Introduction to living cell and cell theory, Lecture notes of Cell Biology

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FACULTY OF AGRICULTURAL SCIENCES AND ALLIED INDUSTRIES

INTRODUCTORY BIOLOGY UGR-

LECTURE- 04

INTRODUCTION

 Just as a building is made up of bricks, the bodies of all plants and animals are made up of cells, i.e., all living beings show cellular organisation. Some organisms like Amoeba, Paramecium, bacteria, Chlamydomonas, etc. are made up of one cell (unicellular) only. There are a large number of other organisms like plants, animals, human beings etc.which are made up of millions of cells (multicellular).  Cell is the structural and functional unit of all living beings.  Robert Hooke (1665) discovered hollow cavities (empty boxes) like compartments in a very thin slice of cork (cell wall) with his crude microscope and named them as cellulae or cells.  Anton Von Leeuwenhoek first saw and described a live cell.  Robert Brown later discovered and named the nucleus in a cell. J. E. Purkinje, used the term protoplasm for the living substance present inside the cell.  The invention of the microscope and its improvement leading to the electron microscope revealed all the structural details of the cell.  Electron microscope : This was developed by Max Knoll and Ernst Ruska (1931) in Germany. It is a large sized instrument which has an internal vacuum, high voltage (50,000 – 1,00,000 volts), a cooling system, a fast beam of electrons (0.54 Å wavelength), a cathode filament of tungsten and electromagnetic lens (which has a coil of wire enclosed in soft iron casing) for focusing. It can magnify objects upto 2,00,000 times (now possible upto 2,50,000 – 4,00,000). The resolving power of electron microscope is 10 Å which is 100 times more than the light microscope. Study of living cells cannot be done through this microscope because of high voltage, which is required to operate it, as that can kill the living materials.

CELL THEORY

 The actual credit for cell theory goes to two scientists, a German Botanist M.J. Schleiden (1838) and a British Zoologist T. Schwann (1839). They gave the concept "all living organisms are composed of cells" and products of cells. Schleiden and Schwann together formulated the cell theory but this theory did not explain how new cells were formed.  Viruses, viroids and prions are exceptions to the cell theory as they are obligate parasites (subcellular in nature).  Rudolf Virchow (1855) first explained that cells divide 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 states that–  All living organisms are composed of cells and products of cells.  All cells arise from pre-existing cells.  Membrane bound cell organelles of the protoplasm do not survive along or outside the protoplasm.  All cells have similar fundamental structure and metabolic reactions.  Genetic information is stored as DNA in the chromosomes present in the nucleus.

CELL

Differences between Plant and Animal cells 

 Differences between prokaryotic and eukaryotic cells are:

CELL ENVELOPE AND ITS MODIFICATIONS

 The cell envelope consists of a tightly bound three layered structure i.e., the outermost glycocalyx followed by the cell wall and then the plasma membrane.  Bacteria can be classified into two groups on the basis of the differences in the cell envelope and the manner in which they respond to the staining procedure developed by Gram viz., those that take up the gram stain are Gram positive bacteria (e.g., Streptococcus, Staphylococcus, Bacillus, Mycobacterium, Streptomyces etc.) and the others that do not are called Gram negative bacteria (e.g., Salmonella, Pseudomonas, Escherichia coli, Rhizobium, Helicobacter etc.)  In a large number of bacteria, a thick slime capsule is present outside the cell wall which is made of polysaccharides and nitrogenous substances. It provides protection against phagocytosis and antibiotics.  The plasma membrane is semi-permeable in nature and interacts with the outer surface. This membrane is structurally similar to that of the eukaryotes.

 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. Osmosis is the term used to refer specifically to the diffusion of water across a differentially or semi-permeable membrane.  As the polar molecules cannot pass through the non-polar 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 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.  Bulk transport : It is transport of large quantities of micromolecules, macromolecules and food particles through the membrane. It is accompanied by formation of transport or carrier vesicles. o Pinocytosis : It is bulk intake of fluid, ions and molecules through development of small endocytotic vesicles of 100 – 200 nm in diameter. ATP, Ca 2+ , fibrillar protein (clathrin) and contractile protein (actin) are required. o Phagocytosis : It is cell eating or ingestion of large particles by living cells, e.g., white blood corpuscles (neutrophils, monocytes), Kupffer’s cells of liver, reticular cells of spleen, histiocytes of connective tissues, macrophages, Amoeba and some other protists, feeding cells of sponges and coelenterates. Plasma membrane has receptors. As soon as the food particle comes in contact with the receptor site, the edges of the latter evaginate, form a vesicle which pinches off as phagosome.  Functions o It provides mechanical strength as well as acts as a protective layer. o Plasma membrane is responsible for the transportation of materials, molecules, ions etc. o Diffusion of gases (O 2 and CO 2 ) takes place through plasma membrane by simple and facilitated diffusion.

