Cell Theory, Scientific Method & Microscopy – Quick Review

Biology

  • Scientific study of life and living organisms.

Theories on the Origin of Life

  • Abiogenesis – life arises from non-living matter (Aristotle; believed until 17^{th} century).
  • Biogenesis – life originates only from pre-existing life; validated by Redi, Spallanzani, Pasteur.

Cell Theory

  • Formulated by Schleiden & Schwann (microscope improvements enabled observations).
  • Three postulates:
    • All organisms are composed of cells.
    • Cell is the basic unit of life.
    • All cells arise from pre-existing cells.

Disproof of Spontaneous Generation (Key Experiments)

  • Redi – covered vs. uncovered meat → maggots only in open jars.
  • Needham – boiled broth; growth occurred (experimental flaw).
  • Spallanzani – repeated with sealed flasks → no growth; showed contamination source.
  • Pasteur – S-shaped flask; microbes trapped in neck, broth remained sterile until neck broken.

Scientific Method & Reasoning

  • Steps: Observation → Question → Hypothesis → Experiment → Results → Conclusion → Peer Review → Theory.
  • Inductive reasoning: specific observations ➜ general principle.
  • Deductive reasoning: general principle ➜ specific prediction.
  • Experimental design: control variables, adequate sample size, statistical analysis.

Microscopy: History

  • Romans fashioned early lenses (first century).
  • Zacharias Janssen – first compound microscope (late 16^{th} century).
  • Robert Hooke – coined “cell” (cork observations, 17^{th} century).
  • Antonie van Leeuwenhoek – father of microbiology; high-magnification single-lens scopes; first to observe bacteria, protozoa, blood, sperm, eggs; linked fertilization to egg + sperm.

Types of Microscopes (Key Uses)

  • Compound – visible light, 40–1000×; routine labs.
  • Dissecting/Stereo – low magnification, 3-D surface view.
  • Electron – electron beams for ultrastructure (viruses, organelles).
  • Digital – built-in camera & screen; image capture.
  • Fluorescence – UV/excitation light to view glowing tagged structures.

Compound Microscope: Major Parts & Functions

  • Eyepiece (ocular) – final 10×–15× magnification.
  • Body & eyepiece tubes – align optics.
  • Objective lenses – 4×, 10×, 40×, 100× (oil) magnifications.
  • Nosepiece – rotates objectives.
  • Stage & clips – hold slide.
  • Coarse focus – rapid stage movement.
  • Fine focus – precise adjustment.
  • Condenser/diaphragm – light concentration & contrast.
  • Light source/mirror – illumination.
  • Arm & base – support and carrying points.

Using a Compound Microscope (Essential Sequence)

  • Carry with one hand on arm, other under base.
  • Place slide; secure with stage clips.
  • Start with low-power objective.
  • Coarse focus → image appears.
  • Fine focus → sharpen.
  • Adjust light for clarity.
  • Switch to higher power if required; refocus with fine knob only.
  • After viewing: return to low power, lower stage, remove slide, power off, cover instrument.