming the grana. Thus, a granum consists of a series of

membrane discs packed back to back, like a stack of coins. However, each disc is interconnected at an angle to all

other discs in a granum by tubules called frets or thylakoid lamellae. By branching, a fret connects a disc to each of

the other discs in turn.

Thylakoids provide a large membrane area to hold the photosynthetic pigments and enzymes. Thylakoids containing

chlorophyll (photosynthetic apparatus) for photosynthesis, permit separation of the light reactions that occur there

from the dark reactions in the chloroplast stroma that fix CO2 to synthesize sugars, starch, fatty acids and some

proteins. Grana vary in size from 0.3 to 1.7 µm in various species. Membrane lamellae of the grana contain protein,

and lipid layers, and in between these two is present the chlorophyll layer. The chloroplasts may contain 40-60 grana

in their stroma. The thylakoid lamellae are composed of alternating layers of lipids and aqueous proteins. There is a

layer of chlorophyll and carotenoid molecules situated between the protein and lipid layers. The chlorophyll

molecules are arranged in such a way that their hydrophilic heads extend into the aqueous protein layer while the

lipophilic tails are embedded in the lipid layer.

The pigments are organized into numerous photosynthetic units called quantasomes. Park and Pon (1968)

discovered These are small paracrystalline spheroid bodies inside the membranes of thylakoids of the grana.

Quantasomes may be plates or elipsoids. measuring about 200 Å in diameter and 100 Å thick, arranged in a regular,

lateral array. Each quantasome contains about 230 to 300 chlorophyll molecules. Quantasomes are capable of

trapping light energy and converting it into chemical energy (ATP) during the photochemical reactions (light

reaction) of photosynthesis.

Photosynthesis has two stages. Grana are the site of the light reactions (phase-I) in photosynthesis. Stroma is the

site of the dark reactions (phase-II) of photosynthesis. In the first stage, the light-dependent reactions capture

sunlight through chlorophyll and carotenoids to form ATP, the energy currency of the cell and NADPH, which carries

electrons. In the second stage consists of the light-independent or dark reactions, also known as the Calvin cycle, the

electrons carried by NADPH convert inorganic carbon dioxide and to an organic molecule in the form of a

carbohydrate, a process known as CO2 fixation. Carbohydrates and other organic molecules can be stored and used

at a later time for energy.

Pigments in photosynthesis :

The most common photosynthetc pigments present in higher plants and green algae are:

(i) Chlorophyll-a (blue-green) = C55H72O5N4Mg

(ii) Chlorophyll-b (yellow-green) = C55H70O6N4Mg

(iii) Carotenoids - Carotenes (orange-red) = C40 H56

(iv) Xanthophylls (yellow) = C40H56O2

For photosynthesis, these pigments can absorb and use light belonging to the visible spectrum only. chlorophyll-a

show maximum light absorption in the blue-violet and chlorophyll-b in the red regions of the visible range of

wavelengths of light. Carotenoids absorb light in the blue and blue-green regions. They also protect the chlorophyll

from undergoing photo-oxidation when exposed to very high intensity light. Chlorophyll-a is the essential pigment in

photosynthesis, because only chlorophyll-a can utilize the absorbed light energy for the synthesis of chemical energy

ATP. Other pigments act as accessory pigments. They collect the light energy and transfer it to chlorophyll-a for

photosynthesis.

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Functions

i. Chloroplasts are the part of plant and algal cells that carry out photosynthesis, the process of converting

light energy to energy stored in the form of sugar and other organic molecules that the plant or alga uses as

food.

ii. The most important function of chloroplast is to make food by the process of photosynthesis. Food is

prepared in the form of sugars. During the process of photosynthesis sugar and oxygen are made using light

energy, water, and carbon dioxide.

iii. Chloroplasts, like the mitochondria use the potential energy of the H+ ions or the hydrogen ion gradient to

generate energy in the form of ATP.

iv. Production of NAPDH2 and evolution of oxygen through the process of photolysis of water

v. Chloroplasts are essential for the growth and survival of plants and photosynthetic algae.

vi. In plants all the cells participate in plant immune response as they lack specialized immune cells.

vii. The chloroplasts with the nucleus and cell membrane and ER are the key organelles of pathogen defense.

