Biology
CELL BIOLOGY WHAT IS A CELL? All organisms are composed of cells. Some are composed of a single cell and are called unicellular organisms while others, like us, composed of many cells, are called multicellular organisms. Unicellular organisms are capable of (i) independent existence and (ii) performing the essential functions of life. Anything less than a complete structure of a cell does not ensure independent living. Hence, cell is the fundamental structural and functional unit of all living organisms. First cell discovered by - Robert Hook in Cork Anton Von Leeuwenhoek first saw and described a live cell. Robert Brown later discovered the nucleus. The invention of the microscope and its improvement leading to the electron microscope revealed all the structural details of the cell. CELL THEORY In 1838, Malthias Schleiden, a botanist, examined a large number of plants and observed that all plants are composed of different kinds of cells which form the tissues of the plant. At about the same time, Theodore. Schwann (1839), Zoologist, studied different types of animal cells and reported that cells had a thin outer layer which is today known as the ‘plasma membrane’. He also concluded, based on his studies on plant tissues, that the presence of cell wall is a unique character of the plant cells. Schwann proposed the hypothesis that the bodies of animals and plants are composed of cells and products of cells. Schleiden and Schwann together formulated the cell theory. This theory however, did not explain as to how new cells were formed. Rudolf Virchow (1855) first explained that cells divided and new cells are formed from pre-existing cells (Omnis cellula-e cellula). He modified the hypothesis of Schleiden and Schwann to give the cell theory a final shape. Cell theory as understood today is: (i)
all living organisms are composed of cells and products of cells.
(ii)
all cells arise from pre-existing cells.
AN OVERVIEW OF CELL
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The onion cell which is a typical plant cell, has a distinct cell wall as its outer boundary and just within it is the cell membrane.
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Cells that have membrane bound nuclei are called eukaryotic whereas cells that lack a membrane bound nucleus are prokaryotic. In both prokaryotic and eukaryotic cells, a semi-fluid matrix called cytoplasm occupies the volume of the cell. The cytoplasm is the main arena of cellular activities in both the plant and animal cells. Various chemical reactions occur in it to keep the cell in the ‘living state’. Besides the nucleus, the eukaryotic cells have other membrane bound distinct structures called organelles like the endoplasmic reticulum (ER), the golgi complex, lysosomes, mitochondria, microbodies. The prokaryotic cells lack such membrane bound organelles. Ribosomes are non-membrane bound organelles found in all cells – both eukaryotic as well as prokaryotic cell. Within the cell, ribosomes are found not only in the cytoplasm but also within the two organelles – chloroplasts (in plants) and mitochondria and on rough ER. Animal cells contain another non-membrane bound organelle called centriole which helps in cell division. 1
Pre-Medical
CELL STRUCTURE
Cell boundaries
Cytoplasm
Nucleus
Cell wall Glycocalyx Cell membrane Hyaloplasm
Cell organelles Mitochondria
Trophoplasm
Non living cell inclusions (Deutoplasm)
Golgibody
Reserve products
Lysosomes
Carbohydrate
E. Reticulum
Nitrogenous products
Plastids
Fats & oil
Cilia, Flagella
Excretory products
Centrosome
Alkaloids
Ribosome
Essential oil
Microbodies
Gums Glycoside Latex Organic acid Tannin Mineral crystal Secretory Products Colouring matter Nectar
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Resin
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Biology SIZE AND SHAPE OF CELL Size : • Cell differ greatly in size, shape and activites. • Mycoplasma (Smallest cells) : Only 0.3 mm in length [PPLO (pleuro pheumonia like organisms) is a type of mycoplama having the size about 0.1 mm)] • Bacteria = 3 to 5 mm • Largest isolated single cell = egg of an ostrich. • Human red blood cell = 7.0 mm in diameter • Nerve cell = longest cell Shape : • The shape of the cell may vary with the funtion they perform. •
They may be disc-like, polygonal, columnar, cuboid, thread like or even irregular.
Red blood cells (round and biconcave)
White blood cells (amoeboid)
Columnar epithelial cells (long and narrow)
Nerve cell (Branched and long)
A tracheid (elongated)
Mesophyll cells (round and oval)
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Diagram showing different shapes of the cells
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PROKARYOTIC CELLS •
The prokaryotic cells are represented by bacteria, blue-green algae, mycoplasma and PPLO (Pleuro Pneumonia Like Organisms). They are generally smaller and multiply more rapidly than the eukaryotic cells.
EUKARYOTIC CELLS • The eukaryotes include all the protists, plants, animals and fungi. In eukaryotic cells there is an extensive compartmentalisation of cytoplasm through the presence of membrane bound organelles. • Eukaryotic cells possess an organised nucleus with a nuclear envelope. In addition, eukaryotic cells have a variety of complex locomotory and cytoskeletal structures. Their genetic material is organised into chromosomes. •
All eukaryotic cells are not identical. Plant and animal cells are different as the former possess cell walls, plastids and a large central vacuole which are absent in animal cells. On the other hand, animal cells have centrioles which are absent in higher plant cells. 3
Pre-Medical Microvilli Golgi apparatus Plasma membrane Centriole Smooth endoplasmic reticulum
Peroxiome Lysosome
Nuclear envelope
Ribosomes
Nucleolus
Mitochondrion Rough endoplasmic reticulum Cytoplasm
Nucleus
Animal cell
Smooth endoplasmic reticulum
Rough endoplasmic reticulum Lysosome
Plasmodesmata
Nucleus Nucleolus
Microtubule
Nuclear envelope Plasma membrane Vacuole Middle lamella Cell wall Peroxisome
Mitochondrion Cytoplasm
Chloroplast
Ribosomes
Plant cell
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Golgi apparatus
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Biology
BIOMEMBRANES OR CELL–MEMBRANE All the living cells are covered by a thin, delicate, elastic, selectively–permeable and living boundry, which is called as – cell membrane (by Nageli & Kramer) or plasmalemma (By J.Q. plowe) or bio membrane or plasma membrane. Biochemical investigation clearly revealed that the cell membranes possess lipid, protein and carbohydrate. The ratio of protein and lipid varies considerably in different cell types. In human beings, the membrane of the erythrocyte has approximately 52 per cent protein and 40 per cent lipids Lipids = 40% (Phospholipid, Cholestrol, Glycolipids) Proteins = 58-59% (Arginine, Lysine rich) Carbohydrates = 1-2% STRUCTURE OF BIOMEMBRANES : (1)
Sandwitch or Trilamellar model :- By Davson & Danielli (1935). According to this model, the plasma–membrane is made up of three layers in which a bimolecular layer of lipid is sandwitched between two single layers of proteins. According to this model each protein layer is 20Å thick and bilayer of phospholipid is 35Å thick. Thus total thickness is 75Å (PLLP – structure, 75–100Å average) Phospholipid molecule called as amphipathic molecule due to presence of two type of parts (hydrophillic head and hydrophobic tail). Hydrophilic head of the phospholipid binds with protein layer by hydrogen and ionic bonds. Hydrophobic tail of phospholipid are attached to each other by vanderwall force.
Bio-molecular layer of phospholipids (35Å)
Protein layer (20 Å)
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(2)
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Polar hydrophilic head
Nonpolar hydrophobic tail
Protein layer (20 Å)
Pores (0.35 nm)
Unit membrane model :- By Robertson 1959. According to this model all the cellular and organeller membranes are structurally & functionally similar (difference in chemically & size). Both of the above models are rejected because they fails to explain the elasticity and selective permeability of plasmalemma. The detailed structure of the membrane was studied only after the advent of the electron microscope in the 1950s. Meanwhile, chemical studies on the cell membrane, especially in human red blood cells (RBCs), enabled the scientists to deduce the possible structure of plasma membrane.
