Dark and light reactions are the process of Photosynthesis. Both Dark and light reaction of photosynthesis undergoes reduction and oxidation. In the reduction process, CO₂ is transformed into glucose, and in the oxidation process, oxygen is removed in the presence of H₂O. Dark reaction is completed in three steps (carboxylation, reduction, and regeneration). The pigment chlorophyll is found in the chloroplast of plant cells. So, the dark and light reactions are the two major processes that occur in the chloroplast.

Difference between Light and Dark Reaction

The main differences between Light and Dark Reactions are given below:

What is Light Reaction?

The light-dependent reactions of photosynthesis occur in the thylakoids, in the little compartments of the chloroplasts that contain pigment. Photosystems, which are protein-pigment complexes, absorb light. A collective stack would be a granum, multiple stacks would be grana.

In the light-dependent reactions, light is captured and water (which is a reactant in the photosynthesis equation) is “split.”It means H₂0 is split to get electrons, protons, and oxygen. So, oxygen is also a product of light-dependent reactions. The light-dependent reactions also produce ATP and NADPH. Both the ATP and the NADPH will be needed for the next process: the light-independent reactions, also known as the Calvin Cycle or Dark Reaction.

Also Read: Photosynthesis

Equation of light reaction:

2H₂O + 2NADP⁺ + 3ADP + 3Pi → O₂ + 2NADPH + 3ATP

What is Dark reaction?

The dark reaction of photosynthesis is a light-independent reaction, explained by Melville Kelvin. It occurs in the presence or absence of light. In this reaction, carbon dioxide enters and is taken in through pores that are often but not always on the bottom of leaves, and those pores are called stomata. Plants can open and close their stomata.

The carbon dioxide gas enters the stomata and the CO₂ is be fixed here. Fixed, means that with the additional help of a major enzyme, the inorganic carbon dioxide is changed to a more usable organic form. The ATP that comes from the light-dependent reactions will act as an energy currency for the Calvin Cycle.

Dark reaction is also known as the Calvin cycle, and it is completed in three steps that are:

• Carboxylation: The enzyme rubisco catalyzes this reaction, which produces 3PG (3-phosphoglycerate).
• Reduction: Consumes the ATP and NADPH produced by photosynthetic electron transport to convert the 3-carbon acid to triose phosphate.
• Regeneration: Rubisco works as an oxygenase at high temperatures and low CO₂ concentrations, resulting in photorespiration. The removal of rubisco reduces net CO₂ fixation.

Equation of dark reaction:

6CO₂ +18ATP+12NADPH + H⁺ → C₆H₁₂O₆+ 6H₂O+18ADP+18Pi+12NADP⁺

Also Read:

• Photosynthetic Pigments
• Cyclic and Non-cyclic Photo-phosphorylation
• Difference Between Photosynthesis And Respiration

FAQs on Dark and Light Reactions

1. What are the 4 Steps of the Light Reaction?

Light-dependent reaction involves four important stages – absorption of light energy, splitting of water molecules, release of oxygen, and formation of energy-carrying molecules – ATP and NADPH.

Q2: What is the Dark or Light Independent Reaction?

Dark reaction is also called carbon-fixing reaction. It is a light-independent process in which sugar molecules are formed from the carbon dioxide and water molecules. It occurs in the stroma of the chloroplast.

Q3: Where do Light and Dark Reactions Occur?

Light reactions occur in grana thylakoids whereas dark reactions occur in stroma of chloroplasts.

Q4: Where did Dark Reaction take Place?

Dark reaction takes place in the stroma of the chloroplasts. In this phase of photosynthesis, sunlight is not required.

Q5: What is the Difference Between the Dark and Light Reaction?

The light reaction is the initial stage of photosynthesis which traps light energy to produce ATP and NADPH. Dark reaction is the second step of photosynthesis which utilizes the energy from ATP and NADPH to produce glucose.