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Cell • Regarded as the basic structural and functional unit of life. • Cell is a mass of cytoplasm enclosed by a plasma membrane consisting of certain organelles besides the nucleus or nuclear material, capable of self replicating. Development of Cell Theory The development of optical lenses and their combination in compound microscope led to the establishment of the cell theory. In 17th Century • Robert Hooke (1665) first observed honeycomb like structures in a thin slice of cork and he called them as ‘cells’. • Leewenhock (1674) discovered free cells and observed some organizations within cells, particularly nucleus. In 19th Century • In 1838-39, Scheilden and Schwann both postulated that the cell is the basic unit of structure and function in a living organism. • Schwann studied both animal and plant tissues. His work with connective tissues (bone and cartilage) led him to modify the evolving theory by including the idea that living things are made up of cells and its products. • Virchow confirmed Nageli’s principle of cellular basis of life’s continuity and stated in latin “omnis cellula e cellula”. • Loius Pasteur (1865) gave experimental evidence to support Virchow’s extension of the cell theory. Cell Theory The modern version of cell theory states that: 1. All living organisms (plants, animals & microbes) are made up of one or more cells and cell products. 2. All metabolic reactions in unicellular and multicellular reactions takes place in cell. 3. Cells originate only from the pre existing cells by division and duplication. 4. The smallest clearly defined unit of life is the cell. Compartmentalisation within the cells • Many membrane bound compartments present in the cytoplasm are the membrane bound organelles. • Part of cytoplasm excluding the compartments are called cytosol. • Microfilaments, centrioles, microtubules, intermediate filaments & ribosomes are not considered compartments. • Lack of intracellular compartmentalisation shows the primitive nature of the cell. Types of Cells • Cells are of two types Eukaryotic and Prokaryotic • Based on i. Organisation of membranes ii. Variety of cytoplasmic membrane iii. Complexity of nuclear region • The terms were suggested by Hans Ris in 1960s. Prokaryotic Cells 1. Most primitive single membrane cells, 2. Present in bacteria and blue green algae and they are known as prokaryotes. 3. Comparatively small, 1-10 µm 4. Consists of three main coverings:Coverings Outer covering Cytoplasm Nuclear region Outer Covering Cell Membrane Cell wall Slimy capsule Cell Memrane • • • • • • • Thin and Flexible Composed of lipids and proteins Controls the movement of molecules in & out of the cell In some bacteria mesosomes are present Photosynthetic lamellae are present Also thought to play a role in replication Prokaryotic cell plan is not compartmentalised Cell Wall • • > > Range in thickness from 1.5-100µm GRAM POSITIVE Bacteria Single thick layer of peptidoglycan and teichoic acid Peptidoglycan is the complex of oligosaccharide chains and short peptides also called mucopeptide or murein > Teichoic acid consists of glycerol or ribitol based subunits linked by phosphate groups • GRAM NEGATIVE Bacteria Peptidoglycan Periplasmic space Outer membrane > Three layers> Peptidoglycan layer is thin(25Å) &lies next to plama membrane > Periplasmic space contains proteins & lies outside peptidoglycan > Outer membrane consist of a lipid bilayer which is permeable and contains porins. Slimy Capsule • Consists largely of polysaccharides but may have polypeptides and other compounds also. • It protects the cell against dessication, antibiotics & virus attacks. Cytoplasm • Contains proteins, lipids, glycogen and inorganic ions; enzymes for biosynthetic reactions, tRNA and mRNA. • Ribosome is the only cytoplasmic organelle found in prokaryotic cell (20-30nm); forms polyribosomes at the time of protein synthesis. It is 70 S & composed of 23 S, 16 S & 5 S rRNAs and 50 different proteins • Cytoplasm do not show streaming movements. Phagocytosis, Pinocytosis & exocytosis do not occur. • May contain deposits of polysaccharides or inorganic phosphates Nucleoid • Not enclosed by a nuclear envelope, such a nuclear material is called nucleoid • Single chromosome that is greatly coiled which shows many special features: ₋ Short, simple & attached to cell membrane at 1 point ₋ Single copy of chromosome ₋ dsDNA, circular not associated with proteins ₋ Amount of DNA is 0.23 nm in length ₋ Transcription & translation occur in the cytoplasm; mRNA does not require processing ₋ Absence of nucleolus & formation of