6. Cell & Endosymbiotic Theory

The Cell (Theories)

6.1 Cell Historical Context

  • Early Microscope Developments:

    • 1595: Janssen Brothers (Hans & Zacharias) invented the first compound microscope.

    • 1665: Robert Hooke observed cells for the first time in a cork sample and termed them "cellulae."

    • 1674: Anton van Leeuwenhoek improved the microscope, observing bacteria (the first living organisms) and referred to them as "animalcules."

6.2 Cell Theory Postulates

  • Foundational Statements:

    • 1838: Theodor Schwann and Matthias Schleiden declared:

      • "All plants are made up of cells."

      • "All animals are made up of cells."

    • 1858: Rudolf Virchow stated: "All cells come from other cells."

6.3 Postulates of Cell Theory

  • Postulate 1: "All living organisms are made up of one or more cells."

  • Postulate 2: "The cell is the basic unit of all living organisms (and all its metabolic functions)."

  • Postulate 3: "All cells come from pre-existing cells."

6.4 Types of Cells

  • Prokaryotic Cells:

    • Smaller than eukaryotic cells;

    • Older evolutionary lineage;

    • Circular DNA chromosome, located freely in the cell (no nuclear membrane);

    • Can be either heterotrophs or autotrophs

    • Represented by bacteria and archaea.

    • Characteristics:

      • Usually possess a cell membrane and cell wall.

      • Vary in shape (Cocci, Bacilli, Spirilli, Vibrio).

      • Reproduce asexually only.

  • Eukaryotic Cells:

    • Larger than prokaryotic cells;

    • DNA is organized in linear chromosomes within a defined nucleus;

    • Can also be heterotrophs or autotrophs;

    • Represented by the domains Animalia, Plantae, Fungi, and Protista.

    • Characteristics:

      • Have internal membranous structures (organelles);

      • May have a cell wall (found in plants and fungi);

      • Can reproduce both sexually and asexually.

Endosymbiotic Theory

Overview

  • Proposed by Lynn Margulis,

  • Endosymbiotic Theory: Eukaryotic cells originated from a symbiotic relationship which became permanent, resulting in one cell.

    • Explains the origin of mitochondria and chloroplasts.

Key Events

  1. First Endosymbiotic Event:

    • A small prokaryotic cell (proteobacterium) performing aerobic respiration was engulfed by a larger cell, resulting in a symbiotic relationship where both benefited.

    • This led to the formation of mitochondria.

  2. Second Endosymbiotic Event:

    • A small autotrophic cell (cyanobacterium) that performs photosynthesis was also engulfed, leading to the creation of chloroplasts.

Implications

  • Endosymbiotic theory illustrates how complex life evolved from simpler forms through cooperative relationships, embedding vital features like energy production into the eukaryotic cells.