endosymbiotic theory (lecture 3/4)

Question 1

what is endosymbiotic theory?

Answer 1

(margulis, 1966)

• endosymbionts evolved into organelles
• e.g. chloroplasts and mitochondria

Question 2

what marks an organelle from an endosymbiont?

Answer 2

• organelles have reduced genome
• most of their cells transferred to host cell genome
• transport mechanisms for essential resources like proteins synthesised by host cell
• host cells control cell division

Question 3

what are key features of mitochondria and plastids?

Answer 3

mitochondria and plastids:

• formed through process similar to binary fission, a feature of prokaryotes
• surrounded by 2+ membranes, the innermost is structurally different to others e.g. has specific transporters
• contain dna different to nucleus and similar to that of bacteria e.g. small and circular
• proteins encoded in the nucleus are transported to the organelle

mitochondria:
- several enzymes and transport systems similar to bacteria

plastids:
- internal structure and biochemistry of plastids e.g. chlorophyll and thykaloids v similar to cyanobacteria

Question 4

how have endosymbiotic plastids evolved?

Answer 4

• one major primary endosymbiosis event where primordial cell captured photosynthetic cyanobacteria
• all chloroplasts descended from this
• plants essentially highly complex and specialised green algae
• secondary endosymbiotic events - diatioms
• tertiary endosymbioatic events - complex dinoflagellates

Question 5

who is in control in organelle endosymbioses? (lowe et al 2016)

Answer 5

lowe et al (2016) on paramecium busaria & their chlorella

• can grow without
• cost to having chlorella at low lights, increased growth with chlorella in high light
• can control chlorella load through spitting out or digesting
• hedges bets at low lights bc light levels more varying
• but limits chlorella load when nutrient levels are low
• nutrients control host response to endosymbiont load