A cell is a structure made up of several small units called organelles, chemicals, molecules and liquids. These all constituents are present in it and work in a way to complete a particular function. The cell shape, size and the constituents vary as per its position and function. Cell defines an organism’s existence and its functions. Hence it is called the structural and functional unit of all the living organisms. Lowey and Sikewitz defined a cell as “the unit of biological activity container and nucleus that has the ability to reproduce or divide in a medium free from other living organisms.
Cytology ⇒ The science dedicated to the Study of Cell Structure.
Cell biology ⇒ The Study of all the structures and functions along with the reproduction of
History of Cell Study
Various scientist all over the world discovered the cell along with its structure and functions. The major highlights of the work as per the scientist is as follows:
Robert Hook (1665) was an English botanist, first to name the term “Cell” in his book “Micrographia”. He observed thin sections of cork (dead cells) under a compound microscope and found compartment like divisions in a honeycomb. Cells is a Latin word cellula which means a hollow space. The Greek word is cella = Small hollow space or Chamber. A. van Leeuwenhoek (1674) called as Animalculist as he was first to study a living cell (bacteria, RBC) and called them as “Animalcule”.
N. Grew (1682) proposed a concept for the cell. It explains how a cell is a unit of structure for several organisms. Robert Brown (1831) observed and reported the presence of a nucleus in the root cells of orchids. Dujardin (1836) observed a semifluid substance surrounding the nucleus in muscle cells and called it as sarcode. A cell has a nucleus which has a living substance around it called as protoplasm. Purkinje (1839) found it in an animal cell while Von Mohl in a plant cell. Huxley called Protoplasm as the physical basis of life. Hammerling called nucleus as the brain of the cell or master or controlling center of a cell.
The scientists who formulated and laid the cell theory are M. J. Schleiden (1838) – German Botanist and T. Schwann (1839) – British Zoologist. Matthias Schleiden studied and tested a large number of plants and observed that all plants are composed of different kinds of cells which form the tissues of the plant.Schwann studied several animal cells and marked a conclusion that all cells have a thin outer layer as their limit or boundary which is presently known as the ‘plasma membrane’. He also studied plant tissues and concluded that the cell wall is a unique character found in all the plant cells. Schwann proposed the hypothesis from all his study that the cell, its products and its constituents form the bodies of animals and plants.
Schleiden and Schwann together formulated the cell theory. This theory, however, was incomplete as it failed to explain the process of new cells formation. Rudolf Virchow (1855) first explained that the cells are divided so as to form new cells from pre-existing cells (Omnis cellula-e cellula). He modified Schleiden and Schwann theory and completed the cell theory.
Conclusions of the theory:
All living organisms are made from cells. The cell thus is the unit of structure and function of cells. All cells have a similar basic structure and function. Each cell has a membrane which separates it from the surrounding. An organism’s function is a result of cellular activities and interactions.
Exceptions of cell theory – As all theories have few left outs even this one has:
Viruses are not included in the theory because viruses lack typical cell properties and organization. Bacteria and cyanobacteria lack an organized nucleus. There are cells that lack nuclei like RBC and sieve tubes. According to modern scientists, organisms that lack cellular basis like Monera and Protista, Xanthophytes (Vaucheria) Phycomycetes (Rhizopus) are the exceptions of cell theory.
Diversity among the Cells
Cells show diversity in size, shape and their activities. Following are a few examples of cell sizes variations:
Longest plant cell
Largest isolated single cell
Fibers of Ramie (Boehmeria nivea)
Mycoplasma gallicepticum– 0.3 mm
Cells vary greatly in their shape as well. The cell can have shapes that are disc-like, polygonal, columnar cuboid, thread-like or even irregular. The shape of the cell changes as per the function they are destined to perform.
The insight of a cell
The cell has an exhaustive different world in it which has certain internal compartments called the organelles limited by a membrane such as mitochondria, Golgi bodies, plastids, lysosomes, nucleus, etc. These membrane-bound organelles help the cell to maintain the separation among all the different chemical reactions occurring within the cell all the time. There are organelles which lack membrane around it like centrioles and ribosomes. The cells of bacteria and blue-green algae (prokaryotes) do not have membrane-bound organelles (one envelope system) hence, no compartments. Eukaryotic cells have membrane-bound organelles.
