Cell: The Unit of Life

1. Cell – Introduction & Key Discoveries

A. Major Biological Discoveries

• G.N. Ramachandran: Triple helical structure of collagen (inspired by Linus Pauling).
• Matthias Schleiden (1838): German botanist, concluded plants are composed of cells.
• Theodore Schwann (1839): German zoologist, observed plasma membrane and noted cell wall is unique to plant cells.
• Rudolf Virchow (1855): Stated “Omnis cellula-e cellula” (All cells arise from pre-existing cells). Extended Cell Theory.
• Singer & Nicolson (1972): Proposed Fluid Mosaic Model of plasma membrane.
• Camillo Golgi (1898): Discovered Golgi body.
• George Palade (1953): Discovered Ribosomes.
• Robert Brown (1831): Discovered Nucleus.
• Flemming: Named stained chromatin in nucleus.
• J.E. Purkinje: Coined the term Protoplasm.

B. Cell Theory & Cell Types

Cell Theory: Proposed by Schleiden & Schwann, modified by Virchow. Exceptions: Viruses, Viroids, Prions.

• Smallest Cell: Mycoplasma (0.3 µm)
• Bacteria Size: 3 – 5 µm
• RBC Size: 7 µm (Biconcave & Round)
• Longest Cell: Neuron
• Largest Cell: Ostrich Egg

2. Prokaryotic Cells

Includes Bacteria, Blue-Green Algae (BGA), Mycoplasma. Multiply rapidly, smaller in size.

Cell Envelope Structures

1. Glycocalyx (Outermost): Loose sheath = Slime layer; Thick and tough = Capsule. Protects cell from host immune system.
2. Cell Wall: Made of Peptidoglycan (murein). Provides shape and structural support, prevents cell from bursting or collapsing.
3. Plasma Membrane: Selectively permeable, lipid bilayer structure.

• Mesosome: Infoldings of plasma membrane. Helps in cell wall formation, DNA replication, distribution to daughter cells, respiration, and increasing enzymatic surface area.
• Polysome / Polyribosome: Multiple 70S ribosomes attached to a single mRNA strand for translation.
• Chromatophore: Pigment-containing membrane extensions in photosynthetic prokaryotes (e.g., BGA, Purple/Green Sulphur bacteria).

3. Gram Staining & Eukaryotic Comparison

A. Gram Staining Technique (Christian Gram)

Differentiates bacteria based on cell wall composition using Crystal Violet stain, Iodine, Alcohol wash, and Safranin counterstain.

B. Prokaryote vs. Eukaryote Differences

• Prokaryotes: Nuclear envelope absent; G-C rich circular DNA without histones; 70S ribosomes; membrane-bound organelles absent.
• Eukaryotes: Distinct nuclear envelope present; Linear DNA with histones (A-T rich); 80S ribosomes (cytoplasm); Membrane-bound organelle system present.

C. Cell Wall Variations

• Algae: Made of Cellulose, Galactans, Mannans, and Minerals ($CaCO_3$).
• Plants: Cellulose, Hemicellulose, Pectins, and Proteins.
• Middle Lamella: Made of Calcium/Magnesium pectate; cements neighboring cells together.

4. Plasma Membrane & Endomembrane System

A. Plasma Membrane Structure & Transport

• Fluid Mosaic Model (Singer & Nicolson): Quasi-fluid nature of lipids enables lateral movement of proteins within the overall bilayer.
• Lipid Composition: Phosphoglycerides with polar head facing outwards (hydrophilic) and non-polar saturated hydrocarbon tail facing inwards (hydrophobic). Human RBC membrane has 52% protein and 40% lipid.
• Membrane Proteins: Integral (Intrinsic / Transmembrane) and Peripheral (Extrinsic – easy to extract).
• Transport Types:
  • Passive Transport: Along concentration gradient without energy expenditure (e.g., Simple Diffusion & Osmosis).
  • Facilitated Diffusion: Neutral/polar molecules cross via membrane carrier proteins without energy requirement.
  • Active Transport: Against concentration gradient using ATP (e.g., $Na^+ / K^+$ Pump).
  • Bulk Transport: Endocytosis (Phagocytosis = cell eating, Pinocytosis = cell drinking) & Exocytosis (Cell vomiting).

