List the periods in which there were mass extinctions, and if possible explain why they occurred

"Big Five" extinction events are
1. Ordovician-Silurian - caused by long ice age and gamma ray burst that followed.

2. Late Devonian - speculated to be caused by impact of asteroids. Caused by glaciation.

3. Permian-Triassic;
maybe one or a combination of these events. (plate tectonics, an impact event, a supernova, extreme volcanism, and the release of frozen methane hydrate from the ocean beds to cause a greenhouse effect). 4. Triassic-Jurassic;
- possible causes are gradual climate change, asteroid impact, or massive volcanic eruptions. 5. Cretaceous-Tertiary. - caused by the darkness brought about by the impact based dust. It made photosynthesis impossible and because the animals are herbivores, they starved to death.

Proteins are the workhorses of the cell, controlling virtually every reaction within as well as providing structure and serving as signals to other cells. proteins are long chains of  amino acids  , and the exact sequence of the amino acids determines the final structure and function of the protein. instructions for that sequence are encoded in  genes  . to make a particular protein, a messenger ribonucleic acid (mrna) copy is made from the gene (in the process called  transcription  ), and the mrna is transported to the  ribosome  . protein synthesis, also called  translation  , begins when the two ribosomal subunits link onto the mrna. this step, called initiation, is followed by elongation, in which successive amino acids are added to the growing chain, brought in by transfer rnas (trnas). in this step, the ribosome reads the  nucleotides  of mrna three by three, in units called  codons  , and matches each to three nucleotides on the trna, called the anticodon. finally, during termination, the ribosome unbinds from the mrna, and the  amino acid  chain goes on to be processed and folded to make the final, functional protein

Lipid because of the high percentage of carbon to carbon bonds, which store more energy than other bonds.