CELL WALL

 Cell wall was discovered by Robert Hooke while observing the cell walls in cork tissue. It is the outermost, rigid, protective, non-living and supportive layer found in all plant cells, bacteria, cyanobacteria and some protists. It is not found in animal cells.  A cell wall is organized at telophase stage of cell division. Fragments of ER and vesicles of golgi body align at the equator, termed as phragmoplast, later which forms the cell plate.  Algae have cell wall made of cellulose, galactans, mannans and minerals like calcium carbonate, while in other plants it consists of cellulose, hemicellulose, pectins and proteins.  In cell wall of a young plant cell, the primary wall is capable of growth, which gradually diminishes as the cell matures and the secondary wall is formed on the inner (towards membrane) side of the cell.  Middle lamella is the outermost region which is found as a common cementing layer between two cells. It is formed of calcium and magnesium pectate.  Pits are formed in lignified cell wall. They occur in sclerenchyma, vessels and tracheids. Tracheids in gymnosperms have maximum number of bordered pits.  A number of plasmodesmata or cytoplasmic strands are present in pit through which the cytoplasm of one cell is in contact with another.

CYTOPLASM

 The substance occur around the nucleus and inside the plasma membrane containing various organelles and inclusions is called cytoplasm.  It forms about half of the cell’s volume and about 90% of it is water.  It contains ions, biomolecules, such as sugar, amino acid, nucleotide, tRNA, enzymes, vitamins, etc.  Cytoplasmic organelles are plastid, lysosome, sphaerosome, peroxisome, glyoxysomes, mitochondria, ribosome, centrosomes, flagella or cilia etc.

ENDOMEMBRANE SYSTEM

The endomembrane system includes endoplasmic reticulum (ER), golgi complex, lysosomes and vacuoles. Since the functions of the mitochondria, chloroplast and peroxisomes are not coordinated with the above components, these are not considered as part of the endomembrane system.

ENDOPLASMIC RETICULUM (ER)

 A network or reticulum of tiny tubular structures scattered in the cytoplasm that is called the endoplasmic reticulum (ER).  The ER is present in almost all eukaryotic cells. A few cells such as ova, embryonic cells, and mature RBCs, however, lack ER. It is also absent in prokaryotic cell. In rapidly dividing cells, endoplasmic reticulum is poorly developed.  The ER is made up of three components. All the three structures are bound by a single unit membrane.  Cisternae : These are flattened, unbranched, sac like structures. They lie in stacks (piles) parallel to one another. They bear ribosomes.  Vesicles : These are oval or rounded, vacuole like elements, scattered in cytoplasm. These are also studded with ribosomes.  Tubules : Wider, tubular, branched elements mainly present near the cell membrane. They are free from ribosomes. These are more in lipid forming cells. 

 The endoplasmic reticulum bearing ribosomes on their surface is called Rough Endoplasmic Reticulum (RER). In the absence of ribosomes, they appear smooth and are called Smooth Endoplasmic Reticulum (SER). RER is frequently observed in the cells actively involved in protein synthesis and secretion. The smooth endoplasmic reticulum is the major site for synthesis of lipid. In animal cells, lipid-like steroidal hormones are synthesised in SER. Differences between Rough ER (Granular) and Smooth ER (Agranular)

 Glycosidation of lipids i.e., addition of oligosaccharides to produce glycolipids.  Glycosylation of proteins i.e., addition of carbohydrate to produce glycoproteins.  Helps in the formation of primary lysosomes.

