Cell - The Unit of Life

2.1 Core concepts

• Cell is the fundamental structural and functional unit of all living organisms; anything less than a complete cell cannot ensure independent living (NCERT §8.1, p. 87). Antonie Von Leeuwenhoek first saw and described a live cell; Robert Brown later discovered the nucleus (NCERT §8.1, p. 87).
• Cell theory: Matthias Schleiden (1838, German botanist) — all plants are composed of cells forming tissues; Theodore Schwann (1839, German zoologist) — animal cells have a thin outer "plasma membrane" and cell wall is unique to plants; Rudolf Virchow (1855) added that all cells arise from pre-existing cells (Omnis cellula-e cellula). Modern theory: (i) all living organisms are composed of cells and products of cells, (ii) all cells arise from pre-existing cells (NCERT §8.2, p. 87–88).
• Eukaryotic vs prokaryotic: eukaryotic cells have membrane-bound nuclei and organelles; prokaryotic cells lack membrane-bound nucleus and organelles (except ribosomes). Cytoplasm is the main arena of cellular activity in both (NCERT §8.3, p. 88).
• Size & shape range: Mycoplasmas (smallest cells) are 0.3 µm; bacteria 3–5 µm; human RBCs ~7.0 µm; ostrich egg is the largest isolated single cell; nerve cells are among the longest. Cells may be disc-like, polygonal, columnar, cuboidal, thread-like or irregular (NCERT §8.3, p. 88–89, Figure 8.1).
• Prokaryotes include bacteria, blue-green algae, mycoplasma and PPLO (Pleuro Pneumonia Like Organisms). Four bacterial shapes: bacillus (rod), coccus (spherical), vibrio (comma), spirillum (spiral) (NCERT §8.4, p. 89). Genetic material is naked circular DNA; many bacteria also carry plasmids (small extra circular DNA) which confer phenotypic traits like antibiotic resistance (NCERT §8.4, p. 90).
• Cell envelope (Gram +/−): outermost glycocalyx → cell wall → plasma membrane, acting as a single protective unit; bacteria taking Gram stain are Gram positive, others Gram negative. Glycocalyx may be a loose slime layer or a thick tough capsule (NCERT §8.4.1, p. 90). Mesosomes are infoldings of plasma membrane (vesicles, tubules, lamellae) that aid cell-wall formation, DNA replication & distribution, respiration, secretion, and increase surface area (NCERT §8.4.1, p. 90–91). Chromatophores are pigment-bearing membranous extensions in cyanobacteria (NCERT §8.4.1, p. 91).
• Bacterial flagellum has three parts — filament, hook and basal body; filament is the longest portion extending outward (NCERT §8.4.1, p. 91). Pili are elongated tubular protein structures and fimbriae are small bristle-like fibres; neither helps in motility — fimbriae help attachment to rocks/host tissues (NCERT §8.4.1, p. 91).
• Prokaryotic ribosomes are 70S (50S + 30S subunits), ~15 nm × 20 nm, sites of protein synthesis; several ribosomes on a single mRNA form a polyribosome/polysome (NCERT §8.4.2, p. 91). Inclusion bodies are non-membrane-bound reserve stores in cytoplasm — phosphate granules, cyanophycean granules, glycogen granules; gas vacuoles occur in blue-green, purple and green photosynthetic bacteria (NCERT §8.4.2, p. 91).
• Plasma membrane (fluid mosaic model, Singer & Nicolson, 1972): phospholipid bilayer with polar heads outside and hydrophobic tails inside, plus cholesterol; integral proteins are partially/totally buried, peripheral lie on the surface; quasi-fluid lipid allows lateral movement of proteins (NCERT §8.5.1, p. 93–94, Figure 8.4). Human erythrocyte membrane is ~52% protein, ~40% lipid (NCERT §8.5.1, p. 93). Transport: passive transport (no energy — simple diffusion, osmosis for water); active transport uses ATP against gradient, e.g., Na⁺/K⁺ pump (NCERT §8.5.1, p. 94).
• Cell wall (fungi and plants): algal walls of cellulose, galactans, mannans, CaCO₃; other plants — cellulose, hemicellulose, pectins, proteins. Primary wall is growing (young cell); secondary wall forms later on the inner side. Middle lamella (mainly calcium pectate) glues neighbouring cells; plasmodesmata traverse wall + middle lamella connecting adjacent cytoplasms (NCERT §8.5.2, p. 94).
• Endomembrane system = ER + Golgi + lysosomes + vacuoles (mitochondria, chloroplasts, peroxisomes excluded because their functions are not coordinated with these) (NCERT §8.5.3, p. 94–95).
• ER: network of tubular structures; RER bears ribosomes — active in protein synthesis & secretion, continuous with outer nuclear membrane; SER lacks ribosomes — major site of lipid synthesis, makes steroidal hormones in animal cells (NCERT §8.5.3.1, p. 95, Figure 8.5).
