1: Origins of Cells

Question 1

What is ‘spontaneous generation’?

Answer 1

life could appear in non-living material

Question 2

Is there any evidence for spontaneous generation? What is the only way cells can be formed on Earth today?

Answer 2

• no

- division of pre-existing cells

Question 3

What can the principle that cells can only be formed from pre-existing cells be used for?

Answer 3

used to trace life back to its origins

Question 4

How are all the cells in a human/ other multicellular organism formed?

Answer 4

by repeated cell division, starting with a single cell formed by reproduction

Question 5

What should happen if we trace cell life back through time? Why?

Hence, what is one of the biggest questions in biology?

Answer 5

• we must reach the first cells
• because life has not always existed on earth
• to understand how the first living cells evolved from non-living matter and why spontaneous generation could take place then but not now.

Question 6

Give a ‘remarkable fact’, a piece of evidence, that suggests strongly that all life evolved from the same original cells. What is this piece of evidence called?

Answer 6

• 64 codons of the genetic code have the same meanings in the cells of all organisms, apart from minor variations
• universality of the genetic code

Question 7

Who verified the principle that cells only come from pre-existing cells beyond reasonable doubt? When?

Answer 7

• Louis Pasteur

- 1862

Question 8

Outline Louis Pasteur’s famous experiment.

Answer 8

• placed samples of broth in swan-necked flasks
• boiled broth in some flasks to kill any organisms present, but left others unboiled to act as controls
• fungi and other organisms soon appeared in unboiled flasks, but not boiled ones, even after long periods of time
• boiled broths were in contact with air, which it had been suggested was needed for spontaneous generation, yet no spontaneous generation had occurred
• he then snapped the neck of the boiled broth flasks: soon organisms were apparent and they decomposed the broth
• he concluded that swan necks prevented organisms getting into the flasks and that no organisms appeared spontaneously

Question 9

What is symbiosis?

Answer 9

two organisms living together

Question 10

What happens in endosymbiosis?

Answer 10

• larger cell takes in smaller cell by endocytosis
• smaller cell inside a vesicle
• smaller cell not digested, but kept alive because it performs a useful function for the larger cell
• smaller cell divides at least as often as larger cell, so all cells produced by division of larger cell contain one or more of the smaller cells inside its vesicle

Question 11

According to the endosymbiotic theory, how many times did endosymbiosis occur in the origin of eukaryotic cells? Outline all the cases where two organisms merged to become one.

Answer 11

at least twice:

1 - nucleus (anaerobic host) + aerobic bacterium = heterotrophic eukaryotes
- aerobic bacterium became mitochondria

2 - heterotrophic eukaryotes + photosynthetic bacterium = autotrophic photosynthetic eukaryotes
- photosynthetic bacterium became chloroplast

Question 12

What does the endosymbiotic theory explain?

Answer 12

the characteristics of mitochondria and chloroplasts

Question 13

What are the characteristics of mitochondria and chloroplasts that can be explained by the endosymbiotic theory?

Answer 13

“how…

1. they grow and divide like cells
2. they have a naked loop of DNA, like prokaryotes
3. they synthesize some of their own proteins using 70s ribosomes, like prokaryotes
4. they have double membranes, as expected when cells are taken into a vesicle by endocytosis