Mitochondria Mitochondria are known as the powerhouses of the cell. They are organelles that act like a digestive system which takes in nutrients, breaks them down, and creates energy rich molecules for the cell. The biochemical processes of the cell are known as cellular respiration. Many of the reactions involved in cellular respiration happen in the mitochondria. Mitochondria are the working organelles that keep the cell full of energy. Synonyms: Chondriosomes, Sarcosome, Plastosome, Fachsinophilic granules, Bioblast, Power houses, ATP mills, Storage batteries cellular furnace, biochemical machine, cell within a cell, or enuosymbiont in cell. Definition: Mitochondria are the filamentous, self-replicating, double membranous cytoplasmic organelles of eukaryotic cells where cellular respiration takes place. Occurrence: They are the energy transducing organelle found in all aerobic eukaryotic cells. But in mature mammalian RBC mitochondria are lost secondarily. They are also absent in prokaryotic cells where mesosomes act as a substitute of mitochondria. History:

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Kolliker (1850): First discovered mitochondria as granular structures in insect striated flight muscles and called as sarcosomes. Altmann (1894): He called them asbioblast. Benda (1897): First coined the term mitochondria. Meves (1904): First noticed the presence of mitochondria in plant cells of Nymphaea. Kingsbury (1912): First suggested that mitochondria are the sites for cellular respiration. Kennedy and Lehninger (1948- 50): He showed that TCA cycle, oxidative phosphorylation and fatty acid oxidation took place in mitochondria. Number: The number of mitochondria varies from cell to cell. Some cells have several thousand mitochondria while others have none. Muscle cells need a lot of energy so they have loads of mitochondria. Neurons (cells that transmit nerve impulses) don’t need as many. If a cell feels it is not getting enough energy to survive, more mitochondria can be created. Sometimes a mitochondria can grow larger or combine with other mitochondria. It all depends on the needs of the cell. The number of mitochondria in a cell is generally proportional to its energy requirement. Cells of dormant seeds have very few mitochondria. Those of germinating seeds have several mitochondria. In general green plant cells contain less number of mitochondria as compared to non-green plant cells and animal cells. The Trypanosoma, Chlorella and Microsterias contain 1 mitochondrion per cell, but the number is 25 in human sperm cell, 300-400 – in a kidney cell, 500-1000 – in a hepatic cell, 50,000 – giant amoeba , 30000 -300000 – in oocytes of sea urchins and 5,00,000 -in flight muscle cell. Position: The position of mitochondria in a cell depends upon the requirement of energy and amino acids. In unspecialized cells they are randomly distributed throughout the cytoplasm. In absorptive and secretory cells, they lie in the peripheral cytoplasm. During nuclear division, more of mitochondria come to lie around the spindle. Mitochondria are more abundant at the bases of cilia or flagella to provide them energy for movements. In muscle fibres they occur in rows in the regions of light bands in between the contractile elements. Shape and Size: Mitochondria vary in shape and size. Typical mitochondria are generally rod shaped or cylindrical, having length 1-4 /µm and breadth (diameter) 0.2-1.5/µm. In some cases, these may be spherical or oval or filamentous (up to 12µ.m long). All mitochondria of a cell are collective called as condriome and constitutes about 25% of the cell volume. Mitochondria appear yellowish due to riboflavin and rich in Mn. The life span of mitochondria is only 5-10 days. They are continuously produced from the pre-existing mitochondria within the cell and destroyed within the cells. Ultrastructure: The A mitochondrion partly cut open to show two membranes form the envelope Each mitochondrion is bounded by a mitochondrial envelope and encloses two chambers or compartments within it.

