A2.1 Origin of Cells

The earths early atmosphere

• Contained water vapour and higher CO₂ concentrations than exist today

• Lacked free oxygen - no ozone present

• Contained ammonia and methane gases, released from constant bombardment by comets and asteroids

• Conditions created extreme weather events (electrical storms)

What was necessary to form life

• Formation of organic molecules

• Provide the structural and functional components for cells to survive and replicate

• Most biological molecules (proteins, lipids, carbs and nucleic acids) are organic molecules

How these organic compounds formed

• Lack of oxygen + high proportion of reducing gases

  → Earth had a reducing atmosphere

• The reducing gases in the atmosphere would have been able to donate electrons to other molecules

• This enabled chemical reactions to take place, resulting in the formation of more complex carbon compounds

• Modern conditions on earth are very different, and would not allow spontaneous formation of biological molecules

development of carbon compounds formed in the early earth

• Organic molecules that spontaneously formed joined together to form proteins, lipids and nucleic acids

• Some of theses would have developed the ability to self-replicate, and would become packaged into membranes

• This would lead to the formation of cells

Cells as the smallest unit of self sustaining life

They contain all components necessary to carry out all 8 processes of life

Processes of life

• Metabolism → chemical reactions that take place

• Response to stimuli → Responding to changes in external environment

• Homeostasis → Maintenance of constant, internal conditions

• Movement → Having some control over their place and position

• Growth → Increasing in size over a period of time

• Reproduction → Production of offspring

• Excretion → removal of metabolic waste

• Nutrition → Intake or production of nutrients

The spontaneous origin of cells

1. Simple organic molecules were formed

2. Chemical reactions were catalysed

3. Larger organic molecules (RNA and phospholipids) assembled from smaller molecules

4. Some could self replicate (RNA)

5. Formation of a membrane-bound compartment allowed a different internal environment

Miller-Urey experiment

• Provided evidence for origins of carbon compound

• They set up a closed system containing:

  1. Water → simulated ocean

  2. Gas inlet → added reducing gases (methane, ammonia and hydrogen)

  3. Electrical sparks → simulated electrical storms

Miller-Urey results + what they showed

• Organic molecules, including amino acids, were collected after a week of running the experiment

• This proved that non-living synthesis of organic molecules was possible in conditions of early earth

• Did NOT prove organic compounds did originate in this way, only that it is possible

Formation of vesicles and compartmentalization

• Formation of a membrane bound compartment is crucial to the formation of the first cells

• The physical separation enabled the cell to have a chemical environment different to the external one

• Occurred when fatty acids spontaneously merged to form a spherical bi-layer

RNA first hypothersis

1. RNA formed from inorganic sources

2. RNA replicated using ribozymes

3. RNA catalysed protein synthesis

4. Membrane compartmentalisation occurred

5. RNA produced both proteins and DNA inside the cell

6. DNA took over as the main genetic material due to being more stable

7. Proteins took over as catalytic form (enzymes) as they are more variable

Evidence to support RNA first hypothesis

• Short RNA sequences are capable of self-replication

• RNA has catalytic ability → could have acted as both genetic material and enzymes

• Ribozymes in the ribosome are still used to catalyse peptide bond formation during protein synthesis

Other origin of cells theories

• Miller-Urey hypothesis - spontaneous generation of simple organic molecules

• Metabolism first hypothesis - simple metabolic reactions formed complex molecules then cells

• Sulfur world hypothesis - first life forms based on iron-sulfur chemistry

• Lipid world hypothesis - lipid bilayers evolved before RNA, providing protective layer to encapsulate RNA

LUCA

• The last universal common ancestor

• LUCA outcompeted the other life forms leading to their extinction

• LUCA is not the first organism

Evidence for LUCA through the genetic code

Genetic code = universal

• Uses same codons to code for the same amino acids in all organisms

• Suggests evolution from a common ancestor

Evidence for LUCA through phylogenetic analysis

• Previously thought the 3 domains of life split form LUCA

• Phylogenetic analysis changed our understanding, now with the belief that bacteria and archaea arose directly from LUCA, and eukaryotes evolved much later by endosymbiosis

• Scientists found 355, ancient, conserved genes across all 3 domains presumed to be present in LUCA

Evidence for LUCA through fossils

• Scientists have studied stromatolites

• These are fossils found within rocks formed by communities of microorganisms

• Helped to understand the types of organisms that existed and the environment they lived in

What this evidence indicates about LUCA

• Existed 2.5 - 3.5 billion years ago

• Existed in hydrothermal vents - > provided minerals and energy source

• Anaerobic, fitting the lack of oxygen in early atmosphere

• Autotrophic conditions