Cells – Core Structure, Theory & Diagnostic Contents

Essential traits: metabolism, highly ordered structure, growth, hereditary reproduction, environmental responsiveness; living organisms also exhibit development and evolution.

All organisms consist of cells that form the basic structural & functional unit.
Cells arise only from pre-existing cells; spontaneous generation disproven (Pasteur’s swan-neck flask experiments).
Key contributors: Hooke (first observed cork "cells"), Brown (nucleus), Schleiden & Schwann (unified theory), Virchow ("omnis cellula e cellula").

Cell wall: primary (flexible) vs secondary (lignified, rigid); built from cellulose microfibrils + hemicellulose + pectins; walls glued by middle lamella.
Plasma membrane: phospholipid bilayer with intrinsic & extrinsic proteins; fluid-mosaic, selectively permeable; supports diffusion, facilitated diffusion, active transport, exo- & endocytosis.
Nucleus: double membrane with nuclear pores; stores DNA as chromatin; nucleolus makes ribosomal RNA.
Central vacuole: tonoplast membrane; provides turgor, stores ions, pigments, wastes; contains hydrolytic enzymes.
Plastids (double-membrane, own DNA):
• Proplastid → chloroplast (photosynthesis, thylakoids/grana, starch inclusion),
• Amyloplast (starch storage),
• Chromoplast (carotenoid pigments),
• Leucoplast (colorless synthesis/storage),
• Etioplast (dark-grown intermediate).
Mitochondria: inner cristae host respiratory enzymes; own DNA & ribosomes; ATP production.
Endomembrane system: rough ER (protein synthesis), smooth ER (lipid synthesis), dictyosomes/Golgi (modify & package; vesicle trafficking).
Microbodies: peroxisomes (photorespiration detox), glyoxysomes (fat → sugar in germinating seeds).
Ribosomes (free & ER-bound): protein translation.
Cytoskeleton: microtubules (tubulin; cell shape, spindle, vesicle guidance), microfilaments (actin; cytoplasmic streaming, cell plate), centrioles/basal bodies absent from higher plants except as basal bodies for flagellated sperm.

Plasmodesmata traverse walls; create symplast (protoplasm continuum) vs apoplast (cell wall & intercellular air space continuum).
Primary pit fields = clusters of plasmodesmata.

Starch grains: varied shape/size; identified via hilum, striations; turn blue-violet with iodine; important sources – maize, rice, wheat, potato.
Aleurone grains: protein bodies with crystalloids & globoids; picric acid → yellow, Millon’s reagent → red.
Calcium oxalate crystals: forms include sandy, acicular, prisms, rosettes, bundles; insoluble in acetic/alkali, soluble in $\text{HCl}$ and $\text{H<em>2SO</em>4}$ .
Calcium carbonate (cystoliths): effervesce with acids; $60\%$ $\text{H<em>2SO</em>4}$ yields needle calcium sulfate.
Fixed oils & fats: refractile droplets; soluble in ether/alcohol, black with osmic acid, red with alkanna tincture; saponified by ammonia + potash mix.
Mucilage: hydrates in water, stains pink with ruthenium red or alkaline corallin.
Tannins & resins: often vacuolar deposits; confer astringency & defence; visible as dark granules/masses.

Simple diffusion for small non-polar.
Facilitated diffusion via channel/carrier proteins.
Active transport (molecular pumps) builds steep ion gradients (e.g., \text{K^+} in roots).
Vesicle-mediated flow: exocytosis (secretion), endocytosis (uptake); continuous membrane recycling maintains surface area.

Cell specialization underlies multicellularity; division of labour increases efficiency but creates interdependence.
Developmental signals move via hormones, plasmodesmata, or secreted molecules, guiding tissue patterning.