(3)
Fluid mosaic model : By Singer & Nicolson (1972) This is latest & most widely accepted model for the structure of plasmalemma. According to fluid mosaic model proteins are arranged in phospholipid layer as mosaic pattern. Thus membrane is termed as "protein icebergh in a sea of phospholipid" or "Gulab Jamun (protein) in a concenterated solution (phospholipid) of sugar". 5
Pre-Medical (1)
Phospholipids : Phospholipid is the main component of cell membrane because it forms continous structural frame of cell membrane. Main type of phospholipids are phosphatidyl serine, phosphatidyl choline (Lecithin), P-ethanol amine (cephalin) The studies showed that the cell membrane is composed of lipids that are arranged in a bilayer. Also, the lipids are arranged within the membrane with the polar head towards the outer sides and the hydrophobic tails towards the inner part.This ensures that the nonpolar tail of saturated hydrocarbons is protected from the aqueous environment. The lipid component of the membrane mainly consists of phosphoglycerides. Phospholipid layer provides fluidity to plasma membrane because phospholipids are rich in unsaturated fatty acid which are liquid in nature. The Quasifluid nature of lipid enable lateral movement of protein with in the overall bilayer. This ability to move within the membrane is measured as its, fluidity. The fluid nature of the membrance is also important in various function like cell growth, formation of intercellular junction, endocytosis, cell division etc. Cholesterol is also present in plasma membrane. Cholesterol are more rigid than phospholipid. So it helps in stability of membrane structure. Cholesterol is absent in membrane of prokaryote. Thus Hopanoides (Pentacyclic sterol) provides stability to prokaryotic cell membrane.
(2)
Proteins : Two types of protein are present in plasma membrane. (On the basis of ease of extraction) (a)
Integral or intrinsic protein These protein are tightly binds with phospholipid. Thus, they can not easily removed from membrane. Integral proteins are of 2 types : (i)
Partially buried
(ii)
Totally buried
Some integral proteins which are totally buried through the complete thickness of membrane. These type of protein are called as tunnel (channel) protein which provide a passage for movement of water soluble material across the membrane. Peripheral or extrinsic protein These are superficially arranged on outer side and can be seperate easily. These protein have enzymatic activity. Spectrin are helical type of extrinsic protein founds on cytosolic face (towards cytoplasm) of membrane and attached to intrinsic protein. Spectrins are part of cytoskeleton.
However phospholipid bilayer has fluid property but no evidence of flip flop mechanism for protein molecule (Flip Flop means exchange of molecules from one monolayer with those in the monolayer on the other side). Rotational diffusion and lateral diffusion of protein and lipids is possible in membrane.
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(b)
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Biology Protein
Sugar
Lipid bilayer
Integral protein
Cholesterol Fluid mosalic model of plasma membrane
Outside the cell
Pore formed by integral protein
Glycoprotein (Oligosaccharides)
Glycolipid (Oligosaccharides)
Phospholipids
Peripheral protein (Extrinsic)
Non cytosolic half of bilayer Phospholipid bilayer
Cytosolic half of bilayer Tunnel protein
Cholesterol
Spectrin
Peripheral protein Inside the cell
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Fluid-mosaic molel of membrane
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Oligosaccharides of the glycolipids & glycoproteins on the outer surface of plasma membranes are involved in cell to cell recognition mechanism. Best example of cell recognition is fertilisation, (where sperm & egg recognize to each other) and blood - Antigens. Plasma membrane have approximate 30 types of enzyme in which ATPase (ATP hydrolysing) is more important. ATPase enzyme helps in active transport of materials. Plasma membrane is an asymmetrical structure because carbohydrate is presents on outer surface and spectrin protein is present only on inner surface of plasma membrane.
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Pre-Medical TRANSPORT THROUGH PLASMA MEMBRANE •
One of the most important functions of the plasma membrane is the transport of the molecules across it. The membrane is selectively permeable to some molecules present on either side of it. Many molecules can move briefly across the membrane without any requirement of energy and this is called the passive transport.
•
Neutral solutes may move across the membrane by the process of simple diffusion along the concentration gradient, i.e., from higher concentration to the lower. Water may also move across this membrane from higher to lower concentration. Movement of water by diffusion is called osmosis.
•
As the polar molecules cannot pass through the nonpolar lipid bilayer, they require a carrier protein of the membrane to facilitate their transport across the membrane.
•
A few ions or molecules are transported across the membrane against their concentration gradient, i.e., from lower to the higher concentration. Such a transport is an energy dependent process, in which ATP is utilised and is called active transport, e.g., Na+/K+ Pump. Molecules to be transported
Channel protein
lipid bilayer
Carrier protein
Simple diffusion
Channel mediated diffusion Passive transport (facilitated diffusion)
(ii)
Active transport
Endocytosis (Bulk transport) (a)
Pinocytosis or Cell Drinking :- Ingestion of liquid material by plasmalemma in the form of vesicles or bag like structure(Pinosome) is called pinocytosis.
(b)
Phagocytosis or Cell eating :- Ingestion of solid complex materials by membranes in the form of vesicles (Phagosome) is called Phagocytosis.
Exocytosis/Emiocytosis/Cell vomitting :- Egestion of waste materials from cell through plasma membrane.
CELL WALL A non-living rigid structure called the cell wall forms an outer covering for the plasma membrane of Bacteria Fungi, Algae and Plants. Cell wall not only gives shape to the cell and protects the cell from mechanical damage and infection, it also helps in cell-to-cell interaction and provides barrier to undesirable macromolecules. Algae have cell wall, made of cellulose, galactans, mannans and minerals like calcium carbonate. In other plants it consists of cellulose, hemicellulose, pectins and proteins. Bacterial cell wall mainly composed or Peptidoglycans (Polysaccharide + amino acid). The cell wall of Fungi are composed of Chitin and Polysaccharides. 8
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(i)
En er gy
Carrier mediated diffusion
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Biology Archaebacteria differ from other bacteria in having a different cell wall structure and this feature is responsible for their survival in extreme condition. Dinoflagellates cell wall has stiff cellulose plate on the outer surface. In Euglenoids instead of a cell wall, the have a protein rich layer called pellicle which makes their body flexible.
Cell wall
Primary wall
:
Outermost layer, thin elastic, diminishes as cell matures, capable of growth, composed of cellulose, hemicellulose & pectin.
Secondary Wall
:
Rigid, thick & composed of cellulose, hemi–cellulose, Pectin.
(S1, S2, S3)
(absent in meristem cells) inner side of cell (toward CM)
Tertiary wall
:
Present only in tracheids of Gymnosperm. Composed of hemi cellulose & xylan.
Middle lamella
:
Common layer between two cells.
Middle lamella is consist of Ca & Mg pectates (Plant cement).Amount of Ca is more.
Primary wall
Cellulose is a main constituent of cell wall but addition to cellulose – Hemicellulose, cutin, Pectin, Lignin, Suberin are also presents in cell wall.
Middle lamella Three layered secondary wall
Cell wall worked as frame or protective layer of cell . Cellulose microfibrils and macrofibrils arranged in layers to form skeleton of cell wall. In between these layers ot her subs tances like pectin,hemicellulose may be present. These form matrix of cell wall.
Tertiary wall
A few cells showing gross structure of cell wall
Middle lamella is cement material between two adjacent cells in multi cellular plants or outermost layer of cell wall. (primary wall is consider as outermost layer in a cell) Martinez and palamo (1970) discovered cell-coat in animal cells, which is known as Glycocalyx. [Made by sialic acid, mucin and hyaluronic acid (animal cement)]. Cell wall materials (Cellulose, Hemicellulose, Pectin, lignin) are synthesized in plant golgibodies or dictyosomes. Material of lipid nature (cutin and suberin) are synthesized in sphaerosome. NODE2\E:\DATA\2014\SMP\BIO\SET-01\01-CELL-BIO\ENG\01-CELL-BIO.P65
Formation of cell wall occurs by two methods :-
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(1)
Intussusception :- This is a deposition of wall material in the form of fine grains.
(2)
Apposition :- Deposition of layers.
Primary wall is formed mainly by intussusception, while secondary wall formed by both methods. Growth of already formed cell wall occurs by only intussusception. A special protein called expansin helps in growth of cell wall by loosing the cellulose microfibril and addition of new cell wall material takes place in the space. Thus expansin is called as "cell wall loosening factor". PLASMODESMATA :– Name proposed by Strasburger (1901).These are cytoplasmic connections between two adjacent plant cells. Plasmodesmata are characteristic of multi-cellular plants and they maintain continuity of cytoplasm between adjacent cells. E.R. tubules (Desmotubules) help to maintain continuity of cytoplasm. 9
Pre-Medical Primary wall (1)
Secondary wall
Cellulose microfibrils are arranged
(1)
Microfibrils are parellel to long axis of cell.
in a dispersed manner. (2)
Hemicellulose more (50%)
(2)
Hemicellulose less (25%)
(3)
Primary wall have lipids (5-10%)
(3)
Proteins and lipids either absent or in
and proteins (5%)
little amount.
