Cell biology

In short

Cell biology is the study of the cell as the structural and functional unit of life — its membranes, organelles, cytoskeleton, division cycle, death and the signalling that coordinates all of it. For a biotechnology engineer it is the operating manual of the production host: every mammalian cell line making a monoclonal antibody, every yeast fermenting ethanol and every stem-cell therapy depends on controlling cell growth, viability and secretion. It carries high weight in GATE Biotechnology (BT) and in university Cell Biology papers, usually as organelle function, transport calculations, cell-cycle checkpoints and microscopy. In Indian industry it is tested directly at interview by Biocon, Serum Institute of India, Bharat Biotech, Syngene, Dr. Reddy's, Panacea Biotec and Thermo Fisher/Merck application-scientist roles, and in CSIR-UGC NET, ICMR-JRF and DBT-BET screening.

Explained

Cell theory, cell types and the microscopes that reveal them

• Cell theory (Schleiden, Schwann 1838–39; Virchow 1855): all organisms are made of cells, the cell is the basic unit of structure and function, and new cells arise only from pre-existing cells (*omnis cellula e cellula*).
• Modern additions: cells carry heritable information as DNA, and all cells have broadly the same basic chemistry and energy currency (ATP).
• Prokaryote: no nuclear envelope, no membrane-bound organelles, 70S ribosomes, a single circular genome in a nucleoid, 0.2–2 µm. Eukaryote: true nucleus, compartmentalised organelles, 80S ribosomes, linear chromosomes with histones, 10–100 µm.
• Surface-area-to-volume ratio limits cell size: as radius rises, volume grows as r³ but area only as r², so diffusion cannot supply a large cell — hence small prokaryotes, microvilli, and flattened or elongated eukaryotic shapes.
• Resolution limit d = 0.61λ/(n·sinθ) ≈ 0.2 µm for a light microscope — organelles below this need electron microscopy (TEM ~0.2 nm for ultrastructure; SEM for surfaces).
• Techniques to know: phase contrast (unstained live cells), fluorescence and confocal (GFP-tagged proteins, optical sectioning), flow cytometry (per-cell fluorescence, DNA content), differential and density-gradient centrifugation (organelle fractionation), immunofluorescence and Western blotting for localisation.

Plasma membrane, transport and the cell wall

• Fluid mosaic model (Singer and Nicolson, 1972): a phospholipid bilayer about 7.5–10 nm thick with integral (transmembrane) and peripheral proteins floating in it, glycolipids and glycoproteins outward-facing, cholesterol between the tails.
• Lipid asymmetry: phosphatidylcholine and sphingomyelin outside; phosphatidylserine and phosphatidylethanolamine inside — PS flipping outward is an early apoptosis signal (the basis of the annexin-V assay).
• Cholesterol buffers fluidity: it stiffens the membrane above the transition temperature and prevents tight packing below it. Bacteria use hopanoids; membrane fluidity rises with unsaturated and shorter fatty-acid chains and with temperature.
• Passive transport: simple diffusion (O₂, CO₂, steroids), facilitated diffusion via channels (aquaporins, ion channels) or carriers (GLUT1), and osmosis — water moving to the higher solute concentration. No ATP; moves down the gradient only.
• Active transport: primary pumps hydrolyse ATP — Na⁺/K⁺-ATPase (3 Na⁺ out : 2 K⁺ in, electrogenic), Ca²⁺-ATPase (SERCA), H⁺/K⁺-ATPase, ABC transporters (P-glycoprotein — a cause of multidrug resistance). Secondary transport (symport/antiport, e.g. SGLT1 glucose–Na⁺) rides an existing ion gradient.
• Bulk transport: phagocytosis, pinocytosis, clathrin-mediated receptor-mediated endocytosis (LDL receptor), caveolae, and exocytosis (constitutive or regulated).
• Membrane potential arises from unequal ion distribution plus selective permeability; resting potential of a typical mammalian cell is about −60 to −90 mV, set largely by K⁺.
• Cell walls: peptidoglycan (bacteria; target of penicillin), pseudomurein (archaea), chitin (fungi), cellulose–hemicellulose–pectin (plants) — relevant to lysis strategies in downstream processing.
• Cell junctions: tight junctions (occludin, claudin — seal), desmosomes and adherens junctions (mechanical, via cadherins), gap junctions (connexons — direct cytoplasmic coupling), plasmodesmata in plants; integrins link cytoskeleton to the extracellular matrix.

