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Cell Structure and Types

Aug 6, 2025

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Overview

This lecture provides a comprehensive summary of Chapter 1: The Cell - Unit of Life, covering key NCERT lines, cell types, cell theory, cell structures, and important differences between prokaryotic and eukaryotic cells, along with associated exam questions.

Introduction to Cells

  • The cell is the basic unit of life.
  • Living things (animate) possess growth, respiration, reproduction, and self-consciousness; non-living (inanimate) do not.
  • Organisms can be unicellular (single cell does all functions) or multicellular (specialized cells and organ systems).

History and Cell Theory

  • Robert Hooke discovered cells (from dead plant tissue).
  • Anton van Leeuwenhoek identified living cells.
  • Robert Brown discovered the nucleus.
  • Schleiden (plants) and Schwann (animals) proposed all living beings are made of cells (cell theory).
  • Rudolf Virchow: "Omnis cellula e cellula"—cells arise from pre-existing cells.

Types of Cells and Cell Structure

  • Unicellular organisms: single cell does all functions (e.g., Amoeba).
  • Multicellular organisms: cells specialize and form tissues, organs, organ systems.
  • Key cell shapes: RBC (biconcave, O₂ transport), WBC (amoeboid, immunity), nerve cell (long, signal transmission), tracheids (elongated, water transport), mesophyll (oval, photosynthesis).
  • Smallest cell: Mycoplasma (0.3 µm); largest: Ostrich egg cell.

Plant vs Animal Cells

  • Plant cells: have cell walls (cellulose), large vacuoles, chloroplasts.
  • Animal cells: lack cell walls, smaller vacuoles, no chloroplasts.

Prokaryotic vs Eukaryotic Cells

  • Prokaryotes: simple, no membrane-bound organelles, nucleoid (no true nucleus), e.g., bacteria.
  • Eukaryotes: complex, have membrane-bound organelles, true nucleus, e.g., plants, animals, fungi, protists.
  • Prokaryotes have 70S ribosomes; eukaryotes have 80S (except mitochondria/chloroplasts—70S).

Prokaryotic Cell Structure

  • Cell envelope: outer glycocalyx (capsule/slime layer), middle cell wall (peptidoglycan in bacteria), inner plasma membrane (selectively permeable).
  • Appendages: flagella (movement), pili/fimbriae (attachment).
  • Plasmids: extra-chromosomal DNA, antibiotic resistance genes.
  • Shapes: bacillus (rod), coccus (spherical), vibrio (comma), spirillum (spiral).
  • Inclusion bodies: storage granules (phosphate, glycogen, gas vacuoles).

Eukaryotic Cell Organelles

  • Plasma membrane: lipid bilayer with proteins (fluid mosaic model), selectively permeable.
  • Cell wall (plants/fungi): rigid, made of cellulose/chitin.
  • Endomembrane system:
    • Rough ER (ribosomes, protein synthesis)
    • Smooth ER (lipid synthesis)
    • Golgi apparatus (modification, packaging)
    • Lysosomes (hydrolytic enzymes, digestion)
    • Vacuoles (storage, large in plants)
  • Mitochondria: double membrane, site of ATP production ("powerhouse"), own DNA/ribosomes.
  • Plastids (plants):
    • Chloroplasts (photosynthesis, green)
    • Chromoplasts (colored pigments)
    • Leucoplasts (storage, colorless)
  • Ribosomes: protein synthesis (70S in prokaryotes/mitochondria/chloroplasts, 80S in eukaryotic cytoplasm).
  • Cytoskeleton: microtubules, actin, intermediate filaments (shape, movement).
  • Centrosome/centriole (animals): spindle formation in cell division.
  • Nucleus: nuclear envelope (double membrane), nucleolus (rRNA synthesis), chromatin (DNA+proteins), chromosomes (condensed chromatin), nuclear pores (exchange).

Key Terms & Definitions

  • Cell — Basic structural and functional unit of life.
  • Prokaryote — Organism with no membrane-bound nucleus or organelles.
  • Eukaryote — Organism with true nucleus and membrane-bound organelles.
  • Organelle — Specialized cell structure performing specific functions.
  • Ribosome — Protein synthesis machinery.
  • Plasmid — Extra-chromosomal, circular DNA in bacteria.
  • Chromatin — DNA + protein complex in nucleus.
  • Lysosome — Organelle with digestive enzymes ("suicidal bag").
  • Mitochondria — Organelle generating ATP via aerobic respiration.
  • Plastid — Plant cell organelle (chloroplast, chromoplast, leucoplast).
  • Centromere — Primary constriction of chromosome where chromatids attach.
  • Histone — Protein aiding in DNA packaging.
  • Fluid mosaic model — Plasma membrane structure of lipid bilayer with embedded proteins.

Action Items / Next Steps

  • Review and memorize NCERT diagrams and tables (cell structure, types).
  • Read all lines and questions in the NCERT textbook for this chapter.
  • Study provided diagrams and make brief revision notes.
  • Practice writing answers to NCERT back questions, especially those on nucleolus, mitochondria, and plasma membrane.
  • Access and review the provided PDF solutions for NCERT questions via the class app or description link.

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