(4)
Form by Intussusception
(4)
By both methods.
(5)
Primary wall is universal layer
(5)
Absent in meristem cells
CYTOPLASM Term "Cytoplasm", was given by Strasburger for the part of cell, presents between the nucleus and cell membrane. Cytoplasm can be devided into two parts. Ground plasm / Hyaloplasm / Cytosol ® Liquid matrix of cytoplasm except organelles Trophoplasm ® Part of cytoplasm containing organelles & non living Inclusions.
CELL ORGANELLES Permanent Metabolically active and living structures of cytoplasm are called organelles.
ENDOMEMBRANE SYSTEM While each of the membranous organelles is distinct in terms of its structure and function, many of these are considered together as an endomembrane system because their functions are coordinated. The endomembrance system include endoplasmic reticulum (ER), golgi complex, lysosomes and vacuoles. Since the functions of the mitochondria, choroplast and peroxisomes are not coordinated with the above components, these are not considered as part of the endomembrane system.
ENDOPLASMIC RETICULUM Electron microscopic studies of eukaryotic cells reveal the presence of a network of reticulum of tiny tubular Components of E.R. :– (1)
Cisternae - These are long flattened and unbranched units arranged in stacks.
(2)
Vesicles - These are oval membrane bound structures.
(3)
Tubules - These are irregular, often branched tubes bounded by membrane. Tubules may free or associated with cisternae.
Structure of E.R. is like the golgi body but in E.R. cisternae, vesicles and tubules are isolated in cytoplasm and these do not form complex. Golgi body is localised cell organelle while E.R. is widespread in cytoplasm. E.R. is often termed as “System of Membranes” ER divide the intracellular space into two distinct compartment i.e. Luminal (inside ER) and extra luminal (cytoplasm) compartments. 10
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structures scattered in the cytoplasm that is called the endoplasmic reticulum (ER)
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Biology Rough E.R. (Granular) (1)
Smooth E.R. (Agranular)
80s ribosomes binds by their larger
(1)
Ribosomes and Ribophorins absent
subunit, with the help of two glycoproteins (Ribophorin I and II On the surface of Rough E.R.)
More stable structure
(2)
Less stable structure
(3)
Mainly composed of cisternae and vesicles
(3)
Mainly composed of tubules.
(4)
Abundantly occurs in cells which are actively engaged in protein synthesis and secretion. e.g. liver, pancreas, goblet cells.
(4)
Abundantly occurs in cells concerned with glycogen and lipid metabolism. In animal cell lipid like steroidal hormones are synthesied in SER. e.g. Adipose tissue, Interstitial cells, muscles,Glycogen storing liver cells, and adrenal cortex.
Lumen Outer membrane Inner membrane
Nucleus
Nuclear envelope
(2)
Nucleus Nuclear pore
Endoplasmic reticulum
Rough endoplasmic reticulum
Cell membrane
Endoplasmic reticulum as seen in section
Cisternae
Vesicles
Tubules
Parts of Endoplasmic reticulum
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Ribosomes
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Ribosome
Lumen
Cytosol
Lumen (A)
(B)
(A) : Rough endoplasmic reticulum (B) : Smooth endoplasmic reticulum
Smooth Endoplasmic reticulum
Endoplasmic reticulum
MODIFICATIONS OF E.R. (1)
Sarcoplasmic Reticulum (S.R.) :– These smooth E.R. occurs in skeletal and cardiac muscles. S.R. Stores Ca+2 and energy rich compounds required for muscle contraction.
(2)
Ergastoplasm :– When the ribosomes are accumulated on the small parallel cisternae of E.R., then called Ergastoplasm. Ergastoplasm of nerve cells is called as Nissl's bodies.
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Pre-Medical (3)
Myeloid Bodies :– Myeloid bodies are the specialised smooth E.R. which found in pigmented epithelial cells of the retina. Myeloid body is light sensitive structure and may be involved in pigment migration.
(4)
Microsomes - These are pieces of E.R. with associated ribosomal particles. These can be obtained by fragementation and high speed centrifugation of cell. They do not exist as such in the living cell. Scientist used microsome for invitro protein synthesis study.
ENZYMES OF E.R. Sucrases, NADH diphosphatase, Gulcose-6-phosphatase, NADH-cytochrome-C-reductase, Mg+2 activated ATPase, Nucleotide diphosphatase, Ascorbic acid synthatase are enzymes of E.R. FUNCTIONS OF E.R. (1)
Mechanical support :– Microfilaments, Microtubules and E.R. forms endoskeleton of cell.
(2)
Intracellular exchange :– E.R. forms intracellular conducting system. Transport of materials in cytoplasm from one place to another may occurs through the E.R. At some places E.R. is also connected to P.M. So E.R. can secrete the materials outside the cell.
(3)
Rough E.R. :– Provides site for the protein synthesis, because rough E.R., has ribosomes on its surface.
(4)
Lipid Synthesis :– Lipids (cholesterol & phospholipids) synthesized by the agranular portion of E.R. (Smooth E.R.). The major lipids synthesized by S. E. R. are phospholipids and cholesterol.
(5)
Cellular metabolism :– The membranes of the reticulum provides an increased surface for metabolic activities within the cytoplasm.
(6)
Formation of nuclear membrane :– Fragmented vesicles of disintegrated nuclear membrane and ER elements arranged around the chromosomes to form a new nuclear membrane during cell division.
(7)
Formation of lysosomes, Golgi body & some Micro bodies. All the organelles are form by E.R. which have membrane except chloroplast and mitochondria (semi autonomous organelles)
(8)
Detoxification :– Smooth ER concerned with detoxification of drugs, pollutants and steroids. Cytochrome P450 in E.R. act as enzyme which function in detoxification of drugs and other toxins E.R. provides the precursor of secretory material to golgi body.
GOLGI COMPLEX Camillo Golgi (1898) first observed densely stained reticular structure near the nucleus. These were later named Gogi bodies after him. Golgi body also named as w
Golgi body
w
Dalton complex
w
Golgi complex
w
Lipochondria ( rich in lipids)
w
Baker's body
w
Idiosome
w
Dictyosome (plant golgi body)
w
Trophospongium
The cytoplasm surrounding Golgi body have fewer or no other organelles. It is called Golgi ground substance or zone of exclusion. 12
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(9)
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Biology STRUCTURE Golgi complex is made up of four parts – (1)
Cisternae :– These are flat disc shaped, sacs like structure many cistenae are arranged in a stack (parallel to each other). Diameter 0.5 mm to 1.0 mm. Dense opaque material inside cisternae is called Nodes. Varied number of cisternae are present in Golgi complex. Convex surface of cisternae which is towards the nucleus is called cis- face or forming face. Concave surface of cisternae which is towards the membrane is called Transface or maturing face. The cis and trans faces of the organelle are entirely different but inner connected.
(2)
Tubules :– These are branched and irregular tube like structures associated with cisternae.
(3)
Vacuoles :– Large spherical structures associated to tubules.
(4)
Vesicles :– Spherical structures arise by budding from tubules. Vesicles are filled with secretory materials. Trans or Maturing face
Discharged vesicles
Cisternae
Cisternae
Vesicles Fusing with cis face Tubules Cis or Forming face
Transition vesicles
Golgi Apparatus Golgi apparatus FUNCTIONS (1)
Cell Secretion :– Chief function of golgi body is secretion (export) of macromolecules.
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Secretion involve three steps :
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(a)
Golgi body recieves the materials from E.R. through it's cis - face.
(b)
These materials are chemically modified by golgi body. (For e.g. glycosidation (glycosylation) of proteins and lipids takes place in golgi body and it yields glycoprotiens and glycolipids).
