1.5: Origin of cells Flashcards
The first cells must have arisen from non-living material
Non-living synthesis
The theory that living cells arose from non-living matter
This process is theorised to have occurred over four key stages:
There was non-living synthesis of simple organic molecules (from primordial inorganic molecules)
These simple organic molecules became assembled into more complex polymers
Certain polymers formed the capacity to self-replicate (enabling inheritance)
These molecules became packaged into membranes with an internal chemistry different from their surroundings (protobionts)
Miller Urey Experiment
Miller and Urey recreated the conditions of early Ear(Figure 2) by including a reducing atmosphere (low oxygen) with high radiation levels, high temperatures and electrical storms. After running the experiment for a week, some simple amino acids and complex oily hydrocarbons were found in the reaction mixture. This experiment proved that non-living synthesis of simple organic molecules was possible.
Biogenesis
describes the principle that living things only arise from other living things by reproduction (not spontaneous generation)
Pasteur’s experiment of biogenesis
Broths were stored in vessels that contained long tubings (swan neck ducts) that did not allow external dust particles to pass
The broths were boiled to kill any micro-organisms present in the growth medium (sterilisation)
Growth only occurred in the broth if the flask was broken open, exposing the contents to contaminants from the outside
From this it was concluded that emergent bacterial growth came from external contaminants and did not spontaneously occur
The endosymbiotic theory
The endosymbiotic theory explains the origin of eukaryotic cells.
This theory supports the idea that mitochondria and chloroplasts were themselves prokaryotes that were taken in by larger prokaryotes by endocytosis. Instead of being digested and broken down, these cells remained inside the host cells. Cells that could carry out aerobic respiration and hence provide energy to their host cell (which were probably anaerobes who did not need oxygen) evolved into mitochondria. Prokaryotic cells that could convert light energy to chemical energy (probably cyanobacteria) became chloroplasts and passed on sugars produced during photosynthesis to the host cell.
Evidence for endosymbiosis
Membranes (double membrane-bound)
Antibiotics (susceptibility)
Division (mode of replication)=binary fission
DNA (presence and structural composition)
Ribosomes (size)=70S
