Cell: The Unit of Life
The cell is the basic structural and functional unit of all life — this chapter establishes cell theory and contrasts the simpler prokaryotic cell with the compartmentalised eukaryotic cell.
A perennial NCERT favourite for Prelims, where match-the-scientist (Schleiden–Schwann–Virchow), prokaryote-versus-eukaryote distinctions, and bacterial structures (plasmids, ribosomes, flagella) are directly asked. For GS-III Science & Technology it underpins biotechnology, recombinant DNA and the antimicrobial-resistance debate, while G.N. Ramachandran recurs in Indian-scientist questions.
Understand the chapter
What Is a Cell, and Reductionist Biology
The cell is the fundamental structural and functional unit of all living organisms; anything less than a complete cell cannot live independently. Organisms are either unicellular (one cell performing every life function) or multicellular. To decode life's processes, biologists adopt a physico-chemical, cell-free approach called Reductionist Biology, which describes physiological processes in molecular terms.
- Unicellular organisms can exist independently and perform all essential life functions.
- Reductionist Biology applies physics & chemistry to explain digestion, excretion, memory, defense, etc., molecularly.
- It also explains abnormal processes occurring in diseased conditions.
Cell Theory and Its Architects
Matthias Schleiden (1838), a German botanist, found all plants are made of cells; Theodore Schwann (1839), a German zoologist, studied animal cells, identified the 'plasma membrane', and noted that a cell wall is unique to plant cells. Together they proposed that bodies of plants and animals are composed of cells and products of cells. Their theory could not explain how new cells arise — Rudolf Virchow (1855) supplied 'Omnis cellula-e cellula' (cells arise from pre-existing cells), giving cell theory its final shape.
- Tenet 1: all living organisms are composed of cells and products of cells.
- Tenet 2: all cells arise from pre-existing cells (Virchow).
- Antonie von Leeuwenhoek first described a live cell; Robert Brown discovered the nucleus.
Prokaryotic vs Eukaryotic Cells
Cells with a membrane-bound nucleus are eukaryotic; those lacking it are prokaryotic. Both share a plasma membrane, cytoplasm and ribosomes, but only eukaryotes have membrane-bound organelles (ER, Golgi, lysosomes, mitochondria, vacuoles) and chromosomes within an enveloped nucleus. Prokaryotes — bacteria, blue-green algae, mycoplasma and PPLO — are smaller, multiply faster, and carry naked genetic material.
- Eukaryotes: protists, plants, animals, fungi — compartmentalised cytoplasm, organised nucleus.
- Ribosomes (non-membrane-bound) occur in BOTH; also in mitochondria, chloroplasts and on rough ER.
- Centrosome (animal cells) is a non-membrane organelle that aids cell division.
- Smallest cell: Mycoplasma; largest isolated single cell: ostrich egg.
Inside the Prokaryotic Cell
The bacterial cell envelope is a tightly bound three-layered unit — glycocalyx (loose slime layer or tough capsule), cell wall (gives shape, prevents bursting), and plasma membrane. Bacteria are classed as Gram-positive or Gram-negative by their response to the Gram stain. The mesosome — infoldings of the plasma membrane — aids cell-wall formation, DNA replication, respiration and secretion, while flagella provide motility; pili and fimbriae do not.
- Prokaryotic ribosome = 70S (50S + 30S), ~15 nm x 20 nm, the site of protein synthesis; chain on mRNA = polysome.
- Plasmids: small extra circular DNA giving traits like antibiotic resistance; used to monitor bacterial transformation.
- Flagellum parts: filament (longest), hook, basal body; chromatophores (in cyanobacteria) hold pigments.
- Inclusion bodies (phosphate, cyanophycean, glycogen granules; gas vacuoles) store reserves and are membrane-free.
