Cell: The Unit of Life (NEET)

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Key Discoveries & Scientists Robert Hooke (1665): Discovered and coined "cell" (dead cork cells). Anton von Leeuwenhoek (1667): First saw and described a living cell (improved microscope). Robert Brown (1831): Discovered the nucleus. M. Schleiden (1838, German Botanist): All plants are composed of different kinds of cells. Theodor Schwann (1839, British Zoologist): All animals are made of cells, and he noted the presence of a plasma membrane. He also concluded that the cell wall is a unique character of plant cells. Schleiden & Schwann (1839): Proposed Cell Theory. Rudolf Virchow (1855): " Omnis cellula e cellula " (all cells arise from pre-existing cells); modified Cell Theory. Camillo Golgi (1898): Discovered Golgi apparatus. Electron Microscope (1950s): Enabled detailed study of cell structures. G.N. Palade (1953): Discovered ribosomes. Singer & Nicolson (1972): Proposed Fluid Mosaic Model of cell membrane. Cell Sizes Perinuclear space: $10-50 nm$ Ribosomes: $15-20 nm$ Viruses: $0.02-0.2 \mu m$ ($20-200 nm$) PPLO (Mycoplasma): $0.1 \mu m$ Mycoplasma: $0.3 \mu m$ (length) Mitochondria: $0.2-1.0 \mu m$ (diameter); $1.0-4.0 \mu m$ (length) Cisternae of Golgi body: $0.5-1.0 \mu m$ (diameter) Typical Bacteria: $1-2 \mu m$ Bacteria: $3-5 \mu m$ (General) Chloroplast: $2-4 \mu m$ (width); $5-10 \mu m$ (length) Human Red Blood Cells (RBCs): $7.0 \mu m$ (diameter) Typical Eukaryotic Cell: $10-20 \mu m$ Largest isolated single cell: Ostrich egg. Longest cells: Nerve cells. Prokaryotic Cells Genetic material: single circular DNA (nucleoid). Plasmid: Small, circular DNA outside genomic DNA, provides unique phenotypic characters (e.g., antibiotic resistance). Cell wall present (except Mycoplasma). Cell Envelope: Chemically complex protective layers. Consists of Glycocalyx, Cell wall, Plasma membrane. Gram Staining: Bacteria classified into Gram positive and Gram negative based on cell envelope and reaction to Gram stain. Glycocalyx: Outermost layer, either as a loose sheath (slime layer) or thick and tough (capsule). Cell Wall: Prevents bursting, provides structural support. Made of peptidoglycan. Plasma Membrane: Selectively permeable, acts as the delimiting structure of the cell. Similar to eukaryotes. Mesosomes: Infoldings of plasma membrane. Appear as vesicles, tubules, and lamellae. Functions: cell wall formation, DNA replication, distribution to daughter cells, respiration, secretion, increase surface area. Chromatophores: Pigmented membranous extensions in some prokaryotes (e.g., cyanobacteria). Flagella: Thin filamentous extension from the cell wall. For motility. Composed of filament, hook, basal body. Pili & Fimbriae: Surface structures, not for motility. Pili involved in conjugation. Fimbriae help attach to substratum. Ribosomes: $70S$ (50S and 30S subunits). Size is $15nm \times 20nm$. Site of protein synthesis. Often associated with the plasma membrane. Inclusion Bodies: Non-membranous, stored reserve material. E.g., phosphate granules, cyanophycean granules, glycogen granules. Gas vacuoles: Present in some aquatic bacteria (e.g., blue-green algae, purple and green photosynthetic bacteria); provide buoyancy. Eukaryotic Cells Genetic material: linear DNA, associated with histone proteins. Plant cells: Cell wall, plastids, large central vacuole. Lack centrioles. Animal cells: Lack cell wall, plastids, large central vacuole. Have centrioles. Cell Membrane (Plasma Membrane) Chemical studies on human RBCs enabled the study of membrane structure. Composed mainly of lipids and proteins. Lipids include phospholipids and cholesterol. Proteins: (Integral) and (Peripheral). Lipids are arranged in a bilayer. The polar head faces outwards, and the non-polar hydrophobic