Type of Cells
There are differences among the cells as some have organelles while others lack them. They are prokaryotic cells and eukaryotic cells. This division is made on basis of the differences among major features: Cell organelles (Compartmentalisation), Cytoskeletal structures and Organisation of nuclear material.
Prokaryotic Cells – Bacterium
Antony Von Leeuwenhoek was first to discover bacteria from the teeth scum and stored rainwater which was called as wild animalcules. He called them Dierkens. Later, Ehrenberg named it as Bacteria. Se’Dillot called animalcules as microbes.
Prokaryotes are included in Kingdom Monera and thus are commonly called as Monerans. The common ones are bacteria, cyanobacteria (Blue-green algae), mycoplasma or PPLO (Pleuro-pneumonia like organisms), Spirochaetes and Rickettsiae. Bacteria is the simplest and commonest type of organisms occurring all over in almost all habitats. The habitat is diverse and varied, even found in the hot springs, beneath the icebergs, in the intestine of man, deep in the soil, deep in seawater, etc.
Bacteria have a range of cell sizes. The smallest bacterium is Dialister (0.15 to0.3 µm in diameter). The largest bacterium is Spirochaetes (about 500 µm). Normally the size of Bacillus lies from 0.3 µm to 15 µm.
J. Cohn studied bacteria and identified the following four basic shapes of bacteria:
Coccus: Spherical shaped bacteria. The cells can be Monococcus (single rounded) or diplococcus (two rounded) or Tetracoccus (four rounded) or Streptococcus (chain of cocci) or staphylococcus (a bunch of cocci) or Sarcina (eight-celled cubical mass). Bacillus: Rod shaped bacteria. The bacteria cells may be Monobacillus or Diplobacillus or Streptobacillus or Palisade (bacteria lying parallel to each other). Spirillum: Spirally coiled and flagellated. Vibrio:Comma shaped and flagellated.
Basic Structure of a Bacterial Cell wall
The outermost covering of the cell which provides shape and rigidity. It also protects the cell from major mechanical injuries and bursting or collapsing completely. Electron microscope studies revealed that the cell envelop has three basic layers. Each layer has its own composition and is specialized to carry out specific functions.
(i) Glycocalyx: It is the outermost layer. It has its chemical composition and thickness differing in different bacteria. In most of the bacteria e.g., Escherichia coli, the layer is in the form of a loose mucilaginous covering called as the slimy layer. It protects the bacterium against desiccation, action of phagocytes and helps in adhesion. However, some bacteria have a hard and tough covering which is called as a capsule. It resists phagocytosis and also incorporates virulence to them.
(ii) Cell wall: Middle layer below Glycocalyx is rigid, protective and supportive in function. E. coli and other Gram-negative bacteria have two layered cell wall: the inner layer made up of murein or peptidoglycan which consists of polysaccharides (like acetyl glucosamine – NAG and acetyl muramic acid – NAM) and a tetrapeptide. The outer layer consists of glycolipids. Gram-positive bacteria have a single-layered cell wall which is made up mainly of murein. The difference of cell wall composition divides bacteria into two categories: Gram-positive and Gram-negative bacteria.
(iii) Plasma membrane: The innermost layer is made up of the cell envelope. It is a thin, transparent layer which is a semipermeable membrane. It has lipoproteins and trilaminar layer (3-layers) similar to eukaryotes. It has components and enzymes that are involved in respiration and thus the layer is called as the respiratory membrane. It regulates the movement through the membrane of specific materials between the cytoplasm and extracellular medium. The membrane has certain receptor molecules that detect and respond to the chemicals helping the bacteria to survive.
Mesosomes and Chromatophores
Mesosomes: Characteristic features of prokaryotes. The plasma membrane infolds itself in only Gram-positive and may be in the form of vesicles or tubules or lamellae. The membrane is helpful to bacteria in. In DNA replication and its separation during cell division into daughter cells; increase the surface area for respiration; in cellular secretion which performs like the Golgi body in eukaryotes and in cell wall construction.
Chromatophores: Internal membrane systems that help in increasing the surface area for efficient enzymatic activity and metabolic rate. In cyanobacterial cells, the chromatophores have pigments for photosynthesis.
