IB Notes
The origin of cells
* CELLS CAN ONLY BE FORMED BY DIVISION OF PRE-EXISTING CELLS.
- The trillions of cells found in our body are formed when the previous one divided in half and this can be traced back to the first cell, the zygote. The genetic material in the nucleus would be copied so both cells have a full complement of genes.
- Humans therefore come from pre-existing ancestral species, back to the earliest cells on Earth.
- Scientist case sequence of DNA of a bacterium was synthesized artificially with a few deliberate changes, then transferred to pre-existing cells of a different bacterium and converted into Mycoplasma Mycoides. This cell is not entirely new.
- It is impossible to create entirely new cells from scratch.
*THE FIRST CELLS MUST HAVE ARISEN FROM NON-LIVING MATERIAL.
- Unless cells arrived on Earth from somewhere else in the universe, the first cell must have risen from non-living material. But how can a structure as complex as the cell arise by natural means from non-living material?
- It is argues that such a complex structure cannot arise by evolution, but only in a series of stages over time.
Production of carbon compounds such as sugars and amino acids
- Stanley Miller and Harold Urey passed steam through a mixture of methane, hydrogen and ammonia (represents atmosphere). Electrical discharges used to stimulate lightning.
- Found out that amino acids and other carbon compounds needed for life were produced.
Assembly of carbon compounds into polymers
- First carbon compounds may originate from deep-sea vents. These are cracks in the Earth's surface, characterized by gushing hot water carrying reduced inorganic chemicals like iron sulphide.
- These chemicals act as a supply of energy which could be used in the assembly of carbon compounds into polymers.
Formation of membranes
-Phospholipids and amphipathic carbon compounds would naturally arrange themselves into bilayers (if they were one of the first carbon compounds).
- Bilayers form vesicles resembling the plasma membrane of a small cell and would allow different internal chemistry from that of the surroundings to develop.
Development of a mechanism for inheritance
- Living organisms have genes made of DNA and use enzymes as catalysts.
- Enzymes are needed to replicate DNA to pass on the genes.
- To make enzymes, genes are needed.
- Solution could be RNA (genetic material) which stores information in the same way as DNA but it is self-replicating and can act as a catalyst.
* THE ORIGIN OF EUKARYOTIC CELLS CAN BE EXPLAINED BY THE ENDOSYMBIOTIC THEORY.
- Endosymbiotic theory states that mitochondria were once free prokaryotic organisms that respired aerobically.
- Larger prokaryotes that could only respire anaerobically took them in through endocytosis and allowed it to stay in their cytoplasm.
- If small prokaryotes grew and divided as fast as the larger ones, they could persist inside the larger cells.
- The large organism would benefit by being able to respire aerobically and getting an efficient supply of energy. The small organism would be supplied with food. Natural selection would have favored this endosymbiotic relationship.
- After hundreds of millions of years of evolution, mitochondria has become part of a eukaryotic cell.
- The theory explains the origin of chloroplasts where a prokaryote that had developed photosynthesis would be taken in by a larger cell.
- It is allowed to grow, divide and survive inside the larger cell where both of them will benefit.
- This could have developed into the chloroplasts of the photosynthetic eukaryotes.
- Chloroplast and mitochonidria are not able to live independently anymore but have features to suggest that they once did.
- They have their own genes, on a circular DNA molecule (like prokaryotes).
- They have their own 70S ribosomes of a typical size and shape.
- They transcribe their DNA and use the mRNA to synthesize their own proteins.
- They can only be produced by division of pre-existing mitochondria and chloroplasts.
* EVIDENCE FROM PASTEUR'S EXPERIMENTS THAT SPONTANEOUS GENERATION OF CELLS AND ORGANISMS DOES NOT GROW ON EARTH.
- Louis Pasteur made a broth by boiling water containing yeast and sugar. The experiment worked with other liquids such as urine and milk.
- Many microorganisms and bacteria will grow in it if the broth is left in the open, the neck is broken or the broth is unboiled.
- Swan necks prevented organisms from the air getting into the broth and no organisms appeared spontaneously.
* TESTING THE GENERAL PRINCIPLES THAT UNDERLIE THE NATURAL WORLD. THE PRINCIPLE THAT CELLS ONLY COME FROM PRE-EXISTING CELLS NEEDS TO BE VERIFIED.
- Spontaneous generation is the formation of living organisms from non-living matter.
- Ideas of spontaneous generation were present when cells and microorganisms had not been discovered.
- Francesco Redi showed maggots only appeared in faeces if flies were allowed to come in contact with it.
- Lozzaro Spallanzani showed that organisms grew in boiled soup left in the open, but not when sealed.
- Some still thought spontaneous generation can occur if there was access to air. Louis pasteur establish that they were wrong.
- Cells only come from pre-existing cells because it is too complex. No natural mechanism has been suggested for producing cells from simpler sub-units.
- No example is know of increases in the number of cells in a population, organism or tissue without cell division occuring.
- Viruses are produced from simpler subunits but they do not consist of cells and they can only be produced inside the host cells that they have infected.
