1.5 The origin of cells

Question 1

The three main principles of cell theory:

Answer 1

1. All cells come from pre-existing cells. More specifically: the division of pre-existing cells
2. All organisms are composed of one or more cells
3. Cells are the smallest unit of life

Question 2

Spontaneous generation

Answer 2

• Prior to cell theory, spontaneous generation was largely accepted
• The theory of spontaneous generation states that living things can arise from a “vital principle” in the air.

Question 3

Louis Pasteur

Answer 3

1882-1895
- A famous french microbiologist that gave crucial evidence to support the hypothesis that cells must come from pre-existing cells. This experiment ultimately disproves the theory of spontaneous generation.

Question 4

Pasteur’s experiment

Answer 4

• Pasteur filled a swan-neck flask with broth (which was sterilized) so that the liquid could be exposed to air but not particles (as these would get trapped in the neck of the flask).
• He boiled the liquid, let it sit, and even though it was exposed to air the borth was never contaminated with bacteria and so no living thing grew showing that there aren’t any vital principles in the air that make living things arise.

Question 5

The first cells

Answer 5

The first cells must have arisen from non-living material.

Question 6

Miller-Urey experiment

The first cells

Answer 6

• They simulated conditions of early earth in a closed system and tested the chemical origin of life under those conditions
• A reducing atmosphere (low oxygen levels) with high radiation levels, high temperatures, and electrical storms.
• After running the experiment for one week, some simple amino acids and complex oily hydrocarbons were found in the reaction mixture.
• The Miller-Urey experiment showed that organic compounds could arise from inorganic compounds.

Question 6

The endosymbiotic theory

Answer 6

The theory of endosymbiosis helps explain the evolution of eukaryotic cells by looking at the origin of mitochondria and chloroplasts.
- About 2 million years ago a bacterial cell took up residence inside a prokaryotic cell through endocytosis
- Instead of being broken down, both cells formed a symbiotic/mutualistic relationship in which both organisms lived in contact with each other.
- The bacterial cell went through a series of changes to ultimately become a mitochondrion (or a chloroplast)
- The prokaryote helped the bacteria (either a mitochondria or chloroplast) by providing protection while the bacteria provided the eukaryote with ATP.

Question 7

Evidence supporting the endosymbiotic theory

Answer 7

Mitochondria and chloroplasts:
- Have double membranes, as expected for cells taken in by endocytosis
- Have their own 70s ribosomes
- Have their own genetic material (circular naked DNA)
- Divide by binary fission like prokaryotes
- Are susceptible to antibiotics (indicates they have bacterial origins)