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Origin of Cells

for 1.5 in the new IB Biology syllabus

Angela De Jong

on 25 September 2017

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Transcript of Origin of Cells

The Origin of Cells
packaging of these molecules into membranes with an internal chemistry different from the surrounding environment
all living things are made of one or more cells... this is the first postulate of the cell theory
Requirements for Spontaneous Generation
non-living synthesis of simple organic molecules
amino acids, simple sugars, etc.
assembly of these molecules into macromolecules
origin of self-replicating molecules that make inheritance possible
DNA & RNA make information storage possible
The first cells must have arisen from non-living material.
spontaneous generation of life!
1. All living things are made up of cells.
2. Cells are the basic units of structure and function in living things.
3. Living cells come only from other living cells.
The Cell Theory
Stanley Miller and Harold Urey investigated spontaneous generation in 1953. They set up an apparatus similar to this:
Miller-Urey Experiment
By continuously boiling and condensing the water, the mixture was kept circulating.
Miller-Urey Experiment
Miller & Urey hypothesized that the mixture of gases they'd used (no oxygen!!) resembled the atmosphere of the early Earth. The spark simulated lightning.
When this experiment was repeated with the addition of carbon dioxide, a greater variety of organic molecules were formed.
At the end of a week, several amino acids and other organic molecules were present.
methane, ammonia, water vapour & hydrogen gas
Pasteur's Experiment
Louis Pasteur designed an experiment to test whether sterile nutrient broth could spontaneously generate microbial life.
To do this, he set up two experiments.
In both, Pasteur added nutrient broth to flasks, bent the necks of the flasks into S shapes, and then boiled the broth to kill any existing microbes.
We know that cellular membranes are phospholipid bi-layers with proteins. Their main function is to separate the inside of the cell from the outside environment, and to control the passage of materials into and out of the cell.
Origin of Membranes
Alexander Oparin suggested that different types of coacervates might have formed in Earth's primordial sea and subsequently been subject to natural selection, leading to the origin of Life.
The microspheres accumulated substances inside themselves and attracted lipids to the outside, forming a lipid-protein layer.The combination of these microspheres with ribozymes would have formed very primitive cells. It is believed that this occurred 3.8 b.y.a..
In 1957, Sidney Fox heated amino acids in the absence of water. The amino acids polymerized into protein chains. When water was added and the mixture cooled, small, stable microspheres (a.k.a. coacervates) were formed.
= the origins of eukaryotic cells
Endosymbiotic Theory
In 1966, Lynn Margulis proposed that mitochondria were once independent organisms. These pre-mitochondria were engulfed by a bacterium (by phagocytosis) and formed a symbiotic relationship with the host cell, receiving food from the host, and supplying the host with ATP. Chloroplasts formed when a cell that had mitochondria engulfed an autotrophic cell.
chloroplasts and mitochondria have a double membrane, which would have been formed by the process of phagocytosis
chloroplasts and mitochondria have their own DNA and ribosomes
ribosomes and DNA in chloroplasts and mitochondria are very similar in structure to bacterial ribosomes and DNA
Evidence for the Endosymbiotic Theory
In the early Earth, there was very little oxygen in the atmosphere. Most of it was locked up in compounds. One such compound, water, can be split by UV radiation, which could account for the minute quantities of oxygen in the atmosphere at that time.
What about oxygen?
Around 2.5 billion years ago, after prokaryotes had lived for 2 billion years, a form of chlorophyll appeared in bacteria that allowed photosynthesis, which we know releases oxygen gas.
The rising oxygen levels allowed for certain changes to the Earth's chemistry:
breakdown of compounds into oxidised mineral sediments and carbon dioxide
formation of the ozone layer, which filtered out UV radiation and stopped production of the more noxious compounds
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