ชีววิทยาของเซลล์

CELLS

fundamental units of life. Eukaryotic cells have a nucleus (animal, plant, fungi, protoctist, yeast); larger and more complicated than prokaryotes; ribosome 80S (70S in mitochondria + chloroplast). Prokaryotic cells have no nucleus (bacteria, archaea); mostly single-celled; ribosome 70S. 3 domains of life: bacteria, archaea, eukaryotes (phagocytosis and digestion of other prokaryotes evolved into eukaryotic cells). All cells share similar basic chemistry. Viruses are not made of cells (non-living). Svedberg units measure sedimentation rate when centrifuged.

FUNCTIONS OF ORGANELLES

cell compartmentalisation makes cell function better. (1) cytosol — metabolic pathways, protein synthesis, cytoskeleton. (2) nucleus — most DNA. (3) endoplasmic reticulum — synthesises most lipids, protein synthesis. (4) Golgi apparatus — modification, sorting, packing of proteins + lipids for secretion or delivery. (5) lysosomes — intracellular degradation, removal of toxic substances. (6) endosomes — sorting endocytosed material. (7) mitochondria — ATP synthesis by oxidative phosphorylation. (8) chloroplast (plants) — ATP synthesis, carbon fixation. (9) peroxisomes — oxidation of toxic molecules.

NUCLEUS

enclosed within nuclear envelope, genomic DNA packed as chromosomes. Function: express genetic info (gene) as RNA which codes for protein. Some cells (e.g. skeletal muscle cells) are multinucleated.

NUCLEOLUS

usually at centre; where ribosomal RNA is synthesised and combined with proteins to form ribosome subunits, transported to cytoplasm via nuclear pore.

CHROMOSOME PACKING

DNA interacts with proteins forming chromatin. Heterochromatin = highly condensed (darker on EM). Euchromatin = open structure used for transcription. DNA-binding proteins divided into histone + non-histone chromosomal proteins. Histone-DNA complex = nucleosome (8 histone proteins = histone octamer + 147 bp dsDNA). Nucleosomes pack together to form chromatin fibre.

NUCLEAR PORE

composed of large complex proteins (nucleoporins, 30 distinct proteins forming Nuclear Pore Complexes). Small water-soluble molecules diffuse passively. Molecules with Nuclear Localisation Signals (NLSs) are recognised by importins (nuclear import receptors). Exportins bind nuclear export signals plus NPC proteins.

NUCLEAR LAMINA

inner surface of nuclear envelope, protein lining (mostly lamins, type V intermediate filaments). Provides structural support, assists DNA repair, controls cell-cycle events, regulates gene expression. Mutations in lamins cause laminopathies.

STEM CELLS

totipotent (formed shortly after fertilisation, can become all cell types including embryo). Pluripotent (all cell types except embryo + placenta). Multipotent (certain class only, e.g. hematopoietic, mesenchymal).

CELLULAR MEMBRANES

phospholipid bilayer made from lipids + proteins; barrier to most water-soluble molecules. Plasma membrane separates intracellular from external environment. Internal membranes surround organelles.

CELL SPECIALISATION

animal cells have 4 major tissue types: epithelium, connective, nervous, muscle.

Mitochondrion

ATP synthesis by oxidative phosphorylation. Site of cellular respiration; uses oxygen aerobically. Contains its own circular DNA (mtDNA, 16569 bp), inherited maternally. Krebs cycle intermediates: citrate, isocitrate, α-ketoglutarate, succinyl CoA, succinate, fumarate, malate, oxaloacetate. Mitochondrial diseases inherited through maternal line.

Endoplasmic reticulum (ER)

rough ER (with ribosomes) for protein synthesis of membrane and secreted proteins; smooth ER for lipid synthesis, calcium storage, detoxification.

Golgi apparatus

receives proteins from ER, modifies (glycosylation), sorts, and packages them into vesicles for delivery to plasma membrane, lysosomes, or secretion.

Lysosomes

contain hydrolytic enzymes active at low pH (~5); break down macromolecules, worn organelles, and engulfed material. Lysosomal storage diseases (e.g. Tay-Sachs, Gaucher) result from defective lysosomal enzymes.

Peroxisomes

oxidation of fatty acids and toxic molecules; produce hydrogen peroxide which is then broken down by catalase.

Vacuoles

storage organelles for water, ions, nutrients, waste.

