Cell Theory and Microscopy History

History of Microscopy and Cell Discovery

• Zacharis Janssen invented the first microscope in 1590.
• Robert Hooke discovered cells in 1665 by observing cork under a microscope, noting a honeycomb-like structure, which he called "Cella" (Latin for compartments).
• Anton van Leeuwenhoek observed free-living cells in 1675, contributing to cell science.

Cell Theory

• Cells are the structural and functional units of life.
• Structural hierarchy: Cell → Tissues → Organs → Organ Systems → Whole Body.
• Functional units include processes such as excretion, reproduction, respiration, and digestion in unicellular organisms, with division of labor in multicellular organisms.
• Schleiden and Schwann proposed cell theory in the 19th century, including key tenets:
  • All organisms are made of cells.
  • Cells consist of protoplasm containing a nucleus and plasma membrane, with or without a cell wall.
  • New cells arise from pre-existing cells (Rudolph Virchow's modern cell theory).
  • All cells share chemical composition and metabolic functions.
  • Organism functions result from the sum of cell activities.
  • Every cell is totipotent, capable of developing into a complete organism.

Exceptions to Cell Theory

• Viruses lack true cellular structures (no cytoplasm or organelles).
• Bacteria and cyanobacteria do not possess a true nucleus.
• Coenocytic organisms (e.g., Mucor, Rhizopus) have multinucleated bodies without partitions.
• Mature mammalian red blood cells and sieve tube cells lack nuclei.
• Paramecium contains two nuclei.

Cell Characteristics and Functions

• Cells are independently functional and contain complete genetic information.
• Cells convert micro to macro molecules and produce/store energy as ATP.
• They regulate the flow of energy and information, with each cell having a specific life span (RBC - 4 months, skin cells - 2 to 4 weeks, pancreatic cells - 1 year, bone cells - 25 to 30 years).
• Cells perform respiration and facilitate gas exchange.

Energy and Information Flow in Cells

• Energy flow: Sunlight → Green parts (photosynthesis) → Non-green parts (respiration) → Energy release → Transfers to higher trophic levels.
• Intrinsic information flow: DNA → mRNA (Transcription) → Protein (Translation).
• Extrinsic information flow: Stimuli, metal ions, or hormones influence cellular response.

Differences Between Unicellular and Multicellular Organisms

• Unicellular: single-cell organisms (e.g., Amoeba, Chlamydomonas); no division of labor; susceptible to injury.
• Multicellular: composed of multiple cells; division of labor; more resilience to injury and can reproduce multiple times.

Cellular Totipotency

• Defined as the ability of a living, vegetative cell to regenerate into a complete organism.
• Concept introduced by Haberlandt in 1902.
• White cultured tomato roots from a single cell in 1932.
• Steward's experiments demonstrated totipotency using carrot tissue:
  • Phloem cells cultured in a nutrient-rich medium produced clusters.
  • These clusters developed into rooted clumps and eventually full plants when transferred to a natural environment.

Types of Cells

• Prokaryotic Cells: Lack a true nucleus and membrane-bound organelles; possess a nucleoid without a nuclear membrane (e.g., Bacteria, Cyanobacteria).
• Eukaryotic Cells: Have a defined nucleus and membrane-bound organelles, surrounded by a plasma membrane.