Cell: The Unit of Life Notes: Class 11 Biology Chapter 8

Short Note:

• Definition: The cell is the fundamental structural and functional unit of all living organisms. Anton von Leeuwenhoek first saw and described a live cell.

• Cell Theory Formulation:

  - Matthias Schleiden (1838): Observed that all plants are composed of different kinds of cells.

  - Theodore Schwann (1839): Reported that animal cells have a thin outer layer (plasma membrane) and stated that plant cells uniquely possess a cell wall.

  - Rudolf Virchow (1855): Modified cell theory by adding "Omnis cellula-e cellula" (all cells arise from pre-existing cells).

• Modern Cell Theory Postulates:

  1. All living organisms are composed of cells and products of cells.

  2. All cells arise from pre-existing cells through cell division.

Short Note:

• Represented by Bacteria, Blue-green algae, Mycoplasma (PPLO - Pleuro Pneumonia Like Organisms). Multiply rapidly and lack a membrane-bound nucleus or membrane-bound organelles.

• Cell Envelope: Consists of a tightly bound 3-layered structure: outermost Glycocalyx (slime layer/capsule), middle Cell Wall, and innermost Plasma Membrane.

• Mesosomes: Special membranous structures formed by extensions of plasma membrane into the cell. Function in cell wall formation, DNA replication, respiration, and secretion.

• Genetic Material: Single circular genomic DNA not enclosed by a nuclear membrane (Naked DNA/Nucleoid). Many bacteria also have small circular extrachromosomal DNA called Plasmids (confer resistance to antibiotics).

• Ribosomes & Inclusion Bodies: 70S ribosomes (50S + 30S subunits) site of protein synthesis. Reserve materials stored as non-membrane bound inclusion bodies (e.g., phosphate granules, cyanophycean granules, glycogen granules).

Short Note:

• Characterized by extensive compartmentalization of cytoplasm through membrane-bound organelles and a well-organized nucleus.

• Plasma Membrane: Based on the Fluid Mosaic Model (Singer and Nicolson, 1972). Phospholipid bilayer with embedded proteins (integral and peripheral) and carbohydrates. Shows quasi-fluid nature allowing lateral movement of proteins.

• Endomembrane System: Coordinated organelles including:

  - Endoplasmic Reticulum (ER): RER (bearing ribosomes, protein synthesis) and SER (lipid and steroid synthesis).

  - Golgi Apparatus: Cis and trans faces; functions in packaging, modification, and secretion of proteins/lipids.

  - Lysosomes: Hydrolytic enzyme-rich vesicles ("suicide bags") active at acidic pH.

  - Vacuoles: Membrane-bound space (tonoplast) storing water, sap, and excretory products.

• Energy Transducers:

  - Mitochondria: Double-membraned "powerhouse of the cell", site of aerobic respiration producing ATP. Contains 70S ribosomes and circular DNA.

  - Plastids: Chloroplasts (photosynthesis, contains chlorophyll), Chromoplasts (fat-soluble pigments like carotene/xanthophyll), Leucoplasts (colorless storage plastids like Amyloplasts, Elaioplasts, Aleuroplasts).

• Ribosomes: 80S type (60S + 40S subunits) in eukaryotic cytoplasm, non-membrane bound, discovered by George Palade.

Short Note:

• First described by Robert Brown (1831). Stained material named Chromatin by Flemming.

• Structure: Double-layered nuclear envelope with nuclear pores, nucleoplasm, nucleolus (site of active rRNA synthesis), and chromatin network.

• Chromosome Classification (based on Centromere position):

  1. Metacentric: Centromere in the middle forming two equal arms.

  2. Sub-metacentric: Centromere slightly away from middle, forming one shorter arm and one longer arm.

  3. Acrocentric: Centromere situated close to its end, forming one extremely short and one very long arm.

  4. Telocentric: Terminal centromere at the tip of chromosome.