1.5 The Origin of Cells

Question 1

Pre-existing cells

Answer 1

Cells can only be formed by division of pre-existing cells.
e.g. prokaryotic cells - binary fission
eukaryotic cells - mitosis (genetically identical diploid daughter cells) + meiosis (generates haploid gametes)

Question 2

Spontaneous generation

Answer 2

Cell division to form new cells from pre-existing cells replaced the concept of spontaneous generation (where cells were formed from inanimate matter)

Question 3

Pasteur’s experiments

Answer 3

Evidence from Pasteur’s experiments showed that spontaneous generation of cells and organisms does not now occur on Earth.

• Pasteur prepared nutrient broth
• Put equal amounts of broth into 2 long-necked flasks (straight neck and s shaped neck)
• Boiled both broths to kill any living matter in the liquid
• Left to sit at room temp. + exposed to air (opened mouthed flask)
• After several weeks: broth in straight necked flask was discoloured and cloudy; the s necked flask stayed the same
• The germs were able to go down the straight necked flask
• If spontaneous generation was a real phenomenon, the broth in curved necked flask would be reinfected

Question 4

Endosymbiotic Theory

Answer 4

The origin of eukaryotic cells can be explained by the endosymbiotic theory.

• there is evidence that mitochondria + chloroplasts were one primitive free-living bacteria cells
• symbiosis = 2 different species benefit from living + working together; endo = inside
• endosymbiosis: symbiosis with one specie living inside the other
• endosymbiotic theory - host cell and the bacteria ingested through endocytosis could easily become dependent on each other, resulting in a permanent relationship
• could persist indefinitely inside those cells as long as mitochondria living side the cytoplasm of the larger cell divided at the same rate
• smaller cell is provided food and protection by the larger cell while the larger cells supplied with energy from the smaller mitochondria (through aerobic respiration)
• over evolution, mitochondria and chloroplast have become more specialised and can’t live outside the cell