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(a) Mitochondrial envelope: It consists to two membranes called outer membrane and inner membrane (each 60-75 Ắ thick). Both the membranes come in contact with each other at several places called adhesion sites or contact zones. The outer membrane is smooth but porous. Due to the presence of integral proteins called porins, it is permeable to a number of metabolites. It contains 40% lipids and 60% proteins. A few enzymes connected with lipid synthesis are located in the membrane. The mitochondrion excluding the outer membrane is called mitoplast. The inner membrane is semipermeable. It is permeable to only some metabolites. It is rich in double phospholipid called cardiolipin (having four fatty acids) which makes the membrane impermeable to ions. It is highly convoluted to form a series of in-folding called cristae or mitochondrial crests. Each crista encloses intracristal spaces which is continuous with the outer chamber. The cristae greatly increase the surface areas of inner membrane. The inner membrane consists of 75% proteins and 25% lipids. It is rich in enzymes of respirators chain and a variety of transport protein is. The cristae are generally arranged like baffles, at right angles to the longitudinal axis of the mitochondrion. They are tubular (most plant cells) or plate like (most animal cells) or vesicle-like (e.g., Euglena). A crista encloses a space that is continuation of the outer chamber. The density of cristae indicates the intensity of respiration. (b) Mitochondrial chambers: In between two membranes a narrow space (about 6-10 nm wide, present called outer chamber or inter-membrane space or peri-mitochondrial space. The central wider space enclosed by the inner membrane is called inner chamber or mitochondrial matrix. Outer chamber It is 60-100 A wide. It extends into the spaces of the cristae. The outer chamber is filled with a watery fluid and contains enzymes like adenylate kinase and nucleoside diphosphokinase. Inner chamber It forms the core of the mitochondrion. The inner chamber contains a semi-fluid matrix. The matrix is filled with a homogenous, granular, dense, jelly like material. It contains- DNAs (2-6 copies), Mitoribosomes (55 to 70S, so resemble that of prokaryotype) protein synthesis occur on the mitochondrial ribosomes are different from that of cytoplasmic ribosomes Mitochondria synthesise some of their own structural proteins. However, most of the mitochondrial proteins are synthesised under instructions from cell nucleus), granules of inorganic salts, enzymes for the kerb`s cycle or citric acid cycle (TCA cycle) and for the oxidation of pyruvate and fatty acids.

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(c) Oxysomes: The matrix side of the inner membrane and cristae bear numerous tennis racket like particles present called oxysomes. They are also known as elementary particles, Parson’s particle, Fernandez-Moran particle, F0F1-particles, F0F1-ATPase, H+ – ATPase, ATP synthetase or ATP synthase. A mitochondrion contains 1 x 104 – 1 x 105 elementary particle (about 104 -105 oxysomes regularly placed at the intervals of l0 nm). Oxysomes comprise about 15% of the total inner membrane protein. Each oxysome is a multi-polypeptide complex consists of 3 parts: (i) Head or F1 particle or soluble ATPase. Has diameter of 8.5nm (ii) Base or F0 subunit. (iii) A stalk that connects F1 subunit with the F0 subunit. Mitochondrial DNA It is double stranded, circular may be linear sometimes. Since Mitochondria have their own DNA they can replicate independently- self replicating. Mitochondrial DNA produces its own mRNA, tRNA and rRNA specific to mitochondria. Mt DNA directs synthesise some of the enzymes required for their functioning. The genome of human mitochondria contains 16569 base pairs of DNA organised in a closed circle. These encodes for 2 rRNA, 22 tRNA, 13 polypep participating in building several protein complexes embedded in the inner mito membrane. Apart from this mitochondria also need other proteins which are synthesized in the cytosol under the control of nuclear DNA then imported in to the mitochondria. Mitochondria replicate much like bacterial cell. When they become too large, they undergo fission. This involves first the replication of mt DNA, then a furrowing of the inner and then the outer membrane finally pinching off the mitochondrian into two daughter mitochondria. In mammals 99% of mt DNA is inherited from the mother. This is because the sperm carries its mitochondria around a portion of its tail and has only about 100 mitochondria compared to egg, which has around 10,000. As the cells develop more and more of mt DNA are diluted out. Hence less than 0.01% of the mt DNA is paternal. This means that any mutations in the mt DNA is passed from the mother to child. Bipaternal inheritance is also observed in yeast. Some diseases caused by inheritanceof mutated mt DNA

