1.5 The Origin of Cells

Question 1

Origin of cells + cell division

Answer 1

• All cells come from pre-existing cells

- The first cells must have come from non-living materials

Question 2

Origin of the first cells

Answer 2

• The natural water cycle creates amino acids and other carbon compounds needed for life
• Deep sea vents created the first carbon compounds into polymers
• Amphipathic carbon compounds naturally formed membranes
• RNA was genetic material in early cell evolution as it can self-replicate and catalyse

Question 3

What is endosymbiotic theory

Answer 3

• Endosymbiotic theory: mitochondria were once free-living prokaryotes that could undergo aerobic respiration, larger cells that could only anaerobically respirate took them in by endocytosis. The larger cells didn’t kill the prokaryotes but kept them alive in their cytoplasm and they persisted over hundreds of millions of years.
• Natural selection favoured endosymbiotic cells
• Endosymbiotic theory explains chloroplasts

Question 4

Features that suggest mitochondria and chloroplast were independent prokaryotes

Answer 4

• They have genes on circular DNA molecule
• They have 70S ribosomes
• They transcribe DNA and use mRNA to synthesise proteins
• They can be produced by division of pre-existing mitochondria and chloroplasts

Question 5

What is endosymbiosis

Answer 5

• Endosymbiosis explains the evolution of eukaryotic cells

Question 6

Spontaneous generation and Pasteur’s experiments

- Evidence from Pasteur’s experiments that spontaneous generation of cells and organisms does not now occur on earth

Answer 6

Louis Pasteur made a nutrient broth by boiling
water containing yeast and sugar. He showed that
if this broth was kept in a sealed flask, it remained
unchanged, and no fungi or other organisms
appeared. He then passed air though a pad of
cotton wool in a tube, to filter out microscopic
particles from the air, including bacteria and the
spores of fungi. If the pad of cotton wool was
placed in broth in a sealed flask, within 36 hours,
there were large number of microorganisms in
the broth and mould grew over its surface.
- By using swan-necked flasks he found that air could not get in when they were twisted into different positions, concluding that without air spontaenous production of organisms was not possible.