Introduction
What makes a thing living? The presence of the cell, the basic unit of life. Unicellular organisms (i) exist independently and (ii) perform all essential functions of life; anything less than a complete cell cannot live independently. Hence the cell is the fundamental structural and functional unit of all living organisms. This chapter covers cell theory, prokaryotic and eukaryotic cells, and each organelle's structure and function.
Key Concepts
8.1 What is a Cell?
Antonie von Leeuwenhoek first saw and described a live cell. Robert Brown later discovered the nucleus.The microscope, and later the electron microscope, revealed the cell's structural details.8.2 Cell Theory
Matthias Schleiden (1838), German botanist: all plants are made of different kinds of cells forming tissues.Theodore Schwann (1839), German zoologist: animal cells have a thin outer layer (plasma membrane); the cell wall is unique to plant cells; proposed that bodies of animals and plants are made of cells and their products.Schleiden and Schwann formulated the cell theory, but it did not explain how new cells form.Rudolf Virchow (1855): cells divide and new cells come from pre-existing cells – *Omnis cellula-e cellula*.Modern cell theory: (i) all living organisms are composed of cells and products of cells; (ii) all cells arise from pre-existing cells.8.3 An Overview of Cell
Onion cell (plant): cell wall outside, cell membrane within. Human cheek cell: outer membrane only.Nucleus contains chromosomes with DNA. Membrane-bound nucleus → eukaryotic; no membrane-bound nucleus → prokaryotic.Cytoplasm: semi-fluid matrix, main arena of cellular activities.Eukaryotic membrane-bound organelles: ER, Golgi complex, lysosomes, mitochondria, microbodies, vacuoles.Ribosomes are non-membrane-bound and occur in all cells – in cytoplasm, chloroplasts, mitochondria and on rough ER. Animal cells also have a non-membrane-bound centrosome that helps in cell division.Size: Mycoplasma is the smallest cell (about 0.3 µm); bacteria 3–5 µm; the largest isolated single cell is the ostrich egg; human RBCs are about 7.0 µm in diameter; nerve cells are among the longest.8.4 Prokaryotic Cells
Bacteria, blue-green algae, mycoplasma and PPLO (Pleuro Pneumonia Like Organisms); smaller and multiply faster than eukaryotic cells.Bacterial shapes: bacillus (rod), coccus (spherical), vibrio (comma), spirillum (spiral).Cell wall present except in mycoplasma. No well-defined nucleus; genetic material naked. Many bacteria have small circular DNA outside the genome called plasmids, giving traits like antibiotic resistance.Only ribosomes as organelles; unique inclusions; mesosome = infolding of the cell membrane.#### 8.4.1 Cell Envelope and its Modifications
Three layers: outer glycocalyx, cell wall, plasma membrane, acting as one protective unit.Gram positive bacteria take up Gram stain; Gram negative do not.Glycocalyx: loose slime layer or thick tough capsule.Cell wall gives shape and strong structural support.Mesosome (vesicles, tubules, lamellae) helps in cell wall formation, DNA replication and distribution to daughter cells, respiration, secretion, and increases membrane surface area and enzyme content.Chromatophores in cyanobacteria contain pigments.Flagellum has three parts: filament, hook and basal body. Pili (tubular, special protein) and fimbriae (small bristles; help attach to rocks and host tissues) are not used for motility.#### 8.4.2 Ribosomes and Inclusion Bodies
Prokaryotic ribosomes are associated with the plasma membrane; about 15 nm × 20 nm; subunits 50S + 30S = 70S.Many ribosomes on one mRNA = polysome, translating mRNA into proteins.Inclusion bodies (no membrane): phosphate granules, cyanophycean granules, glycogen granules. Gas vacuoles occur in blue-green, purple and green photosynthetic bacteria.8.5 Eukaryotic Cells