The bacterial cell surface show one or more thread-like structures extending outwards from the cell membrane which are called as flagella. Each flagellum is made up of single strand i.e. myofibrillar. The strand consists of flagellin protein. Flagella have a shaft or basal body, hook and longest part a filament. The major function is locomotion of the bacteria.
Pili / Fimbriae
Certain bacteria e.g., Escherichia coli, also have minute hair-like, small and thin structures called fimbriae or long tubular structures called as pili. Pili is made from pilin protein. In male E. coli, these pili are called as sex pili as the pili help in the attachment to the female bacterium during sexual reproduction (conjugation). Fimbriae help in adhesion of the bacteria to the rocky substratum or to the host’s tissues.
The cells make up the protists, plants, animals and fungi. The structure has eukaryotic cellular organization different from the prokaryotic cellular organization with respect to several factors.
Biomembranes or Cell Membranes
Living cells have thin, delicate, elastic, selectively permeable and living boundary or cover called as the cell membrane (by Nageli and Cramer) or plasmalemma (By J. Q. Plower) or biomembrane or plasma membrane.
Structure of Biomembranes
Sandwich or Trilamellar model – proposed by Davson and Danielli (1935). The plasma membrane includes three layers: a bimolecular layer of lipid is sandwiched between two layers of proteins. Each protein layer is 20 Å while phospholipid bilayer is 35 Å. Thus the total thickness of the membrane is 75Å (PLLP- structure, 75-100 Å average).
A phospholipid molecule is also called as amphipathic molecule as it has two different parts: hydrophilic (polar head) and hydrophobic (nonpolar tail). Hydrophilic head binds with the protein layer through hydrogen and ionic bonds. Hydrophobic tail is attached with the Vander wall forces.
Unit membrane model – proposed by Robertson -1959. All the cellular and organelle membranes have similar structure and function (difference in chemical and size). All the above models fail to explain the cell wall Fluidity and selective permeability which is why they are not accepted all over.
Fluid mosaic model: Singer and Nicolson (1972). Chemical studies in human red blood cell membrane (RBCs), revealed the possible structure of the plasma membrane. The most widely accepted model as the structure of plasmalemma is well explained. Proteins are present in the phospholipid layer in the mosaic pattern. Thus, the membrane is termed as protein iceberg in a sea of phospholipid.
The fluid nature of the membrane is important as it helps in various functions like cell growth, the formation of intercellular junctions, secretion, endocytosis, cell division etc.
Most important functions are the transport of the molecules across it. The membrane is selectively permeable to molecules which the cell requires and are present on inner as well as outer sides of the membrane. Thus, it is called as the semipermeable membrane. There are molecules that can move across the membrane without any requirement of energy by the process called as the passive transport.
Neutral solutes travel across the membrane through simple diffusion dependent on the concentration gradient, i.e., from higher concentration to the lower. The process called diffusion. Water moves with the same process across this membrane (from higher to lower concentration). Movement of water is called as osmosis. Apart from these non–polar molecules, there are polar molecules as well which, fail to pass through the non-polar lipid bilayer, there are carrier proteins in the membrane to facilitate the transport across the membrane.
A few molecules move across the membrane against the concentration gradient, i.e., from lower to the higher concentration. The transport is facilitated with an energy-dependent process, where ATP is utilized. The process is called active transport, e.g., Na+/K+ Pump.
Robert Hook discovered Cell wall. The plant cell has outer most layer dead and permeable boundary called the cell wall. Plant cell wall consists of cellulose, hemicelluloses, pectins and proteins. Algae cell wall is made up of cellulose, galactans, mannans and minerals like calcium carbonate. Cellulose, microfibril and macrofibrils are arranged in layers so that they form the skeleton of a cell wall. There are pectin and hemicellulose in between these layers that form the matrix of the cell wall.
Cell wall substances (cellulose, hemicellulose, pectin, and lignin) are synthesized in the cell of plant Golgi bodies or dictyosomes. Lipids (cutin and suberin) are synthesized in the sphaerosome. Martinez and Paloma (1970) discovered the cell coat in animal cells, which is now called as Glycocalyx. [Made by sialic acid, mucin and hyaluronic acid (animal cement)].
Plasmodesmata – Strasburger proposed the name (1901). The cytoplasmic connections between the two adjacent plant cells. Plasmodesmata are the characteristic feature of multi-cellular plants and also maintain the continuity of cytoplasm among the adjacent cells. E.R. tubules (Desmotubules) helps in the formation of continuity.