Leigh Syndrome: It presents with seizures, memory loss, and respiratory failure. Leber’s Hereditary Optic Neuropathy: There is a progressive loss of vision due to nerve damage. It leads to blindness

in both eyes. Wolff-Parkinson-White Syndrome: It is a disease of the heart in which conduction defects occur. Diabetes and Deafness- This is a combination of both diabetes mellitus and deafness that occurs due to

mitochondrial disease Functions of Mitochondria: Mitochondria are associated with the following functions: (i). Mitochondria are miniature biochemical factories where food stuffs or respiratory substrates are completely oxidized to carbon dioxide and water (Cellular respiration). The energy liberated in the process is stored in the form of ATP the chief immediate source of chemical energy. ATP comes out of mitochondria and helps perform various energy requiring processes of the cell like muscle contraction, nerve impulse conduction, biosynthesis, membrane transport, cell division, movement, etc. On this account, the mitochondria are often described as the “power houses”, or “storage batteries” or “ATP mills” or “cellular furnace” of the cell. Mitochondria tend to assemble where energy is required. (ii) They provide intermediates for the synthesis of important biomolecules, chlorophyll, cytochromes, steroids, alkaloids, etc. (iii) The matrix or inner chamber of the mitochondria has enzymes for the synthesis of fatty acids. Enzymes required for the elongation of fatty acids have been reported in the outer mitochondrial chamber.

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(iv) Some amino acids are also formed in mitochondria. Synthesis of many amino acids occurs in the mitochondria. The first formed amino acids are glutamic acid and aspartic acid. They are synthesized from a-ketoglutaric acid and oxaloacetic acid respectively. Other amino acids are produced by transformation and transamination or transfer of amino group (—NH2) from glutamic acid and aspartic acid. (v) Mitochondria regulate the calcium ion concentration in the cell by storing and releasing Ca2 +as and when required. The calcium ions in turn regulate many biochemical activities in the cell. (v) They help in β oxidation of Fatty acids (vi) The mitochondria also play important role in the process of apoptosis or programmed cell death. The mitochondria also help in building certain parts of blood and hormones like testosterone and estrogen. (vii)The liver cells mitochondria have enzymes that detoxify ammonia.

Endoplasmic Reticulum. Eukaryotic cell is a complex structure and has a number of membrane bound compartments embedded in the cytoplasm these compartments are called cell organelles. In cytoplasmic matrix there iis a vast network of closed and open cavities in the form of membrane bound tubules, vesicles and flattened sacs. This membranous organisation travelling the matrix is called cytoplasmic vesicular system of which ER form only a part. Other part of this system are nuclear envelope, endosome, lysosome and golgi complex

Definition: Within the cytoplasm of most animal cells there is an extensive network (reticulum) of membrane-limited channels, collectively called the endoplasmic reticulum (or ER). The endoplasmic reticulum is a name derived from the fact that in the light microscope it looks like a “net in the cytoplasm.” The endoplasmic reticulum is only present in the eukaryotic cells. However, the occurrence of the endoplasmic reticulum varies from cell to cell. For example, the erythrocytes (RBC), egg and embryonic cells lack in the endoplasmic reticulum. Some portion of ER membranes remains continuous with the plasma membrane and the nuclear envelope. These ER are commonly seen in nerve cells where it is called as Nissl bodies and in secretory cells, where it is called as Ergastoplasm. Garnier in 1897, called ER as Ergastoplasm. When Keith R Porter, Albert Claude and Ernest F Fullum examined this under electrn micro scope during late 1940s he observed an elaborate network of membrane delimited channels. In cultured cells, the network was less abundant in the periphery of the cell (Ectoplasm), than in the interior (Endoplasm) so Porter called these network Endoplasmic reticulum. Two types of ER 1. Smooth Endoplasmic Reticulum

• They are also called as the agranular endoplasmic reticulum. • possesses smooth walls because the ribosomes are not attached to its membranes. • occurs mostly in those cells, which are involved in the metabolism of lipids (including steroids) and glycogen. • Eg. adipose cells, interstitial cells, glycogen storing cells of the liver, conduction fibers of heart,

spermatocytes, and leucocytes.

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2. Rough Endoplasmic Reticulum

• It possesses rough walls because the ribosomes remain attached to its membranes. • On their membranes, rough ER (RER) contains certain ribosome specific, transmembrane glycoproteins,

called ribophorins I and II, to which are attached the ribosomes while engaged in polypeptide synthesis. • found abundantly in those cells which are active in protein syntheses such as pancreatic cells, plasma cells,

goblet cells, and liver cells.

