Biology IB HL - 1.5 The Origin of Cells

The theory that living cells arose from non-living matter is known as abiogenesis

simple organic molecules

complex polymers

self-replicating polymers

molecules packaged in membranes

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)

The non-living synthesis of simple organic molecules has been demonstrated by the Miller-Urey experiment

Stanley Miller and Harold Urey recreated the postulated conditions of pre-biotic Earth using a closed system of flasks and tubes

high temperatures

reducing atmosphere

electrical discharge

Water was boiled to vapour to reflect the high temperatures common to Earth’s original conditions

The vapour was mixed with a variety of gases (including H2, CH4, NH3) to create a reducing atmosphere (no oxygen)

This mixture was then exposed to an electrical discharge (simulating the effects of lightning as an energy source for reactions)

The mixture was then allowed to cool (concentrating components) and left for a period of ~1 week

After this time, the condensed mixture was analysed and found to contain traces of simple organic molecules

The chemical processes that contributed to the initial formation of biological life required specific conditions to proceed

This included a reducing atmosphere and high temperatures (>100ºC) or electrical discharges to catalyse chemical reactions

These conditions do not commonly exist on modern Earth and hence living cells cannot arise independently by abiogenesis

Instead, cells can only be formed by the division of pre-existing cells (biogenesis)

Biogenesis describes the principle that living things only arise from other living things by reproduction (not spontaneous generation)

The law of biogenesis is largely attributed to Louis Pasteur

demonstrated that emergent bacterial growth in nutrient broths was due to contamination by pre-existing cells

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

An endosymbiont is a cell which lives inside another cell with mutual benefit

Eukaryotic cells are believed to have evolved from early prokaryotes that were engulfed by phagocytosis

The engulfed prokaryotic cell remained undigested as it contributed new functionality to the engulfing cell (e.g. photosynthesis)

Over generations, the engulfed cell lost some of its independent utility and became a supplemental organelle

Mitochondria and chloroplasts are both organelles suggested to have arisen via endosymbiosis

Membranes (double membrane bound)

Antibiotics (susceptibility)

Division (mode of replication)

DNA (presence and structural composition)

Ribosomes (size)

some organelles have double membranes (outer membrane may be vesicular in origin)

Susceptible to antibiotics - indicates organelles may have bacterial origins

Reproduction occurs via a fission-like process

has own DNA which is naked and circular like prokaryotic DNA structure

Have ribosomes which are 70s in size (identical to prokaryotic ribosomes)