Endosomes

vesicles formed during endocytosis; sort endocytosed material to lysosomes for degradation or back to the plasma membrane for recycling.

Autophagosomes

engulf damaged organelles and deliver them to lysosomes for degradation (autophagy).

CYTOSKELETON

aggregate structure of three filament types: microtubules, microfilaments (actin), intermediate filaments, all linked by plectin. Functions: dynamic scaffold, internal framework, transport highways, force-generating apparatus, mRNA anchoring, cell division.

MICROTUBULES (MT)

biggest of the three (~25 nm). Formed of α-tubulin and β-tubulin dimers polymerised into 13 protofilaments in same orientation. Polar: (+) end fast-growing, (−) end slow-growing. Energy: GTP. Often anchored in centrosome with (+) end projecting into cytoplasm. Functions: maintain cell shape, chromosome movement during mitosis (mitotic spindle), vesicle/organelle transport via kinesins and dyneins (motor proteins using ATP), structural support in cilia and flagella. Cilia = many, short, alternating power/recovery strokes. Flagella = few, long, undulatory movement. Cilia/flagella have 9+2 microtubule arrangement.

MT-affecting drugs: (1) colchicine — binds tubulin and inhibits assembly; used in karyotyping. (2) nocodazole — inhibits MT assembly (reversible). (3) vinblastine, vincristine (Vinca alkaloids) — cause tubulin aggregation; used in chemotherapy. (4) paclitaxel (taxol) — stabilises microtubules and inhibits cell division; anticancer drug for breast and ovarian cancers.

MICROFILAMENTS / ACTIN FILAMENTS

smallest (7 nm). Two intertwined strands of F-actin polymerised from G-actin monomers using ATP. (+) end = barbed end (fast-growing, 5–10× faster). (−) end = pointed end. Treadmilling: monomer added at (+) end while another removed at (−) end. Functions: cell shape, microvilli, muscle contraction (actin-myosin sliding controlled by tropomyosin/troponin/Ca2+), cleavage furrow during cytokinesis, amoeboid movement via pseudopodia. Rho GTPase family regulates actin: Cdc42 → filopodia; Rac → lamellipodia; Rho → stress fibres. Cytochalasins (fungal toxin) bind (+) end and block elongation. Phalloidin stains actin.

INTERMEDIATE FILAMENTS (IF)

8–12 nm, medium size. Not polarised; more permanent than MT and MF. Four major types: (1) keratin (epithelial cells), (2) neurofilaments (neurons), (3) vimentin (fibroblasts, glial, muscle), (4) nuclear lamina (all nucleated cells). Functions: reinforce cell shape, fix organelle location, bear tension. Diseases from mutant keratins: Epidermolytic ichthyosis, Harlequin-type ichthyosis (ABCA12), Epidermolysis bullosa simplex, Tree Man Syndrome (epidermodysplasia verruciformis). Neurofilament defects: Amyotrophic Lateral Sclerosis (ALS, Lou Gehrig's disease).

CELL MEMBRANE

composed of (1) membrane lipids, (2) membrane proteins, (3) carbohydrates (glycoprotein + glycolipid).

MEMBRANE LIPIDS

major lipids are phospholipids: phosphoglycerides (glycerol backbone) or sphingomyelin (sphingosine backbone). Amphipathic (polar head + 2 nonpolar fatty acyl chains) — critical for bilayer formation. Cholesterol stabilises membrane at 37°C (up to 50% of lipids).

MEMBRANE PROTEINS

integral (transmembrane) or peripheral. Functions: receptors, transporters (pores, ion channels, carriers). Aquaporins (AQPs) discovered by Peter Agre 1992 — homotetramer; each monomer has 6 membrane-spanning domains + central water pore. AQP2 defects: too little → Diabetes Insipidus; too much → SIADH.

ION CHANNELS

gated mechanisms: ligand-gated, voltage-gated, receptor-activated, volume-sensitive, mechanical stress-induced. CFTR (cystic fibrosis transmembrane conductance regulator) — ligand-gated Cl− channel of ABC transporter family activated by cAMP. Defect causes cystic fibrosis (thick mucus, recurrent infections). Cholera = CFTR over-activation.