Structure of ER The membrane of the endoplasmic reticulum is 50 to 60 Aº thickness and fluid mosaic like the unit membrane of the plasma membrane. The membranes of the endoplasmic reticulum are found to contain many kinds of enzymes that are needed for various important synthetic activities. The most important enzymes are the stearases, NADH-cytochrome C reductase, NADH diaphorase, glucose-6-phosphatase, and Mg++ activated ATPase. The membrane of endoplasmic reticulum remains continuous with the membranes of the plasma membrane, nuclear membrane, and Golgi apparatus. The cavity of the endoplasmic reticulum is well developed and acts as a passage for the secretory products. The endoplasmic reticulum may occur in the following three forms:

1. Lamellar form or cisternae 2. Vesicular form or vesicle and 3. Tubular form or tubules.

The Cisternae RER usually exists as cisternae that occur in those cells which have synthetic roles as the cells of the

pancreas, notochord, and brain. The cisternae are long, flattened, sac-like, unbranched tubules having a diameter of 40 to 50 μm. They remain arranged parallelly in bundles or stakes.

The Vesicles

The vesicles are oval; membrane-bound vacuolar structures having a diameter of 25 to 500 μm. They often remain isolated in the cytoplasm and occur in most cells but especially abundant in the SER.

The Tubules

The tubules are branched structures forming the reticular system along with the cisternae and vesicles. They usually have a diameter from 50 to 190 μm and occur almost in all the cells. Tubular form of ER is often found in SER and is dynamic in nature, i.e., it is associated with membrane

movements, fission and fusion between membranes of cytocavity network.

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Functions Functions of smooth ER include lipid metabolism (both catabolism and anabolism; they synthesize a variety

of phospholipids, cholesterol, and steroids). Glycogenolysis (degradation of glycogen; glycogen being polymerized in the cytosol). Drug detoxification (by the help of the cytochrome P-450). The endoplasmic reticulum provides an ultrastructural skeletal framework to the cell and gives mechanical

support to the colloidal cytoplasmic matrix. The exchange of molecules by the process of osmosis, diffusion and active transport occurs through the

membranes of the endoplasmic reticulum. The endoplasmic reticulum is the main component of the endomembrane system, also called the

cytoplasmic vacuolar system or cytocavity network. The endoplasmic membranes contain many enzymes that perform various synthetic and metabolic activities.

Further, the endoplasmic reticulum provides an increased surface for various enzymatic reactions. The endoplasmic reticulum acts as an intracellular circulatory or transporting system. As a growing secretory polypeptide emerges from the ribosome, it passes through the RER membrane and

gets accumulated in the lumen of RER. Here, the polypeptide chains undergo tailoring, maturation, and molecular folding to form functional secondary or tertiary protein molecules.

RER pinches off certain tiny protein-filled vesicles which ultimately get fused to cis Golgi. The ER membranes are found to conduct intra-cellular impulses. For example, the sarcoplasmic reticulum

transmits impulses from the surface membrane into the deep region of the muscle fibers. The ER membranes form the new nuclear envelope after each nuclear division. The SER contains several key enzymes that catalyze the synthesis of cholesterol which is also a precursor

substance for the biosynthesis of two types of compounds— the steroid hormones and bile acids. RER also synthesize membrane proteins and glycoproteins which are cotranslationally inserted into the

rough ER membranes. Thus, the endoplasmic reticulum is the site of the biogenesis of cellular membranes

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Ribosmes These are Molecular machines for protein synthesis or protein factories. The ribosome word is derived – ‘ribo’ from ribonucleic acid and ‘somes’ from the Greek word ‘soma’ which means ‘body. Ribosomes were first observed in the mid 1950s by Robinson and Brown in plant cell and George Emil Palade in animal cells as dense granules. The term Ribsomes was given by Richard B Roberts. Albert Claude, Christian de Duve, George Emil Palade were jointly awarded the Nobel prize for the discovery. Venkatraman Ramakrishnan, Thomas A Steitz and Ada E Yonath were awarded Nobel prize for determining the detailed structure and mechanism of the ribosome. All living cell has ribosomes; except erythrocyte. A Prokaryotic cell contains 10,000 ribosmes each has Molecular Weight (M.W) of 3million Da and Eukaryotic cell has 10M of ribosomes each has M.W 5million Da

Types Depending upon the Place of occurrence,

• Cytoplasmic ribosome: present in the cytoplasm • organelle ribosome: present in the organelle

Cytoplasmic ribosmes may be • Free ribosmoes: these are found free floating in the cytosol. In cytosol it may occur singly as monosome or

occur in groups and rossetes called as polyribosome/polysome. Proteins are which are synthesized in them are released in to the cytosol and used within the cell.