All protists, plants, animals and fungi; extensive compartmentalisation by membrane-bound organelles, an organised nucleus, and cytoskeletal and locomotory structures.Plant cells have cell walls, plastids and a large central vacuole; animal cells have centrioles (absent in almost all plant cells).#### 8.5.1 Cell Membrane
Mainly lipids and proteins; phospholipids in a bilayer with polar heads outwards and hydrophobic tails inwards; also cholesterol and carbohydrate.Human RBC membrane: about 52% protein and 40% lipids.Proteins: integral (partly or wholly buried) and peripheral (on surface).Fluid mosaic model by Singer and Nicolson (1972): quasi-fluid lipid allows lateral movement of proteins (fluidity) – important for growth, junctions, secretion, endocytosis, cell division.Transport: passive (simple diffusion of neutral solutes along gradient; osmosis for water; polar molecules need carrier proteins) and active (against gradient using ATP, e.g., Na⁺/K⁺ pump).#### 8.5.2 Cell Wall
Non-living rigid covering in fungi and plants; gives shape, protects from mechanical damage and infection, aids cell–cell interaction.Algae: cellulose, galactans, mannans, calcium carbonate. Other plants: cellulose, hemicellulose, pectins, proteins.Primary wall (young, can grow) → secondary wall forms on the inner side. Middle lamella (mainly calcium pectate) glues neighbouring cells. Plasmodesmata connect cytoplasm of neighbouring cells.#### 8.5.3 Endomembrane System
ER, Golgi complex, lysosomes and vacuoles (coordinated functions). Mitochondria, chloroplasts and peroxisomes are not part of it.ER: network of tubules dividing space into luminal and extra-luminal compartments. RER (with ribosomes) – protein synthesis and secretion, continuous with outer nuclear membrane. SER – major site of lipid synthesis; in animal cells, steroidal hormones.Golgi apparatus (Camillo Golgi, 1898): flat disc-shaped cisternae (0.5–1.0 µm), stacked; convex cis (forming) face and concave trans (maturing) face. Packages materials; modifies proteins from ER; important site of formation of glycoproteins and glycolipids.Lysosomes: vesicles formed by Golgi packaging; rich in hydrolytic enzymes (lipases, proteases, carbohydrases) active at acidic pH; digest carbohydrates, proteins, lipids and nucleic acids.Vacuoles: bound by a single membrane, the tonoplast; up to 90% of plant cell volume; tonoplast moves ions against gradient into the vacuole. Contractile vacuole in Amoeba – osmoregulation and excretion; food vacuoles in protists.#### 8.5.4 Mitochondria
Sausage-shaped or cylindrical; diameter 0.2–1.0 µm (average 0.5 µm), length 1.0–4.1 µm.Double membrane; outer membrane smooth boundary; inner membrane folds into cristae (increase surface area); inner compartment filled with matrix.Sites of aerobic respiration, producing ATP → "power houses" of the cell.Matrix has single circular DNA, a few RNAs, 70S ribosomes; mitochondria divide by fission.#### 8.5.5 Plastids
In all plant cells and euglenoids. Chloroplasts (chlorophyll, carotenoids – photosynthesis), chromoplasts (fat-soluble carotenoids – yellow, orange, red), leucoplasts (colourless storage): amyloplasts (starch, e.g., potato), elaioplasts (oils and fats), aleuroplasts (proteins).Chloroplasts mostly in leaf mesophyll; length 5–10 µm, width 2–4 µm; 1 per cell in Chlamydomonas, 20–40 per mesophyll cell.Double membrane (inner less permeable); stroma contains thylakoids stacked as grana, linked by stroma lamellae; thylakoid space = lumen. Stroma has enzymes, circular DNA and 70S ribosomes; chlorophyll in thylakoids.#### 8.5.6 Ribosomes
First observed by George Palade (1953); made of RNA and proteins; no membrane.Eukaryotic 80S (60S + 40S); prokaryotic 70S (50S + 30S). S = Svedberg unit (sedimentation coefficient, indirect measure of density and size).#### 8.5.7 Cytoskeleton
