NCERT Line by Line: Cell - The Unit of Life

Introduction to Cell

  • Fundamental structural and functional unit of LIFE.

  • Two types of organisms based on cellular structure:

    • Unicellular Organisms: Composed of a single cell. Examples: Bacteria, Amoeba.

    • Multicellular Organisms: Composed of many cells. Examples: Animals.

Cellular Characteristics

  • Cellular organization is a defining feature of living organisms.

  • Only unicellular organisms can ensure independent living.

  • Anton Von Leeuwenhoek first observed and described a live cell.

  • Robert Brown discovered the nucleus in 1831.

Cell Theory

  • Formulated by Schleiden (1838) and Schwann (1839), later modified by Rudolf Virchow (1855).

  • Key points summarized:

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

    2. All cells arise from pre-existing cells.

  • Virchow introduced the phrase: “Omnis cellula-e cellula” which means new cells arise from existing cells.

  • Viruses are not considered living and are exceptions to the cell theory.

Overview of Cell

  • Nucleus: Dense structure containing chromosomes with genetic material (DNA).

  • Cytoplasm: Semi-fluid matrix where cellular activities occur, keeping the cell in a living state.

  • Cell Membrane: Outer boundary of the cell, defining its structure.

  • Cell Wall: Present outside the cell membrane in plant cells; absent in animal cells.

  • Organelles differ between prokaryotic and eukaryotic cells:

    • Eukaryotic Cells: Membrane-bound organelles (well-defined).

    • Prokaryotic Cells: Membrane-less structure (not well-defined).

Endomembrane System

  • Includes Endoplasmic Reticulum (ER), Golgi Apparatus, Lysosomes, and Vacuoles.

  • Mitochondria, Chloroplasts, and Peroxisomes are not part of the endomembrane system.

Components of Cells

  • Cell Organelles:

    • Double Membranous: Nucleus, Mitochondria, Plastids.

    • Single Membranous: Lysosomes, Microbodies, Golgi complex, ER, Vacuoles.

    • Non-Membranous: Ribosomes and Centrosomes.

Prokaryotic Cell

  • Represented by members of the kingdom Monera.

  • They are typically 10 x smaller than eukaryotic cells.

  • Examples include E.coli, which divides every 20 minutes.

  • Prokaryotes have a rigid cell wall, primarily composed of peptidoglycan.

  • Genetic material consists of genomic DNA (single chromosome/circular DNA) and plasmid DNA.

Shapes of Prokaryotic Cells

  • Coccus: Round shape.

  • Bacillus: Rod shape.

  • Vibrio: Comma shape.

  • Spirillum: Spiral shape.

Ribosomes

definition

  • Smallest organelle and a site for protein synthesis.

  • Types:

    • 70S: In prokaryotes (subunits: 50S and 30S).

    • 80S: In eukaryotes (subunits: 60S and 40S).

Eukaryotic Cells

  • Defined by the presence of a well-defined nucleus and membrane-bound organelles.

  • Examples: Protista, Fungi, Plantae, Animalia.

Cell Membrane

  • Composed mainly of lipids and proteins (Fluid Mosaic Model).

  • Contains cholesterol for stabilization.

  • Major lipids include phospholipids, which have hydrophilic heads and hydrophobic tails.

Cell Wall

  • A non-living, rigid structure composed of cellulose in plants, peptidoglycan in bacteria, and chitin in fungi.

  • Middle Lamella: Cement layer of calcium-pectate that holds neighboring cells together.

Vacuoles

  • Single membranous structure containing water and sap, which occupies up to 90% of the cell's volume.

Mitochondria

  • Double membranous organelle site for aerobic respiration producing ATP.

  • Contains circular DNA and can replicate through binary fission.

  • Matrix contains enzymes important for metabolic processes.

Plastids

  • Found in plants; types include chloroplasts (photosynthesis) and leucoplasts (food storage).

Cytoskeleton

  • Network of proteinaceous structures that provide mechanical support, shape, and motility to cells (includes microfilaments and microtubules).

Nucleus

  • Contains DNA-forming chromatin and is the site for RNA synthesis.

  • Surrounded by a nuclear envelope with pores that control material movement.

  • Nucleolus produces ribosomal RNA (rRNA).

Chromosomes

  • Structures made up of chromatin that become visible during cell division.

  • Classified based on centromere position: metacentric, sub-metacen

    tric, acrocentric, telocentric.

Summary of Cell Theory and Structures

  • Cells are fundamental units of life and reproduce, contributing to organism growth and development.

  • Understanding cell structure leads to insights into function and roles in ecosystems.

Critical Points

  • Clear distinctions between prokaryotic and eukaryotic cells.

  • Importance of microscopy in understanding cellular structures.

  • Recognition of cellular components roles in biological processes.

  • Continuous advancements in cell biology research further our knowledge of life processes.