Fig. 1.

Simplified overview of an algal cell in the light, showing the processes in which oxygen and energy in the form of ATP are either consumed or produced. In the chloroplast, light is fixed (1), yielding O₂, NADPH, and ATP. These are needed for the fixation of carbon dioxide by Rubisco into glyceraldehyde 3-phosphate (GAP) (2). GAP can be regenerated into Ribulose 1,5-bisphosphate using ATP (3) or can be transported to the cytosol (4) to be converted into building blocks for biomass (5). The oxygenase activity of Rubisco can also fix O₂, forming glycolate (6). This process is called photorespiration. Energy is consumed to convert glycolate into 3-phosphoglycerate (3-PGA) and eventually into GAP (7), so it can enter the central carbon metabolism. During this process, CO₂ and ammonium () are ‘lost’ and need to be re-fixed elsewhere in the metabolism, costing more energy. Energy in the form of ATP is yielded through the glycolysis and TCA cycle (8). Electrons are carried via NADH and FADH₂ to the electron transport chain located in the membrane of the mitochondria (9), yielding more ATP by taking up O₂