Abiogenesis
Living organisms are self-sustaining, meaning they are able to maintain the conditions needed to continue their existence
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These conditions include: metabolism, reproduction, sensitivity (responsiveness), homeostasis, excretion, nutrition and growth (movement)
According to the cell theory, the cell is the smallest unit capable of self-sustaining life (they can undertake all the functions of life)
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Smaller units, such as viruses, are not considered living because they cannot carry out all life functions independently
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Viruses lack metabolism and must rely on metabolic events in a host cell to generate its component parts
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A virus can therefore not reproduce autonomously and must infect a cell in order to replicate
Viral Infection
Scanning EM
Transmission EM
Spontaneous Generation
Cells are highly complex structures that can currently only be produced via the division of pre-existing cells
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However the first cells must have spontaneously arisen from non-living material (abiogenesis)
This process is theorised to have occurred over four key stages:
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Catalysis: Simple organic molecules were synthesised from primordial inorganic molecules
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Self-Assembly: More complex polymers were constructed from these simple organic molecules
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Self-Replication: Certain polymers formed the capacity to be duplicated (enabling inheritance)
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Compartmentalisation: These molecules became packaged into membranes with unique internal chemistry
The theory of abiogenesis has been difficult for scientists to test for a number of reasons:
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The exact conditions on pre-biotic Earth no longer exist and cannot be replicated under controlled settings
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The first protocells did not form fossils and so there is limited evidence on which to base hypotheses