TRANSPORTERS

conformational change. Examples: GLUT1 uniporter (glucose); SGLT (Na+/glucose co-transporter; SGLT inhibitors used in diabetes); Na+/K+ ATPase (3 Na+ out, 2 K+ in, ATP-dependent — maintains gradients and intracellular −ve charge); Ca2+ ATPase (PMCA); H+/K+ ATPase (gastric proton pump). 1° active transport uses ATP directly; 2° active transport uses ion gradient (Na+) established by 1°.

MEMBRANE POTENTIAL

Resting Membrane Potential (RMP): non-excitable cells −20 to −40 mV; excitable cells −70 to −90 mV. Determined by electrochemical gradient, Gibbs-Donnan effect, and Na+/K+ ATPase activity. K+ is dominant on RMP because membrane more permeable to K+ (closer to EK = −90 mV than ENa = +60 mV). Gibbs-Donnan effect — impermeant proteins in cell would cause swelling; Na+/K+ pump counteracts by pumping Na+ out.

MECHANISMS OF TRANSPORT

(1) simple diffusion (small uncharged polar, gases). (2) Facilitated diffusion (protein carriers, no ATP). (3) Active transport: primary (ATP hydrolysis), secondary (Na+ gradient), tertiary. (4) Transcytosis: endocytosis (pinocytosis, phagocytosis, receptor-mediated via clathrin/dynamin), exocytosis (constitutive or regulated). (5) Osmosis — water across semi-permeable membrane down osmotic-pressure gradient. Tonicity (effect on cell volume) ≠ osmolarity (solute concentration).

ACTION POTENTIAL

graded → threshold → all-or-none. Coordinated voltage-gated Na+ and K+ channels. Refractory periods: absolute (no AP), relative (AP needs stronger stimulus). Cardiac myocytes: longer absolute refractory due to Ca2+ plateau — ensures contraction completes before next AP.

CLINICAL

Tetrodotoxin (pufferfish) blocks Na+ channels → paralysis, respiratory failure. Local anaesthetics block Na+ channels → no pain transmission. Fick's first law: diffusion rate depends on concentration difference, area, distance, and diffusion coefficient.

TYPES OF SIGNALLING MOLECULES

proteins (insulin, glucagon), steroids (testosterone, estradiol, cortisol), amines (acetylcholine, thyroxine, catecholamines), dissolved gases (NO, CO). Two components: ligand (extracellular signal) + receptor (transmembrane protein, except for steroids which are intracellular).

FORMS OF INTERCELLULAR SIGNALLING

(1) contact-dependent (direct membrane contact, e.g. Notch signalling, immune responses). (2) Paracrine (local mediators on nearby cells; nerve-nerve, nerve-muscle, cytokines). (2.1) Autocrine — signals to same cell type or self (cancer, immune). (3) Endocrine — hormones into bloodstream; long-distance, slow, high target sensitivity. (4) Synaptic — neurotransmitter at synapse; long-range, very fast, low sensitivity (so higher concentration needed than endocrine).

THREE LARGEST CLASSES OF CELL SURFACE RECEPTORS

(1) Ion-channel-coupled (ionotropic) receptors. Open/close on neurotransmitter binding; rapid synaptic signalling. (2) G-protein-coupled receptors (GPCRs) — largest family; half of known drugs target them. Trimeric G-protein with α, β, γ subunits. (3) Enzyme-coupled receptors — inactive as monomers, dimerise on ligand binding. Types include Receptor Serine/Threonine Kinases (RSTKs, bind TGF-β family) and Receptor Tyrosine Kinases (RTKs).

GPCR PATHWAY (cAMP)

(1) signal binds GPCR → conformational change → activation. (2) Activated Gα activates adenylyl cyclase. (3) AC converts ATP → cAMP. (4) cAMP binds regulatory subunit of inactive PKA → dissociates. (5) Active PKA phosphorylates CREB. (6) Phospho-CREB recruits CBP → transcription. Example: glucagon → cAMP → PKA → phosphorylates glycogen phosphorylase → glycogen → glucose.

GPCR PATHWAY (IP3/Ca2+)

phospholipase C-β cleaves PIP2 → IP3 + DAG. IP3 opens IP3-gated Ca2+ channel in ER → ↑ cytosolic Ca2+. DAG activates PKC. Ca2+/calmodulin activates CaM-Kinase II → phosphorylates many targets.

RTK PATHWAY

(1) ligand brings two RTK monomers together → dimerise. (2) Cross-phosphorylation of tyrosine residues. (3) Phosphotyrosines = docking sites for signalling proteins. Downstream: MAP-kinase pathway (Ras → Raf → MEK → ERK) for proliferation/differentiation; PI3-kinase pathway (PIP3 → PDK1 + Akt; Akt promotes survival by inhibiting Bad). mTOR involved.

