Cell: The Unit of Life

Cell is the fundamental structural and functional unit of all living organisms. Cytology is the study of cell structure.

• Matthias Schleiden (1838): German botanist. Observed that all plants are composed of different kinds of cells.
• Theodore Schwann (1839): British Zoologist. Reported that cells had a thin outer layer (plasma membrane) and plant cells have cell walls.
• Rudolf Virchow (1855): "Omnis cellula-e cellula" (New cells are formed from pre-existing cells). Modified the hypothesis of Schleiden and Schwann.

Represented by Bacteria, Blue-green algae, Mycoplasma and PPLO. Generally smaller and multiply rapidly.

Cell Envelope & Modifications

• Glycocalyx: Outermost. Slime layer (loose sheath) or Capsule (thick & tough).
• Cell Wall: Structural support.
• Plasma Membrane: Selectively permeable. Only structure common to prokaryotes and eukaryotes (functionally).
• Mesosome: Infoldings of plasma membrane (vesicles, tubules, lamellae). Functions: Cell wall formation, DNA replication, Respiration/Secretion.

Other Features

• Genetic Material: Naked (no nuclear membrane). Circular DNA (Genomic DNA) + Plasmids (Small circular DNA, antibiotic resistance).
• Ribosomes: 70S type (50S + 30S). Site of protein synthesis.
• Inclusion Bodies: Reserve material. Phosphate granules, Cyanophycean granules, Glycogen granules. Gas vacuoles (buoyancy) in Blue-green/Purple/Green photosynthetic bacteria.

Fluid Mosaic Model (1972)

Proposed by Singer and Nicolson.

• Lipids: Phospholipids arranged in a bilayer (Polar heads out, Hydrophobic tails in). Also Cholesterol.
• Proteins: Integral (buried) and Peripheral (surface). Ratio varies (Human RBC: 52% Protein, 40% Lipid).
• Fluidity: Quasi-fluid nature of lipids enables lateral movement of proteins. Important for growth, secretion, endocytosis.
• Transport: Passive (Diffusion/Osmosis) and Active (Na+/K+ pump, requires ATP).

Cell Wall

• Algae: Cellulose, Galactans, Mannans, Calcium carbonate.
• Plants: Cellulose, Hemicellulose, Pectins, Proteins.
• Middle Lamella: Calcium pectate (glues cells together).
• Plasmodesmata: Connect cytoplasm of neighbouring cells.

Components whose functions are coordinated: ER, Golgi, Lysosomes, Vacuoles.

• Endoplasmic Reticulum (ER):
  RER (Rough): Ribosomes attached. Protein synthesis & secretion.
  SER (Smooth): Lipid synthesis, Steroid hormones.
• Golgi Apparatus (Camillo Golgi, 1898):
  Flat disc-shaped sacs (cisternae). Packaging of materials. Modification of proteins (Glycosylation). Site of formation of Glycoproteins and Glycolipids.
• Lysosomes:
  Formed by Golgi. Rich in hydrolytic enzymes (Lipases, Proteases, Carbohydrases). Active at acidic pH. "Suicidal bags".
• Vacuoles:
  Tonoplast (membrane). Sap vacuole (Water, sap, excretory product). In Amoeba: Contractile vacuole (Excretion).

• Ribosomes (George Palade, 1953): Granular structures composed of RNA and proteins. Not membrane bound. Eukaryotic = 80S (60S+40S). Prokaryotic = 70S (50S+30S). 'S' = Svedberg's unit.
• Cytoskeleton: Network of filaments (Microtubules, Microfilaments). Mechanical support, motility.
• Cilia & Flagella: Hair-like outgrowths.
  Axoneme: 9+2 arrangement (9 doublets radials + 2 central singlets). Emerging from basal body (centriole-like).
• Centrosome & Centrioles: Centrosome has two centrioles (perpendicular). 9+0 arrangement (9 even triplets, no central). Spindle fibre formation.
• Nucleus (Robert Brown, 1831):
  Double membrane with Nuclear Pores. Nucleoplasm contains Nucleolus (rRNA synthesis) and Chromatin.
  Chromosomes: DNA + Histones. Based on centromere position:
  • Metacentric: Middle centromere (V-shape).
  • Sub-metacentric: Slightly away from middle (L-shape).
  • Acrocentric: Close to end (J-shape).
  • Telocentric: Terminal centromere (I-shape).

