1. Explain the theory of endosymbiosis. What is the evidence in favour of this t
ID: 174320 • Letter: 1
Question
1. Explain the theory of endosymbiosis. What is the evidence in favour of this theory?
(4 marks) 2. What is the panspermia hypothesis? What evidence supports it?
(6 marks) 3. Compare and contrast the types of molecules transported by simple diffusion, by osmosis, and by facilitated diffusion. How are these types of movement different from active transport?
(4 marks) 4. Currently the earth’s atmosphere has more than 20% oxygen. What is the origin of that oxygen, and how has the content of the atmosphere changed over time?
Explanation / Answer
1. Endosymbiotic theory explains the origination of eukaryotic and and the acquisition of the organelles mitochondria and chloroplast by eukaryotes.
Evolution of mitochondria:
An early ancestral cell which has developed rudimentary endoplasmic reticulum and nuclear envelope by invagination of plasmamembrane accidently engulf an aerobic bacteria. Over the time the bacetria lost the ability to survive independently and thus gets incorporated as an organell (like a symbiont) helping to the host to perform aerobic respiration. This is the formation of mitochondria.
Evolution of chloroplast:
When an ancestral cell accidently engulf a photosynthetic cyanobacterium, and over many generation, it gradually lost the ability to live independently and gets incorporated like a symbiont helping the host cell to perform photosynthesis.
Evidences:
1. Presence of circular DNA in chloroplast and mitochondria which is a characteristic of bacterial DNA.
2. The ribosomes present in chloroplast and mitochondria are 70S ribosome similar to bacterial ribosomes.
3. The mitochondrial and chloroplast DNA divide self autonomously by binary fission like prokaryotes.
4. The bacterial cell, mitochondria and chloroplast have a size of around 10 micrometers.
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