MOLECULAR SWITCHES

(1) GTPase switches: G-proteins, Ras (active GTP-bound vs inactive GDP-bound). RGS proteins convert GTP → GDP to inactivate. (2) Protein kinase / phosphatase pairs.

NITRIC OXIDE (NO)

small hydrophobic gas, passes membranes; very short half-life (5–10 s). Synthesised by NO synthase from arginine. Activates guanylyl cyclase → cGMP → smooth muscle relaxation (vasodilation). Mechanism of nitroglycerin in angina.

Ca2+ AS UBIQUITOUS MEDIATOR

kept low in cytosol by plasma-membrane Ca-ATPase, Na+/Ca2+ exchanger, and ER/mitochondrial sequestration. Released signals trigger muscle contraction, exocytosis (neurotransmitter, hormones), platelet activation.

CELL RESPONSES

sigmoidal vs all-or-none. Feedback loops: positive (amplification) vs negative (homeostasis). Cells deprived of signals undergo apoptosis (suicide program).

ECM

non-cellular component present in all tissues and organs. Highly dynamic structure. Functions: cell adhesion (physical scaffolding), cell-cell communication, tissue morphogenesis, differentiation, homeostasis.

THREE MAJOR COMPONENTS

(1) Proteoglycans (PGs). (2) Fibrous proteins: collagens, elastins, fibronectins, laminins. (3) Non-collagen glycoproteins. Other: ECM proteases, integrins.

PROTEOGLYCANS

glycosaminoglycan (GAG) chains covalently linked to protein core (except hyaluronan). Fill interstitial space as hydrated gel; resist compression; permit diffusion of nutrients, metabolites, hormones. Four GAG groups: hyaluronan (HA), chondroitin/dermatan sulfate, heparan sulfate, keratan sulfate. GAGs are highly negatively charged; unbranched polysaccharides of repeating disaccharide (sulfated amino sugar + uronic acid).

HYALURONAN

simplest GAG. No sulfated sugars, no protein core. Spun directly from cell surface enzyme. Functions: resists compression in joints, space-filler in embryonic development (heart valve/septum formation), wound healing, joint lubricant. Degraded by hyaluronidase.

COLLAGENS

most abundant fibrous protein; most common protein in humans. Made mainly by fibroblasts. Provides scaffolding. ~40 types from 42 genes. Triple helix of three α-chains. Fibrillar collagens (Types I, II, III) resist tensile force. Fibril-associated collagens have flexible non-helical interruptions.

ELASTIN

highly hydrophobic, rich in proline + glycine, not glycosylated. Strong elastic function via covalent cross-links. Found in skin, blood vessels, lungs.

FIBRONECTIN

large glycoprotein; multiple domains with binding sites for matrix macromolecules and cell-surface receptors. Organises matrix and helps cells attach.

LAMININ

primary organiser of basal lamina sheet structure. Three long polypeptide chains. Many laminin types create basal laminae with different properties.

BASAL LAMINA (basement membrane)

thin tough flexible sheet underpinning all epithelia, surrounding muscle, fat, Schwann cells, in kidney glomerulus. Structure: laminin, Type IV collagen, nidogen, perlecan + integrin.

INTEGRINS

transmembrane heterodimers (α + β). Link ECM to cytoskeleton. 24 types (8 α-genes × 18 β-genes). Activated by conformational change. Cell-matrix adhesion.

CELL JUNCTIONS

three classes: anchoring, tight, gap.

TIGHT JUNCTIONS

near apical side. Barrier function preventing paracellular leakage. Two transmembrane proteins: claudins, occludin.

CELL-CELL ANCHORING JUNCTIONS

(1) Adherens junctions — linked to actin via cadherins. (2) Desmosomes — linked to intermediate filaments via desmosomal cadherins; provide mechanical strength.

CELL-MATRIX ANCHORING JUNCTIONS

(1) Actin-linked cell-matrix (focal contact) via integrins. (2) Hemidesmosome — anchors intermediate filaments to ECM via integrin α6β4.

GAP JUNCTIONS

channels of connexins (two connexons) directly connecting cytosols of neighbouring cells. Allow molecules through size-regulated gate. In cardiac muscle, gap junctions provide electrical connections allowing AP propagation — cardiac tissue is a functional syncytium.