Functions of cell wall
O Cell wall gives the cell its shape
O Protective against mechanical damage and infection
O Allows cell-to-cell interaction
O Provides barrier to undesirable macromolecules
The Metabolically active permanent and the living structures present in the cytoplasm are called as organelles.
The membranous organelles in a cell are different in its structure and function. Yet they are considered similar called as an endomembrane system as their functions are interrelated and coordinated. The endomembrane system in a cell has endoplasmic reticulum (ER), Golgi complex, lysosomes and vacuoles. However, the functions of mitochondria, chloroplast and peroxisomes are not in relation to the above components, they are not part of the endomembrane system.
Grainer discovered ER, however, the details were described by Porter, Claude and Fullam. It is absent in prokaryotes while present in eukaryotes. It includes the following parts:
OCisternae – Narrow, long, flattened, double layered and unbranched units which are arranged in stacks. They lie close to the nucleus, interconnected, have 40-50 µm.
OVesicles – Oval, scattered in the cytoplasm, are membrane-bound structures with 25-500 µm.
OTubules – Irregular, tubular, membrane-bounded, present near the cell membrane. Tubules may be free or in association with cisternae.
ER is termed as “System of Membranes” and attached with nuclear membrane and plasma membrane. ER divides the intracellular space into the two distinct compartments namely luminal (inside ER) and extraluminal (outside ER in the Cytoplasm) compartments. This division is essential for cellular life which ensures proper functioning of it.
Microsomes – Fragmentation and high-speed centrifugation of the cell yields E.R. part that are associated ribosomal particles. Living cell otherwise does not has this parts. Scientists use microsome for the study of in vitro protein synthesis.
Functions of the ER are as follows:
O Protein and lipid synthesis.
OMechanical support ER along with microfilaments, microtubules are the endoskeleton of a cell.
OIntracellular exchange ER makes a conducting system inside the cell. Also transports materials from one place to another.
O ER is attached at some places to plasma membrane thus ER can secrete materials outside the cell.
O Smooth ER plays a role in the glycogen synthesis.
ODetoxification smooth ER concerned with detoxification of drugs and steroids.
O Cytochrome P450 present in ER function like an enzyme in detoxification of cell.
O Cellular metabolism ER membranes in a cytoplasm provide an increased surface for metabolic activities. Nuclear membrane development in telophase while the cell is dividing.
O Golgi-body and micro-bodies formation.
Camilla Golgi observed Golgi (1898) in the nerve cells of a barn owl. He called it as “internal reticular apparatus”. Golgi apparatus is also named as Golgi body / Golgi complex, Lipochondria (rich in lipids) and Idiosome (plant Golgi body) Number of Golgi body – absent in prokaryotes; several in eukaryotes, located near the nucleus. The cytoplasm around the Golgi body lacks any other organelles. It is called as Golgi ground substance or Zone of Exclusion. Golgi bodies are pleomorphic organelles as the components of Golgi body change in structure and shape in different cells.
Structure of Golgi Body is:
Cisternae – Unbranched, flat disc-like saccules. 4–8 saccules arranged in a stack like structure that are elongated two layered flat and curved in middle with swollen ends. The diameter is 0.5 µm to 1.0 µm. the dense opaque material inside the cisternae is called as Nodes. Cisternae has a convex surface facing towards the nucleus called as the cis face or forming face. Cisternae has a concave surface facing towards the cell membrane called as the trans face or maturing face. Cis and trans faces are entirely different but interconnected.
Tubules – Branched and irregular tubules that are associated with the cisternae.
Vesicles – Spherical structures from the tubules that have originated through budding. Vesicles have secretory materials.
Functions of Golgi bodies:
Packaging and Secretion of materials – Major function is secretion (export) of macromolecules post packaging. It involves:
O ER transports materials to the Golgi body through the cis face (Golgi apparatus is in close association with the ER).
O Chemically modified as glycoproteins and glycolipids.
O Materials are packed in the vesicles. Then the vesicles from the Trans face are pinched off, and then delivered either in the cell or secreted outside the cell.
O All the macromolecules that are secreted outside the cell, have to move through the Golgi body. So Golgi body is termed as “principal director of macromolecular traffic in cell” or middlemen of the cell.
