🔬 Cell: The Unit of Life Study Notes

08.1 Cell Theory & Discovery

The cell is the fundamental structural and functional unit of every living organism. Robert Hooke (1665) first saw dead cork cells under his microscope and coined the word "cell". Anton von Leeuwenhoek first saw and described living cells (bacteria, protozoa, sperm).

Cell Theory

Proposed by M. Schleiden (German botanist, 1838 — plants) and Theodore Schwann (British zoologist, 1839 — animals). Schwann concluded that the bodies of animals and plants are composed of cells and their products. It was expanded by Rudolf Virchow (1855): Omnis cellula-e cellula — all cells arise from pre-existing cells.

• All living organisms are composed of cells and products of cells.
• The cell is the basic structural & functional unit of life.
• All cells arise from pre-existing cells by division (Virchow).

08.2 Cell Size, Shape & Overview

Cells vary enormously in size, shape and volume — but each is a self-contained unit bounded by a membrane.

Shape may be disc-like (RBC), polygonal (columnar), spindle-shaped (muscle), long & branched (neuron) — generally related to function.

08.3 Prokaryotic vs Eukaryotic Cells

Prokaryotes (bacteria, cyanobacteria/BGA, mycoplasma, PPLO, Archaea) have no true membrane-bound nucleus or organelles. Eukaryotes (protists, plants, animals, fungi) have a true nucleus and membrane-bound organelles.

08.4 Prokaryotic Cell — Structure in Detail

A prokaryotic cell has four main zones: cell envelope (glycocalyx + cell wall + plasma membrane), cytoplasm, nucleoid, and surface appendages.

Cell Envelope

• Glycocalyx: outermost; a loose slime layer or a tough capsule — protection & adhesion.
• Cell wall: peptidoglycan; determines shape & prevents bursting. Basis of Gram staining.
• Plasma membrane: selectively permeable; mesosomes = infoldings that help in respiration, secretion, DNA replication & cell-wall formation.

Other Features

• Ribosomes (70S = 50S + 30S): protein synthesis; form polysomes on mRNA.
• Inclusion bodies: reserve material lying free (e.g. glycogen, gas vacuoles, phosphate granules).
• Flagella: motility; made of filament, hook, basal body. Pili & fimbriae = surface attachment (pili also in conjugation).

08.5 Cell Wall (Eukaryotic — Plant)

A non-living rigid structure outside the plasma membrane in plants, fungi and prokaryotes. Gives shape, mechanical strength and protection and prevents the cell from bursting in a hypotonic medium.

Composition & Layers

• Middle lamella: outermost, mainly calcium pectate; cements adjacent cells together.
• Primary wall: thin, capable of growth; cellulose.
• Secondary wall: on the inner side in mature cells; thick, rigid.

Algal walls = cellulose, galactans, mannans, minerals. Plasmodesmata are cytoplasmic bridges connecting neighbouring cells through the wall.

08.6 Plasma Membrane & Transport

The fluid mosaic model was proposed by Singer & Nicolson (1972). The membrane is a lipid bilayer (phospholipids + cholesterol) with proteins (integral & peripheral) floating in it — "protein icebergs in a lipid sea". The quasi-fluid nature allows lateral movement, growth, endo/exocytosis and cell division.

Transport across the membrane

• Passive (no ATP): simple diffusion (down the gradient), osmosis (water), facilitated diffusion (via carrier proteins).
• Active (needs ATP): moves molecules against the gradient, e.g. the Na⁺/K⁺ pump.

08.7 Endomembrane System (ER, Golgi, Lysosome, Vacuole)

Organelles whose functions are coordinated: endoplasmic reticulum, Golgi apparatus, lysosomes and vacuoles. (Mitochondria, chloroplasts and peroxisomes are excluded.)

The Golgi has a cis (forming/convex) face near the ER and a trans (maturing/concave) face. Material flows ER → Golgi → vesicle → membrane/secretion.

08.8 Mitochondria & Plastids (Energy & Pigments)

Both are double-membrane, semi-autonomous organelles (own circular DNA + 70S ribosomes) — evidence for the endosymbiotic theory.

Mitochondria — "powerhouse of the cell"

• Inner membrane folds into cristae → more surface area for the electron transport chain (ATP synthesis).
• The matrix holds enzymes of the Krebs (TCA) cycle.
• Site of aerobic respiration; produces energy currency ATP.

Plastids (plant cells only)

Inside a chloroplast: stacks of thylakoids = grana (light reactions); fluid stroma (dark reactions/Calvin cycle). Leucoplast types: amyloplast (starch), elaioplast (oils/fats), aleuroplast/proteinoplast (proteins).

08.9 Ribosomes, Cytoskeleton, Cilia, Flagella & Centrosome

Ribosomes (Palade, 1953) are non-membranous ribonucleoprotein factories of protein synthesis. Eukaryotic = 80S (60S + 40S); prokaryotic/organellar = 70S (50S + 30S). 'S' = Svedberg sedimentation unit.

Cytoskeleton

A network of microtubules, microfilaments and intermediate filaments giving shape, mechanical support, motility and intracellular transport.

Cilia & Flagella

• Covered by the plasma membrane; core = axoneme with the 9 + 2 arrangement (9 peripheral doublets + 2 central single microtubules).
• The basal body (kinetosome) has a 9 + 0 arrangement.
• Cilia = many, small (move fluid); Flagella = few, long (move the cell).

Centrosome & Centrioles

Two perpendicular centrioles, each with a 9 + 0 (nine triplet) cartwheel pattern. They form the spindle fibres in animal-cell division and the basal bodies of cilia/flagella.

08.10 The Nucleus & Chromosomes

The nucleus (described by Robert Brown, 1831) controls cell activities and stores hereditary information.

• Nuclear envelope: double membrane with nuclear pores for RNA/protein traffic; outer membrane continues with the ER.
• Nucleoplasm: contains chromatin & the nucleolus.
• Nucleolus: non-membranous; site of rRNA synthesis & ribosome assembly.
• Chromatin: DNA + histone proteins. Euchromatin = loose, active; heterochromatin = condensed, inactive.

Chromosome types (by centromere position)

The centromere bears the kinetochore (disc for spindle attachment). A satellite is a knob beyond a secondary constriction.

08.11 Microbodies & Plant vs Animal Cell

Microbodies are tiny membrane-bound vesicles containing enzymes — e.g. peroxisomes (β-oxidation, H₂O₂ breakdown) and glyoxysomes (fat → sugar in germinating seeds).