Plastic metabolism. Photosynthesis, chemosynthesis
Autotrophs make organic substances from inorganic ones. By energy source they are phototrophs (light energy – green plants, cyanobacteria) or chemotrophs (energy from oxidising inorganic substances). The overall equation of photosynthesis is: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂. The light stage occurs in the thylakoids of chloroplasts: chlorophyll absorbs light, water undergoes photolysis releasing free oxygen, and ATP and NADPH (a hydrogen carrier) form. The dark stage occurs in the stroma: CO₂ is reduced to glucose using the energy of ATP and the hydrogen of NADPH; this stage needs no light directly but depends on the products of the light stage. Most of the oxygen in the atmosphere has come from photosynthesis. Chemosynthesis was discovered in 1887 by S. N. Winogradsky: nitrifying bacteria oxidise ammonia to nitrites and then to nitrates; iron, sulfur and hydrogen bacteria also oxidise their substances, store the energy in ATP and make organic matter from CO₂. These bacteria matter for soil fertility and for the cycling of matter.
“Two stages” as a paper model: students draw a thylakoid and the stroma and place cards for substances entering and leaving each stage.