Chloroplasts are sites of solar energy absorption and subsequent transduction into chemically usable forms. Splitting water molecules and developing a proton motive force of sufficient magnitude to drive ATP synthesis are energy-intensive processes. Consequently, photosynthetic organisms evolved with dual photosystems that work cooperatively and sequentially to extract sufficient quantum energy from parcels of absorbed photons to generate a sufficiently strong electrochemical potential gradient to synthesise the relatively stable, high-energy compounds ATP and NADPH. Such metabolic energy sustains cycles of photosynthetic carbon reduction (PCR) where CO2 is initially assimilated by one of three photosynthetic pathways, namely C3, C4 or CAM, but eventually fixed via a PCR cycle within the stromal compartment of chloroplasts. These photosynthetic pathways are described in the following chapter. Section 2.1 describes C3 photosynthesis, Section 2.2 presents C4 photosynthesis and other photosynthetic modes, and Section 2.3 covers photorespiration.
Thermodynamically, the net outcome of photosynthetic energy transduction must be viewed as long-term storage of energy in the form of a product pair, namely free oxygen and reduced carbon (organic matter), rather than as separate molecules. Plants themselves or indeed any heterotrophic organisms subsequently retrieve such energy via metabolic ‘combustion’ of the organic matter where enzyme-catalysed reactions bring this pair of products together again in the process known as mitochondrial respiration. This is described in Section 2.4.