D1 Origins of Life on Earth

Question 1

Describe four processes needed for the spontaneous origin of life on Earth

Answer 1

The synthesis of simple organic molecules
The assembly of these organic molecules into polymers
The formation of polymers that can self-replicate, enabling inheritance
Packaging of these molecules into membranes with an internal chemistry different from their surroundings

Question 2

Outline the experiments of Miller and Urey into the origin of organic compounds

Answer 2

• Miller and Urey recreated the conditions of pre-biotic Earth in a closed system
  
  • These conditions included a low-oxygen atmosphere, high radiation levels, high temperatures and electrical storms
  • Water was boiled to form vapour and then was mixed with methane, ammonia and hydrogen to form a reducing atmosphere
  • 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 3

State that comets may have delivered organic compounds to Earth

Answer 3

• Comets contain a variety of organic molecules (~20% of a comet’s tail is comprised of organic material)
  
  • Heavy bombardment about 4,000 million years ago may have delivered both organic compounds and water to early Earth

Question 4

State the conditions required for the synthesis of organic molecules

Answer 4

Organic synthesis requires the presence of a range of inorganic molecules and an energy source to combine them into organic forms

Question 5

Discuss possible locations where conditions would have allowed for the synthesis of organic compounds

Answer 5

• Hydrothermal vents - Regions of the ocean floor where superheated water is released from the Earth’s crust
  
  • Volcanoes - Intensive volcanic activity could provide sufficient thermal activity either on land or under the seabed
  • Extra-terrestrial locations - Other planets (e.g. Mars) may have been subjected to appropriate conditions (compound transfer via meteorites)

Question 6

State two properties of RNA that would have allowed it to play a role in the origin of life

Answer 6

RNA canself-replicate

RNA can act as acatalyst

Question 7

Outline how RNA’s self-replicating properties would have allowed it to play a role in the origin of life

Answer 7

• One molecule can form a template for the production of another molecule, following the rules of complementary base pairing
  
  • Can store information in the form of a base sequence

Question 8

Outline how RNA’s catalytic properties would have allowed it to play a role in the origin of life

Answer 8

• Could have taken the role which is now carried out by proteins (enzymes)
  
  • Modern cells use RNA catalysts (called ribozymes) to remove introns from mRNA and help synthesise new RNA molecules

Question 9

Outline the theory of protobionts as preceding living cells

Answer 9

• Living cells may have been preceded by protobionts
  
  • Protobionts are aggregates of abiotically produced organic molecules surrounded by a membrane or membrane-like structure.
  • A membrane creates an internal chemical environment different to the surroundings, which could lead to optimal conditions for life activities.

Question 10

Outline how prokaryotes to the creation of an oxygen-rich atmosphere

Answer 10

• Primordial Earth had a reducing atmosphere that contained very low levels of oxygen gas
  
  • After about ~2 billion years of prokaryotic life, a form of chlorophyll evolved in prokaryotes
  • This allowed for photosynthesis to occur, creating oxygen gas as a by-product via the photolysis of water

Question 11

Outline the result of the creation of an oxygen-rhc atmosphere ~2 billion years ago

Answer 11

• The breakdown of chemicals in the atmosphere and oceans to produce oxidised compounds (e.g. CO2)
  
  • The evolution of organisms capable of performing aerobic respiration
  • The formation of an ozone layer which restricted UV radiation to allow for the proliferation of a wider range of life forms

Question 12

Discuss the endosymbiotic theory for the origin of eukaryotes

Answer 12

• An endosymbiont is a cell which lives inside another cell with mutual benefit
  
  • Eukaryotic cells are believed to have evolved from aerobic prokaryotes that were engulfed by endocytosis
  • Mitochondria and chloroplasts are suggested to have originated by endosymbiosis

Question 13

State three pieces of evidence for the endosymbiotic theory

Answer 13

• Mitochondria and chloroplasts have their own DNA which is naked and circular (like prokaryotes)
  
  • Mitochondria and chloroplasts have 70s ribosomes
  • Mitochondria and chloroplasts are roughly the same size as bacteria