• Golgi apparatus (Camillo Golgi, 1898): stacked flat disc-shaped cisternae (0.5–1.0 µm); convex cis/forming face receives ER vesicles, concave trans/maturing face ships them out; principal site of packaging and of glycoprotein/glycolipid formation (NCERT §8.5.3.2, p. 95–96, Figure 8.6).
• Lysosomes are golgi-derived membrane-bound vesicles rich in hydrolytic enzymes (lipases, proteases, carbohydrases — hydrolases) optimally active at acidic pH; digest carbohydrates, proteins, lipids and nucleic acids (NCERT §8.5.3.3, p. 96).
• Vacuoles are bound by a single membrane called tonoplast; in plant cells can occupy up to 90% of cell volume; tonoplast actively transports ions/materials against gradient. Amoeba has contractile vacuoles for osmoregulation/excretion; protists form food vacuoles by engulfing food (NCERT §8.5.3.4, p. 96).
• Mitochondria: sausage-shaped, 0.2–1.0 µm diameter (avg. 0.5 µm) × 1.0–4.1 µm long; double membrane — outer is continuous limiting boundary, inner forms cristae (infoldings) towards the matrix which increases surface area; matrix has single circular DNA, RNAs, 70S ribosomes; sites of aerobic respiration producing ATP — "power houses of the cell"; divide by fission (NCERT §8.5.4, p. 96–97, Figure 8.7).
• Plastids (in plant cells and euglenoides): chloroplasts (chlorophyll + carotenoid — photosynthesis), chromoplasts (fat-soluble carotene/xanthophyll — yellow/orange/red colour), leucoplasts (colourless storage) — amyloplasts (starch, e.g., potato), elaioplasts (oils/fats), aleuroplasts (proteins) (NCERT §8.5.5, p. 97–98). Chloroplast: 5–10 µm long, 2–4 µm wide; Chlamydomonas has 1, mesophyll cells 20–40; double-membraned; inner space = stroma containing enzymes, circular DNA, 70S ribosomes; flattened thylakoids stacked as grana, with stroma lamellae linking grana; thylakoid lumen inside (NCERT §8.5.5, p. 98, Figure 8.8).
• Ribosomes (George Palade, 1953): RNA + protein, no membrane. Eukaryotic = 80S (60S + 40S); prokaryotic = 70S (50S + 30S). 'S' = Svedberg's sedimentation coefficient (NCERT §8.5.6, p. 98, Figure 8.9).
• Cytoskeleton: network of microtubules, microfilaments and intermediate filaments — mechanical support, motility, shape maintenance (NCERT §8.5.7, p. 98).
• Cilia & flagella: hair-like outgrowths of cell membrane; cilia are short (oar-like), flagella longer (cell movement); eukaryotic flagella are structurally different from prokaryotic ones. Core = axoneme with 9 doublets of peripheral microtubules + 1 central pair = 9+2 array; central pair enclosed by a central sheath connected to peripheral doublets by radial spokes (nine of them); peripheral doublets linked by interdoublet bridges/linkers; arise from basal bodies (centriole-like) (NCERT §8.5.8, p. 99, Figure 8.10).
• Centrosome & centrioles: centrosome has two cylindrical centrioles lying perpendicular to each other in a cartwheel organisation, surrounded by pericentriolar material; each centriole = 9 peripheral fibrils of tubulin, each a triplet; central proteinaceous hub linked to triplets by radial spokes; centrioles form basal bodies of cilia/flagella and the spindle apparatus in animal cell division (NCERT §8.5.9, p. 99–100).
• Nucleus (Robert Brown, 1831): bound by a double-membraned nuclear envelope with perinuclear space (10–50 nm); outer membrane is continuous with ER and bears ribosomes; nuclear pores (formed by fusion of the two membranes) allow RNA/protein exchange. Nucleoplasm holds nucleolus (non-membrane-bound, site of ribosomal RNA synthesis — larger/numerous in protein-active cells) and chromatin (DNA + histones + non-histone proteins + RNA) (NCERT §8.5.10, p. 100, Figure 8.11). A human cell has ~2 m of DNA across 46 (23 pairs) chromosomes (NCERT §8.5.10, p. 101).
• Chromosomes: have a primary constriction — the centromere — with disc-shaped kinetochores; by centromere position — metacentric (middle, equal arms), sub-metacentric (slightly off middle, one short + one long arm), acrocentric (near end, one very short + one very long arm), telocentric (terminal centromere). Secondary constrictions can give a small satellite fragment (NCERT §8.5.10, p. 101–102, Figures 8.12, 8.13).
• Microbodies: membrane-bound minute vesicles containing various enzymes; present in both plant and animal cells (NCERT §8.5.11, p. 102).

