Earth has been bathed with light from the Sun ever since; it is our most important source of energy. Sunlight warms us, causes weather patterns, allows plants to manufacture oxygen and our food from carbon dioxide and water, and it allows us to find our way around in the daytime!

There are two end products of the light reaction of photosynthesis, ATP and NADPH. These molecules are produced during the cyclic and the non-cyclic photophosphorylation reactions.

The exergonic (def) light-dependent reactions of photosynthesis convert light energy into chemical energy, producing ATP and NADPH. These reactions occur in the thylakoids of the chloroplasts.

The light-dependent reactions convert light energy into chemical energy, producing ATP and NADPH.

The overall function of light-dependent reactions, the first stage of photosynthesis, is to convert solar energy into chemical energy in the form of NADPH and ATP, which are used in light-independent reactions and fuel the assembly of sugar molecules.

In the light-dependent reactions, energy from sunlight is absorbed by chlorophyll and that energy is converted into stored chemical energy. In the light-independent reactions, the chemical energy harvested during the light-dependent reactions drives the assembly of sugar molecules from carbon dioxide.

In light – dependent photosynthesis, the photosynthetic pigment absorbs the light energy and convert it into chemical energy, ATP and NADPH2 . So the light-dependent reaction should occur before the light-independent reaction.

The process, known as the Calvin cycle, uses products of the light-dependent reactions (ATP and NADPH) and various enzymes. Therefore, the light-independent reaction cannot happen without the light-dependent reaction.

There are three phases to the light-independent reactions, collectively called the Calvin cycle: carboxylation, reduction reactions, and ribulose 1,5-bisphosphate (RuBP) regeneration. Though it is called the “dark reaction”, the Calvin cycle does not actually occur in the dark or during night time.

In fact, the light independent reactions occur at the same time as the light dependent reactions, because they are driven by the ATP made by the light-dependent reactions. Both systems shut down in the dark. The light-independent reactions can last a little longer if there is ATP remaining in the chloroplast.

chloroplast stroma

The Calvin cycle is also called the dark reactions or light-independent reactions because it’s the part that doesn’t need energy from the sun to happen. So although the cycle itself doesn’t use light energy, it does depend on the light reactions to provide it with the ATP and NADPH.

The energy from ATP (produced during the light reactions) drives the dark reactions of photosynthesis. The term dark reactions does not mean the reactions happen at night or that they require darkness. It means that the reactions can proceed regardless of the amount of light available.

The Calvin cycle is fueled by products from the light reaction, but doesn’t need light. Therefore it is called the dark reaction. Note that both stages are interdependent (see image).

The end product of the Calvin Cycle is a 3-C GP3 molecule and two 3-C G3P molecules must combine to create one 6-C glucose which is two spins around the Calvin Cycle. Where does the ATP AND NADPH used in the Calvin Cycle come from? It is produced in the light reactions of photosynthesis.