Plasma Membrane: Fluid Mosaic Model & Transport
The plasma membrane is chiefly a phospholipid bilayer (polar heads facing outward, hydrophobic tails inward) with cholesterol and proteins. Singer and Nicolson (1972) gave the widely accepted fluid mosaic model, in which the quasi-fluid lipid lets proteins move laterally — this fluidity enables growth, secretion, endocytosis and division. Being selectively permeable, the membrane controls transport across it.
- Passive transport (no energy): simple diffusion down the gradient; osmosis = diffusion of water.
- Active transport (uses ATP): moves molecules against the gradient, e.g., Na+/K+ pump.
- Polar molecules need a carrier protein to cross the nonpolar bilayer.
- Membrane proteins are integral (buried) or peripheral (surface); human RBC membrane is about 52% protein, 40% lipid.
Cell Wall, Junctions & Plant vs Animal Cells
The cell wall is a non-living, rigid outer covering of plant and fungal cells that gives shape, protects against mechanical damage and infection, and allows cell-to-cell interaction. A young cell's primary wall can grow; a secondary wall forms on the inner side as the cell matures. The middle lamella, mainly calcium pectate, glues adjacent cells, and key organelles distinguish plant from animal cells.
- Algal wall: cellulose, galactans, mannans, calcium carbonate; other plants: cellulose, hemicellulose, pectins, proteins.
- Plant-cell exclusives: cell wall, plastids, large central vacuole.
- Animal-cell exclusive: centrioles (centrosome), absent in almost all plant cells.
- Middle lamella = calcium pectate, cementing neighbouring cells.
Key terms
- Cell theory
- All organisms are made of cells and products of cells, and all cells arise from pre-existing cells.
- Prokaryotic cell
- Cell lacking a membrane-bound nucleus and membrane-bound organelles (e.g., bacteria).
- Eukaryotic cell
- Cell with a true membrane-bound nucleus and compartmentalised organelles.
- Mesosome
- Infoldings of the prokaryotic plasma membrane aiding cell-wall formation, DNA replication, respiration and secretion.
- Plasmid
- Small extra-chromosomal circular DNA in bacteria conferring traits such as antibiotic resistance.
- Glycocalyx
- Outermost bacterial envelope layer — a loose slime layer or a tough capsule.
- Fluid mosaic model
- Singer–Nicolson model of proteins moving laterally within a quasi-fluid lipid bilayer.
- Active transport
- ATP-dependent movement of molecules against the concentration gradient, e.g., Na+/K+ pump.
- Middle lamella
- Calcium-pectate layer that cements adjacent plant cells together.
Must-know facts exam-ready
- Cell theory: formulated by Schleiden (1838, botanist) & Schwann (1839, zoologist); finalised by Rudolf Virchow (1855) — 'Omnis cellula-e cellula'.
- Antonie von Leeuwenhoek first described a live cell; Robert Brown discovered the nucleus.
- Prokaryotes = Bacteria, Blue-green algae (cyanobacteria), Mycoplasma, PPLO.
- Mycoplasma is the smallest cell (~0.3 µm) and lacks a cell wall; bacteria are 3–5 µm; human RBC ~7.0 µm; ostrich egg is the largest isolated single cell.
- Prokaryotic ribosome = 70S (50S + 30S subunits), ~15 nm x 20 nm; eukaryotic = 80S; ribosomes are the site of protein synthesis.
- Bacterial flagellum has three parts — filament (longest), hook, basal body; pili and fimbriae are non-motile.
- Cell envelope (outer→inner): glycocalyx → cell wall → plasma membrane; Gram-positive take the Gram stain, Gram-negative do not.
- Four bacterial shapes: bacillus (rod), coccus (spherical), vibrio (comma), spirillum (spiral).
- Fluid mosaic model proposed by Singer & Nicolson (1972); membrane = phospholipid bilayer + cholesterol + proteins.
- Human erythrocyte membrane ≈ 52% protein and 40% lipid; proteins are integral or peripheral.
- Na+/K+ pump is active transport (uses ATP); osmosis = diffusion of water across the membrane.
- G.N. Ramachandran — the 'Ramachandran plot' and the triple-helix structure of collagen (Nature, 1954); founder of the Madras school.