LYSOSOMES

 Lysosomes are microscopic, vesicular structures of the cytoplasm, bounded by a single membrane (lipoproteins) which are involved in intracellular digestive activities, containing hydrolytic enzymes. These are popularly called "suicidal bags".  These were first discovered by a Belgian biochemist, Christian de Duve (1955) in the liver cells and named as "pericanalicular dense bodies".  These are absent in prokaryotes but are present in all eukaryotic animal cells except mammalian RBCs. They have been recorded in fungi, Euglena, cotton and pea seeds.  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 acidic pH. These enzymes are capable of digesting carbohydrates, proteins, lipids and nucleic acids.  Types of lysosomes : On the basis of their contents, four types of lysosomes are recognised :  Primary lysosomes : A newly formed lysosome contains enzymes only. Its enzymes are probably in an inactive state.  Secondary lysosomes : When some material to be digested enters a primary lysosome, the later is named the secondary lysosome, or phagolysosome or digestive vacuole, or heterophagosome.  Tertiary lysosomes/Residual bodies : A secondary lysosome containing indigestible matter is known as the residual bodies or tertiary lysosome.  Autophagosomes/Autolysosomes : A cell may digest its own organelles, such as mitochondria, ER. This process is called autophagy. The acid hydrolases of lysosomes digest the organelles thus, it is called autophagosome.  Functions  Lysosomes of sperm provides enzyme for breaking limiting membrane of egg e.g., hyaluronidase enzyme.  Lysosomes function as trigger of cell division or initiate cell division by digesting repressor molecules.  They also engulf the carcinogens.

VACUOLES

 The vacuole in plants was discovered by Spallanzani. It is a non-living reservoir, bounded by a differentially or selectively permeable membrane, the tonoplast.  These contain water, minerals and anthocyanin pigments.  Some protozoans have contractile vacuoles which enlarge by accumulation of fluid or collapse by expelling them from the cell. The vacuoles may be sap vacuoles, contractile vacuoles or gas vacuoles (pseudo vacuoles).  Functions  Vacuole maintains osmotic relation of cell which is helpful in absorption of water. Turgidity and flaccid stages of a cell are due to the concentrations of sap in the vacuole.

MITOCHONDRIA

 The mitochondria occur singly or in groups and their shape and size (0.5μ to 0.2μ) vary from cell to cell and depends on physiological activity of cell.  Mitochondria is considered as semi-autonomous organelle because it has separate protein synthesizing machinery independent of nuclear control.  These were first observed in striated muscles of insects as granules by Kolliker (1880), he called them "sarcosomes".   Each mitochondrion is a double membrane-bound structure with 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 inner membrane forms a number of infoldings called cristae towards the matrix. The cristae increase the surface area.  Oxysomes have ATPase enzyme molecule and therefore, responsible for ATP synthesis. The reaction of ATP formation is endergonic. These elementary particles are also called F 0 - F 1 particles.  The matrix also possesses single circular DNA molecule, a few RNA molecules, ribosomes (70S) and the components required for the synthesis of proteins. The mitochondria divide by fission.

Shape : These have various shapes like   It is a double membrane structure. Both membranes are smooth. The inner membrane is less permeable than outer but rich in proteins especially carrier proteins.  The inter-membrane space is called the periplastidial space. Inner to membranes, matrix is present, which is divided into two parts– o Grana : Inner plastidial membrane of the chloroplast is invaginated to form a series of parallel membranous sheets, called lamellae, which form a number of oval - shaped closed sacs called thylakoids.  Along the inner side of thylakoid membrane, there are a number of small rounded para-crystalline bodies called quantasomes (a quantasome is the photosynthetic unit). o Stroma : It is transparent, proteinaceous and watery substance. Dark reaction of photosynthesis occurs in this portion. Stroma is almost filled with "RuBisCO" (about 15% of total enzyme, protein) enzyme and CO 2 is accepted by this enzyme. CO 2 assimilation results in carbohydrate formation.  Chlorophyll a : C 55 H 72 O 5 N 4 Mg (with methyl group)  Chlorophyll b : C 55 H 70 O 6 N 4 Mg (with aldehyde group)  Chlorophyll c : C 35 H 32 O 5 N 4 Mg  Chlorophyll d : C 54 H 70 O 6 N 4 Mg