Organelles and the endomembrane system

• Nucleus: double envelope continuous with the rough ER, perforated by nuclear pore complexes (~120 nm, nucleoporins; import via nuclear localisation signals and importins). Contains chromatin (euchromatin active, heterochromatin condensed) and the nucleolus, where rRNA is transcribed and ribosomal subunits assemble.
• Ribosomes: 80S (60S + 40S) in eukaryotic cytosol, 70S (50S + 30S) in prokaryotes, mitochondria and chloroplasts — the reason many antibiotics are selective.
• Rough ER: co-translational entry of proteins carrying a signal peptide (Blobel's signal hypothesis), N-linked glycosylation, disulphide-bond formation, chaperone-assisted folding (BiP, calnexin); misfolding triggers the unfolded-protein response and ER-associated degradation. Smooth ER: lipid and steroid synthesis, drug detoxification (cytochrome P450), Ca²⁺ store; sarcoplasmic reticulum in muscle.
• Golgi apparatus: cis → medial → trans cisternae and the trans-Golgi network; O-linked glycosylation, terminal glycan trimming, sulphation, proteolytic processing and sorting to lysosomes, secretory vesicles or the membrane. Vesicle coats: COPII (ER→Golgi), COPI (retrograde), clathrin (TGN and membrane); SNAREs and Rab GTPases give docking specificity.
• Lysosome: ~40–60 acid hydrolases at pH 4.5–5, maintained by a V-type H⁺-ATPase; mannose-6-phosphate tags enzymes for delivery. Autophagy, heterophagy; lysosomal storage diseases — Tay–Sachs, Gaucher, I-cell disease (defective M6P tagging). Enzyme-replacement therapies for these are a live Indian biopharma area.
• Peroxisome: β-oxidation of very-long-chain fatty acids, H₂O₂ generation by oxidases and its removal by catalase; glyoxysomes in germinating seeds.
• Mitochondrion: outer membrane (porins), inner membrane folded into cristae carrying complexes I–IV plus ATP synthase, intermembrane space, matrix with its own circular DNA, 70S ribosomes and tRNAs. Site of the TCA cycle, oxidative phosphorylation and apoptotic cytochrome-c release; maternally inherited; semi-autonomous.
• Chloroplast: double envelope, stroma, thylakoid grana, own genome — light reactions on thylakoids, Calvin cycle in the stroma.
• Endosymbiotic theory (Margulis): mitochondria and chloroplasts descend from engulfed prokaryotes — evidence is double membranes, circular DNA, 70S ribosomes, binary-fission-like division and sensitivity to bacterial antibiotics.
• Vacuole (plant turgor, storage, pigment), centrosome/centriole (9 triplets), cilia and flagella (9+2 axoneme, dynein-driven; prokaryotic flagella are completely different — a rotating flagellin filament driven by proton-motive force).

Cytoskeleton and cell motility

• Microtubules are polar (− end at centrosome, + end outward) and show dynamic instability; kinesin walks to the + end, dynein to the − end.
• Myosin II on actin drives muscle contraction and the cytokinetic ring; myosin V transports vesicles.
• Motility modes: amoeboid crawling (actin polymerisation at the leading edge, integrin adhesion, rear contraction), ciliary beating, flagellar swimming.

