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Cell Cell the Unit of Life

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Cell Cell the Unit of Life CELL THE UNIT OF LIFE INTRODUCTION: EXCEPTION OF CELL THEORY Cell is a basic unit of life and It is I. Virus/ Virion/ Bacteriophage, considered as structural and functional unit II. viroids of an organism. Robert Hooke (1665) III. prions discovered cell. He first observed the cell in a piece of dead cork cells. He described cell SIZE AND SHAPE OF CELL in his book “Micrographia”. Leeuwenhoek (1674) first observed animal SIZE cell and used the term “Animalcule” for it. Cell differ greatly in size, shape and The study of cell structure is called cytology. activites. The study of cell structure, function & Mycoplasma laidlawii (Smallest cells) reproduction is called Cell biology. Only 0.3 m in length [PPLO (pleuro Robert hooke is known as ‘father of pneumonia like organisms) is a type cytology’. of mycoplasma having the size about Swanson is called the ‘Father of modern 0.1 m)] cytology’ Bacteria 3 to 5 m Largest unicellular Eukaryotic cell - CELL THEORY Acetabularia alga - (10cm) Largest isolated single cell ostrich In 1838, Malthias Schleiden, German egg botanist, examined a large number of Human red blood cell 7.0 m plants and observed that all plants Longest animal cell nerve cell are composed of different kinds of (Neurons) cells which form the tissues of the Longest plant cell ramie plant. At about the same time, (Boehmaria) Theodore Schwann (1839), British Zoologist, studied different types of animal cells and reported that cells EAGLE EYE had a thin outer layer which is today known as the ‘plasma membrane’. He 1. Smallest cell has higher surface area also concluded, based on his studies to volume ratio on plant tissues, that the presence of 2. Virus is acellular and connecting link cell wall is a unique character of the between living and non living. plant cells. Schwann proposed the hypothesis SHAPE that the bodies of animals and plants The shape of the cell may vary with are composed of cells and products the funtion they perform. of cells. They may be disc-like, polygonal, Schleiden and Schwann together columnar, cuboidal, thread like or formulated the cell theory. This even irregular. The shape of the cell theory however, did not explain as to may vary with the function they how new cells were formed. Rudolf Virchow (1855) first explained that perform. 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 PROKARYOTIC CELLS cells. Cell that bears naked genetic material i.e. (ii) all cells arise from pre-existing nucleus is without Nuclear envelop is known cells. as prokaryotic cell. 1 The prokaryotic cells are represented by cell wall and innermost cell bacteria, blue-green algae, mycoplasma and membrane. PPLO (Pleuro Pneumonia like Organisms). (vi) If these envelopes are stained by Gram They may vary greatly in shape and size. The stain (Crystal violet) then they are four basic shapes of bacteria are bacillus called Gram positive bacteria while (rod like), coccus (spherical), vibrio (comma other those don’t have are called Gram shaped) and spirillum (spiral). negative bacteria. (vii) Glycocalyx, a polysacchride envelope forms either loose sheath slime layer or thick and tough structure capsule. (viii) Plasma membrane extensions like vesicles, tubules and lamellae not only help in cell wall formation, DNA replication, and distribution to daughter cells, increase in surface area but also form some specific structure for specific function like mesosome for They have following characters : respiration (analogous to mitochondria) (i) Like eukaryotic cells, they are and chromatophore for photosynthesis different in shape and size but small (analogous to chloroplast). than eukaryotes & divide rapidly. (ix) Cell wall form some filamentous (ii) Naked genetic material is called extensions called flagellum. It consists ‘genophore’ and area in which it is of filament, hook & basal body and present known as Nucleiod. helps in locomotion. Filament is (iii) Genomic DNA is circular & termed as longest portion and extends from the single chromosome. cell surface to the outside. (x) Besides flagella, Pili and Fimbriae are also surface structures of the bacteria but do not play a role in motility. The pili are elongated tubular structures (iv) Beside genomic DNA, small circular made of a special pilin protein. The DNA is also present in many bacteria fimbriae are small bristle like fibres called plasmid. The plasmid DNA sprouting out of the cell in some confers certain unique phenotypic bacteria, they are known to help characters to such bacteria, which attach the bacteria to rocks in streams make them antibiotic resistant & also and also to