Formation of Lysosome – Collective function of Golgi body and ER. Cell wall material synthesis (polysaccharide synthesis). Cell plate formation (Phragmoplast) in the new cell formation. Formation of acrosome during spermiogenesis in male gametes. Formation of the Vitelline membrane of the egg. Endocrine glands that secrete hormones are mediated through the Golgi bodies.
A spherical bag-like structures that has a single unit membrane. Lysosomes have a different type of digestive hydrolytic enzymes are termed as acid hydrolases (lipases, proteases, carbohydrases, nucleases). This acid hydrolyzes its function in acidic medium (pH 5). Lysosome membrane has an active H+ pump mechanism. This mechanism produces acidic pH in the lumen or stomach of the lysosome. Lysosomes have polymorphic structures.
OHeterophagy – Foreign materials that enter the cell are digested through a process called as phagocytosis and pinocytosis
OAutophagy – Old or dead cell organelles are digested in the cell. Autophagy also takes place during starvation of cell.
O Lysosomes of osteoclast called as bone-eating cells, dissolve the unwanted part of bones.
Autolysis – The cell has its life like all living organisms which are destined to death. All lysosomes of a cell sometimes burst such that the cell is dissolved completely. Old cells, unwanted organs of an embryo in the body die through autolysis. Cathepsin of lysosome dissolves the tadpole tail of frog during its metamorphosis. Thus, lysosomes are called as suicidal bags of cell.
OStabilizers are chemicals, which stabilize the membrane of the lysosome to stop its rupture. This process prevents Autolysis and cell death. E.g. cholesterol, chloroquine etc.
OLabilizers are chemicals which increase the fragile nature of lysosome membrane and increase the autolysis possibility, E.g. Progesterone, testosterone, Vitamin A, D, E, K, U, V. radiations, bile salts etc.
Biogenesis of lysosome – Lysosomes originates from GERL – (Golgi associated Endoplasmic Reticulum: the area for Lysosomes to arise). ER → Golgi body → Lysosome
Vacuoles are the single membrane-bound organelles called as tonoplast. They are absent in animal cells and in plants the meristematic cells lack it while permanent tissue has well-developed vacuoles. The vacuoles can increase in size of upto 90 percent volume of the cell in plants. The vacuole has a non-living fluid called as the Cell Sap. It can have few water-soluble pigments like Anthocyanin (blue or violet), Anthoclor (yellow) etc. Best known is β cyanin in beetroot cells. Water and excretory material storage are major functions. Amoeba has the contractile vacuole which is important for excretion. In many cells, as in protists, food vacuoles engulf the food particles which initiates their formation.
Mitochondria (Singular: Mitochondrion)
Mitochondria: Kolliker discovered it in the voluntary muscles. These are present in all eukaryotes and absent in mammalian RBC and prokaryotes. The shape is not constant and is variable, can be granular, fibrillary, spherical, cylindrical as sausage or discoidal. The size is dependent on the metabolic activeness of the cell. Diameter ranges from 0.2–1.0 µm (average 0.5 µm) and Length 1.0–4.1 µm. The number ranges from 1000–1600 per cell which is variable and dependent on the physiological activity of the cells.
Double membranous covering. The phospholipids and cholesterol are high in the outer membrane and low in the inner membrane. Protein content is high in the inner membrane and porins are present in the outer membrane for the exchange. The membranes have 60–75 Å thickness and are separated with 80-100 Å space called as the peri mitochondrial space (outer compartment). The space has a good amount of enzymes that are required for the oxidation of fats.
The outer membrane of mitochondria when removed, then the structure left is called as mitoplast. Inner membrane shows several folded finger-like structures facing inwards called as cristae. This cristae increases the surface area. Fungal cristae are plate-like while Euglenal cristae are vesicle shaped. There is intra–cistral space which is a continuous outer membrane. Inner membrane has studded pin head particles which are called as oxysomes or elementary particles or F1 – F0 particles or ATPase or ATP synthase. The main function is Oxidative phosphorylation in respiration which produces ATP. (104 to 106 in number per mitochondria). These particles were first described by Fernandez Moran (1962).