Cell cycle, division and its control

• Interphase: G1 (growth, decision to divide), S (DNA replication, 2C→4C), G2 (growth, repair check); G0 is a reversible quiescent exit. M phase is mitosis plus cytokinesis. In a 24 h cultured mammalian cell, roughly G1 11 h, S 8 h, G2 4 h, M 1 h (varies with the line).
• Mitosis: prophase (chromosome condensation, spindle forms) → prometaphase (envelope breaks down, kinetochore capture) → metaphase (chromosomes on the equatorial plate) → anaphase (sister chromatids separate after separase cleaves cohesin) → telophase (envelope reforms) → cytokinesis (actin–myosin furrow in animals; cell plate from Golgi vesicles in plants). Result: two genetically identical diploid cells.
• Meiosis: one round of replication, two divisions, four haploid cells. Meiosis I is reductional — prophase I (leptotene, zygotene with synapsis and synaptonemal complex, pachytene with crossing over at chiasmata, diplotene, diakinesis), independent assortment at metaphase I, homologues separate at anaphase I. Meiosis II is equational, like mitosis without replication. Source of genetic variation.
• Checkpoints: G1/S restriction point (DNA damage, size, nutrients — p53 → p21 → Cdk inhibition), G2/M (replication complete and DNA undamaged), spindle-assembly checkpoint at metaphase (Mad2 holds APC/C until every kinetochore is attached).
• Engine: cyclin–Cdk complexes. Cyclin D–Cdk4/6 and cyclin E–Cdk2 in G1/S (Rb phosphorylation releases E2F), cyclin A–Cdk2 in S, cyclin B–Cdk1 (MPF) at G2/M. Cyclins are destroyed by APC/C-directed ubiquitin–proteasome degradation — this irreversibility makes the cycle one-way.
• Oncogenes (gain of function — RAS, MYC, HER2) and tumour suppressors (loss of function — p53, Rb, BRCA1) deregulate this control; p53 is mutated in roughly half of human cancers.
• Cell death: apoptosis is programmed, ATP-dependent and non-inflammatory — intrinsic pathway (mitochondrial cytochrome-c + Apaf-1 → apoptosome → caspase-9) or extrinsic (Fas/TNF receptor → caspase-8), converging on executioner caspases-3/7; Bcl-2 is anti-apoptotic, Bax/Bak pro-apoptotic. Morphology: shrinkage, chromatin condensation, DNA laddering, membrane blebs. Necrosis is passive, swelling, lysis, inflammatory. Autophagy and necroptosis are intermediate routes.

Cell signalling, extracellular matrix and the cultured cell

• Signal types: endocrine (hormones via blood), paracrine (neighbours), autocrine (self — many tumour cells), juxtacrine (contact), synaptic.
• Receptor classes: GPCRs (7-TM, G proteins, cAMP–PKA or IP₃/DAG–Ca²⁺–PKC second messengers), receptor tyrosine kinases (insulin, EGF — dimerise, autophosphorylate, trigger RAS–MAPK and PI3K–Akt), ion-channel receptors (nicotinic acetylcholine), intracellular/nuclear receptors (steroids, thyroid hormone — act as transcription factors), cytokine receptors (JAK–STAT).
• Signalling logic: amplification at each step, specificity by receptor expression, adaptation by receptor desensitisation and internalisation, termination by phosphatases and GTP hydrolysis.
• Extracellular matrix: collagen (tensile), elastin, fibronectin and laminin (adhesion, via integrins), proteoglycans and hyaluronan (hydrated gel); the basis of scaffolds in tissue engineering.
• Cell culture practice (the biotech-specific core): primary culture → finite cell line → continuous/immortal line; adherent (monolayer, contact-inhibited) versus suspension. Anchorage dependence, confluency, passage number, doubling time, split ratio, trypsinisation, cryopreservation in 5–10% DMSO, Hayflick limit (~50 divisions for normal human fibroblasts).
• Media and conditions: DMEM/RPMI/Ham's F-12, 5–10% fetal bovine serum or defined serum-free media, L-glutamine, bicarbonate buffer with phenol red, 37 °C, 5% CO₂, >90% humidity; antibiotic-antimycotic only as a backstop, not a substitute for aseptic technique in a Class II biosafety cabinet.
• Workhorse lines in Indian industry: CHO (recombinant antibodies, Biocon/Dr. Reddy's biosimilars), HEK293 (viral vectors, transient expression), Vero (vaccine production — Serum Institute, Bharat Biotech), MDCK (influenza), Sf9/High Five (insect, baculovirus), hybridomas (monoclonals).
• Viability and counting: trypan blue exclusion on a haemocytometer (Neubauer), MTT/XTT metabolic assays, LDH release, flow-cytometric propidium iodide or 7-AAD staining.
• Pitfalls: mycoplasma contamination (invisible by light microscopy — PCR test), cross-contamination and misidentification (STR profiling is now standard), genetic drift at high passage.
• Indian regulation to know: institutional biosafety committees under the Rules for the Manufacture, Use, Import, Export and Storage of Hazardous Micro-organisms/GMOs (1989) under the Environment (Protection) Act, 1986, overseen by RCGM (DBT) and GEAC (MoEFCC); the *National Guidelines for Stem Cell Research* (ICMR–DBT, latest revision 2017) and ICMR's *National Ethical Guidelines for Biomedical and Health Research Involving Human Participants* (2017); CDSCO and the New Drugs and Clinical Trials Rules, 2019 for cell-based products; laboratory biosafety levels BSL-1 to BSL-4 per DBT guidelines. The Labour Codes in force from 21 November 2025 now cover occupational safety for such laboratories in place of the Factories Act. Check the regulators' sites for the current edition — these guidelines are revised periodically.

