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Notes on Life, Cell Theory, and the Scientific Method

What is life?

  • 1) Order: Living things have a highly organized, complex structure. Ascending order of complexity; one level of biological structure builds on the one before it:

    • atoms → molecules → organelles → cells → tissues → organs → organ systems → organism → population → ecosystem → … → biosphere

    • Example labels shown in transcript: POPULATION, ORGANISM, ORGAN, BIOSPH, TISSUE, CELL, CELL, ORGANELLES, MOLECULE

  • 2) Reproduction (replication): A living thing has the ability to make a copy (exact or near exact) of itself. Involves transmission of genetic material (DNA) from parent to offspring.

    • François Jacob: the "dream of a bacterium is to become two bacteria."

  • 3) Growth + development: Living things have the ability to increase in size and change in appearance and abilities over the period of its lifespan.

  • 4) Metabolism (energy processing): Living things have the ability to capture and transform energy and matter via chemical processes that break down (catabolic) and build (anabolic) molecules. Metabolism is the sum of all chemical processes that occur within an organism.

  • 5) Homeostasis (regulation): All living things are capable of maintaining relatively stable physical and chemical conditions. Regulatory mechanisms maintain them within tolerable ranges.

  • 6) Response to environmental stimuli: Living things show sensitivity to surroundings.

  • 7) Evolutionary adaptation: Living things have the ability to change over time in response to the environment and interactions with other organisms. The theory of evolution is a core theory in Biology.

  • 8) Cellular organization: All living things are composed of one or more cells. Cellular life includes Archaea, Bacteria and Eukarya; non-cellular life may include viruses, viroids, prions, etc.

4 common properties that all cells share (Cell Theory)

  • 1) They are bounded by a thin, flexible, selective barrier called the plasma membrane.

  • 2) They contain DNA, the heritable material that stores and transmits biological information.

  • 3) They have cytoplasm (the fluid portion = cytosol plus all other material bounded by the plasma membrane and excluding the nucleus).

  • 4) They contain ribosomes, the sites of protein synthesis.

Cell Theory: overview and examples

  • All organisms are made of cells. (Schleiden & Schwann, 1839 – Plant and animal cells)

  • All cells come from pre-existing cells. (Rudolph Virchow, 1858; Louis Pasteur experiments 1864 in historical context)

  • The cell is the smallest structural unit capable of performing all the activities of life.

  • Bacteria size note: about 1/5000 millimeters, listed as (2 × 0.5 microns) in transcript; i.e. roughly a few micrometers in length.

  • Sequoia trees can exceed 100 meters tall; a “1 mm” scale landmark equals 1000 μm.

  • Relative scale provided in transcript: e.g., HUMAN HAIR 50–180 μm; FINE BEACH SAND 90 μm; GRAIN OF SALT 60 μm; WHITE BLOOD CELL 25 μm; GRAIN OF POLLEN 15 μm; DUST PARTICLE 10 μm; RED BLOOD CELL 7–8 μm; RESPIRATORY DROPLETS ~several μm (illustrative scale).

Prokaryotes vs Eukaryotes

  • Prokaryotic cells:

    • Prefixes: Pro = before; karyotic = kernel/nucleus

    • DNA is not separated from the rest of the cell in a nucleus

    • Very few cytoplasmic organelles; fewer cell types

    • Domains: Bacteria and Archaea

  • Eukaryotic cells:

    • Prefix: Eu = true

    • DNA is contained within a nucleus

    • Many cytoplasmic organelles and many cell types exist

    • All organisms except bacteria and archaea are eukaryotic

How to do science: the scientific method (major steps)

1) Initial observations and research (literature review)
2) Hypothesis + prediction(s) -> details on next slide
3) Experiment or Observational Study and Results

Theories in biology

  • The Cell Theory: addressed by multiple lines of evidence; cells are the basic unit of life; all organisms are made of cells; all cells come from pre-existing cells.

  • The Chromosomal Theory of Inheritance: inheritance is tied to chromosomes.

  • The Theory of Evolution: explains the origins and diversification of life; core theory in biology.

The cell theory: deeper details

  • The cell is the smallest structural unit capable of performing all activities of life.

  • An organism is any living entity composed of one or more cells.

  • Bacteria size: about 1/5000 millimeters (listed as 2 × 0.5 microns).

The growth and historical experiments underpinning cell theory

  • Pasteur experiments (straight-necked vs swan-necked flasks) demonstrated that sterilization and containment of microorganisms prevented growth, arguing against spontaneous generation and supporting biogenesis:

    • (a) Pasteur experiment with straight-necked flask: nutrient broth placed in a straight-necked flask, boiled to sterilize, then observed for growth.

    • (b) Pasteur experiment with swan-necked flask: nutrient broth placed in a swan-necked flask, boiled to sterilize, and sterility maintained due to the curved neck preventing microbial entry.

    • Conclusion: no cells originated from air in the swan-necked setup; cells arose only from pre-existing cells.

  • Virchow (1858) and Pasteur (1864) contributed to the understanding that all cells come from pre-existing cells.

Relative sizes and scale in biology

  • 1 mm = 1000 μm; 1 μm = 1000 nm. These relations help interpret the sizes of cells and subcellular structures.

  • Example scales from transcript:

    • HUMAN HAIR: 50–180 μm

    • FINE BEACH SAND: ~90 μm

    • GRAIN OF SALT: ~60 μm

    • WHITE BLOOD CELL: ~25 μm

    • GRAIN OF POLLEN: ~15 μm

    • DUST PARTICLE: ~10 μm

    • RED BLOOD CELL: ~7–8 μm

Key connections and implications

  • The eight properties of life connect to the experimental goals of biology education: understanding how organisms maintain order, grow, reproduce, metabolize energy, regulate internal states, respond to stimuli, evolve, and maintain cellular organization.

  • Cellular organization underpins all higher levels of structure (tissues, organs, organ systems, organisms, populations, ecosystems, biosphere).

  • The distinction between prokaryotic and eukaryotic cells informs domains of life, cellular complexity, and the presence of a nucleus and organelles, with broad implications for genetics, development, and disease.

Symbols, notations, and quick references

  • DNA: Deoxyribonucleic acid; hereditary material that stores and transmits biological information.

  • Metabolism: Sum of all chemical processes that occur within an organism, including catabolic (breaking down) and anabolic (building) pathways.

  • Homeostasis: Regulation of internal conditions to maintain stability.

  • Cytoplasm: The interior of the cell excluding the nucleus; includes cytosol and organelles.

  • Ribosomes: The molecular machines where protein synthesis occurs.

Connections to real-world relevance

  • Cell theory underpins modern medicine, microbiology, and biotechnology by explaining why cells are the fundamental units of life and targets for therapies.

  • Understanding evolution and inheritance explains patterns of disease, heredity, and adaptation to changing environments.

Quick glossary

  • Atom, Molecule, Organelle, Cell, Tissue, Organ, Organ System, Organism, Population, Ecosystem, Biosphere

  • Prokaryote, Eukaryote

  • Nucleus, Cytoplasm, Ribosome, Plasma membrane

  • Catabolic, Anabolic

  • Biogenesis vs Spontaneous Generation