tail faces inwards, protected from water (polar). Ratio of protein and lipids varies: In human erythrocytes, the membrane contains approximately $52\%$ protein and $40\%$ lipids. Carbohydrates are also present, often attached to proteins (glycoproteins) or lipids (glycolipids). Fluid Mosaic Model: Quasi-fluid nature of lipids enables lateral movement of proteins within the overall bilayer (fluidity). This fluid nature is important for functions like cell growth, formation of intercellular junctions, secretion, endocytosis, and cell division. Functions: Transport of molecules (passive, active). Passive Transport: Down gradient, no energy. Simple diffusion (neutral solutes), facilitated diffusion (polar molecules via carrier proteins). Osmosis (water). Active Transport: Against gradient, requires energy (ATP). E.g., Na+/K+ pump. Cell Wall (Plant & Fungi) Non-living rigid structure. Functions: Shape, protection, prevents bursting, cell-to-cell interaction. Algae: Cellulose, galactans, mannans, minerals (calcium carbonate). Plants: Cellulose, hemicellulose, pectins, proteins. Primary wall: Young plant cell, capable of growth, outer. Secondary wall: Formed inner to primary wall once cell matures. Middle Lamella: Layer between adjacent cell walls, mainly calcium pectate, glues cells together. Plasmodesmata: Cytoplasmic connections between adjacent plant cells. Endomembrane System Includes Endoplasmic Reticulum (ER), Golgi apparatus, Lysosomes, Vacuoles. Functions are coordinated. Endoplasmic Reticulum (ER) Network of tiny tubular structures scattered in cytoplasm. Divides intracellular space into luminal (inside ER) and extraluminal (cytoplasm) compartments. Rough ER (RER): Has ribosomes on surface. Involved in protein synthesis and secretion. Abundant in cells active in protein synthesis/secretion. Smooth ER (SER): Lacks ribosomes. Site for lipid synthesis and detoxification of drugs/poisons. Golgi Apparatus (Golgi Complex/Body) Consists of flattened, disc-shaped sacs or cisternae ($0.5-1.0 \mu m$ diameter). Cisternae are stacked parallel. Cis (forming) face: Convex, towards the nucleus/ER. Trans (maturing) face: Concave, towards the plasma membrane. Functions: Packaging materials for secretion, site of glycoprotein and glycolipid formation. Modifies, sorts, and packages proteins and lipids. Lysosomes Membrane-bound vesicular structures, formed by Golgi apparatus. Contain hydrolytic enzymes (lipases, proteases, carbohydrases) active at acidic pH. "Suicidal bags" of the cell. Digestion of macromolecules, foreign particles, worn-out organelles. Vacuoles Membrane-bound space in cytoplasm. Contains water, sap, excretory products. Single membrane called tonoplast . Plant cells: Large central vacuole, up to $90\%$ of cell volume. Tonoplast facilitates transport of ions against concentration gradients. Contractile vacuole: Found in some protists (e.g., Amoeba), important for osmoregulation and excretion. Food vacuole: Formed by engulfing food particles in protists, where intracellular digestion occurs. Mitochondria "Powerhouses of the cell." Not easily visible under microscope unless specifically stained. Double-membraned structure. Outer membrane smooth, inner membrane highly folded into cristae . Both outer and inner membranes have their own specific enzymes associated with mitochondrial function. Inner membrane divides lumen into outer compartment and inner matrix. Matrix contains single circular DNA molecule, few RNA molecules, $70S$ ribosomes. Divide by fission. Plastids (Plant cells & Euglenoids) Large, double-membraned. Easily observed under microscope due to their large size. Contain specific pigments. Chloroplasts: Green plastids, contain chlorophyll and carotenoid pigments. Site of photosynthesis. Number: 1 in Chlamydomonas, 20-40 per cell in