Lichtunabhängige Reaktion / Calvin-Zyklus / Dunkelreaktion [Fotosynthese, 2/2] -[Biologie,Oberstufe]

The video introduces the light-independent reaction, also called the Calvin Cycle, secondary reaction, or dark reaction. This metabolic process, carried out by all photosynthetically active organisms like plants, algae, and some bacteria, aims to synthesize glucose, a carbohydrate used to build other essential organic molecules.

The light-independent reaction is the second step of photosynthesis, preceded by the light reaction which converts light energy into chemical energy stored in ATP and NADPH. These two molecules are crucial for the dark reaction to synthesize glucose. The term 'light-independent' can be misleading because the production of ATP and NADPH is directly dependent on sunlight, making the entire process indirectly dependent on light. The reaction occurs in the stroma of chloroplasts.

The Calvin Cycle consists of three main stages: carbon fixation, reduction and sugar synthesis, and the regeneration of the CO2 acceptor molecule, RuBP. Atmospheric CO2 is absorbed by plants through stomata and must be chemically bound, or 'fixed', to synthesize carbohydrates.

The CO2 acceptor molecule is ribulose-1,5-bisphosphate (RuBP). In the first reaction step, CO2 is transferred to RuBP. The enzyme rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase), the most abundant enzyme on Earth, catalyzes this CO2 fixation. This forms an unstable C6 compound, which quickly splits into two molecules of 3-phosphoglycerate (3-PGA), a stable C3 compound.

3-PGA is then reduced in a two-stage reaction to glyceraldehyde-3-phosphate (GAP). This reduction is an energy-intensive process requiring ATP, whose energy was harvested during the light reaction. The energy from ATP phosphorylates 3-PGA, creating 1,3-bisphosphoglycerate. Subsequently, NADPH, also from the light reaction, transfers electrons and a hydrogen proton to 1,3-bisphosphoglycerate, reducing it to GAP. A phosphate group is cleaved off, and NAD+ and ADP are regenerated and can be reused in the light reaction.

GAP (triose phosphate) has two main fates. About one-sixth of it is used for sugar synthesis, such as glucose (a hexose with six carbon atoms) or fructose, and can be used to form larger sugar molecules. The majority of GAP, however, is used to regenerate the RuBP acceptor.

The regeneration of RuBP from GAP involves a complex series of reactions, initially leading to the synthesis of RuMP (ribulose monophosphate), which has only one phosphate group. The final conversion to RuBP requires ATP, which provides the energy and the additional phosphate group.

To produce one glucose molecule (C6H12O6), six molecules of CO2 must be incorporated. This requires six turns of the Calvin Cycle. This energy-intensive process consumes 18 ATP molecules and 12 NADPH molecules in total. The full equation for photosynthesis can then be summarized.

The term 'light-independent reaction' is misleading because the reactions are not entirely independent of light. They rely on ATP and NADPH from the light reaction. Furthermore, light directly stimulates the Calvin Cycle by altering the pH in the stroma, activating specific enzymes involved in the cycle. Light-induced electron transport also causes chemical changes in Calvin Cycle enzymes, activating them.

All green plants utilize the Calvin Cycle, but some have evolved variants or additional reactions to adapt to specific climates. For instance, plants in warmer environments face a challenge: they need open stomata to fix CO2 for the Calvin Cycle but must keep them closed at high temperatures to prevent excessive water loss through transpiration. This leads to specialized adaptations in C4 and CAM plants, allowing them to perform photosynthesis effectively despite these challenges.