Microtubules, microfilaments and intermediate filaments – mechanical support, motility, maintaining cell shape.#### 8.5.8 Cilia and Flagella
Hair-like outgrowths of the membrane; cilia work like oars; flagella are longer. Bacterial flagella are structurally different.Core axoneme: nine peripheral doublets + a central pair = 9+2 array; nine radial spokes; linkers join doublets. Both emerge from basal bodies.#### 8.5.9 Centrosome and Centrioles
Centrosome has two perpendicular centrioles in amorphous pericentriolar material; each has a cartwheel organisation of nine peripheral triplets of tubulin; central hub connected by radial spokes.Centrioles form basal bodies of cilia/flagella and the spindle apparatus in animal cells.#### 8.5.10 Nucleus
First described by Robert Brown (1831); Flemming named the stained material chromatin.Interphase nucleus: chromatin, nuclear matrix, one or more nucleoli.Nuclear envelope: two membranes with a perinuclear space (10–50 nm); outer membrane continuous with ER and bears ribosomes; nuclear pores let RNA and proteins move both ways.Some mature cells lack a nucleus: mammalian erythrocytes, sieve tube cells.Nucleolus: not membrane-bound; site of rRNA synthesis.Chromatin: DNA, histones, non-histone proteins, RNA. A human cell has about 2 m of DNA in 46 (23 pairs) chromosomes.Centromere (primary constriction) holds two chromatids; kinetochores are disc-shaped structures on its sides.By centromere position: metacentric (middle, equal arms), sub-metacentric (slightly off-centre), acrocentric (near end), telocentric (terminal). A secondary constriction gives a satellite.#### 8.5.11 Microbodies
Membrane-bound minute vesicles with enzymes, in both plant and animal cells.Diagrams to Know
Plant cell vs animal cell (cell wall, plastids, large vacuole vs centriole, microvilli).Fluid mosaic model (bilayer, integral and peripheral proteins, cholesterol, sugar).Mitochondrion (outer and inner membrane, cristae, matrix, inter-membrane space).Chloroplast (stroma, grana, thylakoids, stroma lamellae).Cilium/flagellum T.S. (9+2, radial spokes, central sheath).Nucleus (nuclear pores, nucleolus, envelope); chromosome with kinetochore; four chromosome types.Key-Facts Sheet
Schleiden 1838, Schwann 1839, Virchow 1855 (Omnis cellula-e cellula), Robert Brown nucleus 1831, Camillo Golgi 1898, George Palade 1953, Singer & Nicolson 1972.70S (50S + 30S) in prokaryotes, mitochondria, chloroplasts; 80S (60S + 40S) in eukaryotic cytoplasm.Endomembrane system = ER + Golgi + lysosomes + vacuoles.Double-membrane organelles: mitochondria, chloroplasts (and nucleus has a double envelope).9+2 axoneme; centriole = 9 triplets.Exam Focus (from exercises)
Which statement is not correct? – "Robert Brown discovered the cell" is incorrect (he discovered the nucleus).Match: cristae → infoldings in mitochondria; cisternae → disc-shaped sacs in Golgi; thylakoids → flat membranous sacs in stroma.Mesosome and its functions; how neutral and polar molecules cross membranes.Characteristics of prokaryotic cells; nuclear pores; lysosomes vs vacuoles.Classification of chromosomes by centromere position with diagram.Key Terms
Cell (कोशिका), Prokaryote (प्राक्केन्द्रकी), Eukaryote (सुकेन्द्रकी), Plasma membrane (जीवद्रव्य झिल्ली), Cell wall (कोशिका भित्ति), Endoplasmic reticulum (अंतर्द्रव्यी जालिका), Golgi apparatus (गॉल्जी उपकरण), Lysosome (लयनकाय), Vacuole (रिक्तिका), Mitochondria (सूत्रकणिका), Plastid (लवक), Ribosome (राइबोसोम), Nucleolus (केन्द्रिक), Chromosome (गुणसूत्र), Centromere (गुणसूत्र बिंदु)Common Mistakes
Crediting Robert Brown with discovering the cell – he discovered the nucleus.Including mitochondria or chloroplasts in the endomembrane system – they are not included.Writing that mycoplasma has a cell wall – it lacks one.Mixing up 9+2 (cilia axoneme doublets) with 9 triplets (centriole).Confusing acrocentric (near end) with telocentric (terminal) chromosomes.