Key points

• Cell theory - cells are the unit of life and arise only from pre-existing cells.
• Prokaryote vs eukaryote - no nucleus/organelles and 70S ribosomes versus true nucleus, compartments and 80S ribosomes.
• Surface-to-volume ratio - the physical reason cells stay small; falls as 3/r with increasing radius.
• Fluid mosaic model - lipid bilayer with embedded proteins, asymmetric, fluidity tuned by cholesterol and unsaturation.
• Na⁺/K⁺-ATPase - 3 Na⁺ out, 2 K⁺ in per ATP; electrogenic, sets the resting potential, consumes a large share of cellular ATP.
• Signal peptide - the N-terminal tag that routes a protein into the rough ER; its absence keeps synthesis cytosolic.
• Golgi - post-translational modification and the cell's sorting office; vesicle targeting by COPI/COPII/clathrin plus SNAREs and Rabs.
• Lysosome - acid hydrolases at pH ~5 kept acidic by a V-type H⁺-ATPase; mannose-6-phosphate is the delivery tag.
• Mitochondrion - cristae-bound ETC and ATP synthase, own circular DNA, maternally inherited, gatekeeper of apoptosis.
• Endosymbiotic theory - double membrane, circular DNA, 70S ribosomes and antibiotic sensitivity are the evidence.
• Cytoskeleton - actin (7 nm), microtubules (25 nm), intermediate filaments (10 nm); only microtubules and actin are motor tracks.
• Dynamic instability - microtubules continuously grow and shrink; the target of taxol and vinca alkaloids.
• Cell cycle - G1 → S → G2 → M, driven by cyclin–Cdk pairs and made irreversible by cyclin degradation.
• Checkpoints - G1/S (p53), G2/M, and the spindle-assembly checkpoint; their failure underlies aneuploidy and cancer.
• Mitosis vs meiosis - 2 identical diploid cells versus 4 genetically varied haploid cells from one replication.
• Apoptosis - caspase-driven, ATP-requiring, tidy; necrosis is passive, lytic and inflammatory.
• Second messengers - cAMP, IP₃, DAG, Ca²⁺ amplify an extracellular signal into a large intracellular response.
• CHO and Vero cells - the production hosts behind Indian biosimilars and vaccines; mycoplasma and STR checks are mandatory practice.

Formulas and examples

Example 1 — Surface-to-volume ratio and why cells are small. Compare a bacterium of r = 0.5 µm with a eukaryotic cell of r = 10 µm.

• Bacterium: S/V = 3/0.5 = 6 µm⁻¹. Eukaryote: S/V = 3/10 = 0.3 µm⁻¹.
• Ratio = 6/0.3 = 20× more membrane per unit volume in the bacterium.
• Volumes: (4/3)π(0.5)³ = 0.524 µm³ versus (4/3)π(10)³ = 4,189 µm³ — the eukaryotic cell is 8,000× larger in volume but only 400× larger in area. Hence internal membranes (ER, cristae) and transport systems are needed instead of plain diffusion.

Example 2 — Haemocytometer count and viability. Cells are diluted 1:5 with trypan blue (0.1 mL suspension + 0.4 mL dye). Counts in four large corner squares: 84, 92, 78 and 86 total cells, of which 6, 8, 5 and 5 are blue.