(c)
After chemical modifications materials are packed in vesicles. These vesicles are pinched off from trans face of golgi body and discharged out side the cell (Reverse pinocytosis)
The golgi apparatus principally performs the function of packaging materials, to be delivered either to the intra-cellular targets or secreted outside the cell. Materials to be packaged in the form of vesicles from the ER fuse with the cis face of the golgi apparatus and move towards the maturing face. This explains, why the golgi apparatus remains in close association with the endoplasmic reticulum. A number of proteins synthesised by ribosomes on the endoplasmic reticulum are modified in the cisternae of the golgi apparatus before they are released from its trans face. Golgi apparatus is the important site of formation of glycoproteins and glycolipids. 13
Pre-Medical All the macromolecules which are to be sent out side the cell, move through the golgi body. So golgi body is termed as “Director of macromolecular traffic in cell” or middle men of cell. (2)
Synthesis of cell wall Material (Polysaccharide synthesis)
(3)
Cell plate formation (Phragmoplast) during cell formation.
(4)
Formation of acrosome during spermiogenesis. (formation of male gametes)
(5)
Vitelline membrane of egg is secreted by golgi body.
(6)
Formation of Lysosome = It is collective function of golgi body and E.R.
(7)
Secretion of hormones by glands.
LYSOSOME These are membrane bound vesicular structures formed by the process of packaging in the golgi apparatus. The isolated lysosomal vesicles have been found to be very rich in almost all types of hydrolytic enzymes (hydrolases – lipases, proteases, carbohydrases) optimally active at the acidic pH (pH = 5). These enzymes are capable of digesting carbohydrates, proteins, lipids and nucleic acids. With the exception of mammalian RBC they were reported from all cells.
In plant cells large central vacuole functions as Lysosome. So in higher plants lysosomes are less frequent. But number of lysosomes is high in fungi. Periplasmic Space :– space between cell wall and cell membrane in bacteria,may play similar role. Lysosomes are spherical bag like structures 0.05-0.5 m m
(0.1-0.8 mm) which is covered by single unit membrane. They are large sized in Phagocytes
Cathepsin
(WBC) (0.8 to 2 mm). Lysosomes are filled with 50 different type of digestive enzymes termed as Acid hydrolases. These acid hydrolases function in acidic medium ( pH=5).Membrane of lysosome has an active H+ pump mechanism which produce acidic pH in lumen of lysosome.
pH-5
Lysosomes are highly polymorphic cell organelle. Because, during functioning, lysosomes have different morphological and physiological states. Endoplamic reticulum + Golgi body
Primary lysosome or Storage granule
Autophagic vacuole
Secondary lysosome
Food particles taken in by endocytosis
Phagosome
Digestive vacuole
Digested mitochondrion
Residual body
Different types of lysosomes and their orgin
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Plasma membrane
Defecation or Exocytosis of wastes
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Plasma membrane
A lysosome containing a number of acid hydrolases that are active under acidic condition, which is maintained by + an H ATPase in the membrane
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Biology TYPES OF LYSOSOMES (1)
Primary Lysosomes or storage granules - These lysosomes store enzyme Acid Hydrolases in the inactive form. (Enzymes synthesized on ribosomes in cytoplasm) these are newly formed lysosome.
(2)
Digestive vacuoles or Heterophagosomes - These lysosome formed by the fusion of primary lysosomes and phagosomes. These are secondary Lysosomes.
(3)
Residual bodies :– Lysosomes containing undigested material are called residual bodies. These may be eliminated by exocytosis. These are also called as Telolysosomes. (Tertiary lysosomes)
(4)
Autophagic Lysosomes or Cytolysosomes or autophagosomes :– Lysosomes containing cell organelles to be digested are known as Autophagosomes.
FUNCTIONS (1)
(2)
Intracellular digestion :(a)
Heterophagy :– This is digestion of foreign materials received in cell by phagocytosis and pinocytosis.
(b)
Autophagy :– Digestion of old or dead cell organelles. Autophagy also takes place during starvation of cell.
Extracellular digestion :– Lysosomes of osteoclast (bone eating cells) dissolve unwanted part of bones. (Extracellular digestion also occurs by fungal lysosomes.)
(3)
Crinophagy :– Excessive secretory granules of hormone in endocrine gland may be digested by lysosomes. This event is called crinophagy. Thyroglobulin stores in thyroid gland with its follicles and after crinophagy by proteases it produces thyroxine.
(4)
Cellular digestion (Autolysis) :– Sometimes all lysosomes of a cell burst to dissolve the cell completely. (so Lysosome called as suicidal bags of cell). Old cells are removed by autolysis. unwanted organs of embryo
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are destroyed by autolysis Cathepsin of lysosome digests the tail of tadpole of frog during metamorphosis.
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Membrane stabilizers are substances, which stabilize the lysosome membrane and stop its rupture, thus prevents autolysis. e.g. cholesterol, chloroquine, cortisone etc. Membrane labilizers are substances which make the lysosome membrane fragile and increase the chance of autolysis e.g. Progesterone, testosterone, Vitamin A, D, E, K, U.V. radiations, bile salts etc. Biogenesis of Lysosome Lyosomes originates from G E R L - (Golgi associated Endoplasmic Reticulum from which Lysosomes arise). E.R. ¾¾® Golgi body ¾¾® Lysosome
15
Pre-Medical VACUOLES The vacuole is the membrane-bound space found in the cytoplasm. It contains water, sap, excretory product and other materials not useful for the cell. The vacuole is bound by a single membrane called tonoplast. In plant cells the vacuoles can occupy up to 90 per cent of the volume of the cell. In plants, the tonoplast facilitates the transport of a number of ions and other materials against concentration gradients into the vacuole, hence their concentration is significantly higher in the vacuole than in the cytoplasm. In Amoeba the contractile vacuole is important for excretion. In many cells, as in protists, food vacuoles are formed by engulfing the food particles.
MITOCHONDRIA Kolliker (1880) first observed mitochondria as cytoplasmic granules in striped muscles of insects. Altman (1894) established them as cell organelles and called Bioblast. Flemming and Altman was credited for the discovery of mitochondria. Term 'Mitochondria, was given by C.Benda. Diameter 0.2 – 1.0 mm (average 0.5 mm) Length 1.0 – 4.1 mm Number 1000–1600 per cell. Number depends upon physiological activity of cell. One in Microasterias, Chlorella fusca (alga). 5 lakhs mitochondria in an Amoeba Chaos Chaos. All the mitochondria present in a cell are collectively called chondriome. Usually plant cells have fewer mitochondria as compared to animal cell. In higher animals maximum mitochondria are found in flight muscles of birds.
Mitochondria Power house of cell or ATP-mill in cell Cell within cell Cell furanaces or storage batteries Most busy and active organelle in cell Semi autonomous cell organelle.
16
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Mitochondria can make its shape sausage or cylindrical.
E
Biology STRUCTURE : Mitochondria unless specifically stained are not easily visible under the microscope. Mitochondria is covered by double unit membrane. Outer membrane has more phospholipids (Phosphatidyl choline) and cholesterol as compared to inner membrane. Phospholipid in inner membrane is mainly diphosphatidyl glycerol and Inner membrane have more protein. The outer membrane and the inner membrane dividing its lumen distinctly into two aqueous compartments, i.e., the outer compartment and the inner compartment. The inner compartment is called the matrix. The outer membrane forms the continuous limiting boundary of the organelle. The two membranes have their own specific enzymes associated with the mitochondrial function. Each membrane is 60-75 A0 thick and separated by a space (80-100A0 ) called perimitochondrial space. This space have enzymes required for oxidation of fats. If outer membrane of mitochondria is removed then it is called as mitoplast. Outer surface of inner membrane is called C- face while inner surface called M- face. Some sessile particles attached to outer membrane are known as "subunit of parsons". Inner membrane is folded into a number of finger like cristae. In metabolically active mitochondria number of cristae is higher. Many electron carrier cytochromes are arranged in a definite sequence in Inner membrane of mitochondria, which forms Electron transport system (ETS). Inner membrane is studded with pin head particles called oxysomes or elementary particles or F1 – F0 particles ( 104 to 106 in number).These particles first described by Fernandez Moran.