The fundamental structural and functional unit of all living organisms.

First saw and described a live cell.

Discovered the Nucleus.

German Botanist. Stated that all plants are composed of different kinds of cells which form the tissues.

British Zoologist. Reported plasma membrane and stated that cell wall is unique to plant cells.

"Omnis cellula-e cellula". All cells arise from pre-existing cells.

Smallest cells. Only 0.3 µm in length.

The largest isolated single cell.

Red Blood Cells are about 7.0 µm in diameter.

Naked (no nuclear membrane), circular genomic DNA, and often Plasmids.

Small circular DNA outside genomic DNA. Confer unique phenotypes like Antibiotic Resistance.

Key feature of prokaryotes. Infoldings of plasma membrane. DNA replication, Respiration, Secretion.

Outermost layer in bacteria. Slime layer (loose) or Capsule (thick/tough).

Several ribosomes attached to a single mRNA chain. Translate mRNA into proteins.

Found in Blue-green and Purple and Green photosynthetic bacteria. Provide buoyancy.

Proposed by Singer and Nicolson (1972). Quasi-fluid nature of lipids allows protein movement.

Human RBC membrane: 52% Protein and 40% Lipid.

Example of Active Transport (against concentration gradient), utilizes ATP.

Layer made of Calcium Pectate holding different plant cells together.

Cytoplasmic connections between neighbouring plant cells traversing the cell walls.

Includes ER, Golgi, Lysosomes and Vacuoles. Coordinate function.

Rough Endoplasmic Reticulum (with Ribosomes) is the site of Protein synthesis and secretion.

Smooth ER is the major site of Lipid synthesis and steroidal hormones.

First observed Golgi apparatus (densely stained reticular structure near nucleus).

Cis (Forming) and Trans (Maturing) faces. Materials packaged in ER fuse with Cis and exit from Trans.

Golgi apparatus is the site of formation of Glycoproteins and Glycolipids.

Membrane bound vesicular structures. Rich in Hydrolytic enzymes (Lipases, Proteases). Active at Acidic pH.

The membrane bounding the vacuole. Facilitates transport of ions against concentration gradient.

In Amoeba, important for Osmo-regulation and Excretion.

Mitochondria are not visible under microscope unless stained (e.g., Janus Green).

Infoldings of inner mitochondrial membrane. Increase surface area for enzyme action.

Contains single circular DNA, RNA, 70S Ribosomes and components for protein synthesis.

Length: 5-10 µm. Width: 2-4 µm.

Flattened membranous sacs in Stroma. Stacked to form Grana. Contain chlorophyll.

Leucoplasts that store carbohydrates (starch), e.g., Potato.

Leucoplasts that store oils and fats.

Discovered by George Palade (1953). Composed of RNA (rRNA) and proteins. No membrane.

Eukaryotic ribosome. Subunits: 60S (Large) + 40S (Small).

Prokaryotic ribosome (also in Mitochondria/Chloroplasts). Subunits: 50S + 30S.

Network of proteinaceous filaments (microtubules, microfilaments). Mechanical support, motility.

Core of Cilia/Flagella. 9+2 arrangement (9 peripheral doublets + 2 central singlets).

9+0 arrangement (9 peripheral triplets). No central microtubule.

Centrioles form the basal body of Cilia and Flagella.

Site for active ribosomal RNA (rRNA) synthesis. Cell dividing actively has larger/more nucleoli.

Chromosome with centromere in the middle. V-shaped.

Centromere close to one end. J-shaped.

Terminal centromere. I-shaped.

Non-staining secondary constriction at constant location. Small fragment of chromosome.

Space between two nuclear membranes (10 - 50 nm). Barrier between nucleoplasm and cytoplasm.

Also called Polysome. Translation unit. Ribosomes stringed together on mRNA.