TRANSMEMBRANE ADHESION PROTEINS

Integrin (cell-matrix). Cadherin (cell-cell, Ca2+-dependent; Ca2+ rigidifies hinges for binding).

CELL JUNCTIONS & ADHESION

key functions: (1) cell polarity and epithelial vectorial transport; (2) selective barrier; (3) intra- and intercellular communication. Order from apical: tight junction → adherens junction → desmosome → gap junction → hemidesmosome / integrin (basal).

TIGHT JUNCTION

physical and selective barrier. Master regulator of cell polarity. Transmembrane proteins: occludin, claudins (24 members, e.g. claudin-1 barrier; claudin-2 pore), tricellulin, angulin-1/2, JAMs. Peripheral: ZO-1, ZO-2, ZO-3. Two types: barrier-forming (occludin, claudin-1/4, tricellulin, ZO-1) vs pore-forming (claudin-2, claudin-15).

PARACELLULAR PATHWAY DEFECTS

(1) LEAK pathway: defective barrier-forming. (2) PORE pathway: over-active pore-forming. (3) UNRESTRICTED: cell apoptosis. Regulated by post-translational modifications. CK2 phosphorylates occludin and controls pore-forming claudin-2. AMPK regulates assembly and controls leak pathway. MLCK (myosin light chain kinase) at PAMR (perijunctional actomyosin ring) — induced endocytosis of occludin causing leak.

INFLAMMATORY BOWEL DISEASE (IBD)

Crohn's disease, ulcerative colitis. Caused by mucosal immunity imbalance + TJ integrity defects (IL-13 ↑ claudin-2 → pore; TNF → MLCK → occludin endocytosis → leak). Symptoms: persistent diarrhoea, abdominal pain, rectal bleeding, weight loss, fatigue. Therapeutics: aminosalicylates, corticosteroids, anti-TNF (infliximab).

CELL ADHESION MOLECULES (CAMs)

four major families: cadherins, immunoglobulin (Ig) superfamily, integrins, selectins. Homotypic vs heterotypic; homophilic vs heterophilic binding.

ADHERENS JUNCTIONS

classical cadherins (E-, N-, P-cadherins). Cytosolic domains linked via catenin/vinculin/α-actinin to actin. Loss disrupts epithelial organisation → cancer metastasis (epithelial-to-mesenchymal transition, EMT).

DESMOSOMES

desmosomal cadherins (desmoglein, desmocollin) linked via plakins to intermediate filaments. DISEASE: Pemphigus vulgaris — autoantibodies against desmoglein → blisters of skin and mucous membranes.

HEMIDESMOSOMES

integral membrane proteins linked via plakins to keratin intermediate filaments. Principal ECM receptor: integrin α6β4. Anchor epithelial cells to basal lamina.

DISEASES OF ADHESION MOLECULES

(1) Rhinovirus (cold) — RV-C binds CDHR3 cadherin. (2)

COVID-19

SARS-CoV-2 spike binds ACE2 → endocytosis. (3) Platelet adhesion disorders: Glanzmann thrombasthenia (deficient GpIIb-IIIa), Bernard-Soulier syndrome (deficient GpIb), von Willebrand disease (deficient vWF). Gap junction mutations cause neurosensory deafness (Cx26, Cx31), cataracts/heart malformations (Cx43, Cx46, Cx50), X-linked Charcot-Marie-Tooth disease (Cx32).

CELL MOTILITY

surface protrusions for movement, phagocytosis, absorption. Most are actin-based. Microvilli (actin-based, fingerlike, on absorptive epithelium) cross-linked by fimbrin and villin. Stereocilia of auditory hair cells detect sound.

CELL MOVEMENT TYPES

Pseudopodia (moderate width, 3D actin network), Filopodia (thin, from lamellipodia bundles), Lamellipodia (broad sheet at leading edge). Cell migration coordinates these. Inhibition of actin polymerisation blocks formation.

CILIA AND FLAGELLA

MT-based projections. Cilia act as antennae sensing extracellular signals + movement. Flagella for cell movement (sperm). Defects cause >30 ciliopathies including polycystic kidney disease (mutations in PKD1/PKD2 encoding polycystin-1/2; abnormal cAMP signalling activates PKA → proliferation and CFTR-mediated fluid secretion → cysts).

