Theories of Life and Cell Theory Notes on Cell Theory

Definition and Characteristics of Life

• Biology is the study of life and living organisms.
• To be classified as alive, organisms must exhibit five specific characteristics:
  • They need energy, typically obtained from food.
  • They produce waste material.
  • They respond and adapt to their environment.
  • They reproduce.
  • They grow.

Historical Theories on the Origin of Life

• Aristotle (384−322 B.C.): Proposed that life could arise from non-living matter based on observations of maggots in meat and mold on food.
• Spontaneous Generation (Abiogenesis): The theory that life can emerge from non-life under certain conditions.
• Theory of Biogenesis: The discovery and subsequent scientific consensus that life only originates from pre-existing life, supported by the experimental findings of Francesco Redi and Louis Pasteur.

The Development of Cell Theory

• Robert Hooke (17th Century): Used a microscope with $30 \times$ magnification to examine cork, discovering tiny box-like structures he named "cells."
• Anton van Leeuwenhoek: Used a $500 \times$ magnification microscope to study tooth scrapings, identifying single-celled organisms he called "animalcules."
• Schleiden and Schwann (1800s): German scientists who concluded that all plants and animals are composed of cells and that new cells originate from nuclei.
• Rudolf Virchow: A German physicist who established that cells divide to produce more cells, correcting earlier misconceptions about cell formation from non-living materials with the statement: ‘Where a cell exists, there must have been a pre-existing cell.’

Modern Cell Theory Principles

• The established Cell Theory consists of three main conclusions:
  1. All organisms are composed of one or more cells.
  2. The cell is the smallest functional unit of life.
  3. All cells are produced from other cells.

Microscopy and Imaging Technologies

• Simple Microscopes: Early instruments using a single lens with $10 \times$ to $20 \times$ magnification.
• Compound Microscopes: First developed in 1590 by the Janssen brothers; utilizes two lenses.
• Modern Light Microscope: Uses light and glass lenses to magnify specimens. Total magnification is calculated by multiplying the objective lens ($4 \times, 10 \times, \text{ or } 40 \times$) by the ocular lens ($10 \times$), commonly ranging from $40 \times$ to $400 \times$.
• Resolution: The ability of a microscope to provide a visible image with clear detail.
• Transmission Electron Microscope (TEM): Developed in the 1930s by Canadian scientists. Uses electron beams and electromagnets to magnify biological materials up to $100,000 \times$ and non-biological materials up to $1,000,000 \times$. Specimens must be dried, sliced, and coated, which kills the cells.
• Scanning Electron Microscope (SEM): An electron microscope where electrons reflect off the surface of the object rather than passing through it.
• Limitations: High-magnification electron microscopy and chemical stains for light microscopy kill the cells, preventing the study of living cells at high resolution.

Questions & Discussion

• Do Now Problem: Is fire alive? Use the five characteristics of living things (energy use, waste production, environmental response, reproduction, and growth) to argue for or against the classification of fire as a living organism.