8.1: Monosaccharides 8.2: Disaccharides 8.3: Polysaccharides 8.4: Amino Acids 8.5: Peptides 8.6: Proteins 8.7: Enzymes 8.8: Triglycerides 8.9: Phospholipids 8.10: Waxes 8.11: Nucleic Acids 8.12: DNA and RNA 8.13: Genetic Code 8.14: Chemistry and Energetics of the Life Process About a third of the chemical elements cycle through living organisms, which are responsible for massive deposits of silicon, iron, manganese, sulfur, and carbon. Large quantities of methane and nitrous oxide are introduced into the atmosphere by bacterial action, and plants alone inject about 400,000 tons of volatile substances (including some metals) into the atmosphere annually. 8.15: Biogeochemical Evolution Present evidence suggests that blue-green algae, and possibly other primitive microbial forms of life, were flourishing 3 billion years ago. This brackets the origin of life to within one billion years; prior to 4 billion years ago, surface temperatures were probably above the melting point of iron, and there was no atmosphere nor hydrosphere.