the host tissues. responsible for bacterial transformation. (v) Prokaryotic cell envelope consists of three layers–outer glycocalyx, middle EAGLE EYES CELL ENVELOPE ITS MODIFICATIONS Periplasmic Space (Analogous to Most prokaryotic cells, particularly lysosome) present between cell wall the bacterial cells, have a chemically and cell membrane in bacteria. complex cell envelope. The cell envelope consists of a tightly bound 2 three layered structure i.e., the The cell wall determines the shape of outermost glycocalyx followed by the the cell and provides a strong cell wall and then the plasma structural support to prevent the membrane. Although each layer of bacterium from bursting or the envelope performs distinct collapsing. function, they act together as a Inclusion bodies: Reserve material in single protective unit. Bacteria can be prokaryotic cells are stored in the classified into two groups on the cytoplasm in the form of inclusion basis of the differences in the cell bodies. These are not bound by any envelopes and the manner in which membrane system and lie free in the they respond to the staining cytoplasm, e.g., phosphate granules, procedure developed by Gram viz., cyanophycean granules and glycogen those that take up the gram stain granules. Gas vacuoles are found in (crystal violet) are Gram positive blue green and purple and green (Purple Colour) and the others that photosynthetic bacteria. do not are called Gram negative (Red/Pink) bacteria. 3 CELL STRUCTURE Cell Boundaries Nucleus Gycocalyx Cytoplasm (PH =7.2) Cell wall Cell membrane Main area of cellular activities & various chemical reactions occur in it to keep the cell in living state Hyaloplasm/ Cytosol / Intracellular fluid. Trophoplasm/ It is a complex Substance that makes up the Cytoplasmic inclusions liquid portion of the Cytoplasm of cell Ergastic bodies/ Non-living inclusion Cell organelles (metabolically Active content /Deutoplasm The cytoplasm also of the cell) contain substances which are metabolically inactive) Reserve Products Double Single Non Membranous Membranous Membranous Carbohydrates Nitrogenous ENDOMEMBRANE products SYSTEM Nucleus E.R Ribosome Plastid Golgi body Centriole Mitochondria Lysome Vacuole MICROBODIES Glyoxisome peroxisome Sphaerosome 4 Rough endoplasmic Microvilli reticulum Golgi Lysosome AN OVERVIEW OF CELL Smooth apparatus endoplasmic Plasma reticulum membrane Plasmodesmata The onion cell which is a typical plant Nucleus Centriole Nucleolus Smooth Peroxiome Golgi endoplasmic Microtubule apparatus reticulum Lysosome cell, has a distinct cell wall as its Nuclear envelope Nuclear envelope Ribosomes Plasma membrane outer boundary and just within it is Nucleolus Mitochondrion Vacuole Middle lamella Rough Cell wall endoplasmic the cell membrane. Peroxisome Nucleus reticulum Mitochondrion Cytoplasm Cytoplasm Chloroplast Ribosomes Cells that have membrane bound Plant cell Animal cell nuclei are called eukaryotic whereas cells that lack a membrane bound CELL WALL nucleus are prokaryotic. A non-living rigid structure called the In both prokaryotic and eukaryotic cell wall forms an outer covering for cells, a semi-fluid matrix called the plasma membrane of Bacteria cytoplasm occupies the volume of Fungi, Algae and Plants. the cell. Algae have cell wall made up of The cytoplasm is the main area of cellulose, galactans, mannans and cellular activities in both the plant minerals like calcium carbonate. and animal cells. Various chemical In other plants it consists of reactions occur in it to keep the cell cellulose, hemicellulose, pectins and in the ‘living state’. proteins. Besides the nucleus, the eukaryotic Bacterial cell wall mainly composed cells have other membrane bound of Peptidoglycans (Polysaccharide + distinct structures called organelles amino acid). like the endoplasmic reticulum (ER), The cell wall of Fungi are composed the golgi complex, lysosomes, of Chitin and Polysaccharides. mitochondria, microbodies. The Dinoflagellates cell wall has stiff prokaryotic cells lack such membrane cellulose plate on the outer surface bound organelles. (Plated cell wall) Ribosomes are non-membrane bound Cell wall of diatoms is made up of organelles found in all cells – both silica and known as FRUSTULE eukaryotic as well as prokaryotic cell. Within the cell, ribosomes are found not only in the cytoplasm but also EAGLE EYE within the two organelles – 1. Archaebacteria differ from other chloroplasts (in plants) and bacteria in having a different cell wall mitochondria and on rough ER. structure and this feature is Animal cells contain another non- responsible for their survival in membrane bound organelle called extreme condition. centriole which helps
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