• Bound ribosomes- These are found attached to the exterior of the ER which makes it the RER. They produce proteins that are used within the plasma membrane or expelled from the cell via exocytosis.

Structure of Ribosome Each ribosome is Porous, hydrated. Ribosome of higher plants and animals are Oblate, spheroid, prolate in shape. The diameter is 250Å. However the ribosome of bacteria are smaller because they contain fewer amount of proteins than higher animal ribosomes. In the Electron microscopic studies, Negative staining reveals a cleft that divides the ribosome into two parts. They are Larger and smaller sub units (SU). In E coli larger SU is cup or dome shaped and 140-160Å thick. Smaller SU forms cap that applied to the flat surface of the other. It is 90-110Å thick.

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Another property which is common to all ribosomes is their sub unit structure and their Sedimentation constant by ultracentrifugation. This is denoted by S (Svedberg unit). On the basis of sedimentation constant there are two types of ribosomes. Those from the bacteria (Prokaryotic ribosome) usually have a sedimentation constant of 70S and the Molecular mass is 2.7×106. Eukaryotic ribosome has sedimentation co-efficient/constant of 80S with 4×106 MW The two sub units require low concentration of Mg2+ ions for their structural cohesion. If you remove Mg2+ ions it will yield two smaller particles. One is 2/3 and other one is 1/3 of the mass of the original intact ribosome.

Composition The major components of ribosomes are RNA and proteins. In an entire ribosome, rRNA (ribosomal RNA) makes 66% part in prokaryotes and 60% in eukaryotes. In subunits, the proportion of rRNA is larger in larger subunit (70% in prokaryotes and 65% in eukaryotes) than in the smaller subunit (60% in prokaryotes and 50% in eukaryotes). Ribosomes are strongly negative, binding cations and basic dyes. Prokaryotes have 70S ribosomes respectively subunits comprising the small subunit of 30S and the bigger subunit of 50S. Their small subunit has a 16S rRNA subunit (consisting of 1540 nucleotides) bound to 21 proteins. The large subunit is composed of a 5S rRNA subunit (consisting 120 nucleotides), a 23S rRNA subunit (consisting 2900 nucleotides) and bound to 31 proteins. Eukaryotes have 80S ribosomes respectively comprising of small (40S type) and Large (60S type) subunits. The smaller 40S ribosomal subunit is prolate ellipsoid in shape and consists of one molecule of 18S ribosomal RNA (or rRNA) and 30 proteins (named as S1, S2, S3, and so on). The larger 60S ribosomal subunit is round in shape and contains a channel through which growing polypeptide chain makes its exit. It consists of three types of rRNA molecules, i.e., 28S rRNA, 5.8 rRNA and 5S rRNA, and 40 proteins (named as L1, L2, L3 and so on).

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Ultra- structure of ribosomes

Ultra- structure of ribosomes has been studied more extensively in prokaryotes than eukaryotes. Negative staining of ribosomes has led to better understanding of the fine-structure of these organelles. Asymmetrical Model (J. A. Lake, 1981): According to this model, the smaller subunit has a head, a base and a platform (Fig. 3.18). The platform separates the head from the base by a cleft. The cleft is an important functional region the site of codon-anticodon interaction and part of binding site for the initiation factor of protein synthesis. On the other hand, the large subunit consists of a ridge, a central protuberance and a stalk. The first two are separated with the help of a valley.

Functions of Ribosomes

i. They assemble amino acids to form specific proteins, proteins are essential to carry out cellular activities. ii. The process of production of proteins, the deoxyribonucleic acid produces mRNA by the process of DNA

transcription. iii. The genetic message from the mRNA is translated into proteins during DNA translation. iv. The sequences of protein assembly during protein synthesis are specified in the mRNA. v. The mRNA is synthesized in the nucleus and is transported to the cytoplasm for further process of protein

synthesis. vi. In the cytoplasm, the two subunits of ribosomes are bound around the polymers of mRNA; proteins are then

synthesized with the help of transfer RNA. vii. The proteins that are synthesized by the ribosomes present in the cytoplasm are used in the cytoplasm itself.

The proteins produced by the bound ribosomes are transported outside the cell.