1.5 Origin of cells

Question 1

Theory of Spontaneous Generation:

Answer 1

the formation of living organisms from non-living matter
Leaving bread or meat on the counter would ultimately give rise to maggots and other organisms that feed on it…. they just appeared!

Question 2

Theophrastus (Greek philosopher and botanist)

Answer 2

Reported that a plant called Silphium had sprung up from the soil from “nothing” (spontaneously?!)

Question 3

Aristotle

Answer 3

Wrote about insects being formed from the dew falling on leaves or from the hair, flesh, or feces of animals.

Question 4

Paracelsus (German-Swiss botanist and astrologer)

Answer 4

Quoted observations of spontaneous generation of mice, frogs, eels, from water, air or decaying matter

Question 5

Francesco Redi

Answer 5

showed that maggots only developed in rotting meat if flies were allowed to come into contact with it

Question 6

Lazzaro Spallanzani

Answer 6

boiled soup in eight containers, the sealed four and left four open to the air
organisms only grew in those left open to the air

Question 7

Louis Pasteur, first to thoughtfully apply the scientific method

Answer 7

established beyond a reasonable doubt that spontaneous generation of life does not occur

Question 8

Pasteur’s experiment method

Answer 8

Louis Pasteur designed an experiment to test whether sterile nutrient broth could spontaneously generate microbial life.

Method:
Two experiments were setup
In both, Pasteur added nutrient broth to flasks and bent the necks of the flasks into S shapes
Each flask was then heated to boil the broth in order than all existing microbes were killed.
After the broth had been sterilized, Pasteur broke off the swan necks from the flasks in Experiment 1, exposing the nutrient broth within them to air from above.
The flasks in Experiment 2 were left alone.

Question 9

Pasteur’s experiment result and conclusion

Answer 9

Results:
The broth in Experiment 1 turned cloudy whilst the broth in Experiment 2 remained clear
Microbe growth only occurred in Experiment 1

Conclusion:
Pasteur rejected the hypothesis of spontaneous generation as for growth of microbes to occur, a source of contamination was needed.

Question 10

Testing the general principles that underlie the natural world - the principal that cells only come from pre-existing cells:

Answer 10

1. Cells are highly complex structures and no mechanism has been found for producing cells from simpler subunits.
2. All known examples of growth be it of a tissue, an organism or a population, are all a result of cell division.
3. Viruses are produced from simpler subunits, but they do not consist of cells, and they can only be produced inside the host cells that they have infected.
4. Genetic code is universal each of the 64 codons (a codon is a combination of 3 DNA bases) produces the same amino acid in translation, regardless of the organism [2.7.A2]*.
   - The logical deduction is that all cells have arisen as the result of cell division from a single common ancestor.

Question 11

The first cells must have arisen from non-living material.

Answer 11

If we accept that there were times in the history of the Earth when cells did not exist then it is an obvious point that ‘The first cells must have arisen from non-living material’.

however, Some of the key problems are:

1. Non-living synthesis of simple organic molecules, e.g. sugars and amino acids
2. Assembly of these organic molecules into polymers
3. Formation of polymers that can self-replicate (enabling inheritance)
4. Formation of membranes to package the organic molecules

Question 12

Earth’s atmosphere was ‘reducing’ in the early days. It did not contain oxygen gas until after plants started photosynthesising. The atmosphere contained:

Answer 12

Hydrogen
Nitrogen
Water vapour
Methane
Ammonia
Hydrogen sulfide

The gases came from abundant volcanic activity

Question 13

These monomers mixed in the _____________–, shallow oceans laden with chemicals where it is thought that they reacted to form biological molecules

Answer 13

‘primeval soup’

Question 14

Miller and Urey tried to early earth conditions in the lab in 1953

Answer 14

They were trying to demonstrate ‘chemical evolution’, the formation of more complex molecules from simpler stock in the primeval soup

They combined the molecules from the previous page in a closed glass vessel (simulated atmosphere), they heated the water (simulated volcanic activity) and sparked electricity through the gases (simulated lightning)

Question 15

Miller-Urey experiment results

Answer 15

After a week they found:
Thirteen of the twenty naturally occurring amino acids
Around 15% of the carbon was now in organic compounds

Question 16

1. Non-living synthesis of simple organic molecules:

Answer 16

Miller and Urey recreated the conditions of pre-biotic Earth in a closed system.

These conditions included a reducing atmosphere (low oxygen), high radiation levels, high temperatures and electrical storms
Water was boiled to form vapour and then was mixed with methane, ammonia and hydrogen
The mixture of gases was exposed to an electrical discharge (sparks) to simulate lightning

The mixture was then allowed to cool and after one week was found to contain some simple amino acids and complex oily hydrocarbons
Based on these findings, it was concluded that under the hypothesised conditions of pre-biotic Earth, organic molecules could be formed

Question 17

2. Assembly of these organic molecules into polymers:

Answer 17

Miller and Urey’s experiments allowed for the formation of amino acids, but the conditions used also tended to hydrolyse bonds preventing polymers forming.

Deep-sea thermal vents
Fissures in a planet’s surface from which geothermally heated water issues. Vents are commonly found near in volcanically active areas)
Along with heat energy the vents issue a ready supply of reduced inorganic chemicals
Vents provide the right conditions and chemicals to allow organic polymers to arise!

Question 18

3. Formation of membranes to package the organic molecules

Answer 18

Experiments have shown that phospholipids natural assemble into bilayers, if conditions are correct
Formation of the bilayer creates an isolated internal environment.
The formation of an internal environment means that optimal conditions, e.g. for replication or catalysis can be maintained.

Question 19

4. Formation of polymers that can self-replicate (enabling inheritance)

Answer 19

DNA though very stable and effective at storing information is not able to self-replicate – enzymes are required
However RNA can both store information and self-replicate - it can catalyse the formation of copies of itself.
RNA molecules can catalyze reactions (ribozymes)
In ribosomes RNA is found in the catalytic site and plays a role in peptide bond formation

Question 20

Endosymbiotic theory explains

Answer 20

the existence of several organelles of eukaryotes. The theory states that the organelles (e.g. mitochondria and chloroplasts) originated as symbioses between separate single-celled organisms,

Question 21

Development of Mitochondria

Answer 21

An aerobic proteobacterium enters a larger anaerobic prokaryote (possibly as prey or a parasite)
It survives digestion to become a valuable endosymbiont*
The aerobic proteobacterium provides a rich source of ATP to its host enabling it to out-compete other anaerobic prokaryotes
As the host cell grows and divides so does the aerobic proteobacterium therefore subsequent generations automatically contain aerobic proteobacterium.
The aerobic proteobacterium evolves and is assimilated to become a mitochondrion.

Question 22

An endosymbiont is a

Answer 22

cell which lives inside another cell with mutual benefit

The development of chloroplasts would be a very similar process except the benefit to the cell would be glucose/starch instead of ATP

Question 23

Development of the Nucleus

Answer 23

A prokaryote grows in size and develops folds in it’s membrane to maintain an efficient SA:Vol
The infoldings are pinched off forming an internal membrane
The nucleoid region is enclosed in the internal membrane and hence becomes the nucleus

Question 24

The evidence supporting the endosymbiotic theory for mitochondria and chloroplasts:

Answer 24

They have their own DNA (which is naked and circular)

They have ribosomes that are similar to prokaryotes (70S)

They have a double membrane and the inner membrane has proteins similar to prokaryotes

They are roughly the same size as bacteria and are susceptible to the antibiotic chloramphenicol

They transcribe their DNA and use the mRNA to synthesize some of their own proteins.

They can only be produced by division of pre-existing mitochondria and chloroplasts.