Timeline
- 1838Schleiden concludes all plants are composed of cells.
- 1839Schwann studies animal cells, identifies the plasma membrane, and co-formulates cell theory.
- 1855Virchow shows cells arise from pre-existing cells, completing cell theory.
- 1954G.N. Ramachandran publishes the triple-helical structure of collagen in Nature.
- 1972Singer & Nicolson propose the fluid mosaic model of the membrane.
Memory tricks remember it for good
Traps to avoid
- Schleiden = botanist (plants); Schwann = zoologist (animals) — UPSC swaps them; it was Schwann who noted the plasma membrane and that a cell wall is unique to plants.
- 'Omnis cellula-e cellula' / 'cells from pre-existing cells' is Virchow (1855) — Schleiden & Schwann could NOT explain how new cells form.
- Robert Brown discovered the nucleus; Leeuwenhoek only first described a live cell — do not interchange.
- Ribosomes are non-membrane-bound and present in BOTH prokaryotes and eukaryotes — they are not absent in prokaryotes.
- Among surface structures only flagella give motility — pili and fimbriae do not; and Mycoplasma is the bacterium that LACKS a cell wall.
- Centrioles/centrosome belong to animal cells (absent in almost all plant cells), while plastids and a large central vacuole are plant-cell features — easy to flip.
Exam focus
🧠 Prelims angles
- Match-the-scientist: Schleiden, Schwann, Virchow, Brown, Leeuwenhoek, Singer & Nicolson.
- Prokaryote vs eukaryote: membrane-bound nucleus/organelles, naked vs organised DNA, 70S vs 80S ribosomes.
- Non-membrane-bound organelles (ribosomes, centrosome) and where ribosomes occur — cytoplasm, rough ER, mitochondria, chloroplasts.
- Bacterial features: flagellum parts, pili/fimbriae (non-motile), mesosome, plasmid (antibiotic resistance), inclusion bodies, Gram +ve/−ve.
- Plant vs animal cell differences — cell wall, plastids, central vacuole versus centrioles.
- Indian-scientist focus: G.N. Ramachandran — the Ramachandran plot and the collagen triple helix.
✍️ Mains angles GS-III
- Plasmid-borne antibiotic resistance and the rise of antimicrobial resistance (AMR).Link plasmids and horizontal gene transfer to AMR as a public-health and biotech-regulation challenge.
- Reductionist (physico-chemical) biology — its strengths and limits in understanding life.Argue it powers molecular biology and biotech, but note emergent, system-level properties it can miss.
- India's contribution to fundamental structural biology.Use G.N. Ramachandran (Ramachandran plot, collagen) as a flagship example of indigenous basic science.
Last-minute revision tick as you recall
- Cell = fundamental structural & functional unit of life; cells arise only from pre-existing cells (Virchow).
- Cell theory: Schleiden (1838) + Schwann (1839) built it; Virchow (1855) completed it.
- Prokaryotes 'BBMP' (Bacteria, BGA, Mycoplasma, PPLO): no membrane-bound organelles, naked DNA, 70S ribosomes.
- Mycoplasma = smallest cell & no cell wall; ostrich egg = largest single cell.
- Cell envelope outside-in: Glycocalyx → Cell wall → Plasma membrane; Gram +ve take stain, −ve don't.
- Mesosome = infolded membrane (DNA replication, respiration, secretion); flagellum = filament + hook + basal body; pili/fimbriae ≠ motility.
- Plasmid = circular DNA → antibiotic resistance & bacterial transformation.
- Fluid mosaic model — Singer & Nicolson (1972); membrane = lipid bilayer + cholesterol + proteins; active transport via Na+/K+ pump (ATP).
- Plant cell: cell wall, plastids, large vacuole; animal cell: centriole; middle lamella = calcium pectate.
Distilled from NCERT Class 11 · Biology (Class 11) for UPSC. Always cross-check facts with the original NCERT.