• Mean total per square = (84 + 92 + 78 + 86)/4 = 340/4 = 85.
• Total cell density = 85 × 5 × 10⁴ = 4.25 × 10⁶ cells/mL.
• Dead cells: mean blue = (6 + 8 + 5 + 5)/4 = 24/4 = 6 → mean live = 85 − 6 = 79.
• Viability = 79/85 × 100 = 92.9% (above the 90% minimum normally required before seeding a bioreactor).
• Live-cell density = 79 × 5 × 10⁴ = 3.95 × 10⁶ cells/mL. To seed a 200 mL flask at 2 × 10⁵ live cells/mL you need 4 × 10⁷ cells → volume = 4 × 10⁷ / 3.95 × 10⁶ = 10.1 mL of suspension.

Example 3 — Doubling time of a CHO culture. A CHO suspension grows from 2.0 × 10⁵ to 1.6 × 10⁶ cells/mL in 48 h.

• Nt/N0 = 8 → number of doublings n = log₂8 = 3.
• td = 48/3 = 16 h.
• µ = ln8/48 = 2.0794/48 = 0.0433 h⁻¹; check td = 0.693/0.0433 = 16.0 h. ✔
• Projected density after a further 24 h at the same rate: 24/16 = 1.5 doublings → 1.6 × 10⁶ × 2^1.5 = 1.6 × 10⁶ × 2.828 = 4.5 × 10⁶ cells/mL (in practice growth would be slowing towards stationary phase).

Example 4 — Osmotic pressure and tonicity of a wash buffer. 0.9% w/v NaCl (normal saline) at 37 °C; NaCl molar mass 58.44 g/mol, i = 2.

• C = 9 g/L ÷ 58.44 = 0.154 mol/L.
• π = 2 × 0.154 × 0.0821 × 310 = 7.84 atm (≈7.94 bar). Osmolarity = 2 × 154 = 308 mOsm/L — isotonic with plasma (~290–310 mOsm/L), so cells neither swell nor shrink.
• A 0.45% NaCl solution gives 154 mOsm/L — hypotonic; water enters and cells swell (frank haemolysis of red cells only sets in below about 0.4% NaCl, and is complete near 0.3%). A 3% NaCl solution gives ~1,027 mOsm/L — hypertonic; cells crenate.

Example 5 — Nernst potential for K⁺ and the resting membrane. A mammalian cell has [K⁺]in = 140 mM, [K⁺]out = 5 mM at 37 °C, z = +1.

• E_K = 61.5 × log₁₀(5/140) = 61.5 × log₁₀(0.0357) = 61.5 × (−1.447) = −89 mV.
• For Na⁺ ([Na⁺]in = 12 mM, [Na⁺]out = 145 mM): E_Na = 61.5 × log₁₀(145/12) = 61.5 × 1.082 = +67 mV.
• The measured resting potential (≈ −70 mV) lies much nearer E_K because resting K⁺ permeability is far higher — the reason K⁺ dominates the resting potential and Na⁺ the action potential.

Where it is used

• Biopharmaceutical manufacturing: CHO-cell cultures in stirred-tank bioreactors making monoclonal antibodies and insulin at Biocon (Bengaluru), Dr. Reddy's, Intas/Accord, Lupin and Zydus — cell-line development, media optimisation and viable-cell-density monitoring are daily cell-biology work.
• Vaccines: Vero and MDCK cell platforms at Serum Institute of India (Pune) and Bharat Biotech (Hyderabad) for rabies, polio, influenza and COVID vaccines; cell-bank qualification, karyotyping and adventitious-agent testing.
• Drug discovery and toxicology CROs: Syngene, Aurigene, Jubilant Biosys and TCG Lifesciences run cytotoxicity (MTT, LDH), apoptosis (annexin-V, caspase-3), cell-cycle (flow cytometry) and high-content imaging assays.
• Diagnostics and clinical labs: cytopathology and Pap smears, karyotyping and FISH, flow-cytometric CD4 counts and leukaemia immunophenotyping at Metropolis, Dr. Lal PathLabs, SRL and hospital labs.
• Regenerative medicine and research institutes: stem-cell and tissue-engineering work at inStem, NCCS Pune, CCMB Hyderabad, NCBS, THSTI and the IITs/NITs; cord-blood and cell-therapy firms operating under ICMR–DBT stem-cell guidelines and CDSCO rules.
• Job roles: cell-culture scientist, upstream process associate, QC analyst (cell-based potency assays), cytogeneticist, flow-cytometry application specialist, academic research scholar.