mitotic apparatus; cell membrane helps to segregate replicated products. ₋ Prokaryotic chromosomes also called prochromosomes. Plasmids • Small circular DNA besides nucleoid called plasmids • 1000-30,000 bps • Generally code for non essential proteins • Some encode proteins needed by organism to resist antibiotics or other toxic materials Flagella • 150Å thick & 10-15µm long Two main parts Filament Basal body Filament Composed of spiral chains of subunits of protein named flagellin Basal Body •Comprises of shaft & hook •M ring is embedded in the plasma membrane Pilli • Short, rod-like, non-motile processes called pilli/fimriae • Formed of protein pilin • Usually less than 10 nm thick & under 1 µm long • Used for attachment to surfaces Eukaryotic Cell • Double membrane or two-envelope system; primary membrane surrounds the cell and secondary membrane envelope the nucleus and other organelles • SIZE: Range from 10-100 µm size is determined by 2 main factors: i. requirement of nutrients and oxygen ii. Regulating ability of its nucleus Types of cells Size Sporozoites of Plasmodium vivax 2 µm Ostrich egg 175 X 120 mm Acetabularia 100 mm Human cells 20-30 µm • SHAPE: spherical, oval, disc-like, cuboidal, columnar, polygonal, spindle-like, irregular, etc. o Shape is often correlated with its function o Shape depends on: i. Function ii. Age iii. Viscosity iv. Cell wall v. External pressure vi. Internal and external skeleton • NUMBER: – Organisms which have their body made up of a single cell are called unicellular/acellular – Organisms composed of large no. of cells are termed as multicellular organism Eukaryotic Cell Cell Membrane Cytoplasm Nucleus Cell Membrane • Thin, elastic, living covering called cell membrane/plasma membrane/plasmalemma • Gives form and shape to the cell & is composed of a lipid-protein complex • Controls the movement of particles through the cell membrane by the processes endocytosis and exocytosis • Certain protists, fungi, all plant cells have thick rigid non-living covering called cell wall Cytoplasm (cytoplasmic matrix/cytosol/hyaloplasm) • It is a semi-fluid, homogenous, translucent substance present b/w nucleus and plasma membrane • Often show streaming movements/cyclosis • Contains two types of structures: – Organelles – Inclusions Organelles • Endoplasmic Reticulum: network of membrane-bound channels whose main role is to transport materials in the cell. • Ribosomes: minute riboprotein particles lying free or attached to ER & nuclear envelope. 80S ribosomes composed of 28S, 5.8S, 5S & 18S rRNAs and 80 different proteins. 70S in mitochondria & plastids. • Microfilaments: fine protein fibrils often found in bundles in the cytoplasm. It plays a role in all movements of the cell. • Microtubules: fine proteins tubules present in cytplasm, centrioles, cilia , flagella, nerve fibres and mitotic spindle. They bring about intra cellular movements. • Intermediate filaments: solid, unbranched, supporting fibres in the cytoplasm. They are composed of proteins called keratin, desmin & vimentin. • Vacuoles: small, membrane-bound, bubble like sacs contianing cell sap • Golgi Apparatus: pile of flattened sacs often near the centrosome and has mainly the secretory role. • Lysosomes: membrane bound organelle comprising of enzymes for intracellular digestion. • Mitochondria: cylindrical vesicles filled with fluid matrix which convert chemical energy into biologically useful form. e.g. ATP • Plastids: these are the fluid filled vesicles of 3 types- chloroplasts, lecoplasts & chromoplasts. • Centrioles: cylinder like organelles which help arrange mitotic apparatus during cell division. Inclusions They are incapable of growth and considered as non living. • Starch Grains: present in plant cells only. They are common storage organs. • Aleurone Grains: protein materials deposited in plant cells in the outer endosperm cells of cereals. • Glycogen Granules: polysaccharide particles stored in liver & muscle cell of animals. • Fat Droplets: present in fat storing cells(adiposites) of animals, endosperm of castor, coconut oil and cotyledons of groundnut and mustard seeds. • Inorganic Crystals: occur in plant cells. Crystals of calcium & silica are common. Nucleus • Genetic material is covered in a nuclear envelope having pores for exchange of particles. • Chromosomes are long complex & have no connection with the plasma membrane • Two copies of each chromosome hence they are diploid. • DNA is ds, helical &linear having histone & non-histone proteins • Replication of DNA occurs only during the S phase of the cell • Transcription takes place in the nucleus and translation in cytoplasm