Unit 1 notes for IB Biology HL students. I took my exams in July 2022 and got a 7 in Biology HL. The notes are a combination of information from different sources with a main focus on the IB Biology Textbook.
1.5 The origin of cells
Objectives:
❏ Understanding:
❏ Cells can only be formed by division of pre-existing cells
❏ The first cells must have arisen from non-living material
❏ The origin of eukaryotic cells can be explained by the endosymbiotic theory
❏ Applications:
❏ Evidence from Pasteur’s experiments that spontaneous generation of cells and organisms does
not now occur on Earth
❏ Nature of Science
❏ Testing the general principle that underlie the natural world: the principle that cells only come
from pre-existing cells needs to be verified
Cell division and the origin of cells:
- Cells can only be formed by division of pre-existing cells
- Theory formed since 1880
- Evidence for this hypothesis is strong
- If we accept that humans evolved from pre-existing ancestral species, we can trace the
origins of cells back through hundreds of millions of years to the earliest cells on Earth
Spontaneous generation and the origin of cells:
- Spontaneous generation = the formation of living organisms from non-living matter
- Reasons for biologists universally accepting that cells only come from pre-existing cells:
1. A cell is a highly complex structure and no natural mechanism has been suggested for producing
cells from simpler subunits
2. No example is known of increases in the number of cells in a population, organism or tissue
without cell division occurring
3. Viruses are produced from simpler subunits but they do not consist of cells and they can only be
produced inside the host cells that they have infected
Spontaneous generations and Pasteur’s experiments:
- Pasteur’s experiments proved that spontaneous generation of cells and organisms does not now occur on
Earth, showing that present conditions (temperature, pressure, light etc.) do not sustain the process
- Pasteur’s experiment:
- Make a nutrient broth by boiling water containing yeast and sugar
- If the broth was kept in a sealed flask, it remained unchanged and no fungi or other organisms
appeared
- If the pad of a cotton wool (with bacteria and spores of fungi) was placed in the broth in a sealed
flask, within 36 hours, there were a large number of microorganisms in the broth that grew over its
surface
- Most famous of Pasteur’s experiments involved the use of swan-necked flasks
- Method:
- Broths were stored in vessels that contained long tubings (swan neck ducts) that did not
allow external dust particles to pass
- The broths were boiled to kill any microorganisms present in the growth medium
(sterilisation)
, - Growth only occurred in the broth if the flask was broken open, exposing the contents to
contaminants from the outside
- From this it was concluded that emergent bacterial growth came from external
contaminants and did not spontaneously occur
- Conclusion:
1. Swan necks prevented organisms from the air getting into the flasks
2. No organisms appeared spontaneously
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