AMOEBOID CELLS

change shape via pseudopodia (e.g. macrophages); also perform phagocytosis.

CHROMATIN

chromos = colour. DNA + protein. DNA wraps around histones (positively charged) forming nucleosomes which neutralise negative DNA backbone. Histone families: H1, H2A, H2B, H3, H4. Heterochromatin: condensed, dark, gene-poor. Euchromatin: open, light, gene-rich, active in transcription.

CHROMOSOME

very long DNA molecule + associated proteins carrying hereditary info. Counted in mitotic metaphase. Largest human chromosome ~3.2×10^8 bp. Structure dynamic: condenses in M phase, opens for transcription/repair.

HUMAN CHROMOSOME STRUCTURE

total 46 = 22 autosome pairs + sex chromosomes (XX female / XY male). Two arms: p (short, top), q (long, bottom). Each has one centromere. Four types: metacentric (chrom 1, 3, 16, 19, 20), submetacentric (2, 4-12, 17, 18, X), acrocentric (13, 14, 15, 21, 22, Y), telocentric (not in humans).

CENTROMERE

repeated specialised DNA. Two kinetochores (disk-shaped). Joins sister chromatids during metaphase. Attaches duplicated chromosome to mitotic spindle.

TELOMERE

telos = end. Tandem repeat TTAGGG ×thousands. Protects chromosome ends from being mistaken as broken DNA. Telomerase elongates telomeres.

NOMENCLATURE

1956 Tjio & Levan confirmed 46 chromosomes (not 48). Denver Classification 1960. Naming: total chromosome number, sex chromosomes, abnormalities. Example: 46,XX = normal female; 47,XX,+21 = trisomy 21 (Down syndrome).

BANDING TECHNIQUES

Q-banding (quinacrine; AT-rich dark, GC-rich bright). G-banding (Giemsa; AT-rich dark, GC-rich bright; standard). R-banding (reverse of G; GC-rich dark). C-banding (constitutive heterochromatin near centromere).

MOLECULAR CYTOGENETICS

(1)

FISH (1977)

fluorescent probe hybridises to nuclei/metaphase; detects deletions, additions, duplications, translocations; gold standard for many cancers. (2) Multiplex FISH — 24-colour karyotyping. (3) Array Comparative Genomic Hybridisation (aCGH) — detects copy-number changes, gains/losses; sensitive, fast; cannot detect balanced rearrangements.

CHROMOSOMAL ABNORMALITIES

(1) Numerical: polyploidy (triploid 69,XXX; tetraploid 92,XXYY), aneuploidy (monosomy 45,X = Turner; trisomy 47,XX+21 = Down; 47,XXY = Klinefelter). (2) Structural: deletion, inversion (paracentric, pericentric), duplication, insertion, ring, translocation (reciprocal, Robertsonian).

KEY DISEASES

Down syndrome (trisomy 21, 47,XX,+21; non-disjunction in maternal meiosis I; related to maternal age; most common chromosomal disorder). Edwards syndrome (trisomy 18). Patau syndrome (trisomy 13). Cri du chat (5p deletion). Klinefelter (47,XXY; tall, infertile males). Turner (45,X; short stature, infertile females). Burkitt lymphoma (t(8;14)). CML (Philadelphia chromosome, t(9;22), BCR-ABL). AML (t(8;21), t(15;17), inv(16)). Myelodysplastic syndrome (del(5q), del(7q), -7).

MITOCHONDRIAL DNA

circular, 16,569 bp, encodes 57 genes (13 mRNAs, 2 rRNAs, 22 tRNAs). Endosymbiotic origin (bacteria engulfed by ancestral eukaryote — now responsible for aerobic respiration). D-loop = replication/transcription origin (400–800 bp, highly variable between individuals). Maternal inheritance. Mitochondrial diseases follow maternal lineage.

OVERVIEW

Cell division for reproduction (unicellular), growth/development/tissue repair (multicellular). Cells divide instead of growing because: (1) prevent DNA overload; (2) surface-area-to-volume ratio limits material exchange.

CELL CYCLE

INTERPHASE (G0, G1, S, G2) + M PHASE. G0: resting; terminally differentiated cells (neurons, skeletal muscle) stay here permanently. G1: cell growth, protein/organelle synthesis, preparation for S phase; epidermal growth factor regulates G1 restriction point. S: DNA replication. G2: protein synthesis for mitosis, DNA error correction.

