The Origin of cells Subtopic1.5

There is an unbroken chain of life, from the first cells on earth to all cells in organisms alive today T. Dobzhansky (1900 – 1975) once said, "Nothing in biology makes sense unless seen in the light of evolution."

When did the first cell come into being? We still cannot fully answer this question. There are several theories.

Scientist use both a reductionist and a systems approach – when is each approach more appropriate? The reductionist approach attempts to explain phenomena by examining the simplest components in isolation. The systems approach examines how the components of a system work together. Both the reductionist and systems approach could be appropriate depending upon the nature of the investigation.

Spontaneous Generation Spontaneous generation was first proposed by Aristotle around 384-322 BCE. Proposed animals and plants arose by spontaneous generation in soil, water and decaying matter - life originating from non-living matter.  Jean Baptist Van Helmont 1580-1640CE, believed that mice came from dirty rags and rotting grain. Francesco Redi, first to challenge the idea of spontaneous generation 1620-1690CE with the “meat maggot experiment”.

Spontaneous Generation

1.5.1 The origin of cells  Louis Pasteur (1822-1895) debunked spontaneous generation.

Louis Pasteur Lost two children to typhoid fever. Dedicated his life to cure illness. Considered the father of the germ theory – the theory that germs cause diseases, and bacteriologist.

Pasteurization

Back in brazil…

The first cells Was the first cell transported to planet earth from elsewhere in the universe? Maybe, but evidence supporting this hypothesis has not been found yet. Did the first cell come from non-living material? This hypothesis has been found to be more credible following the evidence presented by the Miller- Urey experiment.

Miller-Urey experiment The experiment ran for a week, at the end of the week some simple amino acids and complex oily hydrocarbon were found in the reaction mixture. Importance of their experiment: Non-living synthesis of simple organic molecules was possible.

Conditions for emergence of life For life to come into being, and then to persist, four conditions must be met: 1. Simple organic molecules, such as amino acids, fatty acids and carbohydrates, must be formed. 2. Larger organic molecules, such as phospholipids, RNA and DNA, must be assembled from simpler molecules. 3. Organisms reproduce, so replication of nucleic acids must have been possible. 4. Biochemical reactions require set conditions, such as pH. Therefore, self-contained structures, such as membranes, are necessary.

1.5.2 The endosymbiotic theory There is still a lot of debate on how life originated on the planet. First cells were prokaryotic in structure and appearance. The evolution of eukaryotic cells is much more established.

Endosymbiotic theory Explains the origin of eukaryotic cells. This theory supports that mitochondrion and chloroplast were themselves prokaryotes that were taken by larger prokaryotes by endocytosis. Instead of being digested and broken down, these cells were kept inside the host cells where they carried out respiration and photosynthesis respectively.

Endosymbiotic theory Cells that could carry out aerobic respiration and hence provided energy to their host cell (which were probably anaerobes) developed into mitochondria.  Prokaryotic cells that could convert light energy to chemical energy (photoautotrophs) became chloroplasts and passed on sugars produced during photosynthesis to the host cell. Prokaryotes that engulfed both aerobic and photoautotrophic cells became plant cells, while an animal cell evolved when only an aerobic prokaryote was engulfed.

evidence supporting the endosymbiotic theory. Both mitochondria and chloroplast: Have double membranes as expected in cells taken in by endocytosis. Have circular naked DNA as prokaryotes. Have 70S ribosomes as prokaryotes. Grow and divide like prokaryotic cells. Show similarities in genome to prokaryotes.

Not all organelles have a prokaryotic origin like the mitochondria and chloroplast. Endocytosis usually results in digestion of the organism.