2.2 Definitions to memorise

2.3 Diagrams / processes to remember

• Figure 8.1, p. 89 — different cell shapes: biconcave RBCs, amoeboid WBCs, long-narrow columnar epithelial cells, branched nerve cell, elongated tracheid, round/oval mesophyll cells. Classic distractor set for "Which cell is amoeboid?" type questions.
• Figure 8.2, p. 90 — relative-size comparison: typical eukaryotic cell (10–20 µm) vs typical bacterium (1–2 µm) vs PPLO (~0.1 µm) vs viruses (0.02–0.2 µm).
• Figure 8.3, p. 92 — labelled (a) plant cell with cell wall, middle lamella, plasmodesmata, chloroplast, large vacuole; (b) animal cell with microvilli, centriole, no cell wall/plastid. Note: plant cell shows all the endomembrane organelles too.
• Figure 8.4, p. 93 — fluid mosaic model labels: phospholipid bilayer, integral protein, peripheral protein, cholesterol, sugar (glycoprotein/glycolipid moieties).
• Figure 8.5, p. 95 — nuclear envelope continuous with RER; ribosomes studded on RER; smooth ER without ribosomes; nuclear pore visible.
• Figure 8.6, p. 95 — Golgi cisternae stacked with vesicles pinching off.
• Figure 8.7, p. 97 — mitochondrion (LS): outer membrane, inner membrane, inter-membrane space, matrix, crista.
• Figure 8.8, p. 98 — chloroplast (sectional): outer + inner membranes, stroma, granum (stack of thylakoids), individual thylakoid, stroma lamella.
• Figure 8.9, p. 98 — ribosome shown as larger (60S/50S) + smaller (40S/30S) subunit.
• Figure 8.10, p. 99 — cross-section of cilium/flagellum showing 9+2 array: peripheral doublets, central pair, central sheath, radial spokes, interdoublet bridge, plasma membrane sheath.
• Figure 8.11, p. 100 — nucleus: nuclear membrane (double), nuclear pore, nucleolus, nucleoplasm.
• Figure 8.12, p. 101 — chromosome with two chromatids and a kinetochore at centromere.
• Figure 8.13, p. 101 — four chromosome types side-by-side: metacentric (V-shaped, equal arms, satellite + secondary constriction labelled), sub-metacentric (L-shaped), acrocentric (J-shaped — one very short arm), telocentric (rod-shaped — terminal centromere).

2.4 Common confusions / NTA trap points

• Robert Brown discovered the nucleus, NOT the cell. Leeuwenhoek first saw a live cell. (NCERT §8.1, p. 87 — also tested in NCERT exercise Q1.)
• Mesosome ≠ chromatophore. Both are infoldings of prokaryotic plasma membrane, but chromatophores are pigmented (cyanobacteria); mesosomes are general (NCERT §8.4.1, p. 90–91).
• Pili vs fimbriae vs flagella. Only flagella are motile; pili and fimbriae are non-motile surface structures — fimbriae help attachment (NCERT §8.4.1, p. 91).
• Ribosome S-values. 70S = prokaryotic AND mitochondrial AND chloroplast ribosomes; 80S = eukaryotic cytoplasmic. 'S' is Svedberg sedimentation coefficient — not the sum of subunits (50 + 30 ≠ 80; they're not additive) (NCERT §8.4.2, p. 91; §8.5.6, p. 98).
• Endomembrane system excludes mitochondria, chloroplasts and peroxisomes even though they are membrane-bound (NCERT §8.5.3, p. 95).
• Cristae vs Cisternae vs Thylakoids — classic match-the-following: cristae = infoldings in mitochondria; cisternae = disc-shaped sacs in Golgi; thylakoids = flat membranous sacs in stroma of chloroplast (NCERT exercise Q3, p. 103).
• Tonoplast is the vacuolar membrane, not the plasma membrane (NCERT §8.5.3.4, p. 96).
• Centriole is found in animal cells; centrioles are absent in almost all plant cells. Plant cells have plastids and large vacuoles instead (NCERT §8.5, p. 91).