MITOSIS

5 phases. (1)

PROPHASE

chromosomes condense, nuclear envelope dissolves, mitotic spindle assembles, centrosomes move apart. (2)

PROMETAPHASE

kinetochore formation, microtubule attachment, chromosomes move toward equator. (3)

METAPHASE

chromosomes aligned at equator, kinetochore MTs attach sister chromatids to opposite poles. (4)

ANAPHASE

sister chromatids separate (cohesin degradation), kinetochore MTs shorten, cell elongation. (5)

TELOPHASE

chromosomes arrive at poles, decondense, nuclear envelope reforms, two nuclei.

CYTOKINESIS

contractile ring divides cytoplasm.

MEIOSIS

specialised division for sexual reproduction; two divisions, one round of DNA replication; produces haploid gametes. Only in testes/ovaries.

MEIOSIS I

homologous chromosomes pair (synapsis), crossing over between non-sister chromatids → genetic diversity. (1) Prophase I (synapsis, crossing over). (2) Metaphase I (homologues line up, independent assortment). (3) Anaphase I (homologues separate). (4) Telophase I (two haploid cells each with duplicated chromosomes).

MEIOSIS II

like mitosis but starts haploid. (1) Prophase II. (2) Metaphase II. (3) Anaphase II (sister chromatids separate). (4) Telophase II. Result: 4 non-identical haploid cells. Males: 4 sperm. Females: 1 egg + 3 polar bodies.

NON-DISJUNCTION

failure of homologues (meiosis I) or sister chromatids (meiosis II) to separate. Causes aneuploidy: (1) Down syndrome (trisomy 21). (2) Klinefelter (XXY). (3) Turner (X missing). More common in female meiosis (related to maternal age).

CELL CYCLE CONTROL

checkpoints at G1/S, G2/M, and metaphase-to-anaphase. Cyclin-Cdk complexes drive cycle. Three cyclin classes: (1) G1/S-cyclins (trigger cell-cycle entry); (2) S-cyclins (DNA replication); (3) M-cyclins (entry to mitosis). Cyclins rise and fall cyclically; Cdks remain constant. CDK2 active in DNA replication; CDK1 active in mitosis. Erlotinib is a tyrosine-kinase inhibitor of EGFR used in non-small cell lung cancer.

HOMEOSTASIS

process of stabilising internal environment. Feedback controls: (1) Negative feedback maintains a variable within range (main homeostatic mechanism). (2) Positive feedback amplifies stimulus (push to completion) — blood clotting cascade, childbirth.

CELL MEMBRANE TRANSPORT

Passive: simple diffusion, osmosis, facilitated diffusion. Active: ion pumps, co-transport. Bulk: endocytosis, exocytosis.

CELL VOLUME REGULATION

ICF volume regulated by water osmosis driven by ECF volume and osmolarity. Hypertonic ECF → cell shrinks → Regulatory Volume Increase (RVI): influx of organic osmolytes restores volume. Hypotonic ECF → cell swells → Regulatory Volume Decrease (RVD): efflux of osmolytes.

ECF TONICITY DISORDERS

(1) Hypertonicity from insufficient water intake, excessive water loss, or vasopressin (ADH) deficiency (Diabetes Insipidus). (2) Hypotonicity from too much water intake, renal failure, or excess ADH (SIADH).

FLUID BALANCE ABNORMALITIES

hypernatremia (↑plasma Na+), hyponatremia (↓plasma Na+). Na+ and Cl− account for >90% of ECF solute → indicators of plasma osmolarity. Edema = excess interstitial fluid.

ION REGULATION

Na+/K+ pump establishes Na+ (high outside) and K+ (high inside) gradients; maintains osmotic equilibrium and membrane potential. Without pump, gradients dissipate. RMP −70 mV (closer to EK because membrane more K+-permeable). K+ leak channels dominant. Cl− follows gradients passively. Negative anions trapped inside.

