Endosymbiotic Theory
Endosymbiosis
- Endosymbiosis is where one organism lives within another
- If the relationship is beneficial to both organisms the engulfed organism is not digested
- For endosymbiosis to occur one organism must have engulfed the other by the process of endocytosis
Endosymbiotic theory
- The endosymbiotic theory is used to explain the origin of eukaryotic cells. The evidence provided for this theory comes from the structure of the mitochondria and chloroplasts
- Scientists have suggested that ancestral prokaryote cells evolved into ancestral heterotrophic and autotrophic cells through the following steps:
- Heterotrophic cells:
- To overcome a small SA:V ratio ancestral prokaryote cells developed folds in their membrane. From these infoldings organelles such as the nucleus and rough endoplasmic reticulum formed
- A larger anaerobically respiring prokaryote engulfed a smaller aerobically respiring prokaryote (which is not digested)
- This gave the larger prokaryote a competitive advantage as it had a ready supply of ATP and gradually the cell evolved into the heterotrophic eukaryotes with mitochondria that are present today
- Autotrophic cells:
- At some stage in their evolution, the heterotrophic eukaryotic cell engulfed a smaller photosynthetic prokaryote. This cell provided a competitive advantage as it supplied the heterotropic cell with an alternative source of energy, carbohydrates
- Over time the photosynthetic prokaryote evolved into chloroplasts and the heterotrophic cells into autotrophic eukaryotic cells
The endosymbiotic theory - an explanation for the evolution of eukaryotic cells
Evidence to support the endosymbiotic theory
- The evidence to support the endosymbiotic theory arises from the features that the mitochondria and chloroplasts have in common with prokaryotes:
- Both reproduce by binary fission
- Both contain their own circular, non-membrane bound DNA
- They both transcribe mRNA from their DNA
- They both have 70S ribosomes to synthesise their own proteins
- They both have double membranes
Examiner Tip
Learn how the structure of the mitochondria and chloroplast support the endosymbiotic theory.