PIGMENTS OF CHLOROPLASTS

 Bacteriochlorophyll (C 55 H 74 O 6 Mg) or chlorobium chlorophyll present in photosynthetic bacteria.  Carotenoids : These are hydrocarbons, soluble in organic solvents. These are of two types o Carotenes : C 40 H 56 , derivatives of vitamin A. Carrot coloured α, β, γ carotene, lycopene, etc. β - carotene in the most common. o Xanthophyll : C 40 H 56 O 2 , yellowish in colour, fucoxanthin, violaxanthin. Molar ratio of carotene and xanthophyll in young leaves is 2:1. FUNCTIONS  It is the site of photosynthesis (light and dark reactions).  Photolysis of water, reduction of NADP to NADPH 2 takes place in granum.  Photophosphorylation through cytochrome b 6 f, plastocyanin and plastoquinone etc.  They store starch or factory of synthesis of sugars.

 RIBOSOMES

 The ribosomes are the smallest known organelles without membrane, ribonucleoprotein particles attached either on RER or floating freely in the cytoplasm and are the sites of protein synthesis. TYPES OF RIBOSOMES  70S ribosomes : Found in prokaryotes, mitochondria and plastid of eukaryotes.  80S ribosomes : Found in cytoplasm of eukaryotes. CHEMICAL COMPOSITION OF RIBOSOMES  70S (50S + 30S) (60S + 40S) - 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  Levine and Goodenough (1874) observed 77S ribosomes in fungal mitochondria, 60S ribosomes in animal mitochondria and 55S in mammalian mitochondria.

MICROFILAMENT

These were discovered by Paleviz et. al. (1974).  These are microscopic, long, narrow, cylindrical, non-contractile proteins found only in the eukaryotic cytoplasm. These are present in the microvilli, muscle fibres (called myofilaments) etc. But these are absent in prokaryotes. These are mainly formed of actin (contractile protein).  FUNCTIONS  The microfilaments forms a part of cytoskeleton and change the cell shape, motility and division during development.  The microfilaments bring about directed movements of particles and organelles along them in the cell.

INTERMEDIATE FILAMENTS

 These are supportive elements in the cytoplasm of eukaryotic cells. These are missing in mammalian RBCs.  The IFs are somewhat larger than the microfilaments and are about 10 nm thick. These are solid, unbranched and composed of non-motile structural proteins, such as keratin, desmin, vimentin. FUNCTIONS  These form a part of cytoskeleton that supports the fluid cytosol and maintains the shape of the cell.  These provide support to myofibrils which is essential for their contraction.

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.  Structure : Both cilia and flagella are structurally similar and possess similar parts - basal body, rootlets, basal plate and shaft.  Basal body : It is present below the plasma membrane in cytoplasm. The structure is similar to centrioles made of 9 triplets of microtubules.  Rootlets : Made of microfilament and provide support to the basal body.  Basal plate : It is highly dense and lie above plasma-membrane.  Shaft : It is the hair like projecting part of cilia and flagella which remains outside the cytoplasm. It has 9 doublet of microtubules in radial symmetry. These are called axonema. Each axonema has 11 fibrils, 9 in the periphery and 2 in the centre. The arrangement is called 9 + 2 pattern.  Chemically, the central tubules are formed of dynein protein while the peripheral microtubules are formed of tubulin protein.   FUNCTIONS  These help in locomotion, respiration, cleaning, circulation, feeding, etc.  These show sensitivity to changes in light, temperature and contact.

 Table : Differences between Cilia and Flagella  

 CENTROSOME AND CENTRIOLES

 Centrosome was first discovered by Van Benden (1887) and structure was given by T. Boweri.  Centrosome is an organelle usually containing two cylindrical structures called centrioles. They are surrounded by amorphous pericentriolar materials. Both the centrioles in a centrosome lie perpendicular to each other in which each has an organisation like the cartwheel. They are made up of nine evenly spaced peripheral fibrils of tubulin. Each of the peripheral fibrils is a triplet.The adjacent triplets are also linked. The central part of the centrioles is also proteinaceous and called the hub, which is connected with tubules of the peripheral triplets by radial spokes made of protein.  It has (9 + 0) pattern.  Centriole is not covered by any membrane.  FUNCTIONS  The centrioles help in organising the spindle fibres and astral rays during cell division.  They provide basal bodies which give rise to cilia and flagella.