by energy of oxidation)
F1 Subunit stalk F0 Subunit stalk Inner membrane
Oxidative phosphorylation (formation of ATP
Matrix
Head of Oxysomes or F1 is concerned with
Mitochondrial matrix have enzyme for kreb's cycle (Aerobic respiration). Beside these enzymes matrix have a complete protein synthesis apparatus (Ribosome- 70-s, DNA, few
Outer compartment
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RNA's & enzymes) so mitochondria called as semi
E
autonomous cell organelles. Single, double stranded and circular naked DNA present in mitochondrial matrix. Mitochondrial DNA is 1% of total DNA in a cell. It is rich in G-C content. Mitochondrial DNA can code the synthesis of some types of proteins. Rest of the proteins and enzymes of mitochondria are synthesized under the control of nuclear genes (Semi autonomous nature). Enzymes for replication and transcription of DNA like DNA- polymerase and RNA- polymerase are found in mitochondrial matrix. Mitochondria of mammals have 55s ribosomes (35s,25s sub units) 17
Pre-Medical FUNCTIONS OF MITOCHONDRIA : (1)
Mitochondria are site of aerobic respiration.Most of the oxidative metabolism and ATP production occurs in mitochondria, thus mitochondria are the power house of cell, where organic compounds are broken down to release & store metabolic energy in the the form of ATP molecules. (Resp. metabolism). Outer membrane
Inner membrane Matrix
Inter-membrane space Crista
Structure of mitochondrion (Longitudinal section)
F1particle
Circular mt. Inner mitochondria DNA (G.C. Rich)
Cristae
Outer mitochondrial Ribosome (70s)
Matrix (inner chamber)
Perimitochondrial space (outer chamber)
(2)
Mitochondria help in vitellogenesis in oocytes, Mitochondria of oocytes called Yolk nuclei.
(3)
In cytoplasmic inheritance. Biogenesis of mitochondria – (1) Mitochondria divide by Fission
*
18
(2) Endosymbiotic origin from Purple Sulphur bacteria or prokaryotic cells, because mitochondria are similar to prokaryotic cell (rickettsial bacteria) in – (i)
Structure of DNA and DNA sequences.
(ii)
Type of ribosome (70s).
(iii)
Sensitivity of Antibiotic chloremphenicol.
(iv)
Divided by fission.
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Diagrammatic view of the internal structure of a mitochondrion/chondriosome
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Biology PLASTIDS Plastids are found in all plant cells and in euglenoides. These are easily observed under the microscope as they are large. They bear some specific pigments, thus imparting specific colours to the plants. Based on the presence or absence type of pigments plastids can be classified into chloroplasts, chromoplasts and leucoplasts. TYPES OF PLASTIDS (1)
Chromoplasts :– In chromoplasts fat soluble carotenoid pigments like carotene, xanthophyllas and others are present. This gives yellow, orange or red colour to the part of the plant. Chlorophylls either absent or occur in very less amount. Chromoplasts occurs mainly in pericarp and petals. Red colour of chillies and red tomatoes is due to the red pigment "Lycopene" of chromoplasts,. Chromoplasts also occurs in petals but colour of petals are mainly due to water soluble pigments occur in cell sap. e.g. Anthocyanin - (Blue or violet or red pigment) Anthochlor (yellow pigment).
(2)
Chloroplasts :– The chloroplasts contain chlorophyll and carotenoid pigments which are responsible for trapping light energy essential for photosynthesis.
(3)
Leucoplasts :– The leucoplasts are the colourless plastids of varied shapes and sizes with stored nutrients: Amyloplasts store carbohydrates (starch), e.g., potato; elaioplasts store oils and fats whereas the aleuroplasts store proteins. Pigments and lamellar structure absents in Leucoplasts. Generally occurs in non green and underground plant cells. All types of plastids have common origin from proplastids, sac like non-lamellar structures. Different types of plastids may transform from one form to another. Because genetic meteral is similar. But chromoplasts never transform to chloroplasts.
Aleuroplast
Chloroplast
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Amyloplast
E
Chromoplast
Elaioplast Interconversion of plastids
Number, Shape & Size of chloroplasts : Majority of the chloroplasts of the green plants are found in the mesophyll cells of the leaves. Number varies from 1 per cell of the Chlamydomonas a green alga to 20-40 per cell in the mesophyll. These are lens-shaped, oval, spherical, discoid, or even ribbon shaped. Length and width are also variable. Length = 5-10 mm Width = 2-4 mm 19
Pre-Medical STRUCTURE OF CHLOROPLAST Membrane : Like mitochondria the chloroplast are also double membrane bound. Out of the two, the inner membrane is relative less permeble. (Outer membane contain porins) The space limited by the inner membrane is called the stroma (matrix) ds-Circular DNA Outer membrane Inner membrane
Quantasomes (230 pig. mol.)
Stroma or matrix
Granum
Granum thylakoid
Loculus (Lumen of thylakoid)
Plastoglobuli (Fat droplets)
888
70s Ribosomes Rubisco (Most abundant enz.) RUBP / carboxy dismutase
Fret channel or stroma thylakoid
Chloroplast
Outer membrane Inner membrane Granum Thylakoid Stroma lamella Stroma Sectional view of chloroplast Component of stroma : (a)
Thylakoids : In the stroma a number of organised flatted membranous sacs are present called thylakoids.
thylakoids. Each chloroplast contains about 20-100 granum. Stroma lamellae are flat membranous tubules (Fret channel or Stroma thylakoids) connecting the thylakoids of the different granum. The membrane of the thylokoids enclose a space called a lumen. Chlorophyll (photosynthtic pigments) are present in the thylakoids membrane. A photosynthesis functional unit (Located in thylakoids membrane) contains of about 250 to 400 molecules of various pigments (Chl-a, Chl-b, Carotenes, Xanthophylls etc.) is called as Quantasomes.
20
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Thylakoids are arranged in stacks like the piles of coins called grana (singular : granum) or the intragranal
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Biology (b)
Enzymes : The stroma of the chloroplast contains enzymes required for the synthesis of carbohydrate e.g. starch grains (i.e. enzymes of Calvin cycle or Dark reaction) and protein synthesis. The important enzyme of calvin cycle is Rubisco. Rubisco is the most abundant enzyme on the earth. Rubisco forms 16% proteins of the chloroplast.
(c)
DNA : Stroma contain small double-stranded circular DNA molecules. Chloroplast have more genes as compared to mitochondria (100 or more genes)
(d)
Ribosome : The stroma of chloroplast contain 70s Ribosome. The Ribosome of the chloroplast are smaller (70s) than the cytoplasmic ribosomes (80s)
Special points : Chloroplasts have thier own genetic system & complete protein synthesis machinary (ds - DNA, RNA, Ribosome, Enzymes, Amino acids) thus chloroplasts are also called as semi autonomous organelle of the cell.
FUNCTIONS (1)
Photosynthesis : The chloroplasts trap the light energy of sun and transform it into the chemical energy in the form glucose.
(2)
Balancing of O2 & CO2 in nature.
(3)
Chloroplasts impart in cytoplasmic inheritance.
(4)
Chloroplasts impart the pleasing greenary to the earth.
(5)
Chloroplasts store vitamin K, E, Rubisco protein and Fe etc.
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BIOGENESIS
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(1)
From Proplastid
(2)
From Division of pre-existing plastids.
ORIGIN : Endosymbiotic origin by a cyanobacterium
RIBOSOMES (ENGINE OF CELL) Ribosomes are the granular structures first observed under the electron microscope as dense particles by George Palade (1953). They are composed of ribonucleic acid (RNA) and proteins and are not surrounded by any membrane. Except mammalian RBC all living cells have ribosomes. (Both prokaryotes & Eukaryotes) Ribosomes are smallest cell organelles (150x250 A0 ) Ribosomes are organelle without membranes. Ribosomes are also called as ‘‘Organelle with in an organelle’’ & "Protein factory of cell". 21
Pre-Medical Types of Ribosomes :– (1)
Eukaryotic ribosomes :– 80 s - Occur in cytoplasm of eukaryotic cells.