Space enclosed by the inner membrane is called as Matrix. Mitochondrial matrix is energy produces as they have all the enzymes essential for the Krebs cycle (Aerobic respiration). Also, the matrix has its own complete protein synthesis apparatus (70s Ribosome, DNA and RNA). Thus mitochondria are called as semi-autonomous cell organelles. Some proteins required by the mitochondria are self–synthesized, while others are synthesized from nuclear DNA. DNA is double-stranded circular and naked in the mitochondrial matrix.
Function of mitochondria:
ATP production in Aerobic respiration and in the Krebs cycle. Thus produce the heat required by the cell for its survival (thermogenesis).
Biogenesis of mitochondria – New cells arise from existing ones.
Endosymbiosis origin from Purple Sulphur bacteria or any prokaryotic cells, as the eukaryotic mitochondria are similar to the prokaryotic cell in ways
O Structure of DNA and DNA sequences.
O Type of ribosome (70s).
O Divide by amitosis or fission.
Plastids are called by this name by Haeckel. They are present in all the plant cells and Euglenoids.
Types of Plastids
The major basis is its presence and types of pigments in it.
Chromoplasts: They contain different fat-soluble pigment types (carotenes, Xanthophylls etc.). Chlorophylls are either absent or few present. Chromoplasts are mainly present in the pericarp and petals of flowers, fruits. E.g. Red colour of chillies and red tomatoes have red pigment “Lycopene” of chromoplasts. Lycopene is a pigment included in carotene. The yellowish orange colour of fruits is incorporated as they have α-carotene, β-carotene and γ-carotene. The richest source of β-carotene is carrot which is a precursor of vitamin-A.
Chloroplasts: Green coloured plastids that have chlorophyll and carotenoid pigments.
Leucoplasts (Colourless plastids): Food storing organelles in different forms. E.g. starch (Amyloplasts), fat and oil (Elaioplasts) and protein (Aleuroplasts). Pigments and lamellar structure is absent. Non-green plant cells contain it.
Different types of plastids can interchange their forms from one form to another as the genetic material in all the leucoplasts are similar. However, chromoplasts are never transformed to chloroplasts. E.g. Tomato, Chilly etc.
Structure of Chloroplast
Double membranous cell organelle. 20–40 chloroplast in mesophyll cell of higher plants. Chlamydomonas have one Chloroplast per cell. The outer membrane is more permeable than the inner membrane as it has porins. It contains stroma and grana (thylakoids or lamellae). Stroma is similar to cytoplasm part and it contains circular DNA, RNA, 70-s Ribosomes, and starch grains, enzymes of Calvin cycle or dark reaction of photosynthesis. Stroma also has enzymes to synthesize proteins and carbohydrates.
Thylakoids are membranous flattened sacs placed one above the other like stacks called as granum (Plural grana). 40–60 granum is present in chloroplasts. Fret channel or stromal thylakoids or stroma lamellae is the linking of the two grana. The photosynthetic functional unit, with 230 to 400 various pigment molecules is called as Quantasomes. Quantasomes or photosystem are present in the thylakoid membranes that bear Photosynthetic pigments (chlorophylls). Thylakoid membrane encloses a space called as Lumen.
Chloroplasts have their own genetic system along with the complete protein synthetic set (ds-DNA, RNA, Ribosomes, enzymes, Amino Acids). The chloroplasts are called as the semiautonomous organelle of the cell as Photosynthetic enzymes are synthesized on both the genes of the chloroplast and the nucleus.
Ribosomes (Engine of Cell)
Observed by George Palade (1953). All living cells have ribosomes both prokaryotes and eukaryotes however not in RBC. Smallest cell organelles without outer membranes. Also called as “organelle within an organelle” and “Protein factory of cell”. Ribosomes are different in eukaryotes and prokaryotes:
Eukaryotic ribosomes – 80S = freely floating in the cytoplasm or are attached to ER in eukaryotic cells.
Prokaryotic ribosomes – 70S (15nm x 20 nm) = free floating in the cytoplasm or remain associated with the plasma membrane in prokaryotes. Also found in mitochondria and Chloroplast of eukaryotes.
O S is Svedberg unit or Sedimentation rate: The measure of density and size of ribosomes.
O Each ribosome has two subunits: Larger and smaller subunits.
