3.2 Foundations of Modern Cell Theory

Cell theory forms the foundation of modern biology, explaining how all living things are made of cells. From single-celled organisms to complex multicellular life, cells are the basic units of structure and function.

Scientists like Schleiden, Schwann, and Virchow developed cell theory through careful observation. Their work revealed that cells come from other cells and contain genetic material, setting the stage for our understanding of life's building blocks.

Foundations of Cell Theory

Core principles of cell theory

• All living organisms are composed of one or more cells serve as the fundamental structural and functional units of life
• Cells arise from pre-existing cells through the process of cell division (mitosis or meiosis) as spontaneous generation does not occur, which was demonstrated by Louis Pasteur's swan-neck flask experiment that prevented microorganisms from entering the flask and contaminating the broth
• Rudolf Virchow proposed the concept that cells can only originate from pre-existing cells, summarized by the Latin phrase "Omnis cellula e cellula" meaning "every cell from a cell"
• Cells contain genetic material (DNA) that carries hereditary information necessary for regulating cellular functions and passing genetic traits to daughter cells during cell division
• Matthias Schleiden and Theodor Schwann independently concluded that cells form the basic building blocks of all living organisms
  • Schleiden studied plant tissues and observed that all plants consisted of cells (lettuce leaves)
  • Schwann studied animal tissues and observed that all animals were made of cells (human skin)
• Cells maintain internal balance through homeostasis, regulating their internal environment to maintain stable conditions

Cell structure and function

• Cells are enclosed by a cell membrane, which controls the movement of substances in and out of the cell
• The cytoplasm is the gel-like substance within the cell where cellular processes occur
• Eukaryotic cells contain a nucleus, which houses the genetic material and controls cellular activities
• Various organelles within the cell perform specific functions to support cellular processes

Cellular diversity and specialization

• Cell differentiation allows cells to develop into specialized types with specific functions
• Cell specialization enables different cell types to perform unique roles within multicellular organisms

Evidence for endosymbiotic theory

• Endosymbiotic theory proposes that certain organelles in eukaryotic cells, such as mitochondria and chloroplasts, originated as prokaryotic cells engulfed by a larger host cell (endosymbiosis)
• Mitochondria and chloroplasts contain their own circular DNA, similar to bacterial DNA (plasmids)
• These organelles reproduce independently of the cell through binary fission, a process similar to bacterial cell division (budding)
• Mitochondria and chloroplasts have double membranes, with the inner membrane resembling a bacterial cell membrane (peptidoglycan)
• Ribosomes found in these organelles are similar in size and composition to bacterial ribosomes (70S)
• Certain antibiotics that target bacteria also affect mitochondria and chloroplasts (chloramphenicol)

Germ Theory

Scientists' contributions to germ theory

• Ignaz Semmelweis (1818-1865)
  • Observed that handwashing with chlorinated lime solution significantly reduced cases of puerperal fever in a Vienna hospital maternity ward
  • Proposed that puerperal fever was caused by "decomposing animal matter" transmitted by physicians' hands from autopsies to patients
• Louis Pasteur (1822-1895)
  • Conducted experiments demonstrating that microorganisms cause fermentation and spoilage (wine)
  • Developed techniques for growing, studying, and identifying microbes (pasteurization)
  • Disproved spontaneous generation with his swan-neck flask experiment
  • Developed vaccines for anthrax and rabies
• Joseph Lister (1827-1912)
  • Applied Pasteur's principles to surgical practices
  • Introduced antiseptic techniques, such as using carbolic acid to clean wounds and surgical instruments (phenol)
  • Significantly reduced post-operative infections and mortality rates
• Robert Koch (1843-1910)
  • Developed techniques for isolating and culturing pure bacterial strains (nutrient agar)
  • Established ###Koch's_Postulates_0###, criteria for determining if a specific microbe causes a particular disease:
    1. The microbe must be present in every case of the disease (tuberculosis)
    2. The microbe must be isolated from the diseased host and grown in pure culture
    3. The cultured microbe should cause disease when introduced into a healthy organism (guinea pigs)
    4. The microbe must be re-isolated from the inoculated, diseased experimental host and identified as identical to the original specific causative agent
  • Identified the causative agents of anthrax, tuberculosis, and cholera