(2)
Prokaryotic ribosomes :– 70 s - Occur in cytoplasm and associated with plasma membrane of prokaryotic cell. 70s ribosome also present in mitochondria and chloroplast of eukaryotes. (55 S ribosome present in mitochondria of mammals) S= Svedberg unit or Sedimentation rate. It indirectly is a measure of density and size. Each ribosome composed of two subunits i.e. larger and smaller subunits. 80s = 60s + 40s 70s = 50s + 30s Magnesium ion is essential for the binding the ribosome sub units. Mg+2 form ionic bond with phosphate groups of r- RNA of two subunits. Minimum 0.001 M Mg+2 concentration is required for structural formation of ribosomes. If Mg+2 concentration increased 10 times then ribosome dimer are formed. 80s +80s = 120s (Dimer) 70s + 70s = 100s (Dimer) Chemical Composition of Ribosomes : 70s
–
60% r- RNA + 40% proteins
80s
–
40% r-RNA + 60% proteins
60s
–
r-RNA 28s, 5.8s, 5s
40s
–
r-RNA 18s
50s
–
r-RNA 23s,5s
30s
–
r-RNA 16s
At the time of protein synthesis, several ribosomes become attached to m-RNA with the help of smaller subunits. This structure is called polyribosome or polysome or Ergosome. Ribosomes move along the m-RNA like beads on a string, during protein synthesis. Larger subunit contains peptidyl transferase enzyme (23S rRNA) which helps in the formation of peptide bond during protein synthesis. This is an example of Ribozyme.(Noller 1992) Two sites are found on larger sub units : A- site ® Acceptor site for t-RNA
(ii)
P-site ® site for growing polypeptide chain 33 proteins
21 proteins
Base
Platform Small subunit
40S Subunit
30S Subunit
300Å Length
30S Subunit
290Å Length
Head
Cleft
50S Subunit
60S Subunit 45 proteins
31 proteins Central protuberance
210 Å Width 70S Ribosome
Stalk
Valley
200-240 Å Width 80S Ribosome 70S and 80S Ribosome
Ridge
50s
40S
60S
Large subunit Subunits
Mg
80S
Ribosome
120S Mg Dimer
Two parts of Ribosome
mRNA
Polyribosomes
After synthesis on ribosomes, protein are transported in cytoplasm and organelles. The proper folding and transport of proteins is assisted by specific proteins called Chaperons.
22
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(i)
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Biology CYTOSKELETON An elaborate network of filamentous proteinaceous structures present in the cytoplasm is collectively referred to as the cytoskeleton. The cytoskeleton in a cell are involved in many functions such as mechanical support, motility, maintenance of the shape of the cell.
MICROTUBULES Microtubules are composed of contractile protien, Tubulin. In plants microtubules often found associated with cell wall. Probably these transport cell wall material from Golgi body to outside of cell. During cell division these microtubules form spindle fibers.
MICROFILAMENTS They are composed of contractile protien, Actin which concern with muscle contraction, Microtubules and microfilament are part of cytoskeleton-base of cell.
CILIA AND FLAGELLA Cilia (sing.: cilium) and flagella (sing.: flagellum) are hair-like outgrowths of the cell membrane. Cilia are small structures which work like oars, causing the movement of either the cell or the surrounding fluid. Flagella are comparatively longer and responsible for cell movement. The prokaryotic bacteria also possess flagella but these are structurally different from that of the eukaryotic flagella. Cilia & Flagella are mechanical, hair like cellular appendages and locomotory structure. Flagellar apparatus is consist of following Parts. (a)
Shaft or ciliary part : It is projecting hair like part of ciliary appartus. Cilium is composed of 11 microtubules. (9 doublet + 2 singlet) The electron microscopic study of a cilium or the flagellum show that they are covered with plasma membrane. Their core called the axoneme, possesses a number of microtubules running parallel to the long axis. The axoneme usually has nine doublets of radially arranged peripheral microtubules, and a pair of centrally located microtubules. Such an arrangement of axonemal microtubules is referred to as the 9+2 array. (9 double + 2 singlet) Arms of A tubules consist of an enzymatic protein dynien similar to myosin of muscle cells. Dynien have ability of hydrolysis of ATP & liberates energy for ciliary or flagellar movement.
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*
E
The central tubules are connected by bridges and is also enclosed by a central sheath, which is connected to one of the tubules of each peripheral doublets by radial spoke. Thus there are nine radial spokes. The peripheral doublets are also interconnected by linkers. Both the cilium & flgellum emerge from centriole-like structure called the basal bodies. Inner Inner
Bridge B
Flimmer
A
Dynein arms (AT Pase activity) Peripheral microtubules (doublets)
A B
Interdoublet bridge (B-A linker)
A (Naked/smooth) Whiplash
Tinsel
Central sheath
Central microtubule Radial spoke (Nexin) Types of flagella
Plasma membrane
Diagrammatic representation of internal structure of Cilia or Flagella
23
Pre-Medical (b)
Kinetosome or basal granule or Blepheroplast or Basal body : It is membraneless structure, lies immediately below the plasma membrane. Basal body exhibit cart wheel structure similar to centriole. (9 triplet fibriles connected to a central hub in basal body).
(c)
Rootlet or Rhizoplast : This is a conical bundle of protein fibers which arises from basal body to different directions. Rootlet have dark bands composed of ATPase. Types of Flagella : (1)
Whiplash – When the laterel hair like structures absent.
(2)
Tinsel – When the flagella bears lateral hairs like structure (flimmers)
Cilia and Flagella are simialr in structure but some differences may observed – Cilia
Flagella
1.
The cilia are small in size (5–10mm)
1.
Flagella are long (up to 150 mm)
2.
Number of cilia per cell is very large.
2.
Few in number
3.
Cilia beat in a coordinated manner
3.
Flagella beats independently
(sweeping or pendular move) 4.
(Non coordinated manner)
They take part in locomotion, attachment,
4.
Flagella involved only in locomotion
feeding and sensation.
CENTROSOME & CENTRIOLES Centrosome was discovered by Benden. Boveri named as centrosome. Centrosome is absent in almost all plant cell. Two centrioles (diplosome) located just outside the nucleus and lie at right angle (90°) to each other. Cytoplasm which surrounds centrioles called as " Centrosphere". Centrioles and centrosphere collectively called centrosome or Microcentrum. Each centriole is surrounded by amorphous peri centriolar mass, which is called as massules or crown or satellite. Triplet microtubule
C-Tuble B-Tuble
A-Tuble Primary fibre (Radial spoke) Y-Thickening Secondary fibre
Central hub
Centrioles : (A) A pair of centrioles (Diplosome), (B) T.S. of a centriole
Centrioles are membraneless elongated structure which exhibit cart wheel structure (Just like Basal body of cilia). Basal body is also a type of centriole. Centriole mainly consist of 9 peripheral triplet fibers of tubulin. (9 + 0 arrangement) Centrioles are self duplicating units. Replication of centriole occur in s-phase. Function :– In animal cells centrioles play important role in initiation of cell division by arranging spindle fibres between two poles of cell. The location of centrioles during cell division decides the plane of division. The plane of division is always at right angle to the spindle. Thus centrioles is also termed as "cell centers". 24
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X-Thickening
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Biology MICRO–BODIES The cells of protozoa, fungi, plants, liver and kidney cells contain certain membrane bounded spherical bodies of 0.3 to 1.5 m diameter, filled with enzymes are called as ‘‘Micro–Bodies’’. On the basis of functions microbodies are of following types – (1)
Sphaerosomes :– Sphaerosomes occur only in plant cells. They are major site of lipid storage and synthesis in plants. Sphaerosomes also have lysosome like activity so they also termed as plant lysosomes.
(2)
Electron micrograph of a sphaerosome of groundnut
Peroxisomes or Uricosomes :– In animal cells peroxisomes concerned with peroxide (H2O2)metabolism. Catalase degrade the H2O2into water and oxygen.
Nucleus Peroxisome Matrix
Chloroplast
In plants, peroxisomes occurs in cells of green tissues and concerned with photorespiration (glycolate pathway). Peroxisomes involved in b-oxidation of fatty acids. (3)
Mitochondrion
Electron micrograph of a peroxisome (P) in the leaf of phleum prateuse. (A) Mitochondrion, (B) Chloroplast, (C) Nucleus
Glyoxysomes :– Occurs in oil containing seeds, yeast cells, guard cells etc. Glyoxysomes occurs only in plants especially in fatty seeds (castor seed, ground nut seed etc.), guard cells of stomata and unripe fruits. Glyoxysomes are considered as a highly specialised peroxisomes. Glyoxylate acid cycle takes place in glyoxysomes. This cycle convert fats into carbohydrats.
