Light Reactions vs. Dark Reactions in Photosynthesis

The two terms getting discussed in this article are light and dark photosynthesis reactions, and they have several differences that a reasonable person cannot detect on their own. They have their meaning and working, and that makes for an interesting read. The main difference between all such types gets explained in the following ways. The light-dependent reactions use light energy to make two molecules needed for the next stage of photosynthesis: the energy storage molecule ATP and the reduced electron carrier NADPH. Dark reactions make use of these organic energy molecules (ATP and NADPH). This response cycle is also called Calvin Benison Cycle, and it occurs in the stroma.

Comparison Chart

What is Light Reaction in Photosynthesis?

The light-dependent reactions use light energy to make two molecules needed for the next stage of photosynthesis: the energy storage molecule ATP and the reduced electron carrier NADPH. In plants, the light reactions take place in the thylakoid membranes of organelles called chloroplasts. In photosynthesis, the light-dependent reactions take place on the thylakoid membranes. The inside of the thylakoid membrane is called the lumen, and outside the thylakoid membrane is the stroma, where the light-independent reactions take place. The thylakoid membrane contains some integral membrane protein complexes that catalyze the light responses. There are four major protein complexes in the thylakoid membrane: Photosystem II (PSII), Cytochrome b6f complex, Photosystem I (PSI), and ATP synthase. These four compounds work together to ultimately create the products ATP and NADPH. The two photosystems absorb light energy through pigments—primarily the chlorophylls, which are responsible for the green color of leaves. The light-dependent reactions begin in photosystem II. When a chlorophyll a molecule within the reaction center of PSII absorbs a photon, an electron in this molecule attains a higher energy level. Because this state of an atom is very unstable, the electron is transferred from one to another molecule creating a chain of redox reactions, called an electron transport chain (ETC). The electron flow goes from PSII to cytochrome b6f to PSI. In PSI, the electron gets the energy from another photon. The final electron acceptor is NADP. In oxygenic photosynthesis, the first electron donor is water, creating oxygen as a waste product. In anoxygenic photosynthesis, various electron donors are used. They take more time than other reactions and hence, only occur during day time.

What is Dark Reaction in Photosynthesis?

Dark reactions make use of these organic energy molecules (ATP and NADPH). This response cycle is also called Calvin Benison Cycle, and it occurs in the stroma. ATP provides the energy while NADPH provides the electrons required to fix the CO2(carbon dioxide) into carbohydrates. Photosynthesis starts out using the energy from sunlight to get things started, but it ends with the dark reactions, which don’t need the sunshine to complete sugar production. In the Calvin cycle, ATP and NADPH from the light reactions are used to produce sugars. Photosynthesis in plants takes place in chloroplasts. Photosynthesis includes light-dependent reactions and reactions that are not directly energized by light. In the photosynthetic light reactions, the energy of light is conserved as “high energy” phospho- anhydride bonds of ATP, and as reducing the power of NADPH. The proteins and pigments responsible for the photosynthetic light reaction get associated with the thylakoid (grana disk) membranes. The light reaction pathways will not be presented here. The Calvin Cycle, earlier designated the photosynthetic “dark reactions” pathway, is now referred to as the carbon reactions pathway. In this path, the free energy of cleavage of ~P bonds of ATP, and reducing the power of NADPH, are used to fix and reduce CO2 to form carbohydrate. Enzymes and intermediates of the Calvin Cycle are in the chloroplast stroma, a compartment somewhat analogous to the mitochondrial matrix. These reactions only occur at night time, and hence, get the name.

Key Differences between Light Reactions and Dark Reactions in Photosynthesis

1. The light-dependent reactions use light energy to make two molecules needed for the next stage of photosynthesis: the energy storage molecule ATP and the reduced electron carrier NADPH. Dark reactions make use of these organic energy molecules ATP and NADPH and this response cycle is also called Calvin Benison Cycle, and it occurs in the stroma.
2. The light reaction in photosynthesis always takes place in the grana of the chloroplasts. On the other hand, the dark reactions always take place in the stroma of the chloroplasts.
3. Since the light reactions take place during the daytime, they require the processes such as photosystem 1 and photosystem 2. On the other hand, since the dark reactions do not require any light, they do not have the requirement of photosystems.
4. In the process of light reactions, the photolysis of water occurs and hence, oxygen gets released due to the activities going on. On the other hand, the process of dark reaction, the process of photolysis does not take place and carbon dioxide gets absorbed during the activities.
5. NADPH and ATP are produced during the light reactions, that help in performing other activities and become the basis of dark reactions. On the other hand, NADPH gets reduced and glucose gets produced during the dark reactions.