IB Biology Topic 1.5 The origin of cells

Evidence that cells can only be formed by pre-existing cells

-No mechanism has been found for producing cells from simpler subunits and cells are highly complex -All known examples of growth are the result of cell division -Viruses are produced from simpler subunits, but do not consist of cells, they can only be produced inside the host cells that they have infected. -Genetic code is universal each of the 64 combinations of 3 DNA bases produces the same amino acid in translation, regardless of organism

Evidence for endosymbiosis and mitochondria and chloroplasts

-They are about the same size of most bacterial cells - Divide by fission, as do most bacterial cells -Divide independently of the host cell -Have own ribosomes allowing production of own proteins -Have their own DNA, which more closely resembles the DNA of prokaryotic than of eukaryotic cells -Have two membrane on their exterior, which is consistent with an engulfing process.

Three parts of cell theory

1. All organisms are composed of one or more cells 2. All cells come from pre-existing cells 3. Cells are the basis of life

The assembly of these organic molecules into polymers (evidence for how first cells may have arisen)

Around deep sea vents, possible production site of first carbon compounds. The cracks in the Earth's surface, characterised by gushing hot water carrying reduced chemicals such as a iron sulphide. These chemicals represent readily accessible supplies of energy, a source of energy for the assembly of these carbon compounds into polymers.

Louis Pasteur

French scientist that proved, among other things, that the emergent growth of bacteria in nutrient broths is due to biogenesis, not spontaneous generation.

Miller-Urey Experiment

In 1953, they conducted an experiment to simulate the conditions on Early Earth and successfully showed that organic molecules could be produced from inorganic molecules.

Formation of membrane to package the organic molecules

Naturally, phospholipids or other amphipathic carbon compounds would naturally assemble into bilayers. Form vesicles resembling the plasma membrane of a small cell. Would allow different internal chemistry from that of the surroundings to develop.

Non-living synthesis of simple organic molecules (evidence for how first cells may have arisen)

Steam through a mixture of methane, hydrogen and ammonia. Mixture was thought to be representative of early atmosphere of Earth. Electrical discharges were used to simulate lightning. Found that amino acid and other carbon compounds needed for life were produced.

Organelles and endosymbiosis

The endocytosis of smaller cells by larger cells that could only respire anaerobically resulted in the prokaryote cell allowing the smaller prokaryote (which could aerobically respirate) to continue to live in their cytoplasm. As long as the smaller cell divided as fast as the larger one, they could persist indefinitely. This led to emergence of mitochondria inside eukaryotic cells today. Also explains origin of chloroplasts. Prokaryote that had developed photosynthesis taken in by larger cell, allowed to survive, grow and divide, it could have developed into the chloroplasts of photosynthesis eukaryotes. Again, both of the organisms in the endosymbiotic relationship would have benefited.

primordial soup

The mix of chemicals believed to be existing on Early Earth

Formation of nucleus (endosymbiotic theory)

There would be an original ancestral prokaryotic cell. This cell would demonstrate the evolution of the nucleus as it would have an infolding plasma membrane which would further form the porous double nuclear membrane.

Formation of polymers that can self-replicate (enabling inheritance) (evidence for how first cells may have arisen)

To replicate DNA and be able to pass genes on to offspring, enzymes are needed. The solution to this may have been an earlier phase in evolution where RNA was the genetic material. It can store information in the same way as DNA but is both self-replicating and can act as a catalyst.

Evidence from Pasteur experiment refuted idea of spontaneous generation.

Under favourable conditions, the broth that was inside the flasks was in contact with air which it had been suggested was needed for spontaneous generation, yet no spontaneous generation occurred. It shows that the glass swan necks prevented organisms from air getting into the broth or other liquids and that no organisms appeared spontaneously.

Genetic code

a nearly universal sequence of nucleotides in DNA that determines the specific amino acid sequence in the synthesis of proteins.

Endosymbiotic theory

a theory that some eukaryotic organelles, such as mitochondria and chloroplasts, originated as free-living prokaryotes that invaded primitive eukaryotic cells. It also states that mitochondria were once free living prokaryotic organisms that had developed the process of aerobic cell respiration which is why these organelles perform this function in eukaryotic cells today.

Spontaneous generation

old theory that believed in the formation of living organisms from non-living matter.

Sterilization

technique used for the elimination of microbiological organisms to achieve a sterile microbial environment.