NUCLEUS INTRODUCTION : Nucleus as a cell organelle was first described by Robert Brown as early as 1831. Later the material of the nucleus stained by the basic dyes (Acetocarmine) was given the name chromatin by Flemming.
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"Nucleus is double membrane bound dense protoplasmic body, which controls all cellular metabolism and
E
encloses the genetic information of cell". Nucleus is consider as controller or director of cell. Importance of nucleus in control of heredity, growth and metabolism was experimentally proved by Hammerling. (Experiment was on Acetabularia a single cell largest alga). If the nucleus of a cell is, experimentally removed, then unicellular organism will die after some time. Thus nucleus is very important. Strasburgar stated that :- "Nucleus arises from divison of pre-existing nucleus only. The study of nucleus is known as Karyology. Generally eukaryotic cell contain at least one nucleus but nucleus is absents in mature phloem sieve tube elements and mature RBCs of mammals. (exceptionaly nucleus is present in RBCs of camel & lamma). Dikaryotic (Paramoecium) and multikaryotic cells are also known.
25
Pre-Medical STRUCTURE OF INTERPHASE NUCLEUS : Interphase nucleus : Nucleus of cell when it is not dividing. (i)
Nuclear membrane or nuclear envelope or karyotheca.
(ii)
Nuclear matrix / Nucleoplasm/Karyolymph/Karyoplasm.
(iii)
Chromatin net
(iv)
Nucleolus/little nucleus/Ribosome factory
(i)
Nuclear membrane :- Electron microscopy has revealed
Karyotheca Nucleolus Pore-complex
that the nuclear envelope, which consists of two parallel membranes with a space between (10 to 50 nm) called the perinuclear space. These membrane forms a barrier between the materials present inside the nucleus and that of the cytoplasm.
Heterochromatin False nucleolus Perinuclear space
Nucleus
The outer membrane usually remains continuous with the endoplasmic reticulum and also bears ribosomes on it. At a number of places the nuclear envelope is interrupted by minute pores, which are formed by the fusion of its two membranes.
(ii)
Nucleoplasm or Karyolymph :Nucleoplasm or Nuclear sap is a ground substance of nucleus which is a complex colloidal formed of a number of chemicals like nucleotides, nucleosides, ATPs, proteins & enzymes of RNA & DNA polymerases, endonucleases, minerals, (Ca++, Mg++) etc. Nucleoplasm contain high concentration of Nucleotides in the form of triphosphate. (ATP, GTP, TTP, CTP, UTP) Chromatin net and nucleolus are embeded in nucleoplasm. Nucleoplasm provides site for process of transcription.
(iii)
Chromatin net :- (Term given by Flemming) Interphase nucles has a loose and indistingt network of nucleoprotein fibers called chromatin, which embeded in nucleoplasm. Chromatin net is mainly formed of DNA and histone protein complexes. Chromatin fibres contain genetic information and condensed to form constant number of chromosomes during cell division. During different stages of cell division cells show structured chromosomes in place of nucleus. Chemically chromatin consists of DNA (31%), RNA (2-5%), Histone protein (basic proteins) (36%) and non histone proteins (Acidic proteins) (28%). 20 to 30% part of histone is made up of arginine and lysine amino acids. On the basis of relative amount of arginine and lysin there are five type of Histone protein. (H2A, H2B, H3, H4, H1)
26
Amino acid
Type of histone
Lysin rich
H1
Slightly lysin rich
H2A, H2B
Arginine rich
H3, H4
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These nuclear pores are the passages through which movement of RNA and protein molecules takes place in both directions between the nucleus and the cytoplasm. The size of nuclear pores is, 300 to 1000Ă… diameter. Each nuclear pore is guarded by a octagonal discoid structure of nucleoplasmin protein this structure is called as annulus or Bleb. (Annulus + Pore = Nuclear Pore complex). The inner side of inner nuclear membrane is lined by nuclear lamina. This structure is formed by filaments of lamin protein. Pore complex provides the main channel, between nucleoplsm and cytoplam, while nucleoplasmin regulates nucleocytoplasmic traffic.
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Biology Chromatin net has two type of chromatins. (a)
Euchromatin :- This is lightly stained and diffused part of chromatin. Which is transcriptionally or genetically more active. Generally euchromatin lies at central part of nucleus.
(b)
Heterochromatin :- This is dark stained, thick and condensed part of chromatin this part has more histone and less acidic protein. Heterochromatin is genetically less active chromatin. Heterochromatin occurs near nuclear membrane.
(i)
Constitutive heterochromatin:- Occurs in all cells in all stages e.g. centromeric region.
(ii)
Facultative heterochromatin :- Occurs in some cells in some stages e.g. barr body. Barr body in female cells is a facultative heterochromatic structure. (By M.Barr) Number of Barr body in nucleus of an individual is number of X-chromosome minus one.
Difference between Euchromatin and Heterochromatin. Euchromatin (i)
Consist of thin, extended, light
Heterochromatin (i)
stained part of chromatin.
Consist of thick coild, condensed part of Chromatin and dark stained.
(ii)
Genetically more active chromatin
(ii)
Less active or inert chromatin.
(iv)
Less histone protein
(iv)
More histone protein
(v)
Replicate in early s phase
(v)
Replicate in late s phase
(iv)
Nucleolus :The nucleoplasm also contain nucleous. The nucleoli are spherical and membraneless structure so that the content of nucleous is continous with the rest of the nucleoplasm. It is a site for active ribosomal RNA synthesis. Nucleolus usually attached to chromatin (or chromosomes) at specific site called Nucleolar organiser region/NOR. Number of nucleolus in a nucleus is one. Onion cell has 4, and in oocytes of amphibian has 2000 nucleoli. Human cell has 5 nucleoli.
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Nucleolus disappears during prophase and reappears in telophase.
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Chemistry of nucleolus :Proteins
85%
RNA
10%
DNA
5%
Electron microscope has shown nucleolus to be made of following parts : (Ultrastructure of nucleolus) : (i)
Fibrillar region :- This is central fibrous part of nucleolus, which is consist of mainly rDNA and proteins. (Nucleonema)
(ii)
Granular region :- This is peripheral granular part of nucleolus which is consist of rRNA and proteins.
(iii)
Amorphous matrix or pars amorpha :- This is proteinaceous ground matrix, which contains both fibres and granules. 27
Pre-Medical FUNCTIONS OF NUCLEOLUS : Ribosome formation is the chief role of nucleolus, thus its called as Ribosme factory of cell, the proteins of ribosomes are synthesised in cytoplasm but it diffused in to nucleus and reach at nucleolus. Here r-RNA and ribosomal proteins are assembled to form ribosomes which move to cytoplasm through nuclear pores. Larger and more numerus nucleoli are present in cells actively carrying out protein synthesis. At the some places heterochromatin forms thickned dense granules which are known as karyosomes or chromocentre or false nucleoli.
FUNCTIONS OF NUCLEUS : (i)
Genetic information :- Nucleus contains genetic information in its chromatin. (store house of genetic material)
(ii)
Transmission of genetic information :- Nucleus takes part in transmission of genetical information from parent cell to daughter cell or the one generation to next.
(iii)
In cell-division :- Division of nucleus is pre-requisite to cell division.
(iv)
Control of metabolism :- Nucleus controls metabolism of cell by sending m-RNA in cytosol (Basically biomolecule DNA controls cellular activities through directing synthesis of enzyme).
(v)
Variations :- Variation develops due to change in genetic material of nucleus. (Evolutionary role).
CHROMOSOMES GENERAL INTRODUCTION : At the time of cell division the chromatin material get condensed to form chromosomes, thus chromosome is highly condensed form of the chromatin. Chromosomes are not visible during interphase stage. First of all, chromosomes was observed by Hofmeister (1818) and Karl Nageli in pollen mother cells (PMC) of Tradescantia. Strasburger (1875) described chromosome structure appeared in nucleus during cell division. (Credit of discovery of chromosomes goes to Strasburger)
Generally chromosomes are rod-shaped, elongated or dot like in shape with size of 0.5 to 32m (Trillium plant has longest chromosome) Chromosomes can be best studied at metaphase stage because size of chromosomes is the shortest during metaphase (Shape of chromosome is studied at Anaphase stage) Generally chromosomes in plants are larger than chromosomes of animals, but number of chromosome is high in animals as compared to plants. The number of chromosomes has no relation with any specific feature like size, complexity of organism.