Interview questions

• Q: Why can't a cell simply grow larger instead of dividing? A: Surface area rises as r² while volume rises as r³, so the membrane cannot exchange enough nutrients and waste; the nucleus also cannot service an indefinitely large cytoplasm.
• Q: What does "fluid mosaic" actually mean? A: The bilayer is a two-dimensional fluid in which lipids and most proteins diffuse laterally, and the proteins form a mosaic pattern rather than a continuous sheet.
• Q: How does the Na⁺/K⁺-ATPase differ from facilitated diffusion of glucose by GLUT1? A: The pump uses ATP to move 3 Na⁺ out and 2 K⁺ in against their gradients; GLUT1 is a passive carrier that only speeds movement down the glucose gradient.
• Q: How does a protein destined for secretion differ from a cytosolic protein at the start of translation? A: It carries an N-terminal signal peptide recognised by the signal-recognition particle, which docks the ribosome on the rough ER so translation continues into the ER lumen.
• Q: Why do mitochondria and chloroplasts have their own DNA? A: They are descended from endosymbiotic prokaryotes; they retain circular genomes, 70S ribosomes and double membranes, though most of their original genes have moved to the nucleus.
• Q: What tells the cell to pass the G1/S checkpoint? A: Sufficient size, nutrients and growth-factor signalling with undamaged DNA; cyclin D/E–Cdk phosphorylates Rb to release E2F, while p53 → p21 blocks it if DNA is damaged.
• Q: Why is apoptosis preferable to necrosis in a tissue? A: The membrane stays intact and the fragments are phagocytosed, so no inflammatory intracellular contents are released.
• Q: How does taxol kill dividing cells? A: It stabilises microtubules and blocks dynamic instability, so the spindle cannot segregate chromosomes and the spindle-assembly checkpoint arrests the cell, triggering apoptosis.
• Q: You see cloudy medium with no visible organism under the microscope and poor growth — what do you suspect? A: Mycoplasma contamination, since it is too small to see by light microscopy; confirm by PCR or DAPI staining and discard the culture.
• Q: Why must cell lines be STR-profiled? A: Cross-contamination and misidentification of lines is common; STR profiling confirms identity, and regulators and journals require it for cell banks and published data.
• Q: Why is CHO the preferred host for therapeutic antibodies rather than *E. coli*? A: CHO cells perform human-like N-glycosylation, disulphide bonding and secretion of correctly folded large proteins, which prokaryotes cannot do.
• Q: A cell is placed in distilled water and bursts, while a plant cell in the same water does not — why? A: Both take in water osmotically, but the plant cell's rigid cellulose wall generates turgor pressure that opposes further influx; the animal cell has no wall and lyses.

Common mistakes

• Writing that prokaryotes have "no organelles and no ribosomes" — they lack membrane-bound organelles, but have 70S ribosomes, and often carboxysomes, gas vesicles and mesosome-like structures.
• Confusing facilitated diffusion (carrier-mediated, passive, down-gradient) with active transport, and calling the Na⁺/K⁺ pump electroneutral.
• Mixing up mitosis and meiosis outcomes — crossing over and reduction occur in meiosis I, not in mitosis; four haploid cells, not two.
• In haemocytometer sums, forgetting the 10⁴ factor or the dilution factor, or reporting total count as viable count.
• Saying trypan blue stains live cells — it is excluded by intact membranes, so blue cells are dead.
• Treating a eukaryotic flagellum and a bacterial flagellum as the same structure — one is a 9+2 microtubule bundle beating with dynein, the other a rotating flagellin filament driven by proton-motive force.
• Quoting one fixed cell-cycle duration for all cells, and assuming G0 is a stage of mitosis rather than an exit from the cycle.

Related topics

Biochemistry: biomolecules and enzymes · Molecular biology: replication, transcription and translation · Genetics · Microbiology basics · Animal cell culture and bioprocess technology · Immunology and cell-based assays