INTRACELLULAR Ca2+

kept very low in cytosol. Regulated by: (1) Plasma-membrane channels — Ca-ATPase (PMCA pump), Na+/Ca2+ exchanger (NCX, faster than pump). (2) ER and inner mitochondrial membrane store Ca2+. (3) Ca-binding proteins. Abnormal increase causes cell damage. Calcium-channel blockers are first-line antihypertensives. Local anaesthetics block voltage-gated Na+ channels → no AP, no pain transmission.

pH REGULATION

Acid = H+ donor; base = H+ acceptor. Effects of [H+]: (1) excitability changes — acidosis → CNS depression; alkalosis → CNS over-excitability. (2) Enzyme activity changes (alter protein conformation). (3) [H+] affects K+ levels — acidosis ↓K+ secretion, alkalosis ↑K+ secretion. Body buffers: carbonic acid / HCO3−. pH transporters: acidic (Na+/H+ exchangers, Na+/HCO3− cotransporters), basic (Cl−/HCO3− anion exchangers, Ca2+-ATPases).

GLUCOSE

main cellular energy substrate. Diabetes mellitus: Type 1 = β-cell destruction, no insulin → cells starve, hyperglycaemia → polyuria + dehydration + tissue damage. Type 2 = insulin resistance → pancreas overproduces insulin → eventual β-cell exhaustion → rising glucose.

THERMOREGULATION

body temperature controlled by negative feedback (vasodilation, sweating, shivering). Hyperthermia / heatstroke = decompensation when body temperature > ~40°C and feedback fails.

CELL CYCLE REGULATION

cell-cycle control system uses binary biochemical switches (irreversible). Three major regulatory transitions: (1) Start (G1 checkpoint) in late G1: environment favourable? → enter S phase for DNA replication. (2) G2/M transition: all DNA replicated, environment favourable? → enter mitosis. (3) Metaphase-to-anaphase (M checkpoint): all chromosomes attached to spindle? → trigger anaphase + cytokinesis. M-to-A transition triggered by protein degradation (APC/C), not phosphorylation.

CYCLIN-DEPENDENT KINASES (Cdks)

phosphorylate intracellular proteins to initiate/regulate cell-cycle events. Cdk levels constant; require cyclin binding for activity.

CYCLINS

most important Cdk regulators. Activate Cdk by forming cyclin-Cdk complex; without cyclin, Cdk inactive. Cyclic synthesis/degradation. Four classes: (1) G1/S-cyclins activate Cdks in late G1 → trigger Start. (2) S-cyclins activate Cdks after Start → DNA replication. (3) M-cyclins activate Cdks → mitosis entry. (4) G1-cyclins govern G1/S-cyclins.

CDK ACTIVATION

(1) Inactive Cdk: T-loop blocks active site. (2) Cyclin binding → conformational change, partially active. (3) CAK phosphorylates threonine in T-loop → fully active.

CDK REGULATION

(1) Wee1 kinase phosphorylates two sites above active site → inactivates M-Cdk. Cdc25 phosphatase removes them → reactivates. (2) Cdk Inhibitor Proteins (CKIs) like p27 bind cyclin and Cdk to distort active site and block ATP. Used mainly to control G1/S-Cdk and S-Cdk early in cycle.

APC/C (cyclosome)

anaphase-promoting complex; ubiquitin ligase. Triggers proteolytic destruction in proteasomes → M-to-A transition.

p53 / p21

when DNA damaged, p53 (tumour suppressor) → stimulates p21 (CKI) → binds and inhibits cyclin-Cdk complex → cell cycle arrest in M phase until DNA repaired. p53 mutations common in cancer.

CANCER

disease of uncontrolled cell growth and spread. Cancer cells: (1) reproduce despite normal restraints; (2) invade and colonise other tissues (metastasis). Classified by tissue: carcinomas (epithelial), sarcomas (connective/muscle), leukaemias/lymphomas. Benign tumours don't spread; malignant do. Adenoma (benign glandular) vs adenocarcinoma (malignant).

CANCER CELL PROPERTIES

(1) Loss of contact inhibition (continue dividing even after confluence). (2) Warburg effect (dramatic change in glucose uptake; preference for glycolysis even in aerobic conditions). (3) Breakout of homeostasis (more division, more necrosis than apoptosis). (4) Tumour microenvironment / stroma supports growth. (5) Metastasis.

MUTATIONS & CANCER

tumour cells contain somatic mutations and epigenetic changes. Carcinogens (chemicals, radiation) provoke development. Multi-step accumulation of mutations → clonal evolution (single cell mutates → proliferates → invades basement membrane → metastasises).

TUMOUR PROGRESSION

illustrated by progression in uterine cervix epithelium: dysplasia → carcinoma in situ (still within epithelium) → invasive carcinoma (through basement membrane). Detection often years after onset because cells must accumulate before reaching detectable mass.