1.5 Origin of Cells 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 Nature of science: Testing the general principles that underlie the natural world: the principle that cells only come from pre-existing cells needs to be verified Applications: Evidence from Pasteur’s experiments that spontaneous generation of cells and organisms does not now occur on Earth

1880s Theory cells only produced by division of pre-existing cells Cells in body formed this way Trace our cells back to sperm and egg Sperm and egg made by division in parents Trace back through the whole of history to earliest cells on earth Origin of cells Understanding: Cells can only be formed by division of pre-existing cells

Origin of cells Must reach the first ever cell – what was before that? Unless cells arrived from somewhere else they must have arisen from non-living material How could this have happened? Complete the sheet to describe a possible sequence Understanding: The first cells must have arisen from non-living material

Endosymbiotic Theory Formation of eukaryotes from prokaryotes Mitochondria were once free-living prokaryotes Developed aerobic respiration Larger prokaryotes that could not respire aerobically took them in by endocytosis Continued to live in their cytoplasm Persisted to become the mitochondria in eukaryotes today Understanding: The origin of eukaryotic cells can be explained by the endosymbiotic theory

Endosymbiotic Theory Symbiotic relationship (both benefit) Smaller cell provided with food Larger cell provided with energy Natural selection favored cells that had developed this relationship Can also be applied to chloroplasts Understanding: The origin of eukaryotic cells can be explained by the endosymbiotic theory

Chloroplasts and Mitochondria Although unable to live independently now Evidence they once were: Own genes on a circular DNA molecule like that of prokaryotes Ribosomes same size and shape of those in prokaryotes Synthesize their own proteins Only produced by division of pre-existing mitochondria or chloroplasts Understanding: The origin of eukaryotic cells can be explained by the endosymbiotic theory

Endosymbiotic Theory Spontaneous generation of life Louis Pasteur Who is he? What did he believe? What were his experiments? When did he publish his results? What were his conclusions? Louis Pasteur: French scientist Designed a broth to test whether sterile nutrient broth could spontaneously generate microbial life No spontaneous generation of life Applications: Evidence from Pasteur’s experiments that spontaneous generation of cells and organisms does not now occur on Earth