B. Endoplasmic Reticulum (ER)

Network of tiny tubular structures dividing intracellular space into Luminal (inside ER) and Extra-luminal (cytoplasm) compartments.

• Rough ER (RER): Bears 80S ribosomes on outer surface; active in protein synthesis and secretion (e.g., Pancreatic cells, Brain cells). Attached via Ribophorin I & II.
• Smooth ER (SER): Lacks ribosomes; major site for synthesis of lipids, steroidal hormones (testosterone), and glycogen metabolism/detoxification in liver.

5. Golgi Apparatus, Lysosome, Vacuole & Mitochondria

A. Golgi Body (Camillo Golgi, 1898)

• Consists of flattened disc-shaped sacs called Cisternae (0.5 – 1.0 µm diameter) stacked parallelly. In plant cells, referred to as Dictyosomes.
• Distinct Faces: Cis face (Convex / Forming face toward nucleus) and Trans face (Concave / Maturing face toward plasma membrane).
• Primary Functions: Modification of proteins (Glycosylation) and lipids (Glycolipidation); packaging and sorting materials for secretion; formation of Acrosome in sperm.

B. Lysosomes (“Suicidal Bags”)

• Single membrane-bound vesicular structures formed by packaging in Golgi apparatus.
• Rich in hydrolytic enzymes (lipases, proteases, carbohydrates) active at acidic pH (maintained by active proton pumping).
• Exhibits Polymorphism: Primary lysosomes, Secondary (Heterophagosome), Residual bodies, and Autophagic vacuoles.

C. Vacuoles & Mitochondria

• Vacuole: Bound by single membrane Tonoplast. Occupies up to 90% volume of plant cells. Tonoplast facilitates transport against concentration gradient.
  • Contractile Vacuole: Osmoregulation/excretion in Amoeba.
  • Food Vacuole: Engulfs food particles in Protista.
• Mitochondria (“Powerhouse of Cell”): Double-membrane organelle (Sausage/Cylindrical shaped, 0.2 – 1.0 µm diameter).
  • Outer membrane is smooth; Inner membrane forms infoldings called Cristae to increase surface area.
  • Matrix contains single circular dsDNA, 70S ribosomes, RNA, and Krebs cycle enzymes. Stained with Janus Green.
  • Divides by binary fission. Semi-autonomous organelle.

6. Plastids, Ribosomes & Cytoskeleton

A. Plastids Classification

B. Ribosomes & Cytoskeleton

• Ribosomes (Protein Factories): Non-membrane bound granules made of rRNA and proteins.
  • 70S Type: 50S + 30S subunits (In Prokaryotes, Mitochondria, Chloroplasts). Attached via $Mg^{2+}$ ions.
  • 80S Type: 60S + 40S subunits (In Eukaryotic cytoplasm).
  • ‘S’ = Svedberg’s Unit (Sedimentation Coefficient).
• Cytoskeleton: Elaborate network of filamentous proteinaceous structures in cytoplasm. Composed of Microtubules, Microfilaments, and Intermediate Filaments. Provides mechanical support, motility, and maintenance of cell shape.

7. Cilia, Flagella, Centrosome & Nucleus

A. Cilia & Flagella Arrangement

• Hair-like outgrowth of plasma membrane. Core is called Axoneme.
• (9 + 2) Array Structure: 9 peripheral doublet microtubules + 2 central singlet microtubules connected by radial spokes and interdoublet bridges (Nexin protein).
• Both cilia and flagella emerge from centriole-like structure called Basal body.

B. Centrosome & Centrioles

• Membraneless organelle containing 2 cylindrical structures called Centrioles lying perpendicular to each other.
• (9 + 0) Cartwheel Arrangement: Made of 9 evenly spaced peripheral triplets of Tubulin protein (Total 27 microtubules per centriole). Central region is proteinaceous Hub connected to spokes.

C. Nucleus Structure & Chromatin

• Double membrane-bound organelle with nuclear pores forming passage for RNA and proteins. Perinuclear space is 10 – 50 nm.
• Nucleolus: Non-membrane bound spherical structure in nucleoplasm; active site for ribosomal RNA (rRNA) synthesis.
• Chromatin Types:
  • Euchromatin: Loosely packed, transcriptionally active, stains light.
  • Heterochromatin: Tightly packed, transcriptionally inactive, stains dark.