O 80S = 60S (larger) + 40S (smaller)
O 70S = 50S (larger) + 30S (smaller)
O Ribosomal subunits are bound together by Magnesium ion. 0.001 M Mg+2 concentration is essential for ribosomes to be formed and remain active. Mg+2 concentration increases by 10 times and ribosome dimers are formed.
O 80S+80S =120S (Dimer)
O 70S+70S=100S (Dimer)
O In prokaryotes, several ribosomes get attached to m-RNA during the protein synthesis called as polyribosome or polysome or Ergosome.
O There are fixed and free ribosomes with different protein synthesis: Secretory and lytic proteins – fixed ribosomes; non-secretory proteins – free ribosome.
The minute, fibrous tubules that form an elaborate network made of filamentous proteinaceous structures collectively called as the cytoskeleton. Its main functions are mechanical support, motility, maintenance of the cell shape.
Cilia and Flagella
Cilia (Sing – Cilium) and Flagella (Sing- Flagellum) are microscopic hair or threadlike outgrowths which are locomotory structures. These extend from inner cell membrane layer to outside the cell. Cilia are present in all protozoans, Flagellum or Cilium is covered with a protective sheath which is connected to the plasma membrane. The central part or core which is contractile is composed of 11 microtubules (9 doublets + 2 singlets) called as Axoneme.
Peripherally nine microtubules are present, composed of a pair of small tubules: A-tubule and B-tubule. Arms of A tubules have an enzymatic protein dynein (like myosin of muscle cells). Dynein hydrolyzes ATP such that energy is liberated for movement. The central tubules are bundled together which are enclosed in a central sheath. This sheath is connected to one of the tubules present in each peripheral doublets with the radial spoke. Nine radial spokes in all are present. The peripheral doublets are further interconnected by linkers. Both the cilium and flagellum in the cell membrane emerge from a centriole-like structure which is called the basal bodies.
Centrosome and Centrioles
Centrosome has a pair of centrioles that lie at a right angle (900) outside the nucleus to each other. Centrioles are surrounded by amorphous, protoplasmic plaques called pericentriolar materials or massules. Centriolesareelongatedmembranousstructure that show a cartwheel-like structure in transverse section. There are 9 microtubules on the periphery which is composed of three tubules namely A-tubule, B-tubule and C-tubule. The central part of the centriole is proteinaceous called as “Central Hub”. The arrangement is 9+0 as the centre does not have a tubule. Protein fibres called as primary fibres or spokes connect microtubules to the central hub. Secondary fibres connect microtubules with each other. Primary fibres are thick with layers called as X–thickening. Y-thickenings, lie between X-thickenings and both of them are interconnected. Centrioles are self-duplicating units without the DNA and covering. Centrioles replicate in S phase when the cilia and flagella basal bodies are formed.
Centrioles play an important role in cell division as they form spindle fibres that separate two poles in a nucleus. Centrioles are also termed as “cell centres”. Transformation is possible which give rise to the basal body of cilia and flagella. In a spermatozoan, the two centrioles give rise to axial filament or tail.
Robert Brown studied in detail the orchid root cells and named the nucleus in 1831. A nucleus is called as controller or director of the cell. It controls heredity, growth and metabolism in a cell as experimentally proved by Hammerling. (Experiment was on Acetabularia a single cell largest alga). The eukaryotic cell has at least one nucleus. However, it is absent in prokaryotes, mature phloem sieve tube elements and mature RBCs or erythrocytes of mammals.
Structure of the nucleus shows the presence of:
Nuclear membrane or nuclear envelope or karyotheca, Nucleoplasm / Karyoplasm / Karyolymph, Chromatin net, Nucleolus / Little nucleus / Ribosome factory.
Nuclear membrane: Two unit membranes cover the nucleus, thus it is a double membranous component of the cell. Space between two membranes of the nucleus is known as perinuclear space (10 to 50nm). The outer membrane of the nucleus is connected with ER at several places and ribosomes also found on it. Nuclear membrane has minute nuclear pores which are the result of the two membrane fusion. The nuclear pores have an octagonal discoid structure as the guard for them which is made of nucleoplasmin protein. This pore with protein structure is called as annulus or Bleb (Annulus + Pore = Nuclear Pore complex). Pore complex is the connection for nucleoplasm and cytoplasm, and nucleoplasmin is responsible for nucleocytoplasmic traffic (movement of RNA and proteins). The nuclear membrane is continuous with ER in the telophase of the cell division.