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Term "Chromosome" was proposed by Waldeyer. (Term 'Chromatin, was suggested by Flemming)
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Biology CHROMOSOME NUMBER IN SOME ORGANISMS : Plants
2n
n
Mucor hemelis (Fungi)
2
1
Haplopappus gracilis (Family compositae)
4
2
4
2
Pisum sativum (Pea)
14
7
Maize (Zea mays)
20
10
Wheat (Triticum)
42
21
1260
630
2n
n
Ascaris megalocephala (Round worm)
2
1
Drosophila melanogaster (Fruit fly)
8
4
Chimpanzee/Gorilla
48
24
Homo sapiens
46
23
1600
800
& Brachycome plant Takakia (Bryophyta)
Ophioglossum reticulatum (Pteridophyta) Animals
Aulocantha (a protozoan)
2n = number of chromosome in diploid cell. n = number of chromosome in haploid cell. The number of chromosome is definate for each species. For example every normal human being has 46 chromosomes in each body cell. Gametes of all organisms contain only one of each chromosome. The number of chromosomes in a gamete is called "Genome" or haploid chromosome. (Human 23) ‘‘A complete set (n) of chromosomes (all genes) inherited as a unit from one parent is known as genome,,. A single human cell has approximately 2.2 meter long thread of DNA distributed among its fortly six (23 pairs) chromosomes.
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TYPES OF CHROMOSOMES ON THE BASIS OF POSITION OF CENTROMERE
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Telocentric :- When centromere is terminal or located at the tip of chromosome.
(ii)
Acrocentric :- When the centromere is sub-terminal or located near the tip.
(iii)
Metacentric :- When the centromere is located at mid of the chromosome.
(iv)
Sub metacentric :- When the centromere located near centre or mid point of chromosome.
The ratio of length of the long arm to the short arm of a chromosome is called arm ratio. Arm ratio is maximum in acrocenteric chromosome.
Telocentric Acrocentric
Sub-metacentric Metacentric
Single chromatid diagrams 29
Pre-Medical Satellite Secondary constriction Short arm
Centromere
Centromere
Long arm Types of chromosomes based on the position of centromere
Karyotype Þ Karyotype is external morphology of all Chromosomes of a cell which is specific for each species of living organisms. Karyotype can be studied in metaphase of mitosis. Karyotype includes the number of chromosomes, relative size, position of centromere, length of the arms, secondary constrictions and banding patterns. Idiogram :– Diagrammatic representation of Karyotype. In idiogram chromosomes are arranged in decreasing order of size. Sex chromosomes are placed in last but in idiogram of Drosophila sex chromosomes are placed first. Idiogram is specific for every species. USE OF KARYOTYPING OR IDIOGRAM (i) It suggests primitive or advanced features of an organism. If karyotype shows a large size difference between the smallest and the largest chromosome of the set and having fewer metacentric chromosomes then it is called asymmetric karyotype, which is a relatively advance feature. Symmetric karyotype is primitive feature. (ii) The karyotype of different species are compared and similarities in them represent the evolutionary relationships. (iii) Karyotype is helpful in detection of chromosomal abberrations and polyploidy. (iv) In research of medical genetics Forensic science cytogenetics and Anthropogenetics.
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Pellicle – This is outermost, thin proteinaceous covering or sheath of chromosome. Matrix – This is a liquid nongenetic achromatic ground substance of chromosome, which has different type of enzymes, minerals, water, proteins. Chromatid – At metaphase stage each chromosome is consist of two cylindrical structures - called chromatids. Both sister chromatids are joined together by a common centromere. A chromosome, may have single chromatid (in Anaphase or Telophase) or two chromatid. (as in prophase metaphase) Each chromatid is consist of a single long thread of DNA associated with histone. Non histone proteins and RNA are also present. Sometimes bead like structure are seen on chromatid , which are called as chromomeres.
Chromosome with kinetochore
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1.
kinetochore
STRUCTURE OF CHROMOSOME (Parts which appears in metaphase chromosome)
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Biology 4. *
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Centromere/Kinetochore :- (Primary constriction) Each chromosome (at metaphase) is consist of two chromatids. Both the chromatids of a chromosome are joined or connected by a structure called Centromere. At this point or centromere two protein discs are present which is called Kinetochore. Kinetochores constitute the actual site of attachement of spindles to chromosomes during cell division. At the region of centromere the chromosome is comparatively narrower than remaining part of chromosome, thus it is termed as Primary constriction. Secondary constriction : Besides primary constrictions, other constriction may also occurs on some chromosome, which are known as secondary constriction. These constriction are non staining and found at a constant location. Secondary constriction-I is also known as NOR (Nucleolar organizer region)(13,14,15,21,22 chromosomes in human) Secondary constriction-II is found in the chromosome number 1, 10, 13, 17 & Y chromosomes of human. Primary constriction (Centromere)
Chromonema
Kinetochore (Protein dics)
Matrix Pellicle
Chromomere
SAT
Secondary constriction-I NOR (r-RNA synthesis)
Second constriction II Telomeric DNA (Synth. by Telomerase or RNP) Telomere
Satellite (Trabent) A schematic diagrammatic representation of chromosome
6.
Satellite : part of chromosome remains after the NOR is known as chromosomes satellite/ Trabent.
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Chromosomes with satellite part are called as SAT chromosome (SAT = Sine Acid Thymonucleinico)
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Telomere : Chromosomes have polarity and polar ends of chromosomes are known as Telomeres.
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Telomere prevents fusion of one chromosomes to other chromosome. Telomere rich in Guanine base. (5' -TTAGGG-3')
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Enzyme Telomerase presents in telomere part of chromosome, which is a Ribonucleoprotein. According to Richard kathan (2003) telomeres of chromosomes becomes shorter during ageing process.
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DNA Helix
H1 H2B H2A
H2A H4
H3
11 nm
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ULTRA STRUCTURE OR FINE STRUCTURE OF THE CHROMOSOME
H4 H3
DNA Linker 5.5 nm
Human metaphase chromosome
Schematic diagram of a nucleosome
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Pre-Medical PACKAGING OF DNA HELIX *
Taken the distance between two consecutive base pairs as 0.34 nm (0.34×10–9 m), if the length of DNA double helix in a typical mammalian cell is calculated (simply by multiplying the total number of bp with distance between two consecutive bp, that is, 6.6 ×109 bp ×0.34 ×10-9m/bp), it comes out to be approximately 2.2 metres. A length that is far greater than the dimension of a typical nucleus (approximately 10–6 m).
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DNA
H1 histone
Histone octamer
Core of histone molecules Nucleosome
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In eukaryotes, this organisation is much more complex. There is a set of positively charged, basic proteins called histones. A protein acquires charge depending upon the abundance of amino acids residues with charged side chains. Histones are rich in the basic amino acid residues lysines and arginines. Both the amino acid residues carry positive charges in their side chains. Histones are organised to form a unit of eight molecules called as histone octamer [(H2A, H2B, H3 , H4) ×2]. The negatively charged DNA is wrapped around the positively charged histone octamer to form a structure called nucleosome. A typical nucleosome contains 200 bp of DNA helix. Nucleosomes constitute the repeating unit of a structure in nucleus called chromatin, thread-like stained (coloured) bodies seen in nucleus. The nucleosomes in chromatin are seen as ‘beads-on-string’ structure when viewed under electron microscope (EM). The beads-on-string structure in chromatin is packaged to form chromatin fibers that are further coiled and condensed at metaphase stage of cell division to form chromosomes. Nucleosome = Binding DNA + Octamer core (H2A, H2B, H3, H4 ×2) + Linker DNA + H1 Histone 6 Nuclesome units united (or super coiling) to forms Solenoid structure. H1 histone protein (sealing histone) joined the turns of binding DNA in nucleosome.
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Nucleosome unit have 1.75 or 1
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3 turns of binding DNA. 4
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