mesophyll cells. Inner membrane less permeable than outer. Stroma: Fluid-filled space. Contains enzymes for the synthesis of carbohydrates and proteins, DNA (circular), $70S$ ribosomes. Chlorophyll pigments located in thylakoids. Chromoplasts: Non-green, contain fat-soluble carotenoid pigments (xanthophyll, carotene). E.g., in fruits, petals. Leucoplasts: Colourless plastids, store nutrients. Amyloplasts: Store carbohydrates (starch). E.g., potato. Elaioplasts: Store oils and fats. Aleuroplasts: Store proteins. Semi-autonomous. Divide by fission. Ribosomes Non-membranous, granular structures. Composed of ribosomal RNA (rRNA) and proteins. Site of protein synthesis. Prokaryotic ribosomes: $70S$ (50S + 30S subunits). Eukaryotic ribosomes: $80S$ (60S + 40S subunits). 'S' refers to Svedberg unit (sedimentation coefficient), indirectly indicates density and size. Can exist freely in cytoplasm or attached to ER/nuclear membrane. Polysome/Polyribosome: Multiple ribosomes attached to a single mRNA. Cytoskeleton Network of filamentous proteinaceous structures in cytoplasm. Composed of (microtubules [tubulin], microfilaments [actin], intermediate filaments). Functions: Mechanical support, motility, maintenance of cell shape. Cilia and Flagella Hair-like outgrowths of cell membrane. Cilia: Small, many, oar-like movement. Flagella: Longer, fewer, whip-like movement. Prokaryotic Flagella (structurally different): Extension of the cell wall. Made of flagellin protein. Eukaryotic Flagella: Extension of the cell membrane. Composed of microtubules. Core (axoneme) has $9+2$ arrangement of microtubule doublets radially arranged around a central pair. Central pair connected by a central sheath and bridges. Radial spokes extend from each peripheral doublet to the central sheath. Peripheral doublets are connected by linkers. Emerges from basal body (centriole-like structure, $9+0$ arrangement). Centrosome and Centrioles (Animal Cells) Centrosome: Organelle containing two cylindrical structures called centrioles. Centrioles: Perpendicular to each other. Made of $9$ evenly spaced peripheral fibrils of tubulin. Fibrils are triplets. ($9+0$ arrangement). Each peripheral triplet is linked to adjacent triplets. Central part is proteinaceous, called the hub. Hub is connected to peripheral triplets by protein spokes. The arrangement of triplets and spokes resembles a cartwheel structure. Surrounded by amorphous pericentriolar materials. Forms basal body of cilia/flagella, and spindle fibres during cell division. Nucleus Contains genetic material. Nuclear Envelope: Double membrane, with perinuclear space. Continuous with ER. Has nuclear pores (passage for RNA/protein movement). Nucleoplasm: Fluid matrix. Contains nucleolus and chromatin. Nucleolus: Non-membranous, spherical structure. Site of rRNA synthesis. Larger and more numerous in cells actively synthesizing proteins. Chromatin: Network of nucleoprotein fibres (DNA + histones + non-histone proteins + RNA). Condenses into chromosomes during cell division. Chromosomes: Condensed chromatin. Primary constriction (centromere) divides chromosome into two arms. Kinetochore: Disc-shaped structure at centromere, attachment site for spindle fibres. Metacentric: Centromere in middle, two equal arms. Sub-metacentric: Centromere slightly away from middle, one shorter and one longer arm. Acrocentric: Centromere near end, one very short and one very long arm. Telocentric: Terminal centromere. Satellite (secondary constriction): Some chromosomes have non-staining secondary constrictions, forming a small fragment called satellite. Microbodies Membrane-bound minute vesicles. Contain various enzymes. E.g., Peroxisomes (involved in photorespiration in plants, breakdown of fatty acids), Glyoxysomes (in plants, convert fats to carbohydrates).