Nucleoplasm or Karyolymph: Nucleoplasm (Nuclear sap) is a ground substance or matrix of the nucleus which includes a complex colloidal form of many chemicals like nucleotides, RNA and DNA polymerase, endonucleases, minerals (Ca++, Mg++) etc. Chromatin net and nucleolus are a part of nucleoplasm.
Chromatin net (Term given by Flemming): These are intranuclear, long, threadlike thin fibres, embedded in the nucleoplasm. It is made up of DNA, histone protein, non-histone protein and RNA. Chromatin fibres condense in cell division to collect all the genetic information and form a fixed number of chromosomes. Chemically chromatin has DNA (31%), RNA (2%–5%), Histone protein (36%) and non-histone (28%). 20% to 30% histone includes arginine and lysine amino acids. The relative amount of arginine and lysine change in histones which is the basis for its classification into five types of Histone protein. (H2A, H2B, H3, H4, H1). Acetocarmine (basic dye) staining reveals two type of regions in chromatin net.
Euchromatin – Lightly stained and diffused part which is transcriptionally or genetically more active. Heterochromatin – Dark stained, thick and condensed part of chromatin, having more histone and less acidic protein. This part is genetically less active.
Nucleolus: Nucleolus is one per nucleus. A human cell has five nucleoli. The nucleolus is naked or without any membrane, round or slightly irregular part present in the nucleus. It is attached to chromatin (or chromosomes) at a specific site called as nucleolar organizer region (NOR). Nucleolus is called as the ribosome factory of the cell, as it has the proteins for ribosomes synthesis. r-RNA (synthesized by nucleolus) and ribosomal proteins are assembled in nucleolus to form ribosomes. Active cells for protein synthesis have larger and more numerous nucleoli. r-RNA and protein are synthesized in the cytoplasm for all prokaryotes.
The chromatin material gets condensed into chromosomes during a cell division, thus chromosome is a highly condensed form of the chromatin fibers. Chromosome number is different in different organisms.
Structure of chromosome
A pellicle is outermost, thin proteinaceous sheath as a cover of the chromosome. Matrix is ground substance in the chromosome which has a different type of enzymes, minerals, water, and Proteins. It is a liquid that has no genetic or chromatic substance.
Chromatid: Each chromosome consists of two cylindrical structures during metaphase called as chromatids. Both sister chromatids or longitudinal hands of a chromosome are attached to a common centromere. A chromosome is a single chromatid in Anaphase and two chromatids in prophase and metaphase. Each chromatid has a single long DNA associated with histones.
Centromere / Kinetochore: Each chromosome during metaphase has two half chromosome or two chromatids. Both the chromatids of a chromosome are joined or connected by a structure called as Centromere. At this junction or the centromere, there are two protein discs which are called as Kinetochore. Kinetochores are the actual site of attachment of spindles to chromosomes during cell division. At the region of centromere there is less chromosome comparatively than the remaining part of a chromosome, thus it is termed as Primary constriction.
Satellite: The left out part of a chromosome that remains after the NOR is called as chromosome satellite/ Trabant. Chromosomes with satellite part are called as SAT chromosome (SAT= Sine Acid Thymidine).
Telomere: Chromosomes are polar with polar ends known as Telomere. Telomere prevents fusion of two chromosomes. Human Telomeres are rich in Guanine bases (5’-TTAGGG-3’). According to Richard Kathan (2003), chromosome telomeres are getting shorter with the aging process.
Types of Chromosomes on the Basis of Position of Centromere
Telocentric – When centromere is terminal or located at the tip of a chromosome.
Acrocentric – When the centromere is sub-terminal or located near the tip.
Metacentric – When the centromere is located at mid of the chromosome.
Submetacentric – When the centromere located near the centre or midpoint of the chromosome.
Function of Nucleus
It controls the synthesis of structural proteins and the enzymes and proteins synthesis and thus controls cellular functions. It Has the genetic material intact and protects it. Translation of DNA into ribosomes occur. Genetic variation essential for the evolution is initiated. Cellular differentiation as per its destined function is a result of the nucleus.
Small, Spherical, Single membrane-bound cell organelles that have enzymes in it are called as “Microbodies”. These are present in both plants and animals.