Light Reactions vs. Dark Reactions in Photosynthesis: What's the Difference?

Key Difference

Photosynthesis is the food making process done by the organisms possessing chlorophyllous cells. Most primarily, this food making process is associated with the plants which have chlorophyllous cells in abundance.  Photosynthesis process is a kind of the reverse of the process of respiration that takes place in each and every of the organism. It is always said that the process of photosynthesis requires sunlight to takes place while taking a closer look into the process it can be divided into two phases; light reactions and the other one is dark reactions. As their name suggests, the light reaction is the initial phase of the process of photosynthesis, and it requires light, whereas dark reaction is the later phase of the process of photosynthesis and it doesn’t require light energy. In light reaction, the photolysis of water is done, and the energy molecules like NADPH2 and ATP are formed, whereas in dark reaction reduction of the carbon takes place using the light reactions energy products NADPH2 and ATP.

Comparison Chart

Light Reactions Dark Reactions in Photosynthesis
Definition As their name suggests, the light reaction is the initial phase of the process of photosynthesis, and it requires light. Dark reaction is the later phase of the process of photosynthesis and it doesn’t require light energy.
Main Function In light reaction, the photolysis of water is done, and the energy molecules like NADPH2 and ATP are formed. In dark reaction reduction of the carbon takes place using the light reactions energy products NADPH2 and ATP.
Day & Night Light reactions can take place in the daytime as they require light. Dark reactions can take place can take place 24-hours a day.
Place of Reaction Light reactions take place in the grana of the chloroplast. Dark reactions take place in the stroma of the chloroplast.
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What are Light Reactions?

Light Reactions are the initial phase of the process of photosynthesis, these type of reactions require light for taking place. As the light reactions happen at the initial stage, they produce the complementary products that are further used in the next phase, which is a light independent phase (dark reactions).  The light reactions take place in the grana of the chloroplast amid the presence of the light; it can either be the sunlight or the artificial light. The sunlight is said to be a huge source of energy, so such reactions take place best under sunlight. Two main or the necessary functions done in this phase are the photolysis of water and formation of energy molecules like NADPH2 and ATP.  In the process of photolysis of water, the water molecule is oxidized (loses electrons) and gets split into two hydrogen atoms or protons and an oxygen atom. Following this breakdown of water, under the light, the formation of energy molecules like NADPH2 and ATP takes place. The products gained from this phase are even used in the light independent reactions, though at the further formation it doesn’t require light energy.

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What are Dark Reactions in Photosynthesis?

Dark Reactions is the second phase in the process of photosynthesis following the light reaction. As the name suggests, it doesn’t require light energy to take place. The dark reactions take place in the stroma of the chloroplast; it mainly consists of two types of cyclic reactions Calvin Cycle or C3 Cycle and Hatch-Slack or C4 cycle. The main function happens in this phase is the reduction of carbon dioxide to form carbohydrates. The sugar molecules are synthesized using the carbon dioxide molecules. The carbon dioxide is fixed to carbohydrates using the energy molecules like NADPH2 and ATP. This process as complete is called the carbon fixation process. In this phase, the products from the light reactions phase are taken, and the reduction of carbon dioxide takes place without the presence of light. This phase of photosynthesis can even occur in the night time, whereas light reactions take place in the presence of sunlight.

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Light Reactions vs. Dark Reactions in Photosynthesis

  • As their name suggests, the light reaction is the initial phase of the process of photosynthesis, and it requires light, whereas dark reaction is the later phase of the process of photosynthesis and it doesn’t require light energy.
  • In light reaction, the photolysis of water is done, and the energy molecules like NADPH2 and ATP are formed, whereas in dark reaction reduction of the carbon takes place using the light reactions energy products NADPH2 and ATP.
  • Light reactions can take place in the daytime as they require light, on the other hand, dark reactions can take place can take place 24-hours a day.
  • Light reactions take place in the grana of the chloroplast